Case Studies of Mesozoic Calcareous Nannofossils - Implications for Palaeoecology, Calcareous Nannofossil Morphology and Carbonate Accumulation

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Case studies of Mesozoic calcareous nannofossils - implications for palaeoecology, calcareous nannofossil morphology and carbonate accumulation

Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften der Fakultät für Geowissenschaften der Ruhr-Universität Bochum

vorgelegt von

André Bornemann aus Bochum

Bochum, November 2003

Case studies of Mesozoic calcareous nannofossils - implications for palaeoecology, calcareous nannofossil morphology and carbonate accumulation

Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften der Fakultät für Geowissenschaften der Ruhr-Universität Bochum

vorgelegt von

André Bornemann aus Bochum

Bochum, November 2003 Die vorliegende Arbeit wurde von der Fakultät für Geowissenschaften der Ruhr-Universität Bochum als Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) anerkannt.

1. Gutachter: Prof. Dr. J. Mutterlose 2. Gutachter: Prof. Dr. D. Michalzik 3. Gutachter: Prof. Dr. T. Schmitt

Tag der Disputation: 30. Januar 2004 Table of contents I

Table of contents List of figures and tables ...... IV Abstract ...... VI Kurzfassung ...... VII Acknowledgements ...... IX 1 Introduction ...... 1 1.1 Mesozoic climate ...... 1 1.2 Calcareous nannofossils ...... 2 1.2.1 The role of calcareous nannofossils in the climate system and biogeochemical cycles ...... 2 1.2.2 Use of Mesozoic calcareous nannofossils as palaeoceanographic proxies ...... 4 1.2.3 Calcification and coccolith size variability of modern and ancient coccolithophores ...... 5 1.3 Objectives ...... 6 1.4 Thesis overview ...... 7 2 Mesozoic calcareous nannofossils state of the art (J. Mutterlose, A. Bornemann & J.O. Herrle, submitted to Paläontologische Zeitschrift) ...... 9 Abstract ...... 9 2.1 Introduction ...... 9 2.2 Morphology and systematics ...... 12 2.2.1 Function of coccolith morphology ...... 15 2.3 Evolution ...... 15 2.4 Stratigraphy ...... 17 2.5 Palaeobiogeography ...... 18 2.5.1 High latitudinal assemblages ...... 20 2.5.2 Mid - low latitudinal assemblages ...... 20 2.5.3 Low latitudinal assemblages ...... 20 2.6 Palaeoecology ...... 20 2.6.1 Light ...... 20 2.6.2 Nutrients ...... 20 2.6.3 Temperature ...... 21 2.6.4 Ecological strategies ...... 22 2.7 Palaeoceanographic significance of calcareous nannofossils ...... 23 2.8 Conclusions ...... 27 Acknowledgements ...... 27 3 The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic Cretaceous boundary (A. Bornemann, U. Aschwer & J. Mutterlose, published 2003 in Palaeo- geography Palaeoclimatology Palaeoecology 199, 187-228) ...... 28 Abstract ...... 28 3.1 Introduction ...... 29 3.2 Palaeogeography and palaeoceanography ...... 30 3.3 Palaeoclimate and sea-level changes ...... 31 3.4 Structure and calcification of calcareous nannoplankton ...... 32 3.5 Materials and methods ...... 33 II Table of contents

3.5.1 Localities and stratigraphy ...... 33 3.5.2 Assemblage analysis ...... 34 3.5.3 Morphometrics ...... 35 3.5.4 Volumetrics and nannofossil carbonate accumulation ...... 37 3.6 Results ...... 41 3.6.1 Diversity and absolute abundances ...... 41 3.6.2 Assemblage composition ...... 42 3.6.3 Morphometry ...... 46 3.6.4 Calcareous nannofossil carbonate accumulation of DSDP Site 105 ...... 48 3.7 Discussion ...... 52 3.7.1 Diagenesis and methodological errors ...... 52 3.7.1.1 Preservation of calcareous nannofossils ...... 52 3.7.1.2 Potential error sources for the calculation of nannofossil carbonate and size measurements ...... 53 3.7.1.3 Estimates of linear sedimentation rates ...... 55 3.7.2 Composition of nannofossil assemblages ...... 56 3.7.3 Size variations and calcification of nannofossils ...... 58 3.7.4 The importance of calcareous nannofossils for pelagic carbonate accumulation ...... 60 3.8 Conclusions ...... 62 Acknowledgements ...... 62 4 Reconstruction of short-term palaeoceanographic changes during the formation of the Late Albian Niveau Breistroffer black shales (Oceanic Anoxic Event 1d, SE France) (A. Bornemann, J. Pross, K. Reichelt, J.O. Herrle, Ch. Hemleben & J. Mutterlose, submitted to the Journal of the Geological Society, London) ...... 64 Abstract ...... 64 4.1 Introduction ...... 64 4.2 Location, chronostratigraphy and palaeogeography ...... 67 4.2.1 Location of the studied section ...... 67 4.2.2 Stratigraphy and chronostratigraphic framework ...... 67 4.2.3 Palaeogeography ...... 67 4.3 Methods ...... 68 4.3.1 Geochemistry ...... 68 4.3.2 Calcareous nannofossils ...... 69 4.3.2.1 Calcareous nannofossil nutrient and temperature indices ...... 70 4.3.3 Palynomorphs ...... 70 4.3.4 Planktic foraminifera ...... 71 4.4 Results ...... 71 4.4.1 Geochemistry ...... 71 4.4.2 Calcareous nannofossils ...... 71 4.4.3 Palynomorphs ...... 73 4.4.4 Planktic foraminifera ...... 74 4.5 Discussion ...... 74 4.5.1 Diagenesis and microfossil preservation ...... 74 Table of contents III

4.5.1.1 Stable isotopes ...... 74 4.5.1.2 Calcareous nannofossils ...... 75 4.5.1.3 Palynomorphs ...... 76 4.5.1.4 Planktic foraminifera ...... 76 4.5.2 Reconstructing palaeoenvironmental and palaeoceanographic changes ...... 76 4.5.2.1 Surface water productivity ...... 76 4.5.2.2 Surface water temperature ...... 77 4.5.2.3 Humidity ...... 77 4.5.2.4 Surface water stratification ...... 77 4.5.3 Driving mechanisms for OAE 1d formation in SE France ...... 78 4.5.4 Supraregional palaeoenvironmental signals for OAE 1d formation ...... 81 4.5.5 Comparison between OAE 1d and OAE 1b formation in SE France ...... 82 4.6 Conclusions ...... 82 Acknowledgements ...... 83 5 Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae during the formation of the Late Albian Niveau Breistroffer (SE France): taxonomic and palaeoecological implications (A. Bornemann & J. Mutterlose, submitted to Geobios) ...... 84 Abstract ...... 84 5.1 Introduction ...... 84 5.2 Palaeogeography ...... 86 5.3 Palaeoceanography of the Late Albian OAE 1d in SE France ...... 86 5.4 Materials and methods ...... 87 5.4.1 Location, lithology and stratigraphy of the Col de Palluel section ...... 87 5.4.2 Methods ...... 89 5.4.2.1 Morphometry ...... 89 5.4.2.2 Tools for palaeoenvironmental reconstruction ...... 90 5.5 Results ...... 90 5.5.1 Size variability of Biscutum constans ...... 90 5.5.2 Size variability of Watznaueria barnesae ...... 92 5.6 Discussion ...... 95 5.6.1 Nannofossil preservation ...... 95 5.6.2 Coccolith growth pattern ...... 96 5.6.3 Taxonomic implications ...... 97 5.6.4 Palaeoecological implications ...... 97 5.7 Conclusions ...... 99 Acknowledgements ...... 99 6 Summary and perspectives ...... 100 6.1 Palaeoecology ...... 100 6.2 Calcarous nannofossil morphology ...... 100 6.3 Carbonate accumulation ...... 101 6.4 Perspectives ...... 102 References ...... 103 Tabellarischer Lebenslauf ...... 126 IV List of figures and tables

List of figures and tables (short version of the figure and table captions)

Fig. 1.1. Earth's glacial record and Mesozoic climate changes (sea-level changes, humid-arid cycles). Fig. 1.2. Environments of pelagic carbonate deposition and the role of coccolithophores for the marine carbon cycle. Fig. 1.3. Heterococcolith growth sequence of the recent coccolithophore Emiliania huxleyi. Fig. 2.1. Light-microscope and scanning electron microscope images of holo- and heterococcoliths. Fig. 2.2. Overview of ordinal-level classification of heterococcoliths and nannoliths. Fig. 2.3. Light-microscope and scanning electron microscope images of nannoliths. Fig. 2.4. Stratigraphic ranges (JurassicCretaceous) of Mesozoic calcareous nannofossil families. Fig. 2.5. Palaeobiogeography of recent and early Cretaceous calcareous nannofossils. Fig. 2.6. Palaeoecology of calcareous nannofossils of the Cretaceous. Fig. 2.7. The Nannoconus crisis of the Early Aptian. Fig. 2.8. Productivity of calcareous nannofossils during the Early Albian Ocean Anoxic Event 1b. Fig. 3.1. Palaeogeographic setting of the studied DSDP sites. Fig. 3.2. Stratigraphy and lithology of the DSDP sites investigated. Fig. 3.3. Morphometric parameters of the four studied groups of calcareous nannofossils and the corresponding volume calculations. Fig. 3.4. Reproducibility of morphometric measurements. Fig. 3.5. Calcareous nannofossils, light-microscope images. Fig. 3.6. Calcareous nannofossils, scanning electron microscope images. Fig. 3.7. Vertical distribution pattern of CaCO content and calcareous nannofossils in the Tithonian to 3 Valanginian interval of DSDP Site 105. Fig. 3.8. Vertical distribution pattern of CaCO content and calcareous nannofossils in the Tithonian to 3 Valanginian interval of DSDP Site 534A. Fig. 3.9. Vertical distribution pattern of CaCO content and calcareous nannofossils in the Tithonian to 3 Valanginian interval of DSDP Site 367. Fig. 3.10. Size variations of the genus Watznaueria during the Tithonian to lower Valanginian at DSDP Sites 105, 367 and 534A. Fig. 3.11. Size variations of nannofossil taxa, which are important for the carbonate production in the Tithonian (Conusphaera mexicana, Polycostella beckmannii, Nannoconus spp., Watznaueria spp.). Fig. 3.12. Carbonate accumulation at DSDP Site 105. Fig. 3.13. Scanning electron microscope images of sediments. Fig. 3.14. Visual estimates of different carbonate sources from slides of four samples from DSDP Site 105.

Fig. 3.15. Scatter plots of data from DSDP Site 105: bulk-rock CaCO3 vs. (a) nannofossil absolute abundance; (b) calculated nannofossil carbonate; (c) relative abundance of the studied nannolith taxa. Fig. 3.16. Scatter plot of total coccolith length of Watznaueria and the V-unit length of samples from which size measurements have been performed. Fig. 3.17. Synthesis of tectonic, climatic and biotic events for the latest Jurassic and early Cretaceous. Fig. 4.1. Palaeogeographic reconstruction of SE France for the Albian. List of figures and tables V

Fig. 4.2. Lithologic and stratigraphic framework for the Marnes Bleues Formation and the Niveau Breistrof- fer (Col de Palluel section, SE France) based on ammonites, planktic foraminifera, calcareous nannofossils, lithostratigraphy and carbon isotope data. Fig. 4.3. Age control on the formation of the Niveau Breistroffer black shales is based on a correlation of the carbon isotope record from the Blake Nose Plateau with that from the Col de Palluel section. Fig. 4.4. Results from geochemical and calcareous nannofossil analyses from the main Niveau Breistroffer succession (Col de Palluel section). Fig. 4.5. Nutrient and temperature index based on calcareous nannofossils compared to terrigenous/marine ratio of palynomorphs and the abundance patterns of nannoconids and planktic foraminifera. Fig. 4.6. Absolute abundances of non-saccate pollen versus spores for different black shales of mid-Cre- taceous age from the Vocontian Basin. (a) Niveau Breistroffer, Col de Palluel section. (b) Abso- lute spore and pollen abundances from two different mid-Cretaceous black shale events (Niveau Paquier, Niveau Breistroffer) in the Vocontian Basin. Fig. 4.7. Scatter plots showing the linear relationships and Pearson correlation coefficient between parameters potentially indicating diagenetic alteration of the sample material. Fig. 4.8. Temperature trends during the formation of the Niveau Breistroffer black shales at the Blake Nose Plateau (ODP Site 1052E) and in the Vocontian Basin (Col de Palluel section). Fig. 4.9. Model for deep water formation during periods of black shale respectively marlstone deposition of the main Niveau Breistroffer with schematic mean annual atmospheric circulation pattern for the low latitudes at insolation maximum. Fig. 5.1. Palaeogeographic reconstruction of SE France for the Albian. Fig. 5.2. Location map of the Col de Palluel section in SE France. Fig. 5.3. Lithologic and stratigraphic framework for the Marnes Bleues Formation and the Niveau Breistrof- fer (Col de Palluel section, SE France) based on ammonites, planktic foraminifera, calcareous nannofossils and lithostratigraphy. Fig. 5.4. Ultrastructure, light-microscope micrographs and the measured size parameters for Biscutum constans var. constans/B. constans var. ellipticum and Watznaueria barnesae var. barnesae/W. barnesae var. fossacincta. Fig. 5.5. Scatter plots and frequency histograms of length and width of the central R-unit and the total coccolith of the two morphospecies of Biscutum constans. Fig. 5.6. Box plots of the measured parameters of the two pairs of morphotypes of Watznaueria barnesae and Biscutum constans. Fig. 5.7. Size changes of the two studied nannofossil species during the formation of the Niveau Breistroffer. They are compared to nannofossil based nutrient/temperature indices and oxygen isotope data. Fig. 5.8. Scatter plots and frequency histograms of length and width of the central V-unit and the total coccolith of the two morphospecies of Watznaueria barnesae. Fig. 5.9. Scatter plots and frequency histograms of ellipticity versus outer rim width of the species Biscutum constans and Watznaueria barnesae. Table 1.1. Carbon pools in the major reservoirs on Earth.

Table 3.1. Overview of mean sizes, kS shape factors, volumes and weight of important nannofossil taxa. Table 5.1. Overview of simple statistical parameters for the studied species and morphotypes. Table 5.2. Varimax-rotated principal component analyses of the measured and calculated parameters. VI Abstract

Abstract In todays oceans coccolithophores are one of the most important marine primary producers. They affect the marine carbon cycle by (1) secreting calcitic plates (coccoliths), which are a major component of pelagic carbonates, and (2) by the production of organic matter during photosynthesis. Both processes may influence the oceanic and atmospheric concentrations of the greenhouse gas CO2. Therefore blooms of coccolithophores will affect both the marine carbon cycle and the climate system. Their Mesozoic ancestors, along with other tiny calcitic fossils (= calcareous nannofossils), have presumably influenced the carbon and climate system as well. For the fossil record calcareous nannofossils serve as accurate stratigraphic markers of Cenozoic and Mesozoic marine sediments. Furthermore, they respond sensitively to palaeoenvironmental changes, which allows the use of calcareous nannofossils as palaeoceanographic proxies. During the last 20 years numerous studies have been performed to investigate calcareous nannofossil taxa with respect to their ecological preferences. So far only few efforts have been made to study the morphological variability of nannofossils and to quantify their mass contribution to the carbonate accumulation. The overall objectives of this thesis are to improve our understanding of changes within the nannofossil assemblages and the size variability of selected taxa with respect to palaeoenvironmental changes on different timescales. In order to achieve this two time slices have been studied. (1) Long-term changes on the scale of several million years in the nannofossil assemblage have been investigated from three DSDP sites in the Central Atlantic across the JurassicCretaceous boundary. In addition, the morphological variability of common taxa has been studied and the carbonate mass contribution of calcareous nannofossils to the pelagic sedimentation has been estimated by applying a modern quantitative approach. The studied interval is characterized by an onset in the deposition of pelagic carbonates in the latest Jurassic. This goes along with mass occurrences of strongly calcified nannofossil taxa. At the same time these forms show an increase of size. It is believed that a sea-level fall, a cool-arid climate, oligotrophic surface water conditions and/or presumably low atmospheric pCO2 favoured high abundances and the size increase of these taxa. The opening of the Atlantic-Pacific seaway modified the oceanographic conditions and caused possibly a shift from a nannofossil assemblage dominated by large-sized forms in the mid- to late Tithonian to an assemblage, which consists mainly of small-sized, less calcified coccoliths in the earliest Berriasian. (2) Short-term palaeoceanographic changes on the scale of Milankovitch cycles (~18-100 kyr) have been reconstructed for a Late Albian black shale event in SE France (Oceanic Anoxic Event (OAE) 1d) and their possible influence on coccolith morphology has been evaluated. The palaeoceanographic reconstruction is based on calcareous nannofossil, palynomorph, planktic foraminifera and stable isotope data. According to the results black shale formation in SE France occurs under reduced surface water productivity and a warm-humid climate. It is suggested that an orbitally induced increase in monsoonal activity led to enhanced humidity during periods of black shale formation. The humidity increase caused a decrease in low-latitude deep-water formation and probably an increase in surface water stratification. The combination of both mechanisms caused oxygen consumption in the bottom water that increased the preservation potential of organic matter. Furthermore, a model for the supraregional distribution of the OAE 1d is proposed. Size variability of selected nannofossil species has been investigated in order to test whether size changes can be related to palaeoenvironmental changes and to improve the taxonomic definition of the studied taxa. Kurzfassung VII

Kurzfassung In den heutigen Ozeanen gehören Coccolithophoriden zu den wichtigsten marinen Primärproduzenten. Sie beeinflussen den marinen Kohlenstoff-Kreislauf auf zweierlei Weisen: (1) Durch die Produktion von kalzi- tischen Plättchen (Coccolithen), die eine Hauptkomponente pelagischer Karbonate darstellen, und (2) durch die Bildung von organischem Material während der Photosynthese. Beide Prozesse können die atmo- sphärischen und ozeanischen Konzentrationen des Treibhausgases CO 2 verändern. Blüten von Coccolitho- phoriden beeinflussen demnach sowohl den marinen Kohlenstoff-Kreislauf als auch das Klima-System. Ein ähnlicher Einfluss lässt sich auch für ihre mesozoischen Verwandten, oft mit anderen kalkschaligen Organis- men zu kalkigen Nannofossilien zusammengefasst, vermuten. Im Käno- und Mesozoikum stellen kalkige Nannofossilien ein wichtiges Hilfsmittel zur genauen strati- graphischen Einordnung mariner Sedimente dar. Sie reagieren außerdem empfindlich auf Änderungen der Paläoumweltbedingungen und lassen sich dadurch zu deren Rekonstruktion benutzen. In den letzen 20 Jah- ren wurden viele Studien im Hinblick auf die ökologischen Affinitäten einzelner Arten durchgeführt. Weni- ge Arbeiten haben sich jedoch bisher mit der morphologischen Variabilität von Nannofossilien beschäftigt oder haben versucht, deren Anteil an der pelagischen Karbonatsedimentation zu quantifizieren. Die überge- ordneten Ziele dieser Arbeit sind es, das Verständnis von Änderungen in den Nannofossil-Vergesellschaftungen und die Größenvariabilität ausgewählter Taxa im Hinblick auf vermutete Umweltveränderungen innerhalb unterschiedlicher Zeitskalen zu verbessern. Um dies zu erreichen wurden zwei Zeitscheiben untersucht. (1) Drei DSDP-Bohrungen im Zentral-Atlantik wurden im Hinblick auf Langzeit-Schwankungen (meh- rere Millionen Jahre) in der Zusammensetzung von Nannofossil-Vergesellschaftungen während des Jura Kreide Grenzbereichs bearbeitet. Außerdem wurden die morphologische Variabilität von häufig auftreten- den Nannofossil-Taxa untersucht und der Anteil kalkiger Nannofossilien an der pelagischen Sedimentation mit Hilfe moderner quantitativer Methoden abgeschätzt. Der bearbeitete Zeitabschnitt ist charakterisiert durch das weit verbreitete Einsetzen der pelagischen Karbonatsedimentation im obersten Jura. Dies ging einher mit dem Massenauftreten von stark kalzifizierten Nannofossilien, die zur gleichen Zeit eine Größen- zunahme aufweisen. Möglicherweise wurden das häufige Auftreten und die Größenentwicklung dieser For- men durch einen Meerespiegel-Tiefstand, kühles-arides Klima, oligotrophe Bedingungen und/oder niedri- gem atmosphärischen pCO2 begünstigt. Ein darauf folgender Wechsel zu kleineren Formen, verbunden mit einem generellen Umbruch in der Zusammensetzung der Nannofossil-Vergesellschaftungen an der Basis des Berrias, wird mit Änderungen der Ozeanografie, verursacht durch die Öffnung des Atlantik-Pazifik See- wegs, in Zusammenhang gebracht. (2) Kurzzeitige paläoozeanografische Veränderungen im Frequenzbereich von Milankovitch-Zyklen (~18-100 kJ) wurden für ein Schwarzschiefer-Ereignis in SE Frankreich (Ober-Alb, Oceanic Anoxic Event (OAE) 1d) rekonstruiert und deren möglicher Einfluss auf die Coccolithen-Morphologie untersucht. Die paläoozeanografische Rekonstruktion basiert auf Daten von kalkigen Nannofossilien, Palynomorphen, planktonischen Foraminiferen und stabilen Isotopen. Anhand der Ergebnisse wird vermutet, dass die Schwarz- schiefer-Bildung mit einer geringen Produktivität im Oberflächenwasser und warm-humiden klimatischen Bedingungen einherging. Eine Zunahme der Humidität während Phasen verstärkter monsunaler Aktivität führte vermutlich zu einem Rückgang der Tiefenwasser-Bildung in den niederen Breiten und zu einer ver- stärkten Stratifizierung des Oberflächenwassers. Dadurch wurde die Bodenwasser-Durchmischung redu- ziert und das Erhaltungspotential von organischer Substanz erhöht. Zusätzlich zu dem Modell, welches das VIII Kurzfassung regionale Auftreten dieser Schwarzschiefer erklärt wird ein weiteres für die überregionale Verbreitung des OAE 1d vorgestellt. Größen-Untersuchungen an ausgewählten Nannofossil-Arten wurden durchgeführt um zu testen, ob die Größe der Formen mit den rekonstruierten Umweltbedingungen korrespondiert und um die Taxonomie der untersuchten Taxa genauer zu charakterisieren. Acknowledgements IX

Acknowledgements I am most grateful to Prof. Jörg Mutterlose for supervising this thesis and his continuous support during the last eight years. Special thanks to Dr. Jens O. Herrle (Southampton) for his encouragement of the black shale work and numerous stimulating discussions. Dr. Jörg Pross, Dr. Oliver Friedrich, Kerstin Reichelt (all Tübingen) and Dr. Jens Lehmann (Bremen) are thanked for their valuable help during the Breistroffer project. I would like to thank Drs. Elisabetta Erba (Milan) and Emanuela Mattioli (Lyon) for helpful comments and fruitful discussions. Dr. Rolf Neuser is thanked for his support at the SEM. Dr. Paul A. Wilson (South- ampton) kindly provided the stable isotope dataset from ODP Site 1052E. Gar Esmay is thanked for his sampling assistance of DSDP core material and the organization of the pleasant stay at Lamont-Doherty Earth Observatory. Varies kind of help and support were provided by Heiko Legge, Sylvia Rückheim, Dr. Jens Steffahn, Silke Elsche, Ute Aschwer and Sabine Fesl. Thanks to you all! Above all, I warmly thank my parents and Mireille for their constant encouragement. Sample material for this thesis has been supplied by ODP (Atlantic Ocean) and Dr. Jens Lehmann (Col de Palluel section, SE France). Financial support by the German Research Foundation (DFG, MU 667/19-2, 23-1/2) and the Ruhr-University of Bochum is gratefully acknowledged. X Chapter 1: Introduction 1

1 Introduction 1.1 Mesozoic climate Throughout the Earths history climate has changed on different timescales, driven by a multitude of forcing mechanisms. These include tectonic processes working on the scale of several million years, orbitally induced changes in solar radiation on the scale of Milankovitch cycles (~18-400 kyr) or on even shorter timescales (for an overview see Ruddiman, 2001). These processes can be strengthened or weakened by internal response and feedback mechanisms caused, for instance, by concentration changes of greenhouse gases (e.g. water vapour, methane - CH4, carbon dioxide - CO2), changes in the efficiency of the albedo (ice and snow cover, cloud formation) and heat transport (oceanic and atmospheric circulation pattern). For some periods in the geological record of the Proterozoic and Phanerozoic indications of glaciations or of general cooler conditions have been documented, whereas other periods do not show any indications of cooling or glaciation (Fig. 1.1). These long-term alternations on the scale of tens to hundreds of million years have been interpreted as changes from icehouse to greenhouse climate modes and are believed to be controlled by plate tectonic processes (Fischer, 1982). The Mesozoic era (~65-250 myr) has classically been viewed as a predominantly warm, ice free interval, due to the absence of unequivocal indications of glaciation (Fig. 1.1) and the presence of tropical to subtropical faunas and floras in the high latitudes (e.g. Vakhrameev, 1991). Only in the last two decades rising evidence occurred for relative cool periods with possibly polar ice masses (e.g. Kemper, 1987a; Price, 1999) and glacio-eustatic sea-level changes (e.g. Stoll and Schrag, 1996;

Earth's glacial record Mesozoic climate changes Erathem System Indications of glaciation 0 Stage Sea level Humidity -

CENOZOIC Tertiary high low aridity System Cretaceous Maastrichtian MESOZOIC Jurassic 200 Campanian Triassic Santonian Permian Coniacian Turonian Carboniferous Cenomanian OAE 1d 400 Devonian Silurian Albian Ordovician Antarctica Aptian PALAEOZOIC Cambrian

CRETACEOUS Barremian 600 Hauterivian Sinian Valanginian Berriasian

South America Tithonian 800 Kimmeridgian Europe Oxfordian Callovian Bathonian Age (Ma) 1000 Bajocian Aalenian

Australia Toarcian Africa JURASSIC Pliensbachian North America 1200 Riphean Sinemurian PROTEROZOIC Hettangian Rhaetian Norian

1400 Carnian Ladinian

TRIASSIC Anisian 200 100 0 m

1600 Asia Scythian Humid Alter- nating Arid Fig. 1.1. Earths glacial record and Mesozoic climate changes (sea-level changes after Hardenbol et al., 1998; humid- arid cycles examplarily for western Europe), amended after Price (1999). The study intervals are marked by greyish bars. 2 Chapter 1: Introduction

2000a; Gale et al., 2002). Periods of prevailing cooler (e.g. TithonianHauterivian) or warmer climate (e.g. AptianTuronian) have been identified, which in turn are themselves presumably interrupted by warm snaps (e.g. mid-late Valanginian; Lini et al., 1992) and cold snaps (e.g. AptianAlbian boundary interval; Kemper, 1987a; Weissert and Lini, 1991). Climatic variations during the Late Jurassic and the Cretaceous seem to follow changes in volcanic activity, e.g. the formation of large igneous provinces or changes in the rates of oceanic crust production (e.g. Larson, 1991a, b), whereas rapid warmings in Earths history are often linked to the release of high amounts of CH4 (see Jenkyns, 2003). Both mechanisms, volcanic activity and methane release, led directly (volcanic exhalation) or indirectly (CH4 dissociation) to elevated atmospheric pCO2 levels and may have thereby contributed to a greenhouse climate. Apart from temperature changes variations concerning the weathering regime have been observed, and are interpreted as humid-arid cycles (Fig. 1.1; e.g. Wignall and Ruffell, 1990; Hallam et al., 1991). There is increasing evidence that changes of the climatic and palaeoceanographic conditions takes place also on shorter timescales similar to the Quater- nary record (e.g. Herrle et al., 2003a, b; Hofmann et al., 2003). For an overview of the Mesozoic climate see Price (1999) and Jenkyns (2003), and the therein given references. The oceans play an important role for the global climate system, because of their ability to store both large amounts of heat and CO2. The record of Late Quaternary sediments has shown that glacial/interglacial cycles are well coupled to changes in the carbon exchange rates within the ocean (Broecker and Peng, 1989).

Within these mechanisms CaCO3 secreting organisms such as coccolithophores influence the marine carbon cycle and the climate system (e.g. Elderfield, 2002). Furthermore, these organisms provide a useful tool to reconstruct palaeoenvironmental conditions (e.g. Kinkel et al., 2000; Erba, 1994; Mutterlose, 1996; Herrle, 2003).

1.2 Calcareous nannofossils Calcareous nannofossils [nano (Greek) = dwarf] are fossil remnants of tiny marine, algal protists. Their living representatives are named calcareous nannoplankton. The taxonomically rather heterogeneous group- ings of calcareous nannofossils and -plankton (2 to 63 µm in diameter) include among others coccolithophores, nannoliths and small calcareous dinocysts. Coccolithophores are unicellular algae, which belong to the class Prymnesiophyceae within the division Haptophyta (Green and Jordan, 1994). They are photoautotrophic and inhabit the photic zone (upper 50 to 200 m) of the oceans. This thesis focuses on two groups of calcareous nannofossils: (1) coccoliths, calcitic plates which cover the coccolithophore cell, and (2) nannoliths, biogenic calcitic structures of uncertain affinities, which are probably related to haptophyte algae (Young et al., 1999).

1.2.1 The role of calcareous nannofossils in the climate system and biogeochemical cycles Today coccolithophores contribute significantly to the primary production and are believed to influence the marine carbon cycle and the climate system (Westbroek et al., 1993). Marine biological productivity plays an important role within the global carbon cycle by the formation of organic matter during photosynthesis

(biological carbon pump). This process fixes CO2 in the upper ocean layer and acts thereby as a CO2 sink (Fig. 1.2). The formation of calcareous skeletons by biomineralization (both phyto- and zooplankton) is a short-term source of CO2 in the marine environment (Fig. 1.2). Whereas burial of calcareous skeletons into marine sediments may, however, act as a long-term sink by removing CO2 from the atmospheric and oceanic carbon reservoir (Elderfield, 2002). Moreover, the emission of DMSP (dimethylsulphoniopropionate) af- Chapter 1: Introduction 3

UV light

Albedo Atmosphere

Weathering1,2) UV light Air - sea CO2 (input of Ca2+, DMS- exchange - emission HCO3 )

Biologic formation of 3) 4) Ocean CaCO3 and Corg zone Hemipelagic Photic Upwelling sediments Sinking biogenic & Intermediate & terrigenous material deep water formation Basin to basin transport 1) CaCO3 and Corg Partial CaCO3 dissolution 5) accumulation and Corg decomposition in the water column

1) CaCO3 dissolution & Sedimentary 5) CCD Corg decomposition archive in sediments 1) Total CaCO3 dissolution in the water column Hydrothermal venting

No CaCO3 accumulation

1) 4) CaCO3 weathering/dissolution: Photosynthesis: ® 2+ - ® CaCO3 + CO2 + H2O Ca + 2HCO3 6CO2 + 6H2O C6H12O6 + 6O2

2)Silicate weathering: 5)Respiration/remineralisation: ® 2+ - ® CaSiO3 + 2CO2 + H2O Ca + 2HCO3 + SiO2 C6H12O6 + 6O2 6CO2 + 6H2O

3)Calcification: - 2+ ® 2HCO3 + Ca CaCO3 + CO2 + H2O Fig. 1.2. Environments of pelagic carbonate deposition (amended after Schneider et al., 2000) and the role of coccolithophores for the marine carbon cycle. fects the global climate system. DMSP will be converted by oxidation processes to DMS (dimethyl sul- phide), one of the most important cloud condensation nuclei and will thereby affect the efficiency of the albedo (Westbroek et al., 1993, 1994). These factors underline the importance of calcareous nannofossils for the Earths biogeochemical cycles and the climatesystem. Research on calcareous nannofossils in the past may therefore provide important clues for palaeoceanography and palaeoclimatology.

Modern oceans presumably contain up to 60-times the CO2 of the atmosphere (Broecker and Peng,

1986) and contribute thereby significantly to the global CO2 budget. The marine carbonate system interacts with atmospheric and oceanic pCO2 and is an important part of the global carbon cycle. In todays oceans marine carbonate production occurs in shallow marine settings, dominated by calcareous algae and coral reefs, and in the open ocean (e.g. Milliman and Droxler, 1996). Pelagic carbonates of the open ocean are dominated by calcareous nannoplankton (coccolithophores, calcareous dinocysts), planktic foraminifera and pteropods (e.g. Milliman, 1993; Schiebel, 2002). Sedimentary carbonates represent the largest carbon pool on Earth (Table 1.1; Falkowski et al., 2000). The contribution of different pelagic carbonate producers to the total carbonate sedimentation in modern oceans has recently been quantified and summarized by Schiebel (2002) and Baumann et al. (in press). 4 Chapter 1: Introduction

Table 1.1. Carbon pools in the major reservoirs on Earth (Falkowski et al., 2000).

According to Schiebel (2002) the total planktic foraminiferal contribution to the total CaCO3 budget amounts to 32-80%, coccoliths to 12-32% (Beaufort and Heussner, 1999; Broerse, 2000), pteropods to 10% (Fabry, 1990) and calcareous dinocysts to 3.5%. Data from surface sediments of the South Atlantic compiled by Baumann et al. (in press) suggest rather a large range for the amount contributed by the different groups. Carbonate contribution by coccolithophores and planktic foraminifera varies considerably and is most important in the center of the South Atlantic. Each of these groups can regionally make up 80 wt% of the sediment. Aragonitic pteropods contribute up to 50 wt% of the sediment in some areas of the western South Atlantic and are therefore locally important CaCO3 contributors. The importance of calcareous dinocysts is considered to be only minor (< 4 wt%). Since planktic foraminifera occur in pre-Aptian times only in low abundances and pteropods presumably evolved in the Tertiary, it is reasonable to assume that calcareous nannofossils, apart from calpionellids, have been the most important carbonate contributors to pelagic sediments in the latest Jurassic and earliest Cretaceous. But quantitative data are still missing for this time interval.

1.2.2 Use of Mesozoic calcareous nannofossils as palaeoceanographic proxies Due to their high abundance and fossilization potential, calcareous nannofossils are successfully used as biostratigraphic markers in Meso- and Cenozoic sediments (e.g. Sissingh, 1977; Thierstein, 1971, 1973; Perch-Nielsen, 1985; Bown, 1998). The application of calcareous nannofossils as palaeoceanographic proxies is a very modern issue and interpretations are mainly based on spatial and temporal changes in the nannofossil assemblage composition and abundance. In nowadays oceans vertical and horizontal distribution patterns of calcareous nannoplankton are controlled by nutrient availability (incl. trace elements, vitamins), temperature and salinity (Winter et al., 1994; Brand, 1994). Typical assemblages have been recognized to follow latitudinal gradients, ocean currents and water masses (Winter et al., 1994). In general, todays calcareous nannoplankton seem to prefer more oligotrophic, warm and stratified water masses (K-strategists), such as the oceanic gyre systems, but also r- strategists, opportunistic taxa exist which react rapidly to changes in the trophic system, e.g. in upwelling regions. Similar patterns concerning the palaeobiogeography, nutrient availability, temperature and surface water stratification have also been proposed for Mesozoic nannofossil assemblages (e.g. Roth and Krumbach, 1986; Erba, 1994; Mutterlose, 1996; Mutterlose and Kessels, 2003; Street and Bown, 2000; Lees, 2003). An Chapter 1: Introduction 5 overview and a more detailed discussion of the palaeoceanographic significance of calcareous nannofossils are given in Chapter 2.

1.2.3 Calcification and coccolith size variability of modern and ancient coccolithophores Coccolithophores are the most common living representatives of calcareous nannofossils. Thus their mechanisms for calcification and to form coccoliths are here viewed as an analogon for the studied fossil taxa. Coccolithophores develop two types of base scale plates as a cell wall covering, which are made up of low Mg-calcite (Siesser, 1977): holococcoliths and heterococcoliths. Both can be produced by the same organism and are thought to represent phase changes in a presumably haplo-diplontic life cycle (e.g. Billard, 1994; Cros et al., 2000). Holococcoliths are constructed of simple euhedral crystallites, which are only scarcely preserved in fossil sediments. They calcify in an extracellular position (Rowson et al., 1986) during a presumably haploid, motile phase (Geisen et al., 2002). The calcification of heterococcoliths, which takes place in intracellular vesicles, derives from the Golgi body and is consequently under a strong cellular control (Westbroek et al., 1984; Piernaar, 1994). This occurs during a diploid phase, where haptophytes may be motile or non- motile. For some coccospheres both coccolith types have been observed at the same time, possibly reflecting a transition between haploid-diploid phases (Geisen et al., 2002). Due to their high preservation potential heterococcoliths are the most prominent group of calcareous nannofossils in the Mesozoic. They are formed of a radial array of complex crystal units of variable shape. Calcification of this type of coccoliths starts with the nucleation of a proto-coccolith ring (PCR) of small crystals around the rim of a precursor base scale plate. Subsequently the crystals of the PCR grow by accre- tion in various directions to form the crystal units (Young et al., 1992, 1999; Fig. 1.4). According to Young et al. (1992) heterococcoliths consist of two basic crystal units, a V-unit with vertically oriented crystallographic c-axes of the calcite and an R-unit, where the c-axes are oriented radially. This V/R model is an important criterion for the taxonomic concepts of Mesozoic calcareous nannofossils (Bown and Young, 1997; Young et al., 1997).

Fig. 1.3. Heterococcolith growth sequence of the recent coccolithophore Emiliania huxleyi (adopted from Young et al., 1992, 1997). The diagram shows, how by accretionary growth, the initial simple crystals of the proto-coccolith ring develops into the morphologically complex crystal units of the complete heterococcolith. 6 Chapter 1: Introduction

A control of coccolith size, morphology and calcification by palaeoenvironmental factors is under debate. The most prominent parameters which are believed to control the calcification and/or the size of coccoliths are the temperature and the trophic conditions (e.g. McIntyre et al., 1970; Winter et al., 1994). Observations suggesting a palaeoenvironmental control of the coccolith size are sometimes contradictory and vary from species to species. Young (1990), for instance, observed large forms of the genus Reticulofenestra during cooler periods, and larger morphotypes of Emiliania huxleyi have been reported by Colmenero-Hidalgo (2002) and others during glacial times. Renaud et al. (2002) observed a large morphotype of the recent coccolithophore Calcidiscus leptoporus during spring blooms in the North Atlantic Ocean and Arabian Sea. These settings are characterized by nutrient-rich conditions and low surface water temperatures. The large morphotype of C. leptoporus seems to be also common at low latitudes in a range of intermediate to high temperatures (Knappertsbusch et al., 1997; Renaud and Klaas, 2001; Renaud et al., 2002). Most recently culture experiments of Quinn et al. (2003) suggest that the morphotypes of C. leptoporus do not represent temperature dependent ecophenotypes as suggested, for instance, by Knappertsbusch et al. (1997). A correlation of the morphology of Gephyrocapsa and seawater temperature data has been reported by Bollmann et al. (2002). According to this author larger morphotypes with a high bridge angle are more adapted to higher temperatures, while smaller forms with low bridge angles are more common under moderate temperatures. Culture studies revealed other factors which seem to affect changes of coccolith calcification. These include lower salinities (Green et al., 1998) and changes of seawater pH (Riebesell et al., 2000; Zondervan et al., 2001). Toxic metals, in particular copper and cadmium, are considered to inhibit phytoplankton calcification (Brand et al., 1986). Most studies that have yet been performed are based on cultures, plankton material, sediment traps or surface sediments (e.g. Bollmann, 1997; Knappertsbusch et al., 1997; Riebesell et al., 2000; Baumann and Sprengel, 2001; Renaud et al., 2002). For these samples environmental parameters, such as temperature, salinity and nutrients have been measured from seawater. This allows the direct evidence of a palaeoenvironmental control on coccolith morphology. The results of recent studies on coccolith morphology reveal that a rather general rule does not exist on species- or generic-level linking morphology and environmental conditions. Culture, plankton or sediment trap studies cover only very short time intervals from days to years, whereas the impact of global climatic and oceanic changes on the coccolith size can only be understood on a geological timescale. Morphometric studies on Mesozoic nannofossil taxa are rare and only few datasets are available (e.g. Mattioli and Pittet, 2002; Tremolada and Erba, 2002; Mattioli et al., subm.). They suggest that morphologic changes may go along with temperature changes and nutrient availability, or are controlled by local/regional factors. In addition, morphometric data provide additional information on the taxonomy of calcareous nannofossils, which is exclusively based on morphological criteria (morphospecies). Therefore morphometric studies are essential to improve our taxonomic concepts and to understand the driving mechanism causing the morphological variability of coccolith species. Most recently, it has been shown that size changes of Cenozoic planktic foraminifera can be used to assess palaeoceanographic conditions, such as temperature and primary productivity on different timescales (Schmidt, 2002).

1.3 Objectives This thesis focuses on changes of the assemblage composition of calcareous nannofossils and the size variability of selected nannofossil taxa with respect to palaeoceanographic changes in two case studies. (1) Long- Chapter 1: Introduction 7 term changes on the scale of several million of years have been studied at three DSDP sites from the Atlantic Ocean which cover sediments of the JurassicCretaceous boundary interval (Tithonian to Valanginian). (2) Short-term variations on the scale of Milankovitch cycles (18-100 kyr) have been investigated from a Late Albian black shale event in SE France. This allows us to evaluate the impact of different oceanographic driving mechanisms, acting on different timescales, on the nannofossil assemblage composition and the morphology of selected nannofossil taxa. In addition, the application of calcareous nannofossils as proxies for palaeoceanographic reconstructions should be tested. The JurassicCretaceous boundary in the western Central Atlantic has been studied. This period is characterized by an arid and cool climate at least in the mid and high latitudes (e.g. Wignall and Ruffell, 1990; Price, 1999; see Fig. 1.1). Changes in the oceanographic conditions can be proposed from the opening of a deep Atlantic-Pacific seaway (e.g. Winterer, 1991) and a long-term sea-level lowstand (e.g. Haq et al., 1987). Sedimentologically, the latest Jurassic was characterized by an onset in the deposition of pelagic carbonates which went along with a radiation of calcareous nannofossils (e.g. Roth, 1986, 1989). During this crucial interval changes in the nannofossil assemblage have been studied to gain additional information about the prevailing palaeoceanographic conditions. The size changes of important taxa have been analyzed to test whether a link between the increase in carbonate accumulation, the palaeoenvironmental changes and the nannofossil morphology can be drawn. The development of new techniques allowed to gain quantitative data and gave way for new approaches to evaluate the importance of this phytoplankton group for the pelagic carbonate sedimentation. The second case study focuses on the Late Albian oceanic anoxic event (OAE) 1d in SE France. This black shale event has been studied in co-operation with scientists of the University of Tübingen in order to reconstruct short-term palaeoceanographic changes on the scale of Milankovitch cycles in SE France and to develop a model for its regional and supraregional distribution. Furthermore, size variability of two selected coccolith taxa on the species-level has been studied in order to test whether (1) size changes can be related to palaeoenvironmental changes and (2) to gain additional information on their taxonomic definition.

1.4 Thesis overview Apart from the introduction (Chapter 1) the thesis consists of five further chapters. The Chapters 2 to 5 comprise four manuscripts, which have been submitted for publication or published in international scientific journals. In Chapter 2 (Mesozoic calcareous nannofossils state of the art by J. Mutterlose, A. Bornemann and J.O. Herrle; submitted for publication to Paläontologische Zeitschrift) the state-of-art of palaeontological and palaeoecological aspects of Mesozoic calcareous nannofossils is reviewed. The author of the present thesis is in charge of the morphology/taxonomy section and has also contributed to other parts of the article. Chapter 3 (The impact of calcareous nannofossils on the pelagic carbonate accumulation across the JurassicCretaceous boundary by A. Bornemann, U. Aschwer and J. Mutterlose; published 2003 in Palaeo- geography Palaeoclimatology Palaeoecology 199, 187-228) presents a study of calcareous nannofossils from the JurassicCretaceous boundary from three DSDP sites in the Central Atlantic Ocean. Modern quantitative techniques have been applied to the nannofossil preparation and morphometric studies have been performed. Although volume and mass estimates of the most common nannofossil taxa are presented. This part of the thesis discusses long-term changes in the composition of nannofossil assemblages, the size of common nannofossil groups and the contribution of calcareous nannofossils to the pelagic carbonate accumu- 8 Chapter 1: Introduction lation. This research paper is written by the author of the thesis. He has contributed nannofossil counts of

DSDP Site 105, the complete datasets of morphometric measurements, their statistical evaluation and CaCO3 measurements. In the study presented in Chapter 4 (Reconstruction of short-term palaeoceanographic changes during the formation of the Late Albian Niveau Breistroffer black shales (Oceanic Anoxic Event 1d, SE France) by A. Bornemann, J. Pross, K. Reichelt, J.O. Herrle, Ch. Hemleben and J. Mutterlose; submitted for publication to the Journal of the Geological Society, London) calcareous nannofossils are used to reconstruct short- term palaeoceanographic changes on the scale of Milankovitch cycles of a short-termed black shale event in SE France, named Niveau Breistroffer (OAE 1d, Late Albian). In order to test both the validity of the observed signals and the presumed ecological preferences of the different microfossil taxa, the results of different micropalaeontological and geochemical proxies are compared to each other. Furthermore, a model explaining both the regional and supraregional occurrence of the OAE 1d black shales is presented. This research paper has been written by the author of the thesis who tied together the different types of data. He has gained the presented nannofossil counts and the geochemical data of the studied Niveau Breistroffer black shale interval. In Chapter 5 (Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae from the Late Albian Niveau Breistroffer (SE France): taxonomic and palaeoecological implications by A. Bornemann and J. Mutterlose; submitted for publication to Geobios) the palaeoecological and palaeoceanographic interpretation of the Niveau Breistroffer (Chapter 4) is compared to the herein gained morphometric results of two common nannofossil taxa in order to test whether the coccolith morphology is controlled by short-term changes of the palaeoenvironmental conditions. In addition, the taxonomy of the studied taxa is discussed. The author of the thesis is in charge of the morphometric measurements and has written the article. In Chapter 6 the results of the thesis are summarized. Furthermore, an outlook of future nannofossil studies with respect to palaeoenvironmental change, size variability and carbonate accumulation is given. The manuscripts (Chapters 2-5) have been amended to achieve a uniform format of the thesis and errors have been corrected. Taxonomy of calcareous nannofossils follows standard literature (Perch-Nielsen, 1985; Bown, 1998 and references therein). Sample material and nannofossil slides are housed at the Ruhr-Universität Bochum. Chapter 2: Mesozoic calcareous nannofossils state of the art 9

2 Mesozoic calcareous nannofossils state of the art

Jörg Mutterlose1, André Bornemann1 and Jens O. Herrle2

1Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany 2Geological Institute, ETH-Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland (submitted for publication to Paläontologische Zeitschrift)

Abstract Calcareous nannofossils originated in the Triassic, radiated in the Jurassic and became a dominant component of the marine biosphere from the latest Jurassic onward. They can be considered as one of the most important inventions of the Mesozoic oceans. Their basic morphology allows the differentiation of three different groups: coccoliths, nannoliths and calcispheres (= calcareous dinocysts). Only coccoliths and nannoliths are discussed in this article in some detail. For two fields coccoliths and nannoliths have provided most useful information helping in re-interpreting Mesozoic marine systems: 1. biostratigraphy, 2. palaeoecology/palaeoceanography. Ever since the late 1960s both coccoliths and nannoliths have proven to be useful and reliable zonal markers for biostratigraphic schemes, allowing detailed zonations for both the Jurassic and Cretaceous. Though affected by palaeobiogeographic provincialism, coccoliths and nannoliths have supplied many cosmopolitan biostratigraphic markers. These allow a global correlation of marine sedimentary units both from on-shore sections in the classical European and North American areas and pelagic sequences recovered in the course of the DSDP/ODP programme from the worlds oceans. Thus research on calcareous nannofossils covers both, regional and global aspects. Palaeoecological and palaeoceanographic problems, deciphered by calcareous nannofossils, have been a research topic for the last 15 years, with an ever increasing interest in this group as primary producers in Mesozoic oceans. Apart from dinoflagellates, coccoliths were the most important primary producers in Mesozoic oceans. As such they heavily relied on autecological factors such as light, nutrients, temperature and others. Variations in the assemblage composition of these groups may thus be viewed as a key for understanding palaeoecological, palaeoceanographic and palaeoclimatic changes of the past.

Keywords: calcareous nannofossils; coccoliths; nannoliths; Jurassic; Cretaceous; evolution; stratigraphy; palaeoecology; palaeoceanography

2.1 Introduction Interest in calcareous nannofossils goes back to Ehrenberg (1836, 1840, 1854), who was the first to describe and figure small calcareous elliptical discs from the Cretaceous of the island of Rügen (north Germany). These were believed to be of inorganic origin. Huxley (in Dayman, 1858) created the term coccolith for very curious rounded bodies found in sea floor samples from the Atlantic. He also believed in an inorganic nature of these bodies. Wallich (1861) found coccoliths forming a minute sphere, which he named coccosphere and considered these to be larvae of planktic foraminifera. Based on the presence of flagellae it was Lohmann (1902) who classified coccospheres with chrysonomad algae. A more detailed account of these pioneer days of nannofossil research is given by Siesser (1994). After these and other ground breaking studies a second phase of research started in the late 19th century 10 Chapter 2: Mesozoic calcareous nannofossils state of the art which prevailed into the 1960s. Research of this phase mainly concentrated on two aspects. Oceanographic expeditions recovered plenty of material resulting in biological studies (e.g. Ostenfeld, 1900; Lohmann, 1902). It was also Lohmann (1909) who created the term nannoplankton for small plankton, which passed through the mesh of the phytoplankton net (63 µm). Other studies concentrated on the taxonomy of coccoliths describing both living and fossil taxa (e.g. Arkhangelsky, 1912; Schiller, 1930; Kamptner, 1952; Deflandre, 1952, 1959; Braarud, 1954). It is only with the beginning of the DSDP programme in 1968, that a third phase of research started. The need for fast and reliable biostratigraphic dating allowed the usage of calcareous nannofossils as zonal markers. Various biostratigraphic zonation schemes were developed for the Cenozoic (e.g. Hay and Mohler, 1967; Martini, 1971; Okada and Bukry, 1980) and the Mesozoic (e.g. Thierstein, 1971, 1973, 1976; Sissingh, 1977; Roth, 1978). These were summarised in most useful compilations by Perch-Nielsen (1979, 1985). Ever since then more detailed zonation schemes have been proposed (e.g. Bralower, 1987; Bralower et al., 1989; Mutterlose, 1992; Bown et al., 1998; Jeremiah, 2001). Starting in the 1980s calcareous nannofossils became more and more viewed as extremely useful palaeoecological proxies, helping in deciphering palaeoecological, palaeoceanographic and palaeoclimatic questions (e.g. Roth and Bowdler, 1981; Roth and Krumbach, 1986; Mutterlose, 1987; Watkins, 1986, 1989; Erba, 1991, 1994; Erba et al., 1989, 1992; Williams and Bralower, 1995; Eshet and Almogi-Labin, 1996; Mutterlose, 1992; Mutterlose and Ruffell, 1999; Mutterlose and Kessels, 2000; Street and Bown, 2000; Herrle, 2003; Herrle et al., 2003a, b; Bornemann et al., 2003). This resulted in the current research efforts where calcareous nannofossils supply important clues for surface water temperature, nutrient availability and primary productivity. Recent research efforts also try to understand the role of coccoliths and nannoliths for the global carbon cycle and carbonate budgets (e.g. Bornemann et al., 2003; Erba and Tremolada, subm.). These important developments have not been recepted in most on-shore based palaeontological groups, which still concentrate on classical aspects of palaeontology. Comprehensive overviews of palaeontological aspects of calcareous nannofossils are given by Haq (1978), Brasier (1980), Houghton (1991) and Siesser (1993). The rapid progress of the last 10 years in micropalaeontology with respect to palaeoceanography and palaeoecology requires an update of the state-of-art of this group of marine primary producers. Without a good understanding of this phytoplankton group (and other planktic organisms) we are not able to understand Mesozoic and Cenozoic marine systems, geosphere biosphere interactions (e.g. the role of calcareous nannofossils for the carbon cycle and changes of ocean chemistry) or the evolution of marine biota. The importance of calcareous nannofossils for the marine system is underlined by the fact that the recent coccolithophore Emiliania huxleyi is probably the most abundant eucaryotic species of the marine realm.

Fig. 2.1. Holococcoliths. (1) Orastrum cf. dispar (Varol in Al-Rifaiy et al.) Bown in Kennedy et al., XPL, Barremian; (2) O. cf. dispar, SEM, Barremian; (3) O. cf. dispar (lateral view), XPL, Barremian; (4) O. cf. dispar (lateral view), SEM, Barremian. Heterococcoliths. Eiffelithales: (5) Eiffellithus monechiae Crux, XPL, Albian; (6) Rhagodiscus asper (Stradner) Reinhardt, XPL, Barremian; (7) R. asper, SEM, Aptian; (8) Zeugrhabdotus erectus (Deflandre in Deflandre and Fert) Reinhardt, XPL, Tithonian; (9) Z. cf. erectus, SEM, Aptian. Stephanolithiales: (10) Rotelapillus laffittei (Noël) Noël, XPL, Barremian; (11) R. laffittei, SEM, Barremian. Podorhabdales: (12) Axopodorhabdus dietzmannii (Reinhardt) Wind and Wise, SEM, Aptian. (13) Biscutum ellipticum (Górka) Grün in Grün and Allemann, XPL, Albian; (14) B. ellipticum, PCL, Albian; (15) B. ellipticum, SEM, Albian; (16) Polypodorhabdus madingleyensis, Black, XPL, Barremian; (17) Cretarhabdus conicus, Bramlette and Martini, SEM, Aptian; (18) Prediscosphaera cretacea, XPL, Cenomanian. Watznaueriales: (19) Cyclagelosphaera margerelii Noël (coccosphere), SEM, Berriasian; (20) Watznaueria barnesae (Black) Perch-Nielsen, PCL, Barremian; (21) Watznaueria fossacincta (Black) Bown in Bown and Cooper, SEM, Berriasian; (22) Watznaueria manivitae Bukry, XPL, Tithonian. Arkhangelskiales: (23) Broinsonia enormis (Shumenko) Manivit, XPL, Albian/Cenomanian; (24) Arkhangelskiella cymbiformis Vekshina, XPL, Maastrichtian. XPL, light microscope images (crossed polarizers); PCL, phase contrast light microscope images; SEM, scanning electron microscope images; scale bar: 2 µm. Chapter 2: Mesozoic calcareous nannofossils state of the art 11 12 Chapter 2: Mesozoic calcareous nannofossils state of the art

2.2 Morphology and systematics Calcareous nannofossils [nano (Greek) = dwarf] are fossil remnants of tiny algal protists, the living representatives of this group are named calcareous nannoplankton. The taxonomically rather heterogeneous group of calcareous nannofossils (2 to 63 µm in diameter) includes among others coccoliths, nannoliths, and calcispheres. Coccolithophores are unicellular algae, which belong to the class Prymnesiophyceae within the division Haptophyta (Green and Jordan, 1994). They are photoautotrophic and inhabit the photic zone (upper 50 to 200 m) of the oceans. Coccolithophores develop three types of cell wall coverings (organic scales, holococcoliths and heterococcoliths; Fig. 2.1). Both mineralised plates are made up of low Mg-calcite (Siesser, 1977) and can be produced by the same organism. They are thought to represent phase changes in a haplo- diplontic life cycle (e.g. Billard, 1994; Cros et al., 2000; Geisen et al., 2002). Holococcoliths are constructed of simple euhedral crystals (Fig. 2.1), which are only rarely preserved in Mesozoic sediments. While holococcoliths calcify in an extracellular position (Rowson et al., 1986), the calcification of heterococcoliths takes place in intracellular vesicles derived from the Golgi body (Pienaar, 1994). The calcification of heterococcoliths starts with the nucleation of a proto-coccolith ring (PCR) of small crystals around the rim of a precursor base scale plate. Subsequently the crystals of the PCR grow in various directions to form crystal units (Young et al., 1992, 1999). This usually causes a growth approaching a parallel ellipsis (Young et al., 1996). According to Young et al. (1992) heterococcoliths consist of two basic units of calcite crystals, a V- and an R-unit. In the V-unit the crystallographic c-axes are oriented vertically, in the R-unit, the c-axes are oriented radially. The V/R model is important for the taxonomic concepts of calcareous nannofossils (Bown and Young, 1997; Young et al., 1997). The current classification of calcareous nannofossils is based on a bottom-up grouping (Young and Bown, 1997), where species are organized to genera and genera to families following the similarity of morphology and stratigraphic occurrences. An overview of the different types of Mesozoic heterococcoliths (see Figs. 2.1 and 2.2) is given by Bown and Young (1997). Two basic types of heterococcoliths are named muroliths and placoliths. Muroliths consist of a wall-like rim, which is composed of two crystal-units: the distal/outer cycle (V- unit) and a smaller, subordinate proximal/inner cycle (R-unit). Muroliths include the two orders Eiffellithales (imbricating elements) and Stephanolithiales (non-imbricating elements; Figs. 2.1 and 2.2). Placoliths are made up of a broad and thin rim (shield) and are usually constructed from two superim- posed shields joined by a tube-cycle. Two orders of placoliths are distinguished based on imbrication (Watznaueriales) or either non-imbrication (Podorhabdales) of the elements (Figs. 2.1 and 2.2). A third order Arkhangelskiales can be cautiously assigned to the placoliths. Their rather complex structure has been described as tiered placoliths, which means that they consist of 3 to 5 closely appressed shields (Bown and Young, 1997). The morphology of coccoliths varies considerably, additional morphological features, e.g. circular or elliptical outline, a closed/open central area. The central area can be filled with a bar, a bridge, a cross, a grill, perforated plates or other structures. This allows a generic or species assignment. Systematics at the species level is mainly based on central area features. The second important group of calcareous nannofossils are nannoliths. The term nannolith is widely used as a collective term including calcareous nannoplankton which forms calcitic structures that are quite different from hetero- or holococcoliths (Young and Bown, 1997). Usually this group is defined as nannofossils Chapter 2: Mesozoic calcareous nannofossils state of the art 13 (5) (4) ): raarudosphaera, (2) Eoconusphaera, Lithraphidites, Eprolithus, Farhania, Micula, + Nannoconus ): ): Nannoliths Ceratolithina ): Goniolithaceae Ceratolithoides Lapideacassaceae ): Nannoconaceae (4) (1) Polycyclolithaceae (5) Schizosphaerellaceae Microrhabdulaceae (3) Eoconusphaeraceae (2) shaped elements joined along vertical sutures shaped elements protrude distally central axis Braarudosphaeraceae (1) (3) pentaliths consisting of five segments consisting of five segments pentaliths outline to pentalith is tangential c-axis orientation show a laminar ultrastructure units individual crystal truncated-cone-like morphology constructed of lath- inner core of numerous radial lamellae which elongate, rod-like shape cruciform or circular cross-sections conical, globular or cylindrical forms composed of spirally arranged platelets axial canal or cavity with respect to the is tangentially c-axis orientation c-axis orientation have a tangential elements two vertically appressed wall cycles and a central area central a and cycles wall appressed vertically two Microrhabdulus · · · · · · · · · · · · · Quadrum Conusphaera Micrantholithus (2) Eoconusphaeraceae (e.g. (3) Microrhabdulaceae (e.g. (4) Nannoconaceae ( (5) Polycyclolithaceae (e.g. (1) Braarudosphaeraceae (e.g. B ain: Kamptneriaceae tructure uncert S Arkhangelskiellaceae include transverse bars with bars transverse include proximal net, (near) axial crosses with proximal net and perforate plates crossed by axial or near- axial sutures Arkhangeskiellaceae - light - Arkhangeskiellaceae tiered "placoliths" central area structures XP: Kamptneriaceae - dark, - Kamptneriaceae XP: · · · Placoliths Watznaueriaceae laevogyre shield, the distal anticlockwise in proximal shield imbricating placoliths R-unit dominated reduced V-unit element curvation is imbrication is clockwise in XP: high birefringence obliquity is broadly sinistral · · · · · · · ain: Biscutaceae Calyculaceae Tubodiscaceae tructure uncert Cretarhabdaceae Mazaganellaceae S Axopodorhabdaceae Prediscosphaeraceae (distal) and R-units (distal) (proximal) anticlockwise non-imbricating placoliths equal development of V- element curvation is obliquity is broadly dextral XP: low birefringence PC: high relief · · · · · · Parhabdolithaceae Stephanolithiaceae ?Calciosoleniaceae non-imbricating elements (nearly) vertical non-imbricating muroliths cycle composed of distal in side view: sutures are · · · Muroliths Eiffellithaceae Rhagodiscaceae Chiastozygaceae EIFFELLITHALES STEPHANOLITHIALES PODORHABDALES WATZNAUERIALES ARKHANGELSKIALES

cycle composed of imbricating elements (clockwise) visible imbricating muroliths distal

in side view: sutures are not are sutures view: side in

· ·

ORDERS: FAMILIES: CHARACTERISTICS:

Fig. 2.2. Overview of ordinal-level classification of heterococcoliths and nannoliths (adapted from Bown and Young, 1997, 1998). Abbreviations: XP, crossed polarizers; PC, phase contrast. 14 Chapter 2: Mesozoic calcareous nannofossils state of the art

Fig. 2.3. Nannoliths: (1) Micrantholithus stellatus Aguado in Aguado et al., XPL, Barremian; (2) Micrantholithus hoschulzii (Reinhardt) Thierstein, XPL, Barremian; (3) Conusphaera mexicana Trejo, XPL, Tithonian; (4) C. mexicana, SEM, Tithonian; (5) Conusphaera rothii (Thierstein) Jakubowski, XPL, Barremian; (6) Lithraphidites carniolensis Deflandre, XPL, Barremian; (7) L. carniolensis, SEM, Aptian; (8) Microrhabdulus undosus Perch-Nielsen, SEM, Maastrichtian; (9) Nannoconus abundans Stradner and Grün, XPL, Barremian; (10) N. abundans, SEM, Barremian; (11) Nannconus aff. abundans Brönnimann, XPL, Barremian; (12) Nannoconus inornatus Rutledge and Bown, SEM, Barremian; (13) Rucinolithus terebrodentarius (Thierstein) Roth, XPL, Aptian; (14) Eprolithus floralis (Stradner) Stover, XPL, Albian/Cenomanian; (15) E. floralis (lateral view), XPL, Albian/Cenomanian; (16) Micula decussata Vekshina, XPL, Maastrichtian. XPL, light microscope images (crossed polarizers); SEM, scanning electron microscope images; scale bar: 2 µm. of uncertain affinities, which are probably related to haptophyte algae (Young et al., 1999). Mesozoic nannoliths are believed to be important components of pelagic limestones in the Mesozoic (Erba, 1989, 1994; Bornemann et al., 2003). The most common Mesozoic nannolith families are shown in Figs. 2.2 and 2.3. The last group included in calcareous nannofossils are calcareous dinocysts (calcispheres). They are the subject of two other articles in this volume. Chapter 2: Mesozoic calcareous nannofossils state of the art 15

2.2.1 Function of coccolith morphology The function of coccoliths has been discussed by Young (1994). The most obvious function of coccoliths is protection of the cells against damage and/or predation, e.g. spines are increasing the cell diameter (Young, 1994). Continuous cell coverings may prevent physical damage or bacterial/viral attack to the cell mem- brane (Manton, 1986; Young, 1994). Flotation may also be regulated by coccolith production and morphology as well. Large, heavy coccoliths may contribute to accelerated sinking, whilst the orientation of elongate shaped coccospheres may control buoyancy (Young, 1994). Finally light regulation such as reflection of UV light (Braarud et al., 1952) or refraction of light into the cell (Gartner and Bukry, 1969) are other possible functions of coccoliths. Culture studies suggest that the production of calcitic plates must have an effect on

- biochemistry of coccolithophores allowing the indirect utilization of HCO3 for photosynthesis (Paasche, 1962; Buitenhuis et al., 1999).

2.3 Evolution Questionable objects, attributed to calcareous nannofossils, have been described from the Silurian (Munecke et al., 1999, 2001) and the Carboniferous (e.g. Gartner and Gentile, 1972; Munnecke et al., 2001). These findings are, however, not generally accepted as such. It is thus the generally held view that the evolution of calcareous nannofossils started in the late Triassic. While nannoliths and calcispheres have been reported from the early Carnian of the southern Alps (Janofske, 1990, 1992; Bown, 1998), true coccoliths made their first occurrence slightly later in the Norian of the Northern Calcareous Alps (e.g. Bown, 1998). Two nannoliths, Prinsiosphaera and Eoconusphaera became common in the Norian and Rhaetian, with heterococcoliths being present but rare. Triassic nannofloras are thus of low diversity with common nannoliths and rare coccoliths. The TriassicJurassic boundary extinction event also affected calcareous nannofossils with Prinsiosphaera becoming disappearing. Only one coccolith species Crucirhabdus primulus survived the TriassicJurassic boundary (Bown, 1998). Calcareous nannofossils experienced the most important diversification event in the early Jurassic (Roth, 1987; Bown and Cooper, 1989; Fig. 2.4). The first coccoliths, which originated in the late Triassic, subsequently gave rise to various new taxa in the early Jurassic (HettangianToarcian). Muroliths were the first to appear, followed by the more complex placoliths. By the end of the early Jurassic all calcareous nannofossil families common in the ToarcianKimmeridgian had evolved (Fig. 2.4). The remainder of the Toarcian Kimmeridgian saw the steady diversification of coccoliths and nannoliths. The coccoliths underwent an ever increasing complexity of their calcitic skeletons. Species richness peaked in the late Jurassic (Oxfordian, Kimmeridgian), when assemblages consisted of a total of up to 80 species (Roth, 1987). Abundances of calcareous nannofossils in HettangianCallovian oceans were still relatively low, although schizosphaerellids are abundant in certain intervals of the Toarcian of the Umbrian-Marche Basin. In the latest Jurassic (Tithonian) a new group of nannoliths, the nannoconids, evolved (Fig. 2.4). The genus Nannoconus soon dominated the nannolith assemblages of the Tethys, becoming a common, rock-forming component in the Tithonian Valanginian (Busson and Noël, 1991; Erba, 1994; Bersezio et al., 2002; Bornemann et al., 2003; Erba and Tremolada, subm.). The JurassicCretaceous boundary interval (TithonianBerriasian) is also marked by a distinctive provincialism, showing a clear biogeographic separation of Boreal and Tethyan floras (see Sec- tion 2.5). The earliest Cretaceous (BerriasianBarremian) was a stable phase with relatively few speciation and extinction events (Roth, 1987). Diversity varied from 35-45 species (Roth, 1987). The BerriasianHauterivian 16 Chapter 2: Mesozoic calcareous nannofossils state of the art

65.0 Ma.

Ca.

Sa. Co Tu.

Ce. 98.9 Goniolithaceae

Al.

CRETACEOUS Ap. Lapideacassaceae

EarlyBa. Late

Ha. Kamptneriaceae/ Prediscosphaeraceae

Va. Arkhangelskiellaceae

Be. Ryazanian 144.0 Volgian Ti. Eiffelithaceae Tubodiscaceae Calciosoleniaceae Ki.

Ox. Rhagodiscaceae Microrhabdulaceae 159.4 Polycyclolithaceae

Ca. Nannoconaceae Braarudosphaeraceae Bat.

Baj. Middle Late Aa. 180.1 JURASSIC To. Chiastozygaceae

Pl. Biscutaceae Calyptrosphaeraceae Cretarhabdaceae Parhabdolithaceae Early Si.

? Eoconusphaeraceae Watznaueriaceae Calyculaceae Axopodorhabdaceae

He. Mazaganellaceae

205.7 Stephanolithiaceae Fig. 2.4. Stratigraphic ranges (Jurassic - Cretaceous) of Mesozoic calcareous nannofossil families (modified after Bown and Cooper, 1998; Bown et al., 1998). Chapter 2: Mesozoic calcareous nannofossils state of the art 17 saw the origin of new coccolith and nannolith families (Polycyclolithaceae, Eiffellithaceae, Tubodiscaeae) as well as the speciation of new taxa within already established families. A turnover marks the early Aptian with endemic elements, common in the Boreal Realm, becoming extinct, while new species originated (Roth, 1987; Mutterlose and Böckel, 1998). This is linked to a crisis of the nannoconids, which underwent a significant decline in the earliest Aptian (Erba, 1994). New genera (Braarudosphaera, Flabellites, Hayesites) on the other hand became established. After a final bloom in the mid-Aptian (Nannoconus truitti acme; Mutterlose, 1989) nannoconids decreased in post-Aptian time in abundance and diversity. They became rare in the late Cretaceous, but persisted until the Campanian (Deres and Achéritéguy, 1980). The same pattern applies for Conusphaera, which became extinct earlier in the Aptian (Fig. 2.4). Highly diverse nannofloras of late Albian age continued into the Cenomanian, with diversity peaking in CenomanianTuronian time (Lees, 2003). No significant changes of calcareous nannofossils coincide with the CenomanianTuronian oceanic anoxic event (OAE) 2. The subsequent ConiacianCampanian were periods of stasis without abrupt changes in the assemblage composition. In the Coniacian the nannolith genus Micula originated and eventually became the most common genus in most parts of the worlds oceans, outnumbering Watznaueria, as far the dominant taxon, for the rest of the Cretaceous. After the cosmopolitan world of the CenomanianTuronian the rate of endemism increased once again and peaked in the Campanian (Lees, 2003). A distinctive separation between low-latitudinal and polar assemblages became apparent in the Maastrichtian (Thierstein, 1980; Watkins et al., 1996; Lees, 2003). The CretaceousTertiary boundary event is most clearly reflected by calcareous nannofossils, which suffered severely from the extinction. Gardin (2002) reports from the El Kef section in Tunisia a drop in species richness from about 70 species in the latest Maastrichtian to 30 just above the boundary. Detailed studies of the CretaceousTertiary boundary interval were performed by Pospichal (1994) and Gardin (2002). The Palaeocene saw then the stepwise origination of new species, allowing for one of the most solid biozonation schemes. In Mesozoic time coccoliths and nannoliths became most important rock-forming components twice. The TithonianBerriasian saw the radiation of nannoliths and for the first time in the Earths history carbonate production was substantially influenced by planktic organisms (e.g. Colom, 1955; Bornemann et al., 2003). The plankton carbonate world found its hay-days in the late Cretaceous. Coccoliths became an important biotic constituent of late Cretaceous seas, both in diversity and abundance. This resulted in the widespread distribution of chalky sediments in epicontinental seas all over the globe, describing the late Cretaceous oceans by coccoliths, calcispheres and planktic foraminifera.

2.4 Stratigraphy Since the 1970s calcareous nannofossils have become the most powerful and reliable tool for the biostratigraphic dating of both Mesozoic and Cenozoic sediments. Due to their planktic mode of life calcareous nannofossils are more widely distributed than macropalaeontological markers used commonly for age assignments. Furthermore small sized samples (<0.5 g) supply plenty of material. Though autecological factors control and limit the palaeobiogeographic patterns of calcareous nannofossils (see Section 2.6), there are certain index species common to the Pacific, the Indian Ocean and the Atlantic providing a powerful biostratigraphic zonation. Calcareous nannofossils are, like other planktic organisms, well suited for correlation and age assignments on a global scale. This worldwide application is, however, hindered by the fact that calcareous nannofossils are relatively scarce in shallow neritic settings. Calcareous nannofossils are present in the vast oceans, in epicontinental seas, but they are rare in coastal and reef environments (e.g. Erba et al., 1995). 18 Chapter 2: Mesozoic calcareous nannofossils state of the art

The zonation schemes for the Jurassic and Cretaceous are not based on evolutionary or phylogenetic lineages or on an order of subsequent species. First occurrences (FOs) and last occurrences (LOs) of specific taxa, not related to one another, are the main stratigraphic events. Furtheron acmes (mass occurrences) are being used for age assignments in the Cretaceous, as well as crises of certain taxa (e.g. nannoconids). Triassic nannofossils are of low biostratigraphic value, two biozones have been recognised for the entire CarnianRhaetian (Bralower et al., 1991). The biostratigraphic application of coccoliths and nannoliths for the Jurassic goes back to the 1960s (e.g. Stradner, 1963; Prins, 1969; van Hinte, 1976). Subsequently these zonations were refined (e.g. Haq, 1983; Roth et al., 1983; Crux, 1984; Bown, 1987; Bralower et al., 1989), resulting in world wide applicable schemes. Comprehensive reviews were given by Bown and Cooper (1989) and de Kaenel et al. (1996). A scheme of twenty biozones (NJ 1 - NJ 20; bottom to top; NJ = nannofossils Jurassic) has been proposed for the Tethys, and eighteen biozones (NJ 1 - NJ 18; bottom to top) for the Boreal Realm. This differentiation into two zonation schemes reflects the problem of provincialism throughout the Jurassic. In particular the Hettangian - Toarcian and the Oxfordian - Tithonian show high rates of endemisms and a clear limitation of certain taxa to the Tethys or the Boreal Realm (de Kaenel et al., 1996; Bown and Cooper, 1998). The HettangianSinemurian shows relatively few FOs and LOs, while the PliensbachianBajocian was a period with an increased rate of extinctions and originations of new species. The Bathonian to Kimmeridgian was again a stable period with relatively few LOs and FOs, while the Tithonian saw the onset of new groups of nannoliths (Nannoconus, Conusphaera) resulting in quite a few new originations. First zonation schemes of the Cretaceous were suggested by Thierstein (1971, 1973, 1976) and Sissingh (1977), the latter is, in modified form, still in use today. Sissingh (1977) introduced 25 zones (CC 1 - CC 25; from bottom to top; CC = Cretaceous coccoliths) for the BerriasianMaastrichtian interval and defined the zones by FOs and LOs of specific calcareous nannofossil taxa. This zonation scheme, based on on-shore material from Europe and samples from the North Sea was refined and extended to the NW Atlantic by Roth (1978). Perch-Nielsen (1979, 1985) summarised the then important schemes. Further refinement was obtained when independent schemes were developed for the Boreal Realm and the Tethys. Mutterlose (1992) described and compared different zonation schemes for the Indo-Pacific, the Tethys and the Boreal Realm, being caused by latitudinally controlled biogeographic distribution patterns. More detailed schemes were developed subsequently for the North Sea area (Crux, 1982, 1989; Jakubowski, 1987; Jeremiah, 1996, 2001) resulting in the most recent schemes of Bown et al. (1998) for the Lower Cretaceous and Burnett (1998) for the Upper Cretaceous. Apparently cosmopolitan taxa allow a global correlation, resulting in the basic schemes of the first phase. The more sophisticated schemes of recent years are regionally bound, because they partly use endemic taxa as well as nannofossil abundance patterns. They provide a more detailed zonation, but cannot necessarily be applied globally.

2.5 Palaeobiogeography The first studies dealing with provincialism in the Cretaceous go back to Worsley and Martini (1970), Worsley (1974), Wind (1979), Thierstein (1976, 1981), Deres and Achéritéguy (1980), Crux (1989), Mutterlose (1991, 1992), and Pospichal and Wise (1990). These authors recognized that certain calcareous nannofossil taxa are cosmopolitan, while others display bipolar distribution patterns. Certain taxa are endemic to certain parts of the Mesozoic oceans. The semi-quantitative study of Wind (1979) was the first to use calcareous nannofossils to characterize water mass boundaries defined by distinct differences in temperature, salinity, and nutrient Chapter 2: Mesozoic calcareous nannofossils state of the art 19

60° 60° subarctic G.S spp. temperate

subtropic 30° S.S. 30°

tropic Nannoconus

0° 0°

subtropic

30° 30° Watznaueria barnesae Watznaueria temperate Repagulum parvidentatum Crucibiscutum salebrosum subantarctic

60° 60° sites without C. salebrosum sites with C. salebrosum

90°W 60°W 30°W 0° 30°E Fig. 2.5. Palaeobiogeography of recent (Atlantic Ocean: modified after McIntyre and Bé, 1967) and early Cretaceous calcareous nannofossils (Mutterlose and Kessels, 2000; Street and Bown, 2000). Abbreviations: S.S., Sargasso Sea; G.S. Gulf stream. Black arrows indicate warm surface water currents; white arrows cool surface water currents. levels of the South Atlantic and Indian oceans using the ratios of high- and low-latitude species. Bipolar distribution patterns are documented by Crucibiscutum salebrosum for the Valanginian Hauterivian, and by Seribiscutum primitivum, Repagulum parvidentatum and Sollasites falklandensis for the AptianAlbian interval. These taxa are common in the high latitudes, but absent or rare from the low latitudinal settings (Fig. 2.5). Provincialism of calcareous nannofossils during latest Jurassic and earliest Cretaceous (Tithonian– Valanginian) is well known and has been documented (Cooper, 1989; Mutterlose and Kessels, 2000; Street and Bown, 2000). Certain taxa are restricted to the Tethys (e.g. Nannoconus, Conusphaera), others are known from the Boreal Realm only (e.g. Kokia). Migrations of endemic taxa to other parts of the world oceans are well known. Tethyan taxa migrated to the north when sea-ways, sea-level and surface temperatures were suitable (e.g. Mutterlose, 1989). Episodic migrations of the Tethyan genus Nannoconus into the Boreal Realm in the Valanginian, Hauterivian and mid-Aptian have been used to reconstruct global changes in temperature and sea-level. A migration of boreal taxa to the south on the other hand has been documented as well for periods of sea-level high, e.g. the mid-Valanginian (Melinte and Mutterlose, 2001). A rapid expansion of the cool-water species R. parvidentatum, originally limited to the southern high latitudes, in the Late AptianEarly Albian may indicate global cooling during this period (Herrle and Mutterlose, 2003). The same applies for the latest Cretaceous, where a migration of high-latitudinal calcareous nannofossils (e.g. Kamptnerius magnificus, Ahmuellerella octoradiata) to the low-latitudes has been interpreted as an indicator for a cooling of surface water masses (Lees, 2003). A comparison of abundances of high, mid and low latitudes clearly shows maxima of abundance for each stage in the Tethys. This implies assemblages of lower diversity for the high latitudes and of higher diversity for the lower latitudes. For the early Cretaceous three distinctive, latitudinally bound assemblages have been differentiated (see Fig. 2.5). 20 Chapter 2: Mesozoic calcareous nannofossils state of the art

2.5.1 High latitudinal assemblages These are of low diversity and high abundance with common Watznaueria barnesae, Crucibiscutum salebrosum and Sollasites horticus (Mutterlose and Kessels, 2000). This assemblage is typical for the BerriasianHauterivian of the Barents and Norwegian Sea of the northern hemisphere and the Weddell Sea of the southern hemisphere (Mutterlose and Wise, 1990). The absence of this assemblage in contemporane- ous sediments of the Tethys implies substantial temperature gradients throughout the earliest Cretaceous. This assemblage may correspond to the subarctic floral zone of recent oceans (Fig. 2.5).

2.5.2 Mid - low latitudinal assemblages Diversity in the southern part of the Boreal Realm (North Sea and adjoining areas) is higher than those of the high latitudes with common W. barnesae, Biscutum constans and Zeugrhabdotus spp. Moderately temperatured surface waters, often rich in nutrients, allowed for blooms of r-strategists like B. constans and Zeugrhabdotus spp. (Mutterlose and Kessels, 2000; Bown and Street, 2000). Due to the lack of data it is not yet clear whether a similar floral zone existed in the southern hemisphere.

2.5.3 Low latitudinal assemblages The diverse assemblages of the Tethys are dominated by Watznaueria spp., Rhagodiscus asper, Nannoconus spp., Micrantholithus spp. and Conusphaera spp. These thermophile warm water taxa (Erba, 1987; Mutterlose, 1991; Erba et al., 1992) indicate rather warm surface waters of the tropics and subtropics.

2.6 Palaeoecology Mesozoic calcareous nannofossils were important primary producers only known from marine settings. As such they heavily relied on autecological factors such as light, nutrients and temperature, to a lesser extent on ocean currents, and other ecological parameters like salinity (Fig. 2.6).

2.6.1 Light Recent coccolithophores have chlorophylls a and c, beta-carotene, fucoxanthin, diatoxanthin and diadino- xanthin (Brand, 1994). This makes them similar to diatoms, chrysophytes and dinoflagellates, limiting their distribution to the photic zone, i.e. the uppermost 200 m of the water column. It is possible, however, to differentiate in between shallow and deep dwellers in recent assemblages (Honjo and Okada, 1974; Venrick, 1982). The effect of light on the composition of Mesozoic nannoliths and coccoliths is yet poorly understood. It is assumed that shallow dwellers included in Mesozoic time most heterococcoliths which were supposedly common in the upper photic zone. Deep dwellers like Nannoconus were perhaps adjusted to life in the lower photic zone (Erba, 1994; Herrle, 2003).

2.6.2 Nutrients Nitrates and phosphates are among the most important inorganic nutrients, which are limited within the marine system. Nutrient enrichment is found in current oceans either in upwelling areas, where cold nutrient rich waters encounter the ocean surface, or in areas with high riverine input. In contrast to diatoms, which presumably originated in the mid-Cretaceous (Burckle, 1978), calcareous nannofossils are considered to have preferred meso- to oligotrophic conditions, with generally low nutrient concentrations (Fig. 2.6). The late Cretaceous, which is characterised by widespread chalk and limestone sedimentation, may thus be seen as a nutrient desert. The oligotrophic sluggish warm temperate Greenhouse world of the mid-late Cretaceous Chapter 2: Mesozoic calcareous nannofossils state of the art 21

(AptianSantonian) was obviously the time most suited for a major diversification of coccoliths allowing for enhanced pelagic carbonate sedimentation on the shelves. Though an overall oligotrophic group Mesozoic calcareous nannofossils responded to nutrient variations of surface waters in a specific, species related way. Some Mesozoic species (e.g. Biscutum constans, Zeugrhabdotus erectus) responded to nutrient enrichment by increasing their population size, while diversity decreased. High abundances of these small scaled taxa may indicate an r-selection under mesotrophic conditions. Most Mesozoic species (e.g. Watznaueria barnesae, Rhagodiscus asper) do not increase in abundance in upwelling regions or other areas that have elevated nutrient concentration. These are thus viewed to indicate K-selection and rather stable oligotrophic conditions.

2.6.3 Temperature Various species of Mesozoic calcareous nannofossils have different temperature ranges (e.g. Mutterlose and Kessels, 2000; Street and Bown, 2000; Lees, 2003). Cosmopolitan taxa like Watznaueria barnesae covered a broad temperature range, being common both in the low and the high latitudes throughout most of the Mesozoic (Fig. 2.5). Watznaueria barnesae was common in tropical and subpolar regions and may thus be viewed as a eurythermal taxon (Mutterlose, 1996). Other groups (e.g. Nannoconus, Conusphaera, Micula murus) are most common in low latitudinal settings where they were sometimes rock-forming. Since they are rare in the Boreal Realm they have often been interpreted as Tethyan warm water taxa (e.g. Mutterlose, 1992). The same warm water affinity is attributed to Rhagodiscus asper. Some cold water taxa (e.g. Stephanolithion, Biscutum, Crucibiscutum, Repagulum parvidentatum, Seribiscutum primitivum, Sollasites falklandensis, Ceratolithina, Kamptnerius, Nephrolithus) show limited palaeobiogeographic distribution patterns. These taxa are most common only in the high latitudes during certain intervals of the Cretaceous (Hauterivian, AptianAlbian, Maastrichtian) for which considerable thermal gradients have been postulated (e.g. Burnett et al., 2000; Mutterlose and Kessels, 2000; Street and Bown, 2000; Lees, 2003). A biogeographic expansion of these cold water taxa clearly reflects cold modes of the Jurassic and Cretaceous.

Trophic continuum

low high chlorophyll concentrations

turbulence low high

surface water eutrophic nutrient levels oligotrophic mesotrophic

dinoflagellates coccolithophorids

nannoconids

diatoms

K-mode r-Mode specialists opportunists Fig. 2.6. Palaeoecology of calcareous nannofossils. 22 Chapter 2: Mesozoic calcareous nannofossils state of the art

2.6.4 Ecological strategies Keeping in mind that calcareous nannofossils are dissolution susceptible, they are otherwise an ideal proxy for recording palaeoceanographic conditions of ocean surface waters. Variation of their abundance and composition may reflect changes in palaeoclimate, nutrient supply, detrital input and surface water salinity. Six to seven taxa, which usually make up 80-90% of the nannofossil assemblages, may be used for palaeoecological interpretations. These are mainly based on the following observations • In recent settings a high diversity is indicative of ecologically stable conditions, in particular oligotrophic warm surface waters (McIntyre and Bé, 1967; McIntyre et al., 1970; Brand, 1994). K-selection prevails under such conditions. Low diversity assemblages on the other hand are considered to be typical for ecologically unstable stress conditions and cool surface waters enriched in nutrients (e.g. Okada and Honjo, 1973; Brand, 1994), causing r-selection. • In the fossil record W. barnesae, a species common to abundant throughout the Jurassic and Cretaceous in most settings, is considered to be a eurytopic cosmopolitan species (Mutterlose, 1991). Ecologically W. barnesae may be viewed as a K-strategist. According to various authors (Erba, 1991, 1992a; Erba et al., 1992; Williams and Bralower, 1995; Herrle, 2003; Herrle et al., 2003a; Bornemann et al., subm.) W. barnesae may be seen as an oligotrophic species. • High abundances of Biscutum constans and Zeugrhabdotus erectus are considered to be indicative of high nutrient supply (Roth and Bowdler, 1981; Roth, 1986; Roth and Krumbach, 1986; Watkins, 1986, 1989; Erba, 1987, 1989; Erba et al., 1992). Such a B. constans Zeugrhabdotus spp. assemblage is considered to indicate upwelling of cold water rich in nutrients. Discorhabdus rotatorius is a third species, which may have preferred high fertility conditions (Erba, 1991; Herrle et al., 2003a; Bornemann et al., subm.). • Nannoconus spp. has been interpreted as restricted to the lower photic zone and to be controlled by fluctuations of the depth of the nutricline (Erba, 1994; Herrle, 2003; Bornemann et al., subm.). Conse- quently, changes in abundance of nannoconids and other coccoliths have been used to reconstruct the fertility of surface waters and nutricline dynamics. High abundances of Nannoconus spp. may thus indicate a deep chlorophyll maximum zone (DCM) with an increased productivity of the lower photic zone. Nannoconus spp. is thought to have an ecological affinity comparable to that of recent Florisphaera profunda (Erba, 1994; Herrle, 2003). • Micrantholithus spp. (without Micrantholithus speetonensis), Rhagodiscus asper and Conusphaera spp. are interpreted as warm water taxa (Erba, 1987; Mutterlose, 1991; Erba et al., 1992; Herrle et al., 2003a). Micrantholithus spp. is more common in near shore settings becoming rare in pelagic environments. · A new approach to assess surface water conditions is the use of ratios of nannofossil taxa or groups instead of abundance changes of single taxa. According to Andruleit (1995) ratios of common recent taxa provide a more consistent signal than a single taxon alone during the transition from the sinking community to the sedimentary thanatocoenosis. This approach has been successfully applied to mid- Cretaceous sediments in order to record temperature or nutrient/productivity changes (Gale et al., 2000; Herrle, 2003; Herrle et al., 2003 a, b; Bornemann et al., subm.). They are either based on multi-variate statistical analyses (Herrle et al., 2003a) or on literature data (Gale et al., 2000; Bornemann et al., subm.). Indices applied by Herrle et al. (2003a, b) and Bornemann et al. (subm.) have been tested and verified by comparing the nannofossil signal to those of other microfossil groups and geochemical data (benthic and planktic foraminifera, palynomorphs and oxygen isotopes). They identified Z. erectus and D. ignotus as Chapter 2: Mesozoic calcareous nannofossils state of the art 23

high fertility indicators and W. barnesae as a more oligotrophic form. R. asper/achlyostaurion are believed to indicate warmer surface waters, whereas R. parvidentatum, Seribiscutum spp. and C. salebrosum (among others) presumably reflect cooler conditions.

2.7 Palaeoceanographic significance of calcareous nannofossils Calcareous nannofossil assemblages of Mesozoic age have been widely used to reconstruct palaeoenvironmental and palaeoclimatic conditions. Though more than 95% of the Mesozoic taxa became extinct at the CretaceousTertiary boundary, calcareous nannofossils have supplied important palaeoceanographic information about Mesozoic oceans, water masses, current systems and sedimentary patterns. Current studies use calcareous nannofossils as proxies for understanding three palaeoceanographic and palaeoclimatic aspects: (1) the origin of limestone/marlstone and marlstone/claystone bedding rhythms, (2) the genesis of black shales in the Jurassic and Cretaceous, and (3) pelagic carbonate production pattern. Many Jurassic and Cretaceous pelagic and neritic sequences are characterised by the occurrence of bedding rhythms, which have a thickness of decimetre to metre scale. These rhythms consist of limestone/ marlstone and marlstone/claystone beds in the Tethys (e.g. Cotillon et al., 1980; Cotillon, 1984; Herbert and Fischer, 1986; Erba, 1991; Erba and Premoli Silva, 1994; Fiet et al., 2001; Herrle, 2002; Friedrich et al., 2003; Westphal et al., in press) and of marlstone/claystone beds in the Boreal Realm (Schneider, 1964; Rawson, 1971; Kemper, 1987b; Rawson and Mutterlose, 1983; Mutterlose and Ruffell, 1999). Among other proxies (e.g. clay mineralogy, trace elements, stable isotopes, benthic and planktic foraminifera, palynomorphs, trace-fossils) calcareous nannofossils have been used to understand the origin and the nature of these bedding rhythms. Since less than 4 mg raw material is needed for calcareous nannofossil analyses and can be taken on a high resolution scale in mm-steps (e.g. Herrle and Bollmann, subm.). It is generally believed that particularly calcareous nannofossil fertility and temperature indicators play an important role to reconstruct the palaeoenvironmental settings of these rhythms. Taxa used include Zeugrhabdotus erectus, Discorhabdus ignotus (= D. rotatorius), Biscutum constans and Diazomatholithus lehmanii as high productivity indicators. Watznaueria barnesae is interpreted as an indicator for low surface water productivity. Indicators of cooler surface waters are e.g. Repagulum parvidentatum, Seribiscutum spp. The occurrence of Rhagodiscus asper, in contrast, reflects warmer surface waters (see Section 2.6). Tethyan limestone/marlstone bedding rhythms yield typical assemblages of calcareous nannofossils with common high fertility indicators in the limestone beds and low fertility indicators in the marlstone beds (Premoli Silva et al., 1989, 1999; Erba, 1991; Bellanca et al., 1996; Mattioli, 1997; Galeotti et al., 2003). This pattern applies both for Jurassic and Cretaceous limestone/marlstone rhythms of open marine and neritic low-latitudinal environments. In contrast, Watkins (1989), Fisher and Hay (1999) and Mutterlose and Ruffell (1999) observed the opposite trend for marlstone/claystone rhythms. The carbonate rich marlstones are characterised by low fertility indicators, while the carbonate poor claystones indicate higher surface water fertility. This opposite interpretation of bedding rhythms may be explained by the specific palaeoceanographic settings of the studied sections. Watkins (1989) and Fisher and Hay (1999) studied sections from the Western Interior Basin and Mutterlose and Ruffell (1999) analysed outcrops from the Lower Saxony Basin (NW Germany). These basins are, in comparison to the Tethyan sections, relatively small with a large hinterland. Fertility in these basins was generally perhaps much higher than in the oceanic nutrient deserts of the Tethys. Variation of the freshwater runoff on the scale of Milankovitch cycles (e.g. Eicher and Diner, 1985; Below and Kirsch, 1997; Mutterlose and Böckel, 1998) may have played an addi- 24 Chapter 2: Mesozoic calcareous nannofossils state of the art tional role in these restricted basins. This may have caused cyclic variation of the surface water productivity and a stable stratification of the surface water during the formation of the claystones, going along with suboxic conditions for the deposition of the claystones (Mutterlose and Ruffell, 1999). In contrast, increased mixing of surface waters may have caused a rise in surface water productivity in more open ocean environments or basins which are less affected by freshwater runoff during times of the formation of limestones. However, limestone/marlstone alternations reflect a complex interplay of a great number of palaeoceanographic, palaeoclimatic, palaeogeographic and diagenetic factors (Einsele et al., 1991; Thierstein and Roth, 1991; Westphal et al., in press). Calcareous nannofossils are also useful tools for deciphering the formation of the widespread Jurassic and Cretaceous black shales in the Toarcian (Posidonienschiefer, Alum Shale), Kimmeridgian, Volgian and mid-Cretaceous. Starting with the pioneering studies of Noël and Manivit (1978), Thomsen (1989a, b), Bralower (1988), Erba (1986, 1991) and Premoli-Silva et al. (1989) qualitative and quantitative methods have been developed to utilize the palaeoclimatic and palaeoceanographic potential of calcareous nannofossils. The Toarcian black shales of Italy have so far only been studied using calcareous nannofossils by Mattioli (1997) and Bucefalo Palliani et al. (1998, 2002). The black shale interval of the Toarcian Oceanic Anoxic Event (OAE) is characterised by high abundances of Biscutum dubium, B. finchii and Lotharingius, which may indicate higher surface water productivity during the black shale formation, than during the ambient marlstones (Bucefalo Palliani et al., 1998, 2002). The late Jurassic organic-rich Kimmeridge Clay was thor- oughly studied by Gallois (1976), Gallois and Medd (1979), and Young and Bown (1991) in the last decades. This studies show monospecific coccolith blooms on sub-annual scale in laminated sediments as well as ontogenic sequences of calcareous nannofossils. The excellent preservation of calcareous nannofossils provides a better understanding of the original occurrences and productivity changes on annual scales during the late Jurassic. Similar coccolith-rich laminae have been also described in association with oil shales in the Liassic Posidonienschiefer of northern central Europe (e.g. Müller and Blaschke, 1969). The Volgian sequence of the Russian Platform consists of calcareous claystones and intercalated organic rich shales (up to 10 wt% TOC) overlain by phosphorite beds. The calcareous nannofossil assemblages of these sequences are dominated by Watznaueria spp. Biscutum constans, Zeugrhabdotus erectus and Crucibiscutum salebrosum (in descending abundance). From base to top consecutive mass occurrences of different taxa was observed (Kessels et al., 2003). A W. barnesae-W. fossacincta acme is followed by a W. britannica-W. communis acme, which in turn gives way to a Z. erectus acme, and finally a B. constans acme (including sparse occurrences of C. salebrosum). The observed distribution patterns have been interpreted as a transition from a warmer, low productive, oligotrophic setting and predominating marlstone sedimentation to a cooler, higher productive, eutrophic setting and black shale sedimentation. The mid-Cretaceous is characterised by five periods with extensive black shale sedimentation, called Ocean Anoxic Events (OAE): The early Aptian OAE 1a, the early Albian OAE 1b, the middle late Albian OAE 1c, the late Albian OAE 1d and the CenomanianTuronian OAE 2 (e.g. Schlanger and Jenkyns, 1976; Arthur et al., 1990; Erbacher and Thurow, 1997). Of these, the OAE 1a and the OAE 2 are the most intensively studied black shale intervals of the Cretaceous. They are known from both high- and low-latitudinal settings and all major oceans (see Leckie et al., 2002 for detailed discussion and references). The most striking feature of the OAE 1a is the global crisis of the genus Nannoconus, predating the formation of the OAE 1a black shale interval (Bralower et al., 1993, 1994, 1999; Erba, 1994; Cobianchi et al., 1999; Premoli Silva et al., 1999; Tremolada and Erba, 2002; Herrle and Mutterlose, 2003; Erba and Tremolada, subm.; Fig. 2.7), reflecting a major change in the hemipelagic and pelagic carbonate system. The Chapter 2: Mesozoic calcareous nannofossils state of the art 25

Fig. 2.7. The Nannoconus crisis of the Early Aptian (Erba, 1994). black shale interval of the OAE 1a shows high percentages of the high fertility indicators Biscutum and Zeugrhabdotus (e.g. Coccioni et al., 1992; Premoli Silva et al., 1999; Luciani et al., 2001). Radiolarians and planktic foraminifera (Premoli Silva et al., 1999; Luciani et al., 2001) support the interpretation of the Tethyan OAE 1a as a high productivity event. In the Boreal Realm, the OAE 1a (Fischschiefer) was studied in detail by Bischoff and Mutterlose (1998) and Habermann and Mutterlose (1999). Calcareous nannofossil assemblages show a trophic cyclicity, best displayed by Rhagodiscus spp., Nannoconus spp./Braarudosphaera spp., Biscutum spp./Zygodiscus spp. (Zygodiscus = Zeugrhabdotus) and Watznaueria spp. It is assumed that the succession of different calcareous nannofossil taxa reflects a progression of warmer surface water conditions, followed by changing surface water productivity. The Fischschiefer itself is interpreted as a high productivity event, as indicated by the increasing percentages of Biscutum spp. and Zeugrhabdotus spp. (Habermann and Mutterlose, 1999). The early Albian OAE 1b black shale was studied on a high-resolution (>3.5 kyr) long-term record (~380 kyr) in order to reconstruct palaeoceanographic and palaeoclimatic changes under normal conditions in relation to the OAE 1b black shale formation (Herrle, 2003; Herrle et al., 2003a, b). For reconstructing surface water productivity and temperature, a nutrient index and temperature index has been used to reconstruct surface water productivity and temperature changes. Time series analyses of the nutrient index show fluctuations within the precessional band, whereas variations of the temperature index reflect eccentricity (Fig. 2.8). The precession-controlled fluctuations of the surface water productivity are interpreted to represent a monsoonal climate signal, with the nutrient supply in the surface waters depending on the strength of monsoonal activity. The OAE 1b black shale occurs under increasing warm and humid conditions controlled 26 Chapter 2: Mesozoic calcareous nannofossils state of the art

Fig. 2.8. Temperature and productivity changes during the Early Albian Ocean Anoxic Event 1b (after Herrle et al., 2003a) based on calcareous nannofossil temperature and nutrient indices (see Section 2.6.4). (A) Power spectra of the temperature and nutrient indices on linear scales: A, 124 kyr; B, 57 kyr; C, 23.5 kyr; D, 18.4 kyr. The power spectrum of the nutrient index shows peaks at: E, 20 kyr; F, 16.8 kyr. (B) Smoothed curves represent precessional (nutrient index) and eccentricity (temperature index) cycles. by the eccentricity, whereas surface water productivity increases with the onset of OAE 1b. The OAE 2 is characterized by decreasing calcareous nannofossil diversity and a major decrease of Biscutum spp. and D. ignotus. In contrast, Zeugrhabdotus spp. is present over the whole interval of the OAE 2 succession (Erba, subm.). Based on the high abundances of Zeugrhabdotus spp., the OAE 2 is interpreted as a high productivity event (e.g. Jarvis et al., 1988; Paul et al., 1999; Gale et al., 2000). The increasing abundances of Eprolithus floralis, observed at different high- and low-latitude successions, after the OAE 2, is interpreted as a cooling of surface waters (Bralower, 1988; Lamolda et al., 1994; Erba, subm.). A third field where calcareous nannofossils contribute substantially to the better understanding of marine systems is their contribution to pelagic carbonate production. This issue has been raise only in the last five years and research is still in its infancy. The OAE 1a shows high numbers of the large species Rucinolithus terebrodentarius and Assipetra infracretacea in the Tethyan sections (Erba, 1994; Erba and Tremolada, 2002; Herrle and Mutterlose, 2003; Erba and Tremolada, subm.). It has been hypothesised that rapid decreases of strongly calcified nannoconids are caused by a pronounced increase in atmospheric CO2 (Herrle and Mutterlose, 2003; Bornemann et al., 2003; Erba and Tremolada, subm.), which reduced the calcification rate of calcareous nannoplankton organisms. A similar coupling of reduced calcification rates and increased atmospheric CO2 pressure have recently been shown by Riebesell et al. (2000) and Zondervan et al. (2001) for the recent species Emiliania huxleyi and Gephyrocapsa oceanica. This interpretation is supported by the synchronous drowning of carbonate platforms with reduced growth rates of calcareous reef building caused by the increased atmospheric CO2 contents (e.g. Langdon et al., 2000). Chapter 2: Mesozoic calcareous nannofossils state of the art 27

2.8 Conclusions The state of knowledge of calcareous nannofossils has been described with respect to their morphology, systematics, evolution, biostratigraphy, palaeoecology and palaeoceanography. Both coccoliths and nannoliths supply a high resolution biostratigraphic scheme for both the Jurassic and Cretaceous. Despite the fact that some taxa are endemic to either the Tethys or the Boreal Realm, a global zonation has been developed in recent years, applicable to both on-shore sections and pelagic sites. Palaeoecologically, calcareous nannofossils provide an excellent tool to understand the palaeoclimatic and palaeoceanographic changes of the Jurassic and the Cretaceous. Since coccoliths and nannoliths provide, apart from dinoflagellates, the mass of the recorded marine primary producers in Jurassic and Cretaceous oceans the marine system cannot be interpreted without a good understanding of this group. Furthermore late Jurassicrecent carbonate accumulation patterns are influenced by calcareous nannofossils.

Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft (Mu 667/15) is gratefully acknowledged. We are indebted to David Watkins for useful discussions and comments. 28 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

3 The impact of calcareous nannofossils on the pelagic carbonate accumulation across the JurassicCretaceous boundary

André Bornemann, Ute Aschwer and Jörg Mutterlose

Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany (published 2003 in Palaeogeography Palaeoclimatology Palaeoecology 199, 187-228)

Abstract Calcareous nannofossils were important producers of pelagic carbonates in Mesozoic oceans. In order to better understand the origin of Mesozoic pelagic carbonates we studied upper Jurassic to lower Cretaceous sediments in the Central Atlantic Ocean (DSDP Sites 105, 534A, 367) with respect to their content of calcareous nannofossils. The interval under investigation is characterized by a significant increase in the deposition of carbonate-rich sediments, a trend that goes along with the rapid evolution of calcareous nannofossils. The assemblage composition and the size variation of common taxa were analyzed, and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated. Results reveal two long-term changes of the nannofossil assemblage composition. (1) The early Tithonian coccolith assemblages are of low-diversity (Watznaueria spp., Cyclagelosphaera spp., Zeugrhabdotus spp.), whilst the mid- to late Tithonian assemblage is dominated by nannoliths ( Conusphaera mexicana, Polycostella beckmannii, Nannoconus spp.) and large-sized Watznaueria. (2) The early Berriasian is characterized by a shift from the late Tithonian nannolith-rich assemblage to a highly diverse coccolith assemblage. Morphometric studies of the placolith genus Watznaueria show for all three DSDP sites large-sized forms during the mid- and late Tithonian, followed by a decrease of up to 2 µm for the mean size in the earliest Berriasian. At DSDP Site 105 the studied nannolith taxa show an increase in size during the mid- Tithonian to Berriasian interval. The records of both nannofossil carbonate estimates and the measured bulk-rock carbonate reveal two periods of increase in the nannofossil carbonate record of DSDP Site 105. A first significant increase of carbonate accumulation is observed in the mid- and late Tithonian, probably caused by mass occurrences of strongly calcified taxa (C. mexicana, P. beckmannii, Nannoconus spp., Watznaueria cf. manivitae). This interval is here named Nannofossil Calcification Event (NCE). The second carbonate maximum in the late Berriasian is related to a rise in absolute abundances of nannofossils. The peak is amplified by an overall increase of the sedimentation rate. The calculated accumulation rates of nannofossils, nannofossil carbonate and bulk-rock carbonate for the late Berriasian are on the same scale as values from recent ocean surface sediments. A comparison of nannofossil carbonate values with the bulk-rock carbonate content shows that on average only 27% of the total carbonate can be explained by our nannofossil carbonate estimates. This discrepancy is most likely caused by the high amounts of unidentifiable micrite and fragments of calcareous nannofossils. Other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations.

The NCE occurs during a long-term sea-level fall, dry climate and presumed low pCO2 levels. A decline in abundances of strongly calcified nannofossils coincides with the establishment of the Pacific-Atlantic seaway via Central America, which may have had a substantial impact on the palaeoceanographic situation Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 29 in the Central Atlantic Ocean. These changes are here considered to have at least partly caused the shifts in abundance, size and assemblage composition of calcareous nannofossils observed across the JurassicCreta- ceous boundary interval.

Keywords: Jurassic; Cretaceous; calcareous nannofossils; size analysis; morphometry; carbonate sediments; paleoceanography

3.1 Introduction Calcareous nannoplankton is, apart from planktic foraminifera, the major producer of pelagic carbonates in the recent ocean system (Milliman, 1993; Westbroek et al., 1993). Today this phytoplankton group contrib- utes significantly to primary production and is believed to influence the global climate system in three ways:

(1) by a drawdown of CO2 from the surface layer of the ocean by producing organic matter during photosynthesis (organic carbon pump), (2) by the formation of calcitic coccoliths and their postmortal dissolution or storage in the sedimentary reservoir (carbonate pump), and (3) by emitting DMS (dimethylsulphide). This last process is considered to affect the efficiency of albedo by cloud formation (Westbroek et al., 1993). While the amount of organic matter produced by calcareous nannoplankton and the emission of DMS cannot be reconstructed for the fossil record, new approaches allow the quantification of nannofossil carbonate accumulation. Investigations of sediment traps (e.g. Samtleben and Bickert, 1990; Beaufort and Heussner, 1999; Broerse et al., 2000a, b; Haidar et al., 2000; Sprengel et al., 2000; Young and Ziveri, 2000; Ziveri et al., 2000) and core material (e.g. Flores et al., 1995; Haidar et al., 2000; Kinkel et al., 2000; Ziveri et al., 2000) were performed in order to improve the understanding of temporal variations of the coccolith production and their importance for the total carbonate flux. Recently, first attempts were undertaken to quantify the amount of nannofossil carbonate for Mesozoic carbonates by calculating the volume and absolute abundances of nannofossil species, which are of rock-forming importance (Erba and Tremolada, 2000; Mattioli and Pittet, 2002; Tremolada and Young, 2002). The current state of art suggests that carbonate accumulation in pre-Jurassic time has mainly occurred in epicontinental seas. Substantial amounts of deep-sea carbonate are unknown from the Palaeozoic or the early Mesozoic. The JurassicCretaceous boundary interval is characterized by a widespread increase of the pelagic carbonate production (e.g. Roth, 1986) and major changes in the palaeoenvironmental conditions (e.g. Weissert and Channell, 1989). Enhanced sedimentation of pelagic carbonates has been reported from numerous low-latitudinal sections and wells of Tithonian age, particularly from the Tethys and the Central Atlantic Ocean. In late Jurassic and earliest Cretaceous times, before planktic foraminifera occur in rock- forming proportions, calcareous nannofossils were the most important carbonate contributors to deep-sea sediments. According to numerous authors (e.g. Garrison and Fischer, 1969; Roth, 1986, 1989), the onset of the deposition of pelagic carbonates goes along with a radiation of calcareous nannofossils, in particular with the rapid evolution of nannofossil groups of rock-forming importance (Colom, 1955; Erba, 1989; Bown et al., 1992). In the current paper we present nannofossil data from the JurassicCretaceous boundary interval of the DSDP Sites 105, 534A and 367. Changes in the composition of the calcareous nannofossil assemblages have been used to assess changes in the palaeoceanographic conditions. Size measurements were performed for those nannofossil groups, which were important for the carbonate accumulation due to their size and their abundance. For DSDP Site 105 the amount of sedimentary carbonate, which is composed of calcareous 30 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

nannofossils, was quantified. These results were compared to the CaCO3 bulk-rock data, in order to improve our understanding of the composition of ancient deep-sea carbonates. Data obtained from this study were viewed in a palaeoceanographical and palaeoclimatological context.

3.2 Palaeogeography and palaeoceanography The opening of the Central Atlantic Ocean started in the mid-Jurassic between North America and NW Africa with the oldest sediments being of Callovian age (Sheridan et al., 1983). During the JurassicCreta- ceous boundary interval the Central Atlantic was a NE-SW oriented basin with a maximum length of approx. 6000 km extending from the equator to 30°N (Fig. 3.1). The Central Atlantic was episodically connected with the Pacific Ocean in the west and with the Arctic-Boreal Sea in the north. A seaway to the Tethys existed throughout the studied interval. A clockwise surface water circulation system probably existed in the Central Atlantic in late Jurassic to early Cretaceous time. Among others Berggren and Hollister (1977), Winterer (1991) and Adatte et al. (1996) postulated a circum-equatorial current system. This was initiated in the Central Atlantic in the Late Jurassic caused by the establishment of an Atlantic-Pacific connection in Central America. Plate-tectonic reconstructions of Ross and Scotese (1988) suggest that a permanent connection via the Strait of Panama was established in the M19 magnetochronozone (uppermost Tithonian). Adatte et al. (1996) related changes of clay mineralogy and biota in Mexico to the opening of the Atlantic-Pacific gateway. These changes were dated

60° 30° 0°

30°

105 TETHYS ATLANTIC 534A 367

0°

2000 km PACIFIC ? ?

presumed land presumed oceanic surface shelf area water currents ocean-floor presumed oceanic surface ? presumed gateway (according water currents during intervals to Ross & Scotese 1988) of an open gateway between the Atlantic and the Pacific Ocean position of DSDP sites studied Fig. 3.1. Palaeogeographic setting of the studied DSDP sites. The palaeogeographic reconstruction is taken from Golónka et al. (1994). The hatched area in Central America indicates the possible gateway between the Atlantic and Pacific Ocean according to the plate-tectonic reconstructions of Ross and Scotese (1988). Black lines and arrows show the proposed surface water circulation in the Atlantic during times without or with only a shallow connection to the Pacific via Central America. Across the JurassicCretaceous boundary (magnetochronozone M19; calpionellid zone B; see text) a circum-equatorial current system became established, which possibly weakened the clockwise circulation in the Atlantic Ocean. This scenario is indicated by the dashed lines. Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 31 into the calpionellid zone B, which partly overlaps with the M19 magnetochronozone (Fig. 3.2). The circum- equatorial current caused a westward flow of warm Atlantic water into the Pacific. The water masses probably sank when encountering the cooler, less saline Pacific, just as the modern Mediterranean water forms intermediate water masses when entering the Atlantic Ocean (Brass et al., 1982; Hay, 1995). In the late Jurassic the Central Atlantic was of minor oceanographic importance due to its relatively small size and the restricted geography in comparison to the modern Atlantic Ocean.

3.3 Palaeoclimate and sea-level changes The palaeoclimatic conditions of the latest Jurassic and earliest Cretaceous are still poorly understood. Ac- cording to Frakes (1979) and Hallam (1985) the Jurassic and Cretaceous were periods of great warmth over the globe, whilst Kemper (1987a) postulated an ice age for the Valanginian. For the Late Jurassic Hallam (1985) assumed three climate belts based on the distribution of coals and evaporites: an arid zone extended from 45°N across the equator to 45°S, two seasonally wet climate belts stretching up to a latitude of 60° and a temperate wet zone covering the high latitudes. Mean annual temperatures were significantly higher and latitudinal gradients lower in comparison to the modern climate system. According to Price (1999) the latest Jurassic was an episode of cold and subfreezing polar conditions with possible polar ice sheets approximately about one-third of the size of those at the present day (Valdes et al., 1995). The hypothesis of cooler temperatures and polar ice sheets is supported by an oxygen isotope record from the high latitudes (Podlaha et al., 1998) and by global circulation models (GCM) for the late Jurassic (Sellwood et al., 2000). The GCMs allow ephemeral ice caps during times of minimal seasonal forcing on the southern continents. More recently, rather significant latitudinal gradients have been postulated by Mutterlose et al. (2003). Based on the occurrence of evaporites, bauxites and laterites as well as on clay mineralogy and palynomorphs, dry climate conditions are thought to have prevailed in western Europe and in the Atlantic region in the latest Jurassic (Hallam, 1985; Wignall and Ruffell, 1990; Hallam et al., 1991; Abbink et al., 2001). In earliest Cretaceous times the climatic conditions changed to more humid and coals became more prominent than evaporites in particular in western Europe. During the Late Jurassic to earliest Cretaceous interval the eustatic sea-level reached its maximum in the early Tithonian (Haq et al., 1987), probably caused by increased sea-floor spreading and rifting of the Pangaea continent (Sheridan, 1983, 1997). The Tithonian was characterized by unusual rapid oscillations on the scale of 0.5-1 myr in the eustatic sea-level curve of Haq et al. (1987). According to Hallam (1988), these rapid changes in sea-level recorded by Haq et al. (1987) are possibly an artefact. The late Jurassic part of this sea-level curve has been calibrated in the Moray Firth region, where sea-level fluctuations were possibly controlled by tensional tectonic activity. Hallam (2001) excluded glacioeustasy as a major force in driving sea-level changes, because the potentially small polar ice masses of the late Jurassic, as proposed by the GCMs, could have caused only sea-level changes on a meter-scale (Valdes et al., 1995). But both Haq et al. (1987) and Hallam (1988) agree that the Tithonian was marked by a sea-level high followed by a drop across JurassicCretaceous boundary. This long-term sea-level fall was probably initiated by decreasing spreading rates (Haq et al., 1987; Ziegler, 1990) causing numerous semi-restricted epicontinental seas (Ziegler, 1990; Hardenbol et al., 1998), which in turn favoured the evolution of endemic taxa. Provincialism in Tithonian to Berriasian times has been proposed for many marine floral and faunal groups on a global scale, e.g. belemnites (e.g. Stevens, 1973; Mutterlose, 1988; Doyle, 1992) and ammonites (e.g. Casey, 1973; Hoedemaeker, 1990; Rawson, 32 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

1994), with endemisms peaking in the Berriasian. Similar patterns are also reflected by calcareous nannofossils, which exhibit a distinctive latitudinal distribution (Mutterlose, 1996; Mutterlose and Kessels, 2000; Street and Bown, 2000). The subsequent Valanginian stage was characterized by a widespread transgression (Haq et al., 1987; Ziegler, 1990).

3.4 Structure and calcification of calcareous nannoplankton Calcareous nannoplankton is a taxonomically heterogeneous group which includes calcareous plankton of 2 to 63 µm in diameter with coccolithophores, Thoracosphaera, chrysophytes, etc., but excluding bacterial picoplankton (Lohmann, 1909; Young et al., 1997). In the current paper we studied two groups of calcareous nannoplankton: (1) coccoliths, calcitic plates which cover the coccolithophore cell, and (2) nannoliths. Coccolithophores are unicellular algae, which belong to the phylum Haptophyta. These algae develop two types of base scale plates as a cell wall covering: holococcoliths and heterococcoliths. Both can be produced from the same organism and are thought to represent phase changes in a complex, possibly haplo- diplontic life cycle (Cros et al., 2000; Geisen et al., 2002). Holococcoliths are constructed of simple rhombohedral crystals, which are only scarcely preserved in fossil sediments. While holococcoliths calcify in an extracellular position, the calcification of heterococcoliths takes place in intracellular vesicles, derived from the Golgi body. Calcification starts with the nucleation of a proto-coccolith ring (PCR) of small crystals around the rim of a precursor base scale plate. Subsequently the crystals of the PCR-unit grow in various directions to form the crystal units (Young et al., 1992, 1999). This causes a growth approaching a parallel ellipsis (Young, 1989; Young et al., 1996). Young et al. (1992) developed a general growth model for heterococcoliths, the V/R model. According to this model, heterococcoliths consist of two basic units, a V- and an R-unit. In the V-unit the crystallographic c-axes of the calcite crystals are oriented vertically, in the R-unit these c-axes are oriented radially. Young and Bown (1991) applied this model for the genus Watznaueria, which is very common in the Mesozoic. This genus, which has been intensively studied in the current paper, consists of large R-units, which form both, the inner tube element, and the distal and proximal shield elements. Only the mid-tube element is formed by a V-unit, which is considered to represent the original PCR-unit (Young and Bown, 1991). Under polarizing light the peg-like V-unit is darker than the surrounding R-units, therefore these units can be easily distinguished. A second important group of calcareous nannoplankton are nannoliths, which are calcitic structures quite different from holo- and heterococcoliths. It is thought that nannoliths are also formed by haptophyte algae (Young et al., 1999). Most prominent representatives of this group in recent assemblages are Florisphaera profunda and Braarudosphaera bigelowii. In this paper we focus on three nannolith groups: Conusphaera mexicana, Polycostella beckmannii and Nannoconus spp. These groups are common in the latest Jurassic and we consider them to be of rock-forming importance due to their size and abundance. The factors controlling the calcification of haptophytes remain uncertain; recent studies show that mal- formation of coccoliths occurs under lower salinities (Green et al., 1998) and changes of pH (Riebesell et al., 2000; Zondervan et al., 2001). From cultures it is known that the calcification rate of common recent taxa like Emiliania huxleyi and Gephyrocapsa oceanica decreases with increasing atmospheric pCO2, whilst the production of particulate organic carbon increases (Riebesell et al., 2000; Zondervan et al., 2001). A decrease in the production of carbonate with increasing atmospheric pCO2 has also been suggested for other important carbonate producers like planktic foraminifera (Bijma et al., 1999; Barker and Elderfield, 2002), coralline algae (Gao et al., 1993) and corals (Kleypas et al., 1999; Leclercq et al., 2000). It is therefore Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 33

reasonable to assume that changes of atmospheric pCO2 and seawater pH will affect the global carbon system. The influence of seawater temperature on the calcification and size of coccoliths is still under debate. Numerous authors observed that the coccolith size may increase with falling water-temperature (McIntyre and Bé, 1967; Roth and Berger, 1975; Young, 1990; Renaud et al., 2002). The opposite observation has, however, been described as well (Wilbur and Watabe, 1963; Verbeek, 1989; Bollmann, 1997; Knappertsbusch et al., 1997). Culture studies of Blackwelder et al. (1976) and Weiss et al. (1976) suggest that concentrations of Ca2+, Mg2+ and Sr2+ in seawater may affect the rates of calcification and cell growth of the coccolithophore species Cricosphaera carterae. Toxic metals, in particular copper and cadmium, are considered to inhibit phytoplankton calcification (Brand et al., 1986).

3.5 Material and methods 3.5.1 Localities and stratigraphy In the current paper we studied a sequence of lower Tithonian to Valanginian sediments from three DSDP Sites (105, 534A, 367) in the Central Atlantic (Fig. 3.1). DSDP Site 105: This site is located at 34°54N, 69°10W, east of N America in the Hatteras Basin (Fig. 3.1). The investigated sequence is ~100 m thick and ranges from 587.60 to 487.86 m below seafloor (mbsf), the recovery is moderate (~70%; Fig. 3.2). A detailed description of the lithology is given by Hollister et al.

DSDP DSDP DSDP Tethyan Site 105 Site 534A Site 367 ammonite Calcareous nannofossil

a] zonation Calpionellids zonation bsf] (Hoedemaeker (Remane, (Bralower et al., 1989; bsf] agnetochrono- STAGE et al., 1993) 1986) Hardenbol et al., 1998)

SYSTEM

(Gradstein et al., 1994) al., et (Gradstein Age [M Age M stratigraphy

ples ples

N. bucheri ples

ore no./ ore ore no./ ore

Depth [m Depth Depth [m Depth

M11 N. (T.) callidiscus [mbsf] Depth

recovery recovery Sam Sam zones zones C C Nanofossil Lithology Nanofossil Lithology Sam

Core no./ Core recovery

Lithology Nanofossil 485 zones 1230 965 T. verenae NK- 25 NK- H. trinodosum 80 M11A 490 3 1240 3

M12 970 28 Upper T. verenae 495 81 135 S. verrucosum NK-3 26 1250 NK- M13A E. striatus ? 500 2B 82 M13 1260 995 B. campylotoxus Calpio- E. windii M14 E 83 VALANGINIAN T. pertransiens nellites 505 27 29 C. oblongata 1270 NK- T. otopeta NK- 84 3 Lower 2B 1000 M15 510 NK-2B 1280 NK- P. fenestrata 2A 85 515 Calpione- D 28 1290 llopsis R. wisei 86 1025 F. boissieri 520 30 1300 M16 NK-2A NK- 87 525 29 1 140 R. angustiforata 1310 88 530 NJK RYAZANIAN C 1320 89 -D 1055 31 T. occitanica 535 30 M17 NK-1 90

Middle 1330

LOWER CRETACEOUS D. rectus NJK 27 540 -C Calpionella NK- 91 1080 NK- B N. st. steinmannii 2A 31 1340 2B 545 M18 N. steinmannii minor NJK-D NJK 92 B. jacobi -B N. kamptneri minor 1350 NK- 550 93 1085 2A 32 Lower Upper NJK-C R. laffittei 32 1360 94 NK-1 M19 R. asper 555 NJK Durangites spp. -A 145 Crassi- P. beckmannii NJK-B NK-1 95 1090 A 1370 colaria U. granulosa 560 NJK-D M. microcanthum NJK-C NJK- 33 96 M20 H. noeliae 1380 20B NJK NJK-A 565 B H. chiastia 1105 B. ponti 97 NJ-20B NJK- N. compressus 570 A 1390 Chitinoidella P. beckmannii 20B 34 NJ- 33 M21 VOLGIAN S. fallauxi 20A 99 1110 NJ-20A 575 NJ- 1400 NJ- 20A Middle C. mex. mexicana 100 20 H. (H.) verruciferum/S. semiforme S. bigotii 580 1410 Lower Middle Upper 35 NJ- C. mexicana minor NJ- 101 1115 NJ- 34 M22 TITHONIANS. darwini /V. BERRIASIAN albertinum NJ-19B 19B 19B TITHONIAN BERRIASIAN VALANGINIAN 585 19B 1420 Z. embergeri NJ-19A 102 NJ- 150 36 NJ- H. (H.) hybonotum ? 19A 19A 19A Lower Upper 590 1430 1120 M22A first occurrence UPPER JURASSIC M23 H. (H.) beckeri last occurrence grey to white limestone red marlstone U. K. Fig. 3.2. Stratigraphy and lithology of the DSDP sites investigated. Correlation between the different biostratigraphic schemes are taken from Hardenbol et al. (1998). Greyish lines show the correlation of the stage boundaries of the three DSDP sites. 34 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

(1972), Ogg et al. (1983) and Frank et al. (1999). The lower Tithonian consists of reddish marls (Cat Gap Formation) and is overlain by greyish limestone (Blake Bahama Formation; Fig. 3.2). The Berriasian and lower Valanginian are dominated by bedding rhythms of marls and limestones. A total of 56 samples was studied from this site. DSDP Site 534A: Drilled in the Blake Bahama Basin, Site 534A is situated south of DSDP Site 105, east of Florida (28°20.6N, 75°22.9W; Fig. 3.1). The studied sequence is ~180 m thick (1238.44 to 1420.11 mbsf; Fig. 3.2). The lithology is similar to that of the other two sites, but the limestones are much more lithified. A description and discussion of the succession is given by Sheridan et al. (1983), Robertson and Bliefnick (1983) and Ogg et al. (1983). The recovery is moderate with ~75%. 25 samples were investigated from this sequence. DSDP Site 367: The lithology encountered resembles that of DSDP Site 105, due to its symmetrical position east of the mid-ocean ridge (Jansa et al., 1978). The site is situated in the Cape Verde Basin west off Africa (12°29.2N, 20°02.8W; Fig. 3.1). The investigated interval ranges from 968.12 to 1116.43 mbsf, but shows a poor recovery (~30%; Fig. 3.2). The lithology is described by Lancelot et al. (1978) and is discussed by Gardner et al. (1978), Ogg et al. (1983) and Frank et al. (1999). 38 samples were chosen for the nannofossil study. Biostratigraphic schemes for the JurassicCretaceous boundary interval based on calcareous nannofossils have been proposed for on-shore and pelagic settings by Thierstein (1971, 1973, 1975), Perch-Nielsen (1979, 1985), Roth (1978, 1983), Cooper (1984) and Bralower et al. (1989). We follow the zonation scheme suggested by Bralower et al. (1989; Fig. 3.2), who studied low-latitudinal DSDP sites. This zonation provides a high resolution frame while in addition the bioevents are correlated to magnetostratigraphy, calpionellids and ammonites. Absolute age estimates for bioevents and palaeomagnetic reversals have been adopted from Hardenbol et al. (1998; Fig. 3.2).

3.5.2 Assemblage analysis In order to calculate absolute abundances of calcareous nannofossils (nannofossils g-1 sediment) the random settling technique (Williams and Bralower, 1995; Geisen et al., 1999) was applied for slide preparation. In contrast to simple smear slides, this method provides a homogeneous distribution of the particles on the cover slide and an improved statistical reproducibility. The data obtained were corrected to the total water column within the settling box (Bollmann et al., 1998; Geisen et al., 1999). Abundances from settling slides were determined by counting at least 300 specimens or 200 fields of view, additionally a random traverse of the slide was scanned for rare species. From these counts the species richness S, heterogeneity HS and evenness E were calculated. S is the total number of species observed in a sample. HS was determined by using an information function (Shannon and Weaver, 1949), which character- izes the population by taking into account both the number of species and their relative abundances. E is a measure for how evenly the individuals are distributed among the species present in a given sample. S reflects changes in the number of species due to available habitats, evolution or migration (MacLeod et al.,

2000). Both HS and E are thought to reflect ecological factors, high values are considered to represent more stable or oligotrophic, low values more instable or eutrophic conditions for diagenetically unaltered nannofossil assemblages (Watkins, 1989). Preservation of nannofossils were qualitatively evaluated during counting the assemblage, therefore visual criteria following Roth and Thierstein (1972) and Roth (1983) for etching (E) and overgrowth (O) were used. Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 35

For counting calcareous nannofossils an OLYMPUS BH-2 light-microscope with a magnification of x1500 was used. Light-microscope images were carried out by using an OLYMPUS DP10 digital camera from settling slides; Scanning electronic microscopy (SEM) images were done on a LEO Gemini 1530 at the Ruhr-University of Bochum. Sample material and slides are housed at the Department of Geology, Mineral- ogy and Geophysics, Ruhr-University Bochum.

3.5.3 Morphometrics Morphometric studies were performed on an image analysis system consisting of a CCD camera mounted on an OLYMPUS BH-2 light-microscope and an Apple MacIntosh PowerPC with a SCION LG-3 framegrabber

Watznaueria spp. Conusphaera mexicana

ltrunc. lmax lV-unit lmax cone

lcone 2 µm 2 µm

wmin wV-unit wmax wmax . 3 Vtotal=Vcone+Vtrunc.cone-Vcanal V=ks lmax

Nannoconus spp. Polycostella beckmannii

wcanal

lmax 2 µm

2 µm 3 V=4/3p((d1+d2)/4) -30%

wmin wmax Vtotal=Vtrunc.cone-Vcanal

Fig. 3.3. Morphometric parameters of the four studied groups of calcareous nannofossils and the corresponding volume calculations. Abbreviations: trunc.cone, truncated cone; l, length; w, width. For further explanations see Section 3.5.3. 36 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... card. In addition, the software package ScionImage 1.62 (available at http://www.scioncorp.com) and macro routines written by M. Geisen (AWI-Bremerhaven, Germany) and J. Young (NHM, London; partly available at http://www.nhm.ac.uk/hosted_sites/ina/CODENET/CoccoBiom/) were used. The macro routines were modified and adjusted to our purposes. In a first step the specimens were captured electronically from live images with a macro routine (Young et al., 1996) and copied to a mosaic, which contains 50 specimens. In a second step the specimens were measured manually by using an unpublished macro routine developed by M. Geisen. The precision of size measurements depends on the pixel resolution of the framegrabber card and the magnification of the microscope. In this study the accuracy of size measurements is 0.056 µm per pixel. In this study we investigated changes in size of common nannofossil groups in order to estimate their contribution to the nannofossil carbonate accumulation and to test whether these variations are related towards palaeoenvironmental changes. The genus Watznaueria was chosen, because it is the only taxon, which occurs in high abundances throughout the studied interval of all three DSDP sites. We decided to measure the V-unit (length, width) for 100 specimens per sample (Fig. 3.3), because this unit can be also easily identified on strongly dissolved specimens, whereas the total placolith length can be affected by dissolution. For well preserved and oriented specimens measurements of the total length and width were carried out on at least 50 specimens per sample from the same mosaics. Taxonomically, six species of the genus Watznaueria were distinguished when performing counts and size measurements. Three of these species, W. barnesae, W. fossacincta and W. ovata, were taxonomically separated by the presence and the size of a central opening. A fourth species, Watznaueria manivitae is separated from W. barnesae and W. fossacincta by its larger size; large forms (> 8 µm) with a small or closed central opening were referred to W. cf. manivitae. Species showing a transverse bar were assigned to W.

Watznaueria spp. Watznaueria spp. maximum length, sample 105-34-5,65-66 cm V-unit length, sample 105-34-5,65-66 cm

20 15 A1 A1 N50 sd. 1.49 µm 10 N 50 sd. 0.61 µm N 10 mean 6.23 µm 95%cl.0.41 µm N mean 3.13 µm 95%cl.0.17 µm min. 3.82 µm skew. 0.89 5 min. 1.93 µm skew. 1.22 max. 11.19 µm kurt. -0.72 max. 5.11 µm kurt. 0.68 0 0 20 15 A2 A2 N50 sd. 1.44 µm 10 N 50 sd. 0.52 µm N 10 mean 6.25 µm 95%cl.0.40 µm N mean 3.11 µm 95%cl.0.14 µm min. 3.88 µm skew. 1.80 5 min. 2.12 µm skew. 1.28 max. 11.47 µm kurt. 2.41 max. 4.98 µm kurt. 0.39 0 0 20 15 B1 B1 N50 sd. 1.53 µm 10 N50 sd. 0.62 µm N 10 mean 6.24 µm 95%cl. 0.42 µm N mean 3.22 µm 95%cl. 0.17 µm min. 3.93 µm skew. 0.90 5 min. 2.14 µm skew. 2.13 max. 11.39 µm kurt. -0.82 max. 4.84 µm kurt. 4.61 0 0 20 15 B2 B2 N50 sd. 1.50 µm 10 N 50 sd. 0.63 µm N 10 mean 6.27 µm 95%cl.0.42 µm N mean 3.15 µm 95%cl.0.17 µm min. 3.68 µm skew. 0.90 5 min. 1.91 µm skew. 1.59 max. 12.42 µm kurt. 0.17 max. 5.65 µm kurt. 1.98 0 0 40 30 A1+2 30 A1+2 N 100 sd. 1.46 µm 20 N 100 sd. 0.56 µm N 20 mean 6.24 µm 95%cl.0.29 µm N mean 3.12 µm 95%cl. 0.11 µm 10 min. 3.82 µm skew. 1.30 10 min. 1.93 µm skew. 1.28 max. 11.47 µm kurt. 0.35 max. 5.11 µm kurt. 0.66 0 0 40 B 30 30 1+2 B1+2 N 100 sd. 1.51 µm 20 N 100 sd. 0.62 µm N 20 mean 6.26 µm 95%cl.0.30 µm N mean 3.19 µm 95%cl.0.10 µm 10 min. 3.68 µm skew. 1.09 10 min. 1.91 µm skew. 1.39 max. 12.42 µm kurt. -0.27 max. 5.65 µm kurt. 0.68 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1.52.02.53.03.54.04.55.05.5 Size [µm] Size [µm] Fig. 3.4. Reproducibility of morphometric measurements. Abbreviations: N, number of measurements; min., minimum; max., maximum; sd., standard deviation; 95%cl., 95% confidence limit; skew., skewness; kurt., kurtosis (ex- cess). Histograms are showing the frequencies of different size classes for 50 specimens measured (length V-unit and total length of Watznaueria spp.; sample 105-34-5, 65-66 cm, NJ-20A nannofossil subzone). From two different areas of the slide (A, B) two times 50 specimens have been measured (A , A , B , B ). In order to compare the results to a 1 2 1 2 higher number of measurements two datasets have been combined to achieve 100 measurements (A , B ). For both 1+2 1+2 the V-unit length and the total coccolith length mean values, the 95% confidence limits and the standard deviations show a sufficient reproducibility, whereas parameters describing the shape of the size distribution vary significantly. The reproducibility is only slightly improved by measuring 100 specimens. Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 37 britannica. The sixth species, W. biporta is unequivocally characterized by two large perforations in the central area; this form has been observed only rarely. For the middle Tithonian to lowest Berriasian of DSDP Site 105 size measurements of other common nannofossils were carried out. The maximum length of C. mexicana, P. beckmannii and Nannoconus spp. was measured for 50 specimens per sample (Fig. 3.3). Only those samples were included, in which the particular group exceeds a relative abundance of 3%. Altogether more than 13,000 specimens were measured for this study. The reproducibility of morphometric measurements was tested by measuring four times 50 specimens from the same slide (Fig. 3.4). We found a sufficient reproducibility for the mean, the 95% confidence limit and the standard deviation, whereas parameters describing the shape of the size distribution curve (skewness, kurtosis) show a broad variability. Thus we decided to plot only the mean value for each sample with the corresponding 95% confidence limit and the standard deviation.

3.5.4 Volumetrics and nannofossil carbonate accumulation A combination of the random settling technique and morphometrical investigations on coccoliths and nannoliths allows the calculation of the amount of carbonate supplied by calcareous nannofossils. This is achieved by using a method described by Young and Ziveri (2000) for coccoliths; for nannoliths an approxi- mation to simple geometrical bodies has been applied. The values were compared to the total carbonate content of bulk-rock samples calculated from AAS measurements (Varian SpectrAA 300; Ruhr-University of Bochum). The most useful method to estimate the volume of coccoliths is described by Young and Ziveri (2000).

According to this approach the shape of each body can be described by a shape factor kS. This factor multi- . 3 plied with the cube of the maximum length of the body is giving the volume (V= kS lmax ). These authors published numerous kS factors for recent coccoliths. For Jurassic and Cretaceous species only few values are yet available (Tremolada and Young, 2002). In order to obtain volumetric data cross-sections of the most common coccolith taxa of the late Jurassic and early Cretaceous were analyzed and subsequently shape factors were calculated (see Table 3.1). These taxa include Biscutum constans, Calcicalathina oblongata, Cretarhabdus/Retecapsa spp., Cruciellipsis cuvillieri, Cyclagelosphaera deflandrei, Cyclagelosphaera margerelii, Diazomatolithus lehmanii, Discorhabdus ignotus, Rhagodiscus asper, Tubodiscus verenae, Watznaueria spp., and two groups of both Zeugrhabdotus embergeri and Zeugrhabdotus erectus. One of the two groups of Z. embergeri is representing early forms of this species which have been mainly observed in late Jurassic samples. These forms are smaller (Z. embergeri sp.1; mean length 8.54 µm; Table 3.1) than the representatives of the second group (Z. embergeri sp.2; mean length 10.4 µm; Table 3.1) and less calcified. The two groups of Z. erectus can be easily distinguished. One is abundant in the Jurassic, consists of a broader transverse bar and is larger (Z. erectus sp.1; mean length 4.6 µm; Table 3.1; Fig. 3.5). This form possibly represents an evolutionary linkage to Z. embergeri (Bralower et al., 1989). In contrast, forms which are common in the Cretaceous are usually smaller (Z. erectus sp.2, mean length 3.1 µm; Table 3.1; Fig. 3.5) and have a less distinctive transverse bar. Both have been assigned to the species Z. erectus by Bown and Cooper (1998) and Burnett (1998). Size variations with time were taken into account only for those groups, which provide high amounts of biogenic carbonate (C. mexicana, P. beckmannii, Nannoconus spp., Watznaueria spp.). For all other taxa, the kS factor was determined by using at least three different cross sections of each species. A general value for the volume for these taxa was calculated from the average length of 30 specimens and the mean of the 38 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Fig. 3.5. Calcareous nannofossils, light microscope (crossed polarizers) images. (1, 2) Biscutum constans (Górka) Black, sample 105-28-2, 98-99 cm; (3) Cruciellipsis cuvillieri (Manivit) Thierstein, sample 105-26-2, 104-105 cm; (4) Cyclagelosphaera deflandrei (Manivit) Roth, sample 105-35-3, 47-48 cm; (5) Cyclagelosphaera margerelii Noël, 105-28-2, 98-99 cm; (6) Conusphaera mexicana Trejo ssp. minor Bown and Cooper, sample 105-34-5, 96-97 cm; (7) Conusphaera mexicana Trejo ssp. mexicana Bralower et al., sample 105-33-5, 97-98 cm; (8) Discorhabdus ignotus (Górka) Perch-Nielsen, sample 105-28-2, 98-99 cm; (9) Hexalithus noeliae Loeblich and Tappan, sample 105-29-2, 29-30 cm; (10) Nannoconus kamptneri Brönnimann, sample 105-32-2, 34-35 cm; (11) Polycostella beckmannii Thierstein, sample 105-33-5, 97-98 cm; (12) Speetonia colligata Black, sample 105-25-3, 96-97 cm; (13) Stephanolithion bigotii Deflandre, sample 105-35-3, 47-48 cm; (14) Watznaueria barnesae (Black) Perch-Nielsen with Watznaueria fossacincta (Black) Bown and Cooper, sample 105-34-5, 986-97 cm; (15) Watznaueria biporta Bukry, sample 105-28- 4, 112-113 cm; (16) Watznaueria britannica (Stradner) Reinhardt, sample 105-35-3, 47-48 cm; (17) Watznaueria manivitae Bukry, sample 105-35-3, 47-48 cm; (18) Zeugrhabdotus embergeri (Noël) Perch-Nielsen, sample 105-28-4, 33-34 cm; (19) Zeugrhabdotus erectus sp.1 (Deflandre in Deflandre and Fert) Reinhardt, sample 105-35-4, 65-66 cm and (20) Zeugrhabdotus erectus sp.2 (Deflandre in Deflandre and Fert) Reinhardt, sample 105-28-2, 98-99 cm. Scale bar: 1 µm. Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 39

Fig. 3.6. Calcareous nannofossils, scanning electron microscope images. (1) Axopodorhabdus dietzmannii (Reinhardt) Wind and Wise, sample 105-28-2, 98-99 cm; (2) coccosphere of Cyclagelosphaera margerelii Noël, sample 105-35-4, 65-66 cm; (3) Conusphaera mexicana Trejo ssp. mexicana Bralower et al., sample 105-33-5, 97-98 cm; (4) C. mexicana ssp. mexicana, sample 105-34-1, 104-105 cm; (5) Hexalithus noeliae Loeblich and Tappan, sample 105-28-2, 98-99 cm; (6)-(8) Nannoconus cf. N. kamptneri Brönnimann, sample 105-32-2,34-35 cm; (9, 10) Polycostella beckmannii Thierstein, sample 105-34-1, 104-105 cm; (11) P. beckmannii, sample 105-33-5, 97-98 cm; (12) Staurolithites quadriaculla (Noël) Wilcoxon, sample 105-35-4, 65-66 cm; (13) distal view Watznaueria fossacincta (Black) Bown and Cooper 105-32-2, 34-35 cm; (14) proximal view W. fossacincta, sample 105-34-1, 104-105 cm; (15) coccosphere W. fossacincta, sample 105- 28-2, 98-99 cm; (16) Zeugrhabdotus erectus (Deflandre) Reinhardt, sample 105-35-4, 65- 66 cm. Scale bar: 1 µm.

determined kS factors. The mean, maximum and minimum values of the most common taxa are given in

Table 3.1. All these species together make up 73 to 98% of the nannofossil assemblage. The volume and mass of species which are not mentioned in Table 3.1 have been estimated by approximating the shape and size to forms consisting of a similar shape which have been studied in more detail. The geometry of nannoliths is more simple than those of the coccoliths. For the nannoliths C. mexicana, P. beckmannii and Nannoconus spp. 50 specimens per sample were measured (see Section 3.5.3) and their shape was approximated to a simple geometrical body. 40 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Table 3.1. Overview of mean sizes, k shape factors, volumes and weight of important nannofossil taxa (N: number of S measurements). All size measurements and shape factors k are from samples of DSDP Site 105. The size and mass S estimates of Assipetra infracretacea (1) have been adopted from Tremolada and Young (2002).

The body of conuspheres was divided into two parts: (1) a long truncated cone, and (2) a short flattened cone (Figs. 3.3, 3.5 and 3.6). From this a cylindrical axial canal with a mean diameter of 0.4 µm was subtracted. The diameter of the canal was measured on SEM images and seems to remain more or less constant for all specimens observed under the SEM. Polycostellids are spherical bodies with a central cavity and an obvious porosity (Figs. 3.3, 3.5 and 3.6). 30% of the sphere volume was subtracted due to the porosity and the cavity. High abundances of these two groups are only documented from the Tithonian (Thierstein, 1975; Bralower et al., 1989); during Cretaceous times these groups became very rare or even disappeared. The third group of nannoliths, the genus Nannoconus, is not exclusively abundant in the Tithonian; it became extinct only in the Campanian (Deres and Achéritéguy, 1980). Nannoconids are abundant in the Berriasian to Aptian interval in particular within the western Tethys with crises in the Valanginian and Aptian (Bersezio et al., 2002). The outer shape and the axial canal of Nannoconus were measured under the light-microscope and approximated to a cylinder or a truncated cone (Figs. 3.3, 3.5 and 3.6). It has to be considered that all geometrical parameters show a strong variability due to size variations with time, porosity, dissolution or fragmentation of nannofossils. Young and Ziveri (2000) calculated a cumulative error of up to 50% for their method. Despite of such an error the obtained data are valuable, because they are giving an estimate of both volume and mass of late Mesozoic coccoliths. This will supply more accurate data about the composition of ancient pelagic carbonates and allow for the first time to esti- Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 41 mate changes in the nannofossil carbonate accumulation through the latest Jurassic and earliest Cretaceous. The proportion of the sediment which is made up by calcareous nannofossil carbonate was calculated in the following way: n () (1) N =100 ⋅ ∑ AA ⋅ V ⋅ Ncarb: nannofossil carbonate (wt%); AAi: absolute abundance carb i i cc i=1 of species i (number of nannofossils g-1 sediment); V : mean i volume of species i (1 cm3 = 1012 µm3); density of calcite (2.7 g cm-3). Both datasets, absolute abundances and the amount of nannofossil carbonate are usually biased by dilution. Therefore we calculated accumulation rates (AR) of nannofossils and nannofossil carbonate. We applied the method of van Andel et al. (1975) and used GRAPE wet-density (DW) and porosity (P) data from

DSDP Site 105 (Hollister et al., 1972) to calculate dry-bulk densities (DD) assuming that the total pore space is filled with seawater (density 1.025 g cm-3). In addition, sedimentation rates (SR) were estimated by using absolute age data of nannofossil events (Hardenbol et al., 1998; Fig. 3.2). The following equations were used to calculate dry-bulk densities and accumulation rates: . -3 (2) DD = DW - 1.025 PDD, DW: (g cm ) . . -2 -1 (3) ARN = DD SR AA AR of calcareous nannofossils: (number of nannofossils cm kyr ); AA: absolute abundance of calcareous nannofossils: (number of nannofossils g-1 sed.) . . -2 -1 (4) ARNcarb = DD SR Ncarb /100 AR of nannofossil carbonate: (g cm kyr )

. . -2 -1 (5) ARcarb = DD SR CaCO3bulk /100 AR of bulk-rock carbonate: (g cm kyr )

In order to explain possible differences between nannofossil carbonate and the bulk-rock carbonate content we quantified the proportions of the different carbonate sources (unidentifiable carbonate, nannofossil fragments and counted calcareous nannofossils) by performing visual estimates from settling slides (20 fields of view). This was exemplarily done on four samples from different lithologies at DSDP Site 105; three of these samples have been also studied under the SEM. Size measurements of nannolith taxa and estimates of nannofossil carbonate accumulation were only applied to material from DSDP Site 105. This is the only site investigated which contains a well to moderately preserved nannoflora throughout the studied interval, allowing a detailed investigation with the lowest degree of diagenetic influence on the results.

3.6 Results 3.6.1 Diversity and absolute abundances At DSDP Site 105 (Fig. 3.7) species richness S increases from 20 species (earliest Tithonian) to more than 50

(late Berriasian). The diversity indices show high values in the early Tithonian (heterogeneity HS 3.1; evenness

E 0.88; NJ19-B) and throughout the Berriasian (HS 3.0; E 0.8), a distinctive minimum occurs in the mid- to late Tithonian (HS 2.2; E 0.7; NJ-20A to NJK-B). This decrease is most likely caused by the dominance of only few taxa (C. mexicana, P. beckmannii, Nannoconus spp., Watznaueria spp.). Absolute abundances of calcareous nannofossils from this site vary from 8E+8 nannofossils g-1 sed. (early Tithonian) to 3E+9 nannofossils g-1 sed. (late Berriasian). At DSDP Site 534A (Fig. 3.8) S decreases from 28 (early Tithonian; NJK-A) to 12 species (middle Tithonian). During the latest Tithonian and earliest Cretaceous, S increases continuously to more than 40 species (early Berriasian) and decreases subsequently to 30 species (latest Berriasian). The diversity indices 42 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... reveal similar patterns as those of Site 105, with highest values in the early Tithonian and the Berriasian (H S 2.6; E 0.75) and a minimum in the middle Tithonian (H 2; E 0.6; NJK-A). Absolute abundances are very low S and vary from 8E+7 to 8E+8 nannofossils g-1 sed. In the late Berriasian (NK-2A) the abundance increases in one sample to 3E+9 nannofossils g-1 sed. In the Cape Verde Basin (Site 367; Fig. 3.9) S increases from a minimum of 10 (early Tithonian) to a maximum of 50 species (Valanginian), for HS from 1.4 to more than 3.0 (E 0.58 to 0.82). The absolute abundance is much lower than at DSDP Site 105, it does rarely exceed 5E+8 nannofossils g-1 sed. It is only in the Valanginian that values up to 2E+9 nannofossils g-1 sed. are reached. All three sites show a general increase of species richness and absolute abundances during the interval under investigation. In the sites east of N America (Sites 105, 534A) the diversity (HS, E) shows high values in the early Tithonian and during the Berriasian, whereas the mid- to late Tithonian is characterized by a distinctive minimum. The latter interval is only partly recovered at DSDP Site 367.

3.6.2 Assemblage composition Similar patterns in the composition of the nannofossil assemblages were observed for all three DSDP sites investigated, despite of different states of preservation. The nannofossil assemblages of the Tithonian to Berriasian interval are characterized by long-term changes, on the scale of several million years. These changes allow to differentiate three typical assemblages. The first phase (early to early mid-Tithonian, NJ-19A to lower NJ-20A) is marked by an assemblage dominated by Watznaueria spp. (W. barnesae, W. fossacincta, W. cf. manivitae), C. margerelii and Zeugrhabdotus spp. These taxa reach about 85% of the total relative abundance. Watznaueria makes up to 70% of the assemblage at Site 105 (Fig. 3.7), 60% at Site 534A (Fig. 3.8) and 90% at Site 367 (Fig. 3.9). In addition, DSDP Site 105 is characterized by higher abundances of Biscutum spp. (up to 12%), Site 534A by common D. ignotus (up to 9%) and D. lehmanii (up to 6%). The second phase (mid- to late Tithonian, NJ-20A to lower NK-2A) exhibits a remarkable shift towards a nannolith-dominated assemblage. A flora consisting of Watznaueria spp., C. mexicana, P. beckmannii and Nannoconus spp. replaces the Watznaueria - C. margerelii - Zeugrhabdotus assemblage. At DSDP Sites 105 and 534A C. mexicana has its maximum in the NJ-20A nannofossil subzone, P. beckmannii follows in the upper NJ-20A nannofossil subzone and Nannoconus becomes abundant in NJK-A to NK-1 interval (Site 105: NJK-A to NK-2A; Site 534A: NJK-B to NJK-D). These taxa make up more than 25% of the nannofossil assemblage at their distinctive maxima. At Site 367 the mass occurrences of C. mexicana (40%) and P. beckmannii (10%) are overlapping in the NJ-20 to NJK interval. Nannoconus reaches its maximum with 25% in the NK-2 nannofossil zone. The abundances of P. beckmannii and Nannoconus are lower than those of the two other DSDP sites. It has to be considered that the stratigraphy for this site is not as accurate as that for the others due to poor preservation and recovery gaps. While C. mexicana and P. beckmannii are very abundant only in the mid-Tithonian, Nannoconus spp. remains common through the JurassicCretaceous boundary and shows higher abundances during the Berriasian. The occurrence of P. beckmannii in the Valanginian of DSDP Site 367 possibly indicates redeposition of Tithonian sediments. The JurassicCretaceous boundary approximates the onset of the third phase with Watznaueria spp., B. constans, D. lehmanii, R. asper and D. ignotus becoming abundant. Other species have their first appearance in this interval or became more common, e.g. Rotellapillus laffitei, C. cuvillieri, Lithraphidites carniolensis, Nannoconus kamptneri, Nannoconus steinmannii, R. asper, Speetonia colligata and Umbria granulosa. This third phase is represented by all important taxa, which dominated the nannofossil assemblages in early

Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 43

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NJ- NJ- NK- NK- NK- 20A 19B 19A

NJK-C NJK- NJK- NK-1

S ta g e Val. Berriasian Tithonian DSDP Site 105 DSDP

Fig. 3.7. Vertical distribution pattern of CaCO3 content and calcareous nannofossils in the Tithonian to Valanginian interval of DSDP Site 105. Note different horizontal scales.

44 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

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L ith o lo g y

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NJ- NJ- NJ- NK- NK- NK- 20B 20A 19B 19A NJK NJK NJK NJK

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Berriasian Tithonian Sta g e Val. DSDP Site 534A DSDP

Fig. 3.8. Vertical distribution pattern of CaCO3 content and calcareous nannofossils in the Tithonian to Valanginian interval of DSDP Site 534A. Nannofossil biostratigraphy is taken from Bralower et al. (1989). Note different horizontal scales.

Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 45

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S ta g e Valanginian Berriasian Tithonian DSDP Site 367 DSDP

Fig. 3.9. Vertical distribution pattern of CaCO3 content and calcareous nannofossils in the Tithonian to Valanginian interval of DSDP Site 367. Note different horizontal scales. 46 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Cretaceous times in the Atlantic. Thus it is possible to differentiate between an early Tithonian assemblage (Watznaueria spp. - C. margerelii - Zeugrhabdotus spp.), a mid- to late Tithonian assemblage ( C. mexicana - P. beckmannii - Nannoconus spp. - Watznaueria spp.) and an early Cretaceous assemblage (Watznaueria spp. - B. constans - R. asper, etc.).

3.6.3 Morphometry At Site 105 larger forms of Watznaueria became more common causing an increase in total coccolith length of Watznaueria throughout early to mid-Tithonian times (NJ-19 to NJK-A nannofossil zone). A maximum is reached in the upper Tithonian (NJK-A/B), where large-sized forms of this genus ( W. cf. manivitae) became abundant (Fig. 3.10). This is followed by a sharp decrease from 6.6 to 5.1 µm across the JurassicCretaceous boundary (V-unit 3.3-2.6 µm). A second, less distinctive minimum in size occurs in the middle Berriasian (lower NK-2B). The measurements for Site 534A show a steady course, with largest forms in the upper Tithonian (lower NJK-A). This maximum is followed by a steady decrease in size (total coccolith length 7 to 5 µm, V-unit length 3.3 to 2.5 µm; Fig. 3.10). Measurements from DSDP Site 367 show a decrease in size from 7.3 to 5 µm for the total coccolith length and from 3.4 to 2.5 µm for the V-unit (JurassicCretaceous boundary). It is striking that geographically the sizes of Watznaueria specimens from DSDP Site 367 are up to 1 µm larger (V-unit 0.5 µm) than those of the DSDP Sites 105 and 534A (Fig. 3.10). A more detailed study was carried out for the Tithonian and lower Berriasian of DSDP Site 105. We compared measurements of the total length of Watznaueria to size variation of C. mexicana, P. beckmannii and Nannoconus spp. Conusphaera shows a continuous increase of the average sizes from 2 to 8 µm (Fig. 3.11) from the base of NJ-20A to the NJK-B nannofossil subzone. The upper part of the NJK-B zone is characterized by a distinctive minimum, around 6 µm. In the NJK-C zone the mean size reaches again values around 8 µm. P. beckmannii occurs only in the interval from the NJK-A zone to the NJK-C zone. This species has an average diameter of 4.0 µm from its first occurrence to the basal part of the NJK-B nannofossil zone. It increases to 4.8 µm in the uppermost part of NJK-B and in the subsequent NJK-C zone (Fig. 3.11). The evolutionary development of size of Nannoconus spp. is showing a pattern similar to that of P. beckmannii for the NJK-A to NJK-C zones. The size variation has, however, a broader range. The maximum length of specimens decreases from 6.8 µm (NJ-20B) to less than 4 µm (NJK-B); from NJK-C onwards the length increases to 6 µm and more (Fig. 3.11). These variations in size are reflecting changes in the species composition. Medium-sized forms (N. compressus, N. dolomiticus) are the first nannoconids to appear. The subsequent decrease in size in the upper Tithonian was caused by the dominance of the small form N. infans. In the Berriasian an increase in size is related to the evolution of two common species N. steinmannii and N. kamptneri. Early forms include the small subspecies (< 9 µm; N. steinmannii ssp. minor, N. kamptneri ssp. minor), whilst the later subspecies (N. kamptneri ssp. kamptneri, N. steinmannii ssp. steinmannii) are larger. In all three DSDP sites the upper Tithonian interval is characterized by high abundances of large-sized Watznaueria, resulting in a maximum of size, which is subsequently followed by a sharp decrease of up to 2 µm across the JurassicCretaceous boundary. In addition, the standard deviation of the size mean decreases throughout the Berriasian, reflecting a smaller range in the variance of the size. Generally, during the mid- to late Tithonian all three nannolith groups show a major increase in their size.

Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 47

m])

µ Watznaueria

456789

(length coccolith [

m])

Watznaueria

(length V-unit [ (length V-unit

22.533.54

D ep th [m b sf]

965 970 995 1000 1025 1055 1080 1085 1090 1105 1110 1115 1120

reco ve ry

C ore no ore

./ 28 29 30 31 32 33 34 27

L ith o lo g y

zo ne s

N a n ofossil 3

2A 2B NK- 20

NJ- NK- NJ- NJ- 19B NK- 19A NJK NK-1

S ta ge Valanginian Berriasian Tithonian

DSDP Site 367

m])

Watznaueria

(length coccolith [

m])

Watznaueria

(length V-unit [ (length V-unit

2535456789 22.533.5

D e p th [m th b sf]

1230 1240 1250 1260 1270 1280 1290 1300 1310 1320 1330 1340 1350 1360 1370 1380 1390 1400 1410 1420 1430

re co very

C o re n o

./ 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 99 100 101 102

L itho lo g y

zo n e s

N a no fo ssil 3 1 -B -C -A

-D 2B 2A NJ- NJ- NJ- NK- NK- NK- NJK NJK NK- 20B 19A NJK 20A 19B NJK

S ta g e ihna Berriasian Tithonian Val. DSDP Site 534A

9 m])

8 µ 7 6

5 Watznaueria

4 (length coccolith [

m])

µ Watznaueria

2 2.5 3 3.5 4 4.5 (length V-unit [ (length V-unit

1.5 D e p th [m b sf]

485 490 495 500 505 510 515 520 525 530 535 540 545 550 555 560 565 570 575 580 585 590

re co ve ry

C o re n o

./ 25 26 27 28 29 30 31 32 33 34 35 36

L ith o lo gy

zo n e s

N a n o fo ssil B

3 A NJK-D 20B 2B 2A

NJ- NJ- 19A NK- NK- NK- 20A 19B NJK-

NJK-C

NK-1 NJK-

S ta g e Tithonian Berriasian Val. DSDP Site 105

Fig. 3.10. Size variations of the genus Watznaueria during the Tithonian to lower Valanginian at DSDP Sites 105, 367 and 534A. The V-unit was measured in 100 specimens per sample, the total coccolith length was measured of at least 50 specimens (see text). The thick bar is indicating the 95% confidence limit of the mean and the thin bar the standard deviation. Note that size variations of the total coccolith length correlates with that of the V-unit, which is not affected by dissolution. This suggests that the total size represents the original trend. 48 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Morphometry DSDP Site 105

Conusphaera Polycostella

bsf] Watznaueria spp. mexicana beckmannii Nannoconus spp. (max. length [µm]) (length [µm]) (diameter [µm]) (length [µm])

456789 2 3 4 5 6 7 8 9 3 4 5 3 4 5 6 7 8

Depth [m Depth

Stage zones Nanofossil Core no./recovery Core Lithology 550

32 NK -2A

555

NK -1

NJK-D NJK 560 -C

33 NJK -B

565

NJK -A

570 20-B 34

NJ- 20A 575 Tithonian Berriasian

580 35

NJ- 19B

585

36 NJ- 19A 590 Fig. 3.11. Size variations of nannofossil taxa, which are important for the carbonate production in the Tithonian (Conusphaera mexicana, Polycostella beckmannii, Nannoconus spp., Watznaueria spp.). For the three nannolith taxa 50 specimens of each group were measured per sample. The thick bar is indicating the 95% confidence limit of the mean and the thin bar the standard deviation.

3.6.4 Calcareous nannofossil carbonate accumulation of DSDP Site 105 The bulk-rock carbonate in all investigated DSDP sites is usually less than 40 wt% in the lower Tithonian (Figs. 3.7, 3.8, 3.9 and 3.12). A first increase to ~60 wt% occurs in the NJ-20 nannofossil zone (105, 534A; at 367: ~80 wt%). A second increase to more than 70 wt% CaCO3 has been documented for the early Berriasian of the DSDP Sites 105 and 534A. This latter increase coincides with the lithological change from marlstones to limestones across the JurassicCretaceous boundary. According to our calculations, on average 27% (range: 7.5-76%) of the bulk-rock carbonate is contributed by calcareous nannofossils. The main sources of the nannofossil carbonate are either strongly calcified nannofossils (C. mexicana, P. beckmannii, Nannoconus spp., W. cf. manivitae; mid- to late Tithonian) or high absolute abundances of coccoliths (Berriasian). The amount of carbonate supplied by nannofossils peaks in the mid-Tithonian where high numbers of nannoliths and large-sized Watznaueria co-occur, contributing up to 80% to the nannofossil carbonate (Fig. 3.12). In these samples up to 24 wt% of the Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 49 3 ) 3 CaCO ] -1 , non-CaCO 3 kyr -2 3 [g cm non- CaCO Accumulation rates (bulk-rock CaCO 01234 1.6 ] -1 kyr -2 ] -1 kyr -2 0.8 0.5 NCE accumulation rates [g cm 3 nannofossils cm 10 0.25 0.75 1 (selected nannolith taxa, other nannofossil groups) others CaCO [x10 Nannofossil accumulation rates 0 0 3 sed.]

-1 ]

-1

Linear [cm kyr nannofossils g 9 1

Absolute abundance sedimentation rate sedimentation [x10 0123 0 100 80 others 60 [%] contribution of 40 3 CaCO 20 different nannofossil groups different 0 100 ) 80 3 .manivitae spp. others 60 cf

W. [%] 40

Watznaueria (without other nannofossil taxa bulk-rock CaCO Composition of the

20 nannofossil assemblage others 0 (a) (b) (c) (d) (e) (f) manivitae

cf. 3 spp.)

[wt%]

CaCO bulk-rock) bulk- rock 25 75

important Nannolith taxa important (Conusphaera mexicana, Polycostella beckmanii, Nannoconus Watznaueria

(selected nannolith taxa, other nannofossil groups, others

0 Depth [mbsf] Depth

485 490 495 500 505 510 515 520 525 530 535 540 545 550 555 560 565 570 575 580 585 590

recovery

Core no./ Core 25 26 27 28 29 30 31 32 33 34 35 36

Lithology

zones Nanofossil B A

3 NJK-D 20B 2A 2B

NK-

NJ-

NJ- 19A NK- NK- 20A 19B NJK- NJK- NJK-C NK-1

Tithonian Berriasian Val. Stage DSDP Site 105 DSDP

Fig. 3.12. Carbonate accumulation at DSDP Site 105. (a) Carbonate contents compared to the amount of nannofossil carbonate. (b) Simplification of assemblage composition. (c) Proportions of the same groups as in (b) which they contribute to the total nannofossil carbonate. (d) Absolute abundances (grey line) and linear sedimentation rates (black line with dots). (e) Accumulation rates of nannofossils (grey line) and nannofossil carbonate (black area: nannoliths; white area: other coccoliths; NCE: Nannofossil Calcification Event). (f) Carbonate accumulation rates vs. accumulation rates of non-carbonate material. 50 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Fig. 3.13. Scanning electron microscope images of sediments, white bar = 10 µm. (1) Upper Jurassic sediment (sample 105-35-4, 65-66 cm; NJ-19B nannofossil subzone) with well preserved nannofossils bedded into finely laminated clays. (2,3) Mid-Tithonian marls show high abundances of Conusphaera mexicana and Polycostella beckmannii (sample 105-34-1, 104-105 cm; NJK-A nannofossil subzone)(2), but also areas with diagenetic carbonate and recrystallized or overgrown nannofossils occur (3). (4) Typical nannofossil bearing limestone from the Berriasian (sample 105-28-2, 98-99 cm; NK-2B nannofossil subzone). Nannofossils (Watznaueria spp., Diazomatolithus lehmannii, Cyclagelosphaera margerelii, Biscutum contans) are dominating this facies, central processes and broken pieces of coccoliths are common, but also unidentifiable calcite occurs and makes up ~25% of the sediment (see Fig. 14). Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 51 sediment and 76% of the bulk carbonate content can be linked to calcareous nannofossils. Large specimens of W. cf. manivitae or Nannoconus attain a weight of greater than 1,500 pg per specimen (Table 3.1). These two taxa are up to 1,000-times heavier than smaller, less calcified forms like B. constans or D. ignotus. Lateron in the NK-2B nannofossil subzone the nannofossil carbonate only once reaches high values, caused mainly by high absolute abundances of Watznaueria spp. and other coccoliths. In the Berriasian most of the nannofossil carbonate is contributed by normal-sized Watznaueria and other coccolith taxa (Fig. 3.12). In order to explain the large difference between the calculated nannofossil carbonate and the measured bulk-rock carbonate content SEM studies were performed. These revealed that the early Tithonian successions consist of clay minerals and calcareous nannofossils with mainly Watznaueria and delicate species of Biscutum, Stephanolithion and Zeugrhabdotus. Secondary carbonate has been only rarely observed, whereas fragments of coccoliths are common (Fig. 3.13). In the middle Tithonian the nannofossils are often showing a significant overgrowth, carbonate of unknown origin becomes more common (Fig. 3.13). The discrepancy between the nannofossil carbonate and the bulk carbonate content increases throughout the latest Tithonian and Berriasian (Fig. 3.12). A late Berriasian sample yields high abundances of nannofossil carbonate with high amounts of coccolith fragments (central processes, single elements; Fig. 3.13). Fragments of nannofossils were not taken into account while counting the assemblage and were therefore not included in our calculations. Visual estimates of the different types of carbonate (Fig. 3.14) show that usually only about 50% or less of the carbonate particles can be linked to nearly complete nannofossils. The other 50% are made up by nannofossil fragments or unidentifiable carbonate, mostly micrite. These estimates support the assumption that an increase of carbonate contents goes along with higher amount of nannofossil fragments and non-nannofossil carbonate (Fig. 3.14). The amounts of nannofossil carbonate derived from visual estimates are slightly higher than the calculated values using the volumetric approach. Comparing the nannofossil carbonate record with the bulk-rock carbonate content it becomes apparent that the Tithonian increase of nannofossil carbonate does not accurately correspond to the onset in the bulk- rock record. This is also true for the second increase of the bulk-rock carbonate content in the earliest Berriasian, during which a decline in the amount of nannofossil carbonate occurs. It is, however, obvious

nannofossil carbonate 80 (calculated) nannofossil carbonate (visually estimated) 70 carbonate of nannofossil fragments (visually estimated) 60 unidentifiable carbonate (visually estimated) 50 [wt%] 3 40

CaCO 30 20 10 0 105-35-4, 105-34-1, 105-32-1, 105-28-2, 65-66 cm 104-105 cm 134-135 cm 98-99 cm (Lower Tithonian) (Middle Tithonian) (Lower Berriasian) (Upper Berriasian) Fig. 3.14. Visual estimates of different carbonate sources from settling slides of four samples (20 fields of view) from DSDP Site 105. Estimated nannofossil carbonate is compared to the calculated nannofossil carbonate using the volumetric approach. 52 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... that the general trend in the bulk-rock carbonate record followed that of the mass occurrences of nannoliths in the mid- to late Tithonian and goes along with high absolute abundances of nannofossils in the late Berriasian. Linear sedimentation rates were calculated for each well defined nannofossil zone of DSDP Site 105 (Fig. 3.12). The sedimentation rates in the early Tithonian are 1.1 cm kyr-1 (NJ-19B) followed by a condensed phase with accumulation rates as low as 0.15 cm kyr-1 across the JurassicCretaceous boundary. Highest sedimentation rates were calculated for the Berriasian with 1.4 (NK-2A) and 2.7 cm kyr-1 (NK-2B). The resulting accumulation rates of nannofossil carbonate and bulk-rock carbonate show values of 0.3 and 0.8 g cm-2 kyr-1 respectively in the mid-Tithonian. These values are significantly lower than those calculated for the increase in the Berriasian. This latter increase is accompanied by high absolute abundances of coccoliths. Highest values for nannofossil carbonate accumulation rates are about 1 g cm-2 kyr-1 (nannofossil accumulation rate 1E+10 nannofossils cm-2 kyr-1, NK-2B), whilst values of older sediments range from 0.05 to 0.4 g cm-2 kyr-1 (nannofossil accumulation rate 1 to 4E+9 nannofossils cm-2 kyr-1; Fig. 3.12). Accumulation rates of the non-carbonate fraction follow the course of the carbonate. Carbonate accumulation exhibits high values in the early Tithonian, subsequently followed by two phases of slightly lower accumulation rates. It has to be noted that carbonate accumulation during the NJ-19 nannofossil zone are as high as that of the mid-Tithonian (upper NJ-20 to NJK-A) increase. During this latter interval carbonate accumulation and the non-carbonate fraction are on the same scale. As reflected by the lithology the carbonate accumulation is higher than that of the non-carbonate fraction in the Berriasian. Accumulation rates of carbonate and non-carbonate sediments range from 0.1 to 4 g cm-2 kyr-1 and 0.1 to 2.3 g cm-2 kyr-1, respectively.

3.7 Discussion 3.7.1 Diagenesis and methodological errors 3.7.1.1 Preservation of calcareous nannofossils Dissolution and diagenesis can alter the preservation of calcareous nannofossils und thereby limit their usage as palaeoenvironmental indicators for surface water conditions in the oceans. The most important process which controls preservation is dissolution, which can take place in the water column, at the sediment- water interface and in the sediment. Accumulation is biased by dilution processes and lateral drifting of specimens (e.g. Honjo, 1976; Steinmetz, 1994; Andruleit, 1995). The observed preservation of calcareous nannofossils of the three DSDP sites varies considerably. Nannofossils from Site 105 show an overall good to moderate preservation (E1, O1). In contrast, nannofossils from Site 534A are often overgrown (E1-2, O1-2), the pelagic signal is strongly diluted by carbonate of detrital or diagenetic origin. This results in low absolute abundances of calcareous nannofossils at this site. At Site 367 the nannoflora is mostly moderately to poorly preserved (E2, O1). Generally, overgrowth becomes a more dominant feature in carbonate-rich samples. According to Roth and Krumbach (1986), assemblages consisting of more than 40% of W. barnesae are considered to be significantly altered, a statement that has been questioned. It has to be taken into account that Jurassic and earliest Cretaceous assemblages are less diverse and show higher abundances of Watznaueria than younger assemblages. Furthermore, high abundances of this group can also be controlled by ecological parameters (Williams and Bralower, 1995). We rather suggest that altered samples have to show: (1) high relative abundances of dissolution resistant species like Watznaueria, (2) a high degree of dissolution, which will result in (3) a distinctive decrease of species richness and absolute abundance. Such changes have been Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 53 noted in the mid-Tithonian of Site 534A and in some samples of Site 367. According to our observations the described long-term trends in the nannofossil record can be traced throughout all studied sites. We assume that these relative changes in the assemblage composition reflect original long-term variations in the surface-water conditions.

3.7.1.2 Potential error sources for the calculation of nannofossil carbonate and size measurements Disaggregation of nannofossils is another factor, which can change the composition of the nannofossil assemblage and the amount of identifiable nannofossil carbonate. Unidentifiable micrite has been observed in most samples, becoming more important with the lithological shift to carbonates across the Jurassic Cretaceous boundary (Fig. 3.14). Less than a third of the bulk-rock carbonate can be explained by calcareous nannofossils following the method presented in the current paper (Figs. 3.12 and 3.14). Absolute abundances and the estimated nannofossil carbonate are highest at CaCO values between 40 and 80 wt% (Fig. 3.15a, b); 3 they decrease slightly with increasing bulk-rock carbonate content. This observation is in agreement with observations of numerous authors (e.g. Thierstein and Roth, 1991; Erba, 1991; Mattioli, 1997), who found the best preservation of calcareous nannofossils in samples with a CaCO content between 40 and 60 wt%. 3 This suggests that diagenetic alteration of nannofossils, such as overgrowth, recrystallisation and disaggregation of nannofossils, is more efficient in sediments with a higher carbonate content.

3.5·109 (a) N=56 3.0·109

2.5·109

2.0·109

1.5·109 [nannof./g sed.] 1.0·109 Absolute abundance

0.5·109

0.0·109 0 20 40 60 80 100

CaCO3 [wt%] 25 (b) N=56

20 [wt.%] 3 15

Nannofossil CaCO 5

0 0 20 40 60 80 100

CaCO3 [wt%]

60 (c) N=56

40 spp. [%] P. beckmannii P.

+ 30

20 Fig. 3.15. Scatter plots of data from DSDP Site 105: bulk-rock Nannoconus + CaCO vs. (a) nannofossil absolute abundance; (b) calculated C. mexicana 3 10 nannofossil carbonate; and (c) relative abundance of the nannolith 0 taxa (Conusphaera mexicana, Polycostella beckmannii, Nanno- 0 20406080100conus spp.). CaCO3 [wt%] 54 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Several factors contribute to the remarkable difference between the amount explained by our nannofossil estimates and the measured bulk-rock carbonate content. (1) The applied method of Young and Ziveri (2000) consists of a cumulative error of approximately 50%, (2) erroneous calculation of absolute abundances, (3) high abundances of coccolith fragments, central processes and single elements of nannofossils have not been taken into account when calculating the amount of nannofossil carbonate, (4) diagenetic carbonate or recrystallized nannofossils caused by diagenetic processes, (5) the decay of nannolith taxa like conuspheres and nannoconids in pelagic carbonates is considered to be an important factor in producing micrite (Noël and Busson, 1990); 2+ and (6) the calculation of the CaCO3 values by using Ca from AAS measurements might provide a further error source. We assumed that all Ca was bound to CaCO3. According to our observations it is most likely that the estimates of nannofossil carbonate are biased by high amounts of unidentifiable micrite and recrystallized or broken nannofossils which have been not quantified when studying the assemblage. These two factors may be the most important factors which can explain the discrepancy between the two datasets. Therefore the estimates of nannofossil carbonate have to be considered as minimum values. One important factor which may contribute to unidentifiable micrite is the decay of nannolith taxa such as Conusphaera or Nannoconus. According to Noël and Busson (1990) these two groups are often not strongly lithified in pelagic environments allowing the skeletons to fall apart into smaller elements during diagenesis. Locally they can constitute the bulk of micritic limestones (Noël and Busson, 1990; Busson and Noël, 1991). In particular for nannoconids diagenetic alteration may have affected the length measurements, which have been performed for this group in this study. Single elements are arranged perpendicular to the axial canal, thus disaggregation can change the measured length of these fossils. This has also consequences for some taxonomic concepts of subspecies, which are mainly defined by their size (e.g. N. kamptneri ssp. minor N. kamptneri ssp. kamptneri, N. steinmannii ssp. minor N. steinmannii ssp. steinmannii). Possibly such changes may also account for the slight offset between the nannofossil and bulk-rock carbonate records

4.0 DSDP Site 105 3.8 DSDP Site 367 DSDP Site 534A 3.6

3.4

3.2 spp.

3.0

2.8

Watznaueria 2.6

Mean maximum length V-unit 2.4

2.2 r=0.94 N=112 2.0 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 Mean maximum coccolith length Watznaueria spp. Fig. 3.16. Scatter plot of total coccolith length of Watznaueria and the V-unit length of samples from which size measurements have been performed. Abbreviation: r, Pearsons correlation coefficient. Error bars indicate the 95% confidence limit. Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 55 in the Tithonian and the sporadical occurrence of nannoconids in the Berriasian. Differential degrees of disaggregation through time may in addition have caused changes in the abundance of nannoconids and contributed to the amount of unidentifiable micrite in the sediment. This is also indicated by Fig. 3.15c, where these nannolith groups are dominant at moderate CaCO values. The effect of disaggregation on the 3 measured size is less important for conuspheres because crystals are lath-shaped which are arranged nearly parallel to the axis, thereby the length will not change when they fall apart. The influence of dissolution on the measured length of coccolith taxa, in particular Watznaueria spp., is rather small. As indicated by the positive linear correlation between the sizes for the inner V-unit, which is not changed by dissolution, and the total coccolith length (Fig. 3.16), the original trend of the size variations through time is preserved. Absolute size values, however, may have been changed by dissolution. This may have slightly lowered the volume calculations and sizes given in Table 3.1.

3.7.1.3 Estimates of linear sedimentation rates Developing a sedimentation rate record was hampered by the absence of palaeomagnetic data for DSDP Site 105, therefore we had to use nannofossil bioevents and the corresponding age estimates from Hardenbol et al. (1998). Radiometric age estimates of the stage boundaries were adopted from Gradstein et al. (1994, 1995). According to these authors the absolute age data consist of an uncertainty of up to ±3 myr, which would change any calculated sedimentation rate significantly. However, the same authors pointed out, without giving any estimate of the error, that the uncertainty of the relative stage duration can be considered to be much more precise than the stage boundary age estimates. Absolute ages of bioevents are derived from linear interpolation based on the correlation with magnetostratigraphy. It has to be noted that two assumptions have been made for the assessment of linear sedimentation rates. Firstly nannofossil events have to occur synchronously to the Hardenbol et al. (1998) scale. This is not unlikely, since the herein used part of chronostratigraphic framework of Hardenbol et al. (1998) is mainly based on results from Bralower et al. (1989), who correlated magnetostratigraphic chrons with nannofossil events from DSDP sites of the Atlantic Ocean. Secondly the duration of the stage and the nannofossil zones are only to a minor extent affected by the inaccuracies of radiometric age data. The magnitude of the calculated sedimentation rates are usually within the range of observed recent sedimentation rates for pelagic environments (0.2 - 3 cm kyr-1; Milliman, 1993). Highest numbers observed in the current study are 2.7 cm kyr-1 (NK-2B), which are, when considering sediment compaction, higher than the maximum values given by Milliman (1993). Compaction corrected accumulation rates of carbonate show high values close to the upper limit for pelagic environments. But, the observed magnitude of accumulation rates has been also reported from Tertiary or Quaternary pelagic settings (e.g. Howard and Prell, 1994; Shackleton, 1995). For the studied sediments no obvious indications for redeposition has been observed, but this does not rule out that small-scale reworking processes increased the amount of accumulated sediment. The observed pattern and the magnitude of sediment accumulation rates are in agreement with those calculated by Ehrmann and Thiede (1985, 1986) for the JurassicCretaceous boundary interval in the Atlantic Ocean. According to the error sources mentioned above and high values for carbonate accumulation we assume that the changes in accumulation do at least reflect the original pattern. They give an impression of temporal changes of the accumulation of nannofossils and carbonate in the latest Jurassic and earliest Cretaceous at DSDP Site 105. 56 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

3.7.2 Composition of the nannofossil assemblages Temporal and spatial changes in the distribution and abundance of calcareous nannofossils are controlled by climatic and oceanographic factors. Such changes allow the use of calcareous nannofossils to reconstruct palaeoenvironmental conditions in the Quaternary (e.g. Baumann et al., 1999; Kinkel et al., 2000) as well as in the fossil record (e.g. Mutterlose, 1996; Street and Bown, 2000). Among other factors surface water temperature and nutrient availability are thought to be the most important parameters controlling the distribution and composition of calcareous nannoplankton (e.g. Brand, 1994; Winter et al., 1994). The most common species W. barnesae and related taxa like W. fossacincta and W. cf. manivitae are considered to be dissolution-resistant, cosmopolitan forms. They are thought to indicate more oligotrophic conditions of the surface water (e.g. Roth and Krumbach, 1986; Premoli-Silva et al., 1989; Williams and Bralower, 1995; Pittet and Mattioli, 2002). From higher abundances of these forms during the entire Tithonian to Valanginian interval we infer general low to moderate surface water nutrient levels for the Central Atlantic. Long-term changes in the nutrient content of the surface water are indicated by changes in the abundance of taxa, which may indicate an increase of fertility. In particular B. constans, D. ignotus and Z. erectus have been considered to prefer higher nutrient levels in the oceans surface water (e.g. Premoli-Silva et al., 1989; Coccioni et al., 1992; Erba et al., 1992). These species are quite common in the early Tithonian and the Berriasian, implying slightly higher surface water fertility than in the mid- to late Tithonian interval. The mid- to late Tithonian interval is characterized by high abundances of nannolith taxa, from which Nannoconus is best known. Busson and Noël (1991) interpreted this group as meroplanktic calcareous dinoflagellate cysts, flourishing in shallow-water environments under oligotrophic conditions with low rates of terrigenous supply. Generally, this group is thought to have preferred warm surface waters impoverished in nutrients (Erba, 1987; Coccioni et al., 1992; Mutterlose, 1996). Erba (1994) suggested that Nannoconus inhabits the lower photic zone similar to the recent Florisphaera profunda. According to Molfino and McIntyre (1990) abundance variations of F. profunda are related to changes of the nutricline depth and stability. A shallow nutricline is believed to cause a higher nutrient transfer into the upper photic zone causing blooms of coccoliths and low abundances of F. profunda. A deep nutricline leads to more fertile, mesotrophic conditions in the lower photic zone and thereby to higher abundances of F. profunda. Under these conditions the surface waters are usually impoverished in nutrients. Similar population patterns have been observed by Herrle (2003) for Nannoconus in mid-Cretaceous sediments from the Vocontian Basin. High abundances of nannoconids are accompanied by low numbers of coccoliths during intervals of enhanced stratification. On the other hand times of seasonally increased wind stress caused a rise of the nutricline, with more nutrients being transferred into the surface waters. This is reflected by higher abundances of coccoliths and very low numbers of nannoconids. But there are still significant differences between the ecology of nannoconids and F. profunda. Based on a few oceanic sections it has been inferred that nannoconids are more common in marginal settings of the low latitudes, where they occur in rock-forming proportions in latest Jurassic and earliest Cretaceous sediments (e.g. Colom, 1955; Erba, 1989; Busson and Noël, 1991; Erba, 1994). But nannoconids have also been reported in low abundances from high latitudinal and oceanic environments (e.g. Busson and Noël, 1991; Erba 1994). By contrast, F. profunda proliferates in open-oceanic parts of the tropics and subtropics. Recent work has shown that an increase in the relative abundance of F. profunda is caused by a decrease of the absolute abundance of all other taxa (e.g. Baumann et al., 1999; Haidar et al., 2000; Sprengel et al., 2002). Therefore, their contribution to the carbonate accumulation can be considered to be rather small.

Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 57

(Price et al., 1998) al., et (Price

humid arid humid

cool warm

Climate mode Climate warm cool warm

sites studied sites

silica dominated silica carbonate dominated carbonate Lithology of DSDP of Lithology ] crisis vs. PDB [ 1.5 2.5 platform drowning carb nannoconid C (western Tethys, 13 12

d Weissert et al., 1998) Weissert

Sheridan, 1997) Sheridan, +

(Stewart et al., 1996; al., et (Stewart

volcanism crust production crust

Parána

Oceanic slow spreading slow fast spreading - ? g/g] Sr m [ (Renard, 1986) 200 600 1000 + 'NCE' - carbonate (Nannofossil) accumulation ) ) forms; affinity; Biscutum, Polycostella, Nannoconus, Watznaueria) Conusphaera, (Cretaceous Watznaueria, Watznaueria, Assemblage 2 Nannofossil assemblage Rhagodiscus Assemblage 1 Assemblage 3 Zeugrhabdotus (strongly calcified (Jurassic affinity; Cyclagelosphaera, + Polycostella, Nannoconus Conusphaera, (this study) Size variation Watznaueria

N a n n o c o n u s s p p .

P . b . e c k m a n n ii

C . m . e x ic a n a rare common abundant connection Opening of Atlantic-Pacific

(Haq et al., 1987,

sea-level changes

Long- and short-term

Gradstein et al., 1994)

Hardenbol et al., 1998) al., et Hardenbol

(Bralower et al., 1989; al., et (Bralower zonation ?

NK-3 NK-1

Calc. nannofossil Calc. NK-2B NK-2A NJK-B NJK-D NJK-C NJK-A

NJ-20B NJ-20A NJ-19B NJ-19A

oe Upper Middle Lower Upper Middle Lower U. Lower Upper

.VALANGINIAN BERRIASIAN TITHONIAN K. STAGE

SYSTEM UPPER JURASSIC UPPER LOWER CRETACEOUS LOWER

(G ra d s te in e t a t l., 1 l., 9 9 4 )

s tra tig ra p h

y M23 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21 M22

M22A M11A M13A

M a g n e to c h ro n o -

A g e [M a ] 135 140 150 145 Fig. 3.17. Synthesis of tectonic, climatic and biotic events for the latest Jurassic and early Cretaceous. Area shaded in a light grey represents interval of the proposed opening of the Atlantic-Pacific seaway; NCE is Nannofossil calcification event. 58 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ...

Considering the observations of Erba (1994) and Herrle (2003), we assume that nannoconids inhabited an ecological niche within the lower photic zone; high abundances may indicate a deep nutricline and more oligotrophic surface water conditions. The ecological affinities of C. mexicana and P. beckmannii are unknown, but due to some similarities of their skeletons these groups possibly inhabited an ecological niche similar to that of Nannoconus. We assume that the slight offset of the three nannolith acmes reflects either a competition between these groups for a similar ecological niche or slight differences in their ecological affinities. From these groups Nannoconus was most successful in the light of evolution, perhaps due to its greater variability in size and shape. Mass occurrences of strongly calcified nannoliths occurred during a regressive period (Figs. 3.12 and 3.17). This coincidence may have various causes: (1) particularly nannoconids inhabited marginal oceanic settings in the low-latitudes (Roth and Krumbach, 1986). A sea-level drop would have caused a basinward shift of their preferred environments, a sea-level rise a shift towards the margins. (2) A restriction of water mass exchange over the Strait of Panama during sea-level lowstands would have intensified the back up currents along the eastern coast of the North American continent. This in turn would have caused a deepening of the thermocline/nutricline in the western Central Atlantic favouring an elevated productivity in the lower photic zone and causing high relative abundances of deep-dwelling species (Molfino and McIntyre, 1990). This explanation does also account for the general lower nannolith abundances in the eastern part of the Central Atlantic (DSDP Site 367). (3) Limited seaways to the surrounding oceans and seas favoured the evolution of new taxa endemic to the Central Atlantic and led to provincialism. In particular nannoconids are very common in the western Tethys and the Central Atlantic. They have been only scarcely reported from other parts of the world during the JurassicCretaceous boundary interval. We assume that one or a combination of these explanations led to higher abundances of the studied nannolith groups. The calcareous nannofossil assemblages of the three DSDP sites indicate open-oceanic conditions for the earliest Tithonian to Valanginian interval. Generally more oligotrophic conditions prevailed in the surface waters with a slightly higher fertility in the early Tithonian and the Berriasian. The mid- to late Tithonian assemblages may indicate low nutrient and stable surface water conditions. A climatic change with a northward expansion of the dry climate zone accompanied by cooler temperatures at least in the high latitudes or a long-term sea-level drop across the JurassicCretaceous boundary may have favoured a domination of nannoliths and Watznaueria in the mid- to late Tithonian assemblages. Roth (1989) suggested that the habitat of calcareous nannofossils shifted from shallow shelf environments to the open- ocean in the latest Jurassic (Kuenen Event; Roth, 1989) and caused thereby the increase in the accumulation of pelagic carbonates. The change in the assemblage to higher values of species richness and coccolith abundances coincides with the proposed opening of a deep-water connection between the Atlantic and Pacific Ocean. According to Bralower et al. (1989) the number of first occurrences exceeds the number of last occurrences by a factor of four during the earliest Cretaceous, also Roth (1986, 1989) reported high evolutionary turnover rates during this interval.

3.7.3 Size variations and calcification of nannofossils Causes for size variations and different degrees of calcification of coccoliths are still under discussion. Morphometric studies of calcareous nannofossils were successfully used to decipher evolutionary lineages and to indicate species-level variations by Backman and Hermelin (1986), Young (1990), Bralower and Parrow (1996), and Knappertsbusch (2000). Results of various authors (e.g. Young and Westbroek, 1991; Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 59

Baumann and Meggers, 1996; Bollmann, 1997; Knappertsbusch et al., 1997; Renaud et al., 2002) suggest that size variations are at least partly controlled by changes of the palaeoenvironment or palaeoceanography. So far only few studies focused on Mesozoic calcareous nannofossils. Recently, Mattioli and Pittet (2002) studied the nannolith group Schizosphaerella in Early Jurassic sediments with respect to its change of size and its contribution to the carbonate accumulation. Tremolada and Erba (2002) analyzed size changes of the nannolith groups Assipetra infracretacea and Rucinolithus terebrodentarius during the early Aptian OAE 1a. They related larger morphotypes of these species to increased surface water fertilization. As pointed out earlier (Section 3.4) various factors are thought to control the size of coccoliths, but observations are sometimes contradictory. In the mid- to late Tithonian high abundances of strongly calcified nannoliths co-occur with large-sized Watznaueria, reflecting an increase in the calcification of calcareous nannofossils. We name this shift Nannofossil Calcification Event (NCE). The composition of the nannofossil assemblages proposes oligotrophic surface water conditions. Another hint towards an increase of biogenic calcification is mentioned by Erba (1989) for this interval. According to this author, the radiation of nannoconids occurred synchronously to the change from dominantly hyaline to calcitic tests within the group of calpionellids. Another factor which is considered to control the calcification is the atmospheric pCO and the resulting 2 seawater pH. High pCO increases the primary production of organic matter for living diatoms as well as for 2 coccolithophores (Riebesell et al., 1993; Riebesell et al., 2000). Lower pCO2 levels increase the rate of calcification of modern coccolithophores in culture studies (Riebesell et al., 2000; Zondervan et al., 2001), whereas high pCO inhibits calcification. On the other hand decreasing calcification caused by high pCO 2 2 has a negative feedback on rising atmospheric pCO . In periods of high pCO calcification is reduced and 2 2 less CO is released during the process of biogenic calcification. This is counteracting the decreasing buffer 2 capacities of the oceans (Elderfield, 2002). Models of Berner (1991, 1994) and Tajika (1999) predict decrease of atmospheric pCO levels for the 2 latest Jurassic. This agrees with low spreading rates (Sheridan, 1983, 1997; Fig. 3.17), dry climate (e.g. Hallam et al., 1991; Abbink et al., 2001) and cooler temperatures in the high latitudes (Price, 1999). The synchronity between predicted pCO levels and calcification events from this and other studies leads us to 2 hypothesize that there is a possible link between lower pCO conditions and an increase in calcification of 2 calcareous nannofossils during the mid-Tithonian NCE. A comparable change in the nannofossil calcification occurred in the Eocene. A rise in size and abundance of Reticulofenestra corresponds to the mid-Eocene cooling. This is believed to have gone along with lower values in atmospheric pCO (Backman and Hermelin, 1986; Pearson and Palmer, 2000). 2 Higher rates of ocean crust production and/or volcanic activity were predicted for the Kimmeridgian- early Tithonian (Sheridan, 1983, 1997) and the Valanginian (Larson, 1991b; Channell et al., 1993; Stewart et al., 1996). Such phases are considered to be characterized by elevated pCO levels due to volcanic out 2 gassing. This may have caused greenhouse conditions with an acceleration of the hydrological cycle including more humid climate, elevated run off and intensification of weathering processes causing higher rates of nutrient transfer from the continents into the oceans (Weissert et al., 1998). This may also account for higher abundance of fertility indicating species in the early Tithonian. Both in the Valanginian and in the early Aptian, two intervals of presumed higher pCO conditions, a 2 rapid decrease of nannoconid abundances (nannoconid crises) has been observed in the western Tethys (e.g. Lini et al., 1992; Channell et al., 1993; Erba, 1994; Bersezio et al., 2002). This decrease can be interpreted 60 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... as a reduction of the nannofossil calcification. Drowning events of numerous platforms (e.g. Morocco, Mexico, SE France) in the shallow-water carbonate system co-occur with the Early Cretaceous nannoconid crises (Weissert et al., 1998; Wortmann and Weissert, 2000). This suggests an overall decrease in the biogenic carbonate production during a period of enhanced ocean fertilization and predicted high pCO . Possibly both 2 factors the concentration of nutrients and variations of atmospheric pCO , may have led to the changes in the 2 composition of the nannoflora and the biogenic calcification in the Tithonian and the Valanginian/Aptian. Due to the absence of reliable pCO /pH proxies, our poor understanding of the buffer capacities of the 2 oceans and the influence of seawater pH on the biogenic calcification on geological timescales the interpretation of these factors remain highly speculative. Another important change took place in the latest Tithonian and earliest Berriasian, when the size of Watznaueria decreased and the nannoflora changed towards a more diverse, coccolith-dominated assemblage. This has been observed in all DSDP sites studied indicating major changes in the marine, planktic system. This corresponds to the proposed opening of the Atlantic-Pacific seaway, which may have caused a re- organisation of ocean-atmosphere feedback mechanisms including changes of climate and the circulation pattern of the Central Atlantic.

3.7.4 The importance of calcareous nannofossils for pelagic carbonate accumulation In present oceans coccolithophores are considered to be the most important pelagic carbonate producers apart from planktic foraminifera and pteropods. Their role for the pelagic carbonate production must have been more important in the Jurassic and early Cretaceous when other carbonate producing plankton was rare or had not yet evolved. The only other common plankton group during the Tithonian and Berriasian period which was important as a carbonate producer were calpionellids, which are thought to have preferred neritic environments rather than the open ocean (Bernoulli and Jenkyns, 1974). This study has revealed that calcareous nannofossils occur in rock-forming proportions in the sediment (up to 24 wt%). As shown in Fig. 3.14, about 50% of the carbonate can be explained by complete calcareous nannofossils or fragments of this group. Taking the scarcity of other pelagic carbonate producers into account, calcareous nannofossils were the most important biogenic contributor to the carbonate fraction. According to Schneider et al. (2000), who summarized data of Milliman (1993) and Milliman and Droxler (1996), more than 74% of the recent biogenic carbonate precipitation is contributed by planktic microorganisms, particularly coccolithophores (Westbroek et al., 1989). 26% are produced in shallow water environments. Estimates of carbonate production, dissolution and burial in sediments diverge, however, considerably. While the production of shallow water carbonate amounts only 26%, its part of the total preserved carbonate is, however, larger than 45% (Schneider et al., 2000). 75% of the pelagic carbonate, which has been produced in the upper water-column of the oceans, is dissolved on its way to the sediment or by early diagenetic processes. Milliman (1993) calculated a mean total carbonate flux for the worlds oceans at a water depth of 1000 m of approx. 0.8 g cm-2 kyr-1. For high productive regions in the Atlantic Ocean maxima of 3.3 g cm-2 kyr-1 have been reported for the upwelling zone west of Africa and 1.1 to 1.8 g cm-2 kyr-1 for the western N Atlantic close to the drilling site of DSDP Site 105 (Milliman, 1993). The values obtained for the Tithonian to early Berriasian are comparable to recent low productivity conditions, whilst the high values in the late Berriasian are comparable to times or regions of elevated carbonate accumulation. This implies that the latest Jurassic and earliest Cretaceous carbonate accumulation varied on a similar scale as in todays oceans. Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 61

Quantitative studies of coccolith accumulation in Quaternary sediments have been carried out by e.g. Kinkel et al. (2000) and Ziveri et al. (2000). Kinkel et al. (2000) calculated coccolith accumulation rates between 0.01 - 1.5E+11 nannofossils cm-2 kyr-1 (absolute abundances 2 - 8E+10 nannofossils g-1 sed.) for the equatorial Atlantic Ocean. Ziveri et al. (2000) reported 0.2 - 4E+9 nannofossils cm-2 kyr-1 (absolute abundances 0.1 - 1E+9 nannofossils g-1 sed.) for surface sediments in the Eastern Mediterranean Sea. Mean daily coccolith carbonate flux rates from sediment traps at water depths between 2 - 3 km, which is approximately the palaeo-water depth of DSDP Site 105, show comparable results from the Bay of Biscaye (Beaufort and Heussner, 1999) and the eastern N Atlantic (Broerse et al., 2000b; Sprengel et al., 2000). The values range from 1.1 to 9.9 mg m-2 d-1, which correspond to 0.04 to 0.396 g cm-2 kyr-1. A careful comparison of these recent nannofossil and nannofossil carbonate accumulation rates with our data shows that they also occur on more or less the same scale, when considering that the herein calculated amounts of nannofossils and nannofossil carbonate are minimum values, this would suggest that the accumulation of nannofossil carbonate must have been higher during the JurassicCretaceous boundary interval. The long-term changes in the nannofossil carbonate accumulation are possibly reflected by variations in the Sr concentration of pelagic carbonates (Renard, 1986; Fig. 3.17). Factors controlling the Sr (Sr/Ca) record in pelagic sediments have been controversely discussed during the last years. For a long time such oscillations were thought to be primarily controlled by regressive/transgressive cycles and the ratio of the calcite/aragonite formation (Renard, 1986). Rapid increases in seawater Sr/Ca have been predicted to accompany sea-level falls, exposing aragonitic (Sr-rich) shelf sediments to weathering (Stoll and Schrag, 1996). Sr partitioning in carbonates depends strongly on the speed of crystal growth rate and in the case of modern coccolithophores, where it goes along with changes in the rates of cell division, calcification and growth as recently demonstrated in culture studies (Stoll and Schrag, 2000b). Hence, bulk-rock variations in Sr (Sr/Ca) of nannofossil limestone are possibly driven by changes in coccolith Sr/Ca content. The rapid radiation of calcareous nannofossils and the onset of their rock-forming importance in pelagic settings are closely related to a deepening of the calcite compensation depth (CCD) in the late Jurassic (e.g. Roth, 1983, 1986). Boss and Wilkinson (1991) interpreted the deepening of the CCD primarily as a response to the evolution of calcareous plankton (calcareous nannofossils, calpionellids) and secondarily to other factors like sea-level changes, ocean circulation efficiency and changes in the ratio from cratonic to pelagic carbonate accumulation.

The production of calcium carbonate by marine organisms is a short-term source of CO2 to the marine environment (Westbroek et al., 1994; Holligan and Robertson, 1996), whereas burial of biogenic carbonates into the sediment acts on the other hand as a long-term sink of carbon (e.g. Elderfield, 2002). In fact, sedimentary carbonates are the largest reservoir of carbon on Earth. At the end of the Jurassic the CCD dropped by more than 1 km (Roth, 1983) and the area of carbonate deposition increased of about 25% (Weissert and Channell, 1989). This improved the preservation and deposition of carbonates and thereby their mechanisms as a carbon sink. Weissert and Channell (1989) described a decrease in carbon isotope values across the JurassicCretaceous boundary (Fig. 3.17) reflecting an increase in the ratio of carbonate carbon to organic carbon production or a change in burial rate. The increase of this ratio is also visible in the western Tethys by an increase in platform growth during the Tithonian and Berriasian period (Weissert et al., 1998) and of pelagic carbonate (Roth, 1986; Weissert and Channell, 1989; this study). For the same interval a dry climate and decreasing weathering rates have been proposed derived from clay mineralogy, palynomorphs and phosphorus burial 62 Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... rates (Wignall and Ruffell, 1990; Föllmi, 1995; Abbink et al., 2001). This caused a decrease of terrigenous input into the N Atlantic (Ehrmann and Thiede, 1986; this study; Fig. 3.12) and the western Tethys (Weissert, 1979). Consequently, non-carbonate sedimentation decreased, contributin to the change of the sediment composition from silica dominated reddish marls in the late Jurassic to limestones in the Cretaceous.

3.8 Conclusions Two long-term trends in the nannofossil assemblage composition have been observed. (1) The early Tithonian is characterized by a low-diversity coccolith assemblage (Watznaueria spp., Cyclagelosphaera spp., Zeugrhabdotus spp.), whilst the mid-Tithonian assemblage is dominated by nannoliths ( C. mexicana, P. beckmannii, Nannoconus spp.) and large-sized Watznaueria. (2) The early Berriasian exhibits a shift from nannoliths to a high-diversity coccolith assemblage. Both intervals, the early Tithonian and the Berriasian, show higher abundances of taxa, which may indicate slightly higher surface water fertilities. Morphometric studies of the placolith genus Watznaueria and three common nannolith taxa from DSDP Site 105 show significant variations in size. From the middle Tithonian to the earliest Berriasian the size of Watznaueria decreases by approximately 2 µm, while the size of the nannoliths increases during the same interval. The similarity of trends in the composition of calcareous nannofossils (abundance, diversity) and the size evolution of Watznaueria spp. in palaeogeographically different DSDP sites from the Central Atlantic Ocean suggests that the observed variations reflect a primary signal on an at least regional scale. The quantification of the nannofossil carbonate revealed that on average only 27% of the total carbonate can be explained by calcareous nannofossils. This reminder of the carbonate is most likely attributed by high amounts of unidentifiable micrite and fragments of calcareous nannofossils. Other factors contributing to the error are inaccuracies in the determination of absolute abundances and nannofossil volume calculations. Due to the observed high amounts of nannofossil fragments in addition to the calculated nannofossil carbonate and the lack of other important carbonate producers, it can be stated that nannofossils are the most important biogenic carbonate producer during the studied interval. The record of nannofossil carbonate accumulation is punctuated in the mid-Tithonian by the NCE with mass occurrences of strongly calcified taxa (C. mexicana, P. beckmannii, Nannoconus spp., W. cf. manivitae). These forms occur during an interval, which is environmentally characterized by a sea-level lowstand, a dry climate and more oligotrophic surface water conditions. Lower atmospheric pCO2 during the latest Jurassic may have contributed to the evolution of calcifying organisms (calcareous nannofossils, calpionellids). This event is followed by a sharp decrease in abundance of the nannoliths (in particular C. mexicana and P. beckmannii), higher abundances of coccolith taxa and a size reduction of Watznaueria. These changes possibly went along with the opening of the Strait of Panama leading to the initialisation of an equatorial current system. During the Berriasian (nannofossil) carbonate accumulation rates increased, possibly reflecting a general shift to higher carbonate productivity. Generally, the observed carbonate accumulation rates are within the same range than as those in the recent oceans. Thus the deep sea is thought to become an important sink for

CaCO3 from the latest Jurassic onwards.

Acknowledgements This research used samples and data provided by the Ocean Drilling Program (ODP). B. Horn and H.-L. Legge are thanked for technical assistance. We gratefully acknowledge help of M. Geisen, who introduced Chapter 3: The impact of calcareous nannofossils on the pelagic carbonate accumulation ... 63 us to the image analysis system. This paper benefited from the reviews of E. Erba and an anonymous reviewer. Funding for this research was provided by the Deutsche Forschungsgemeinschaft (MU 667/13-1,-2; MU 667/20-1) and the DAAD (315/PPP). 64 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

4 Reconstruction of short-term palaeoceanographic changes during the formation of the Late Albian Niveau Breistroffer black shales (Oceanic Anoxic Event 1d, SE France)

André Bornemann1, Jörg Pross2,3, Kerstin Reichelt2, Jens O. Herrle2,4, Christoph Hemleben2 and Jörg Mutterlose1

1Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany 2Institut für Geowissenschaften, Eberhard-Karls-Universität Tübingen, Sigwartstr. 10, D-72076 Tübingen, Germany 3Laboratory of Palaeobotany and Palynology, Dept. Geobiology, Universiteit Utrecht, Budapestlaan 4, NL-3584 CD Utrecht, The Netherlands 4Geological Institute, ETH-Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland (submitted for publication to the Journal of the Geological Society, London)

Abstract The Niveau Breistroffer black shale succession in the Vocontian Basin (SE France) represents the regional equivalent of the widespread distributed Late Albian Oceanic Anoxic Event 1d. The studied black shale interval at the Col de Palluel section is 6.28 m thick and comprises four black shale beds with up to 2.5 wt% TOC, which are intercalated into marlstones. Calcareous nannofossil, palynomorph, planktic foraminifera and stable isotope data from the Niveau Breistroffer succession suggest short-term changes from relatively warm and humid climate conditions during black shale formation to a relatively cool and dry climate during marlstone deposition. Enhanced humidity and higher runoff during black shale formation are reflected by an increase in the terrigenous/marine ratio (TMR) of palynomorphs. Short-term changes in surface water productivity are indicated by a nutrient index based on calcareous nannofossils and the abundance pattern of small (63-125 µm) hedbergellid foraminifera. Surface water productivity was reduced during black shale formation and increased during marlstone deposition. Relative temperature changes recorded by a calcareous nannofossil temperature index and oxygen isotope data indicate warmer surface waters during black shale deposition. Warm-humid climate conditions and reduced surface water productivity, as they occurred during black shale formation, were accompanied by increasing abundances of subsurface-dwelling calcareous nannofossils (nannoconids) and planktic foraminifera (rotaliporids). The occurence of subsurface-dwelling plankton may indicate stratified surface water conditions. As the underlying cause for the formation of the Niveau Breistroffer black shales we suggest that an orbitally induced increase in monsoonal activity led to increasing humidity during periods of black shale formation. The humidity increase, in turn, caused a decrease in low-latitude deep-water formation and probably also an increase in surface water stratification. The combination of these two mechanisms caused oxygen consumption in the bottom water that increased the preservation potential of organic matter.

Keywords: black shale; Cretaceous; Albian; calcareous nannofossils; palynomorphs; planktic foraminifera

4.1 Introduction The mid-Cretaceous has often been characterized as a period of prevailing greenhouse conditions. A generally warm and humid climate was probably caused by elevated atmospheric pCO2 levels (e.g. Barron and Washington, 1985; Bice and Norris, 2002) triggered by high rates of ocean crust production and submarine Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 65 volcanism (Larson, 1991a, b). These warm conditions were accompanied by a long-term sea-level rise (Haq et al., 1987), low latitudinal temperature gradients (e.g. Huber et al., 1995) and an accelerated hydrological cycle (e.g. Weissert et al., 1998; Wortmann et al., subm.). Recent studies on the oxygen isotopic composition of benthic and planktic foraminiferal tests suggest that surface and bottom water temperatures were tempo- rally higher than in modern oceans (e.g. Savin, 1977; Erbacher et al., 2001; Huber et al., 2002; Wilson et al., 2002). The sedimentological record of the mid-Cretaceous is punctuated by the repeated occurrence of regionally to supraregionally distributed black shales, named Oceanic Anoxic Events (OAEs; Schlanger and Jenkyns, 1976). The storage of high amounts of organic carbon during OAE formation caused major perturbations of the global carbon cycle as they are recorded, for instance, in the carbon isotopic signature of marine carbonates (e.g. Jenkyns et al., 1994; Weissert et al., 1998). The OAE intervals correspond to positive carbon isotope excursions with amplitudes larger than 1.5 observed in hemi- to pelagic carbonates, platform sediments and terrestrial records (e.g. Grötsch et al., 1998; Weissert et al., 1998; Gröcke et al., 1999). To date, most studies on mid-Cretaceous OAEs have focused on the Early Aptian OAE 1a (e.g. Erba, 1994; Menegatti et al., 1998; Beerling et al., 2002) or the Cenomanian-Turonian OAE 2 (e.g. Arthur et al., 1988; Kuhnt, 1992; Gale et al., 1993; Huber et al., 1999). Fewer efforts have been undertaken to study the formation of sub-OAEs (OAE 1b to 1d). In this study, we focus on the Late Albian OAE 1d. Black shales of this age have been described in detail from SE France (Niveau Breistroffer; Bréhéret, 1988, 1994, 1997; Giraud et al., 2003) and the Atlantic Ocean (Mazagan Plateau - Nederbragt et al., 2001; Blake Nose Plateau - Wilson and Norris, 2001). An overview of the geographic occurrence of the OAE 1d black shales is given by Wilson and Norris (2001). In order to better understand the mechanisms that led to the formation of mid-Cretaceous black shales, different approaches have been followed. Sedimentological studies have provided information on sea-level fluctuations and the depositional environment (e.g. Bréhéret, 1994; Wignall, 1994). Geochemical investigations focused on elementary analyses (e.g. Wortmann et al., 1999), stable isotopes (e.g. Menegatti et al., 1999; Erbacher et al., 2001; Wilson and Norris, 2001; Price and Hart, 2002) and organic geochemistry (e.g. Bréhéret, 1994; Erbacher et al., 1996; Baudin et al., 1998). These studies supplied information on weathering conditions, the chemical composition of seawater and the source of organic matter as well as data on surface water productivity, oceanic carbon cycling and temperatures. Microfossils (see Leckie et al., 2002 for an overview; calcareous nannofossils e.g. Erba, 1992b, 1994; Herrle, 2003; Herrle et al., 2003a, b; Bischoff and Mutterlose, 1998; planktic foraminifera e.g. Galeotti, 1998; Premoli Silva et al., 1999; Nederbragt et al., 2001; benthic foraminifera e.g. Erbacher et al., 1999; Luciani et al., 2001; Friedrich et al., 2003; radiolaria Erbacher and Thurow, 1997; palynomorphs Tribovillard and Gorin, 1991; Hochuli et al., 1999; Herrle et al., 2003a, b) have been used to reconstruct biological, ecological and palaeoceanographic changes in the oceans during black shale formation. The formation of mid-Cretaceous black shales has often been linked to productivity changes caused by variations in nutrient supply (e.g. Pederson and Calvert, 1990; Hochuli et al., 1999; Wilson and Norris, 2001) and/or to increased organic matter preservation in a stratified water column with low bottom-water oxygenation (e.g. Bralower and Thierstein, 1984; Pederson and Calvert, 1990; Erbacher et al., 2001). Stable isotope data from excellently preserved foraminiferal tests suggest that the origin of mid-Cretaceous black shales in the western Atlantic Ocean may be compared to the formation of Mediterranean sapropels (Erbacher et al., 2001) or are related to a collapse of water column stratification (Wilson and Norris, 2001). Recently, 66 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

Herrle et al. (2003a, b) proposed a model according to which the formation of the OAE 1b in the western Tethys was triggered by changes in deep-water formation rates resulting from changes in the monsoonal activity. This is believed to have influenced bottom water oxygenation and thus the preservation potential of organic matter. Similarities between the formation of mid-Cretaceous black shales and that of Pliocene to Quaternary sapropels in the Mediterranean Sea have been stressed by various authors (e.g. Ryan and Cita, 1977; Erbacher et al., 2001; Herrle et al., 2003a). Multi-proxy approaches dealing with the formation of Quaternary black shales, however, indicate that many processes that are responsible for the accumulation of organic matter operate on Milankovitch and even sub-Milankovitch time scales (e.g. van Os et al., 1994). This is in strong contrast to our present insights in mid-Cretaceous black shale formation which are mainly based on studies reaching a relatively low stratigraphic resolution and/or utilizing relatively few proxies only. Hence, fundamental differences in the available datasets hinder a further, in-depth comparison of mid-Cretaceous and late Neogene black shales. This paper presents high-resolution data from calcareous nannofossils, planktic foraminifera, palynomorphs and stable isotopes for the Late Albian OAE 1d. The aim of our study is (1) to calibrate and compare signals between different microfossil groups and geochemical data, (2) to reconstruct short-term palaeoceanographic changes, and (3) to develop a model for the formation of this black shale event on a regional and supraregional scale. We have choosen the Vocontian Basin (SE France; Fig. 4.1) as a study area since this marginal basin reacted very sensitive to climatic and oceanographic changes during the mid-Cretaceous (Herrle et al., 2003a).

Palaeogeography

eroded Albian

Grenoble

Massif Central

Valence ercors V Die Internal zones of the Alps

Col de Gap Palluel

Mt. VENTOUX Digne

Nice eroded

pre-Triassic basement a Marseille e drowned platform facies S gravity-reworked siliciclastics n shallow open-marine facies a n e deep open-marine facies r a 0 50 km M e d i t e r Col de Palluel section

Fig. 4.1. Palaeogeographic reconstruction of SE France for the Albian (modified after Arnaud and Lemoine, 1993). Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 67

4.2 Location, chronostratigraphy and palaeogeography 4.2.1 Location of the studied section The OAE 1d black shales, which are called Niveau Breistroffer in SE France (Bréhéret, 1988), have been studied at the Col de Palluel section (Fig. 4.1) south of the road D994 ca. 5 km east of Rosans, Département Drôme (TK 25 Rosans, Nr. 3239 Ouest, Série Bleues, Lambert III coordinates x: 853 750, y: 3238 425). In the literature, there is some inconsistency on the definition of the Niveau Breistroffer interval in the Vocontian Basin. Bréhéret (1997) described seven black shale bundles (BR1 to BR7) over a 40 m thick sequence as the Niveau Breistroffer. In contrast, Gale et al. (1996) used this name for a bundle of five black shale beds covering a total of 10 m at the Col de Palluel section. In the present paper, we have followed an intermediate approach and named the black shale interval of Gale et al. (1996) main Niveau Breistroffer (Fig. 4.2). It covers the black shale bundles BR2 and BR3 following Bréhéret (1997). The investigated succession is 6.28 m thick and consists of carbonate-rich greyish marlstones and laminated black shales of the main Niveau Breistroffer interval. Pale-dark bedding rhythms are indicated by slight differences in colour which reflect variations of the carbonate and organic carbon content. The intercalated black shale beds are between 0.2 to 0.7 m thick and comprise different degrees of lamination. Com- pressed ammonites and aucellines are common in the black shales.

4.2.2 Stratigraphy and chronostratigraphic framework According to Gale et al. (1996), the main Niveau Breistroffer can be assigned to the Upper Albian Stoliczkaia dispar ammonite zone and Eiffellithus turriseiffelii nannofossil zone (=NC10A after Bralower et al., 1993 or CC9b after Sissingh, 1977; Fig. 4.2). The last appearance datum of the planktic foraminifera Planomalina buxtorfi has been observed 17 m above the top of the main Niveau Breistroffer supporting the assignment to the P. buxtorfi/Rotalipora appenninica planktic foraminiferal subzone (Reichelt et al., subm.). Carbon isotope data from the P. buxtorfi/R. appeninnica foraminiferal subzone show a major negative shift in the lower interval of the main Niveau Breistroffer. This shift is subsequently followed by a major positive excursion with an amplitude of 1.5 (Fig. 4.2) and can be correlated with the planktic foraminifera carbon isotope record from the Blake Nose Plateau (ODP Site 1052E; Wilson and Norris, 2001; Reichelt et al., subm.; Fig. 4.3). Time control for the investigated succession was achieved through the correlation of the carbon isotope record from the Col de Palluel section (Reichelt et al., subm.) to the carbon isotope record of Wilson and Norris (2001) from the Blake Nose Plateau (Fig. 4.3). Based on this correlation, the mean sedimentation rate for the section investigated was calculated at ~4.4 cm kyr-1, the entire section comprises ~159 kyr. Sedimentation rates increased from ~2.7 cm kyr-1 at the base of the section to ~5.2 cm kyr-1 in the upper part. This change in sedimentation rate is also reflected by increasing thickness of the marlstones between the black shale beds. Depending on the proxies studied, sample resolution varied between ~3.2 and ~3.7 kyr (compare Section 4.3).

4.2.3 Palaeogeography During the early Cretaceous, the Vocontian Basin was located at a palaeo-latitude of 25-30°N (Savostin et al., 1986; Hay et al., 1999) and was part of the European continental margin of the Ligurian Tethys (Lemoine et al., 1986). The basin was surrounded by the Massif Central landmass in the northwest and by carbonate platforms in the north and south (Urgonian platform carbonates; Arnaud-Vanneau and Arnaud, 1990). To the east, it was open towards the Tethyan Ocean (Fig. 4.1). 68 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

MARNES BLEUES FORMATION Vocontian Basin MAIN NIVEAU NIVEAU BREISTROFFER BREISTROFFER Col de Palluel

Planktic Calcareous Col de Palluel Ammonite foraminifera nannofossil 13 Substage zonation zonation zonation Key beds Lithology d Cbulk vs. PDB [] [m] LithologySamples Rotalipora [m] 121.5 Mantelliceras globotrunc- Lithology mantelli anoides

L. Cen. 70 53 Rotalipora CC appeninica 9B

65 Stoliczkaia Niveau dispar R. appeninica + Breistroffer P. buxtorfi (OAE 1d) 52 * NC10

Rotalipora 60 appeninica

Upper Albian CC Rotalipora 9A ticinensis 51 Mortoniceras 55 * § inflatum T. subticinensis Petite Ticinella Verole praeticinensis *

NC9 * Euhoplites CC Ticinella 8B 50 * lautus 50 * primula Hoplites NC8C

Mid.-Alb. dentatus

Douvilleiceras Niveau mammillatum Leenhardt 45 Hedbergella CC NC8B Niveau planispira 8A L. tardefurcata Paquier 49 (OAE 1b) Lower Albian Niveau § Hypacant- Kilian 40 hoplites

NC8A Niveau jacobi Jacob Faisceau Fromaget A. nolani Ticinella 48 beajouensis Niveau Nolan 35 * § P. nutfieldensis

CC 7B Hedbergella 30

Upper Aptian trocoidea 47

E. G. algerianus martinoides Niveau Globigerinel- Fallot loides ferreolensis marlstone marlstone (dark) calcareous nodules

NC7ANiveau NC7B/C D. deshayesi Leupoldina cabri CC Goguel marlstone (pale) black shale turbidites

L. Apt. 7A (OAE 1a) NC6 Fig. 4.2. Lithologic and stratigraphic framework for the Marnes Bleues Formation and the Niveau Breistroffer (Col de Palluel section, SE France) based on ammonites (Bréhéret, 1997), planktic foraminifera (Moullade, 1966; Reichelt et al., subm.), calcareous nannofossils (Gale et al., 1996; Herrle and Mutterlose, 2003), lithostratigraphy (Bréhéret, 1997) and carbon isotope data (Reichelt et al., subm.; this study). Isotope data have been smoothed based on the weighted harmonic mean method. Samples marked by * or § have not been studied with respect to palynomorphs (*) or planktic foraminifera (§).

From the Early Aptian to the latest Albian, a ~750 m thick succession of cyclically bedded marl- and limestones was deposited in the center of the basin (Marnes Bleues Formation; Flandrin, 1963). Black shales, limestones and glauconite-bearing turbidites are intercalated in this succession and serve as key beds of lithostratigraphic importance (Bréhéret, 1997; Fig. 4.2). The studied succession at the Col de Palluel section is situated close to the depo-center of the basin.

4.3 Methods

13 18 4.3.1 Geochemistry (CaCO3, TOC, δ Cbulk, δ Obulk)

A total of 50 samples (Fig. 4.2) was measured for CaCO3, TOC and stable isotopes. CaCO 3 contents have been calculated from AAS (Varian SpectrAA 300) measurements. TOC was measured with a DELTRONIK

13 18 coulometer at the Ruhr-University Bochum. Stable isotope preparation (δ Cbulk, δ Obulk) was performed using an off-line preparation technique and measurements were performed using a Finnigan MAT Delta S Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 69 mass spectrometer at the Ruhr-University Bochum. The measurements yielded a precision of ±0.10 and ±0.12 for carbon and oxygen isotopes, respectively.

4.3.2 Calcareous nannofossils The same samples used for geochemistry were investigated for their content of calcareous nannofossils. The random settling technique (Williams and Bralower, 1995; Geisen et al., 1999) was applied for slide preparation. The obtained absolute abundances were corrected to the total water column within the settling box (see Bollmann et al., 1999; Geisen et al., 1999). To detect preservation change s induced by preparation, simple smear slides were prepared and the state of nannofossil preservation was compared to the settling slides. Abundances were determined by counting at least 300 specimens. In addition, one random traverse of the slide was scanned for rare species. Counts were performed using an OLYMPUS BH-2 light microscope with cross polarized-light at a magnification of x1500. The diversity of the nannoflora was characterized by the

parameters of species richness (S) and heterogeneity (HS; Shannon and Weaver, 1949). n

Niveau Breistroffer n

tio tio a

Blake Nose Col de Palluel a

n n

o o tio

ODP Site 1052E (SE France) tio

a a

n n

f. z f. f. z f.

o o

n n

] . z . . z .

r n n y

h 13

y m m

g a 13 13 a d C vs. PDB [] d C vs. PDB [] d C vs. PDB [] g

s pl. forams

ta ra

pl. forams bulk ra

. n . . n .

[M s

]

lc lc

e b

. fo . . fo .

a a

l g u l la 1.01.52.02.5

1.01.52.02.5 0.5 1.0 1.5 2.0 2.5 ith

C P A C S P B L 70 (a) 500 (b) (c) 98.8 CC9c R. globotr.

Lower Cen. 65

99.0 ? CC9b 60 OAE 1d 99.2 R. appenn. 55

99.4 5.2 cm/kyr 50 4.4 cm/kyr (238 kyr) ? (NC10A, UC0) 520 ~ 159 kyr 2.7 cm/kyr 99.6 CC9a R. ticinensis 45 E. turriseiffelii Upper Albian Upper 99.8 buxtorfi R. appenninica/P. 40

100.0 35

100.2 30 540

Fig. 4.3. Age control on the formation of the Niveau Breistroffer black shales is based on a correlation of the carbon isotope record from the Blake Nose Plateau (planktic foraminifera, ODP Site 1052E (a, b); Wilson and Norris, 2001) with that from the Col de Palluel (bulk rock (c); Reichelt et al., subm.). The age model of ODP Site 1052E is based on orbitally tuned neutron log data (see Wilson and Norris, 2001 for details). Based on the assumed correlation, the studied succession of the main Niveau Breistroffer at the Col de Palluel section covers ~159 kyr, with the calculated sedimentation rate increasing from the bottom (2.7 cm/kyr) to the top (5.2 cm/kyr). The calculated average sedimentation rate for the entire section is 4.4 cm/kyr. Stippled area indicates assumed correlation of the studied succession at the Col de Palluel section to the Blake Nose Plateau section. Isotope data have been smoothed based on the weighted harmonic mean method. For explanations of lithological symbols see Fig. 4.2. 70 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

4.3.2.1 Calcareous nannofossil nutrient and temperature indices Temporal and spatial changes in the distribution and abundances of calcareous nannofossils are controlled by climatic and oceanographic conditions. Thus, they allow to reconstruct palaeoenvironmental conditions in the geological record (e.g. Erba et al., 1992; Mutterlose, 1996; Street and Bown, 2000). Surface water temperature and nutrient availability are thought to be the most important parameters controlling the distribution and composition of recent calcareous nannoplankton assemblages (e.g. Brand, 1994; Winter et al., 1994). In this study, short-term changes in surface water nutrient content and temperature were assessed through nannofossil-based nutrient and temperature indices (NI, TI). Based on literature data and an Varimax-rotated R-mode principal component analysis (PCA), Herrle et al. (2003a) developed such indices from a dataset of AptianAlbian nannofossils from the Vocontian Basin. These authors used ratios of index species instead of single taxon abundances to assess changes in surface water nutrient availability and temperature. The literature suggests that Biscutum constans, Discorhabdus ignotus (= D. rotatorius) and Zeugrhabdotus erectus have an affinity to elevated nutrient levels in the surface waters (e.g. Roth and Krumbach, 1986; Premoli-Silva et al., 1989; Erba, 1992b; Erba et al., 1992). According to Erba (1992b) a differentiation in the abundance pattern between these taxa occurs with respect to different nutrient concentrations, with D. ignotus and Z. erectus being adapted to higher nutrient levels than B. constans. Watznaueria barnesae is often interpreted as an indicator of oligotrophic conditions (e.g. Roth and Krumbach, 1996; Erba et al., 1992; Williams and Bralower, 1995). Based on the PCA calculated by Herrle et al. (2003a) D. ignotus and Z. erectus were used as indicators for high and W. barnesae (incl. W. fossacincta) for low nutrient conditions.

D.ignotus + Z.erectus (1) NI = ⋅100 D.ignotus + Z.erectus +W.barnesae Due to taxonomic changes of calcareous nannofossil assemblages from the Early to the Late Albian the TI of Herrle et al. (2003a) was modified based on literature data. Crucibiscutum salebrosum (incl. C. hayii), Repagulum parvidentatum and Tranolithus orionatus were considered as cold water species (e.g. Roth, 1983; Mutterlose and Wise, 1990; Crux, 1991; Mutterlose and Kessels, 2000; Street and Bown, 2000; Herrle et al., 2003a), and Rhagodiscus spp. (R. asper and R. achlyostaurion) as indicators for warmer surface waters (e.g. Crux, 1991; Roth and Krumbach, 1986; Mutterlose 1989; Erba et al., 1992; Herrle et al., 2003a).

C. salebrosum + T.orionatus + R. parvidentatum (2) TI = ⋅100 C. salebrosum + T.orionatus + R. parvidentatum + Rhagodiscus spp.

4.3.3 Palynomorphs Palynomorphs were studied from 43 samples (Fig. 4.2). Sample preparation followed standard palynological preparation techniques (e.g. Wood et al., 1996). Known weights of sample material (between 12 and 19 g) were treated with HCl and HFl. To facilitate the calculation of absolute palynomorph abundances, samples were spiked with Lycopodium marker spores prior to chemical processing. Due to the high amount of amor- phous organic matter in the residues, a short oxidation with HNO3 was performed. For each sample at least 300 palynomorphs were counted from strew mounts. To quantify terrestrial input, the terrigenous/marine ratio (TMR) of palynomorphs was calculated (e.g. Pross, 2001). Absolute spore abundances and the ratio between absolute spore and non-saccate pollen abundances were used as proxies for humidity in the hinterland. To minimize taphonomic effects, saccate pollen, which are especially prone to long-distance aeolian transport, were not considered in the evaluation. Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 71

4.3.4 Planktic foraminifera 47 samples were studied with respect to planktic foraminifera (Fig. 4.2). Sample preparation followed standard techniques (H O , Wick, 1947; tenside, Wissig and Herrig, 1999). Samples were washed over a 63 µm 2 2 sieve and dried for approx. 24 hrs. Subsequently, the sample was soaked in 5 to 10 ml of an ethanol-tenside (REWOQUAD) mixture for 24 to 72 hrs. In a second washing over a 63 µm sieve, the remaining sediment was removed. Residues were split into three fractions (63-125, 125-250 and 250-500 µm). A total of 200 to 300 specimens per fraction were counted from each sample. As small globular forms are thought to have been better adapted to high surface water nutrient levels and an instable environment (Caron and Homewood, 1982; Leckie, 1989; Premoli Silva and Sliter, 1999), we studied abundance changes of Hedbergella spp. within the size fraction 63-125 µm as a representative of this group. In contrast, flattened, keeled forms are thought to have preferred a deeper environment close to the thermocline (Wilson and Norris, 2001; water depth >100 m, Leckie, 1987) and more stable, stratified conditions (Caron and Homewood, 1982; Hart, 1999; Premoli Silva and Sliter, 1999). From this group Rotalipora spp. (>125µm) has been counted.

4.4 Results

13 18 4.4.1 Geochemistry (CaCO3, TOC, δ Cbulk, δ Obulk)

CaCO3 content varies between 47 and 67 wt% (mean: 56 wt%). The record shows a cyclic pattern which does not necessarily trace the lithology. Generally, CaCO3 values are up to 20 wt% higher in the marlstones as compared to the black shales (Fig. 4.4). Distinctive minima occur in the black shale bed at 49.26 m and in the dark marlstone at 50.19 m. TOC contents show a mean of 1.3 wt%, but the values differ significantly between marlstones and black shales. Whereas marlstone samples exhibit TOC contents around 1 wt% (min. 0.4 wt%), black shales consists of up to 2.5 wt% TOC. The carbon isotope record shows fluctuations between 1 and 2.6 (mean: 1.6), with generally lighter values in the marlstone samples. Heavier values for oxygen isotopes (from -4.9 to -3; mean: -4.1; Figs. 4.4 and 4.5) have been observed in the marlstones. Three of the four black shale beds are characterized by decreasing values with minima as low as 4.9. The oxygen isotope record shows a general trend from lighter values in the lower part to heavier ones in the upper part of the succession.

4.4.2 Calcareous nannofossils All samples yielded highly diverse nannofloras with a species richness between 44 and 66 (mean: 58; Fig. 4.4). High species numbers occur generally in the black shale samples, but values remain also high in the marlstones between 49.5 and 52.5 m. Similar to the species richness, the assemblage heterogeneity shows maxima in the lowermost three black shale beds (2.71-3.28, mean: 3.0) and high values in the upper part of the studied succession. Absolute calcareous nannofossil abundances range from 1.2E+9 to 3.2E+9 nannofossils g-1 sediment (mean: 2.1E+9). Highest absolute abundances occur in marlstone samples. Between 60.6 and 76.6% of the total nannofossil assemblage are made up by the eight nannofossil groups mentioned below. Biscutum constans shows relative abundances between 26.9 and 43.4% (mean: 35.6%; Fig. 4.4). D. ignotus and Z. erectus make up for 3.6 to 13.2% (mean: 8.0%) and 0.6 to 5.6% (mean: 3.2%) of the assemblage, respectively. These three taxa show increasing values in the marlstones. Relative abundances of W. barnesae vary between 3.3 and 13.3% (mean: 7.2%) with slightly higher percentages in the black shales. The percentages of Nannoconus spp. (N. elongatus, N. fragilis, N. truitti regularis, N. truitti rectangularis, 72 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

Absolute e

n abundance o 13 18 9

z d C [] d O []

e bulk bulk Species [10 nannof. F

g CaCO3 [wt%] TOC [wt%] vs. PDB vs. PDB richness Heterogeneity g-1 sediment]

. N .

s b

lc [m] u

a Lithology 50 60 70 0 1 2 11.52 -5 -4 -340 50 60 70 2.8 3 3.2 123 S C 53.5

53.0

52.5

52.0

51.5

51.0

(CC 9b) (pars) 50.5

50.0

49.5

Upper Albian (pars) 49.0

Eiffellithus turriseiffelii 48.5

48.0

47.5

47.0

46.5

NUTRIENTS TEMPERATURE

e n

o Biscutum Discorhabdus Zeugrhabdotus Watznaueria Nannoconus Rhagodiscus Crucibiscutum Tranolithus z

e constans ignotus erectus barnesae spp. spp. salebrosum orionatus

F g

ta [%] [%] [%] [%] [%] [%] [%] [%]

. N .

s b

lc [m] u

a Lithology 0 10 20 30 40 0 5 10 15 0 2 4 6 0 5 10 15 0 1 2 3 4 5 0 4 8 12 16 0 2 4 6 0 1 2 S C 53.5

53.0

52.5

52.0

51.5

51.0

(CC 9b) (pars) 50.5

50.0

49.5

Upper Albian (pars) 49.0

Eiffellithus turriseiffelii 48.5

48.0

47.5

47.0

46.5

13 18 Fig. 4.4. Results from geochemical (CaCO3, TOC, δ Cbulk, δ Obulk) and calcareous nannofossil (species richness, diversity, absolute abundance and relative abundances of selected taxa) analyses from the main Niveau Breistroffer succession (Col de Palluel section). For explanations of lithological symbols see Fig. 4.2. Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 73

NUTRIENTS PALYNOMORPHS LOWER PHOTIC ZONE SURFACE WATER

SURFACE WATER COMMUNITY TEMPERATURE e

n TMR o

z (terrigeneous/marine ratio,

e Nutrient index Assemblage composition Nannoconus Rotalipora spp. Temperature index F

g (calc. nannofossils) (palynomorphs) without saccate pollen) spp. [%] (>125 mm) [%] (calc. nannofossils)

. N .

s b

lc [m] 40 60 80 02550751000 0.5 1 1.5 2 2.5 024601 2 50 40 30 20 10 u

a Lithology S C 53.5 N- N+ T- T+ 53.0

52.5

52.0 TI d18O 51.5

51.0

(CC 9b) (pars) 50.5 NI 50.0

49.5

Upper Albian (pars) 49.0

Eiffellithus turriseiffelii 48.5

48.0 hedb. 47.5

47.0

46.5 50 60 70 80 90 100 dinoflagellate cysts -3.0 -3.5 -4.0 -4.5 -5.0 spores 18 Hedbergella spp. d Obulk vs. (63-125 mm) [%] other marine palynomorphs PDB [] pollen other terrestrial palynomorphs Fig. 4.5. Nutrient and temperature index based on calcareous nannofossils compared to terrigenous/marine ratio of palynomorphs and the abundance patterns of nannoconids and planktic foraminifera (small hedbergellids, rotaliporids). Errors bars indicate the binomial standard error (Fatela and Taborda, 2002). Thick lines represent smoothed records based on the weighted harmonic mean method. For explanations of lithological symbols see Fig. 4.2.

N. truitti truitti, Nannoconus sp.) fluctuate between 0 and 4.7% (mean: 1.4%), with maxima occurring in or close to the black shale beds. Nannoconids are not present at the base and the top of the studied succession (Figs. 4.4 and 4.5). Rhagodiscus spp. exhibits relative abundances between 6.3 and 15.5% (mean: 11.0%). Maximum values have been observed in the black shales or close to them. Both C. salebrosum and T. orionatus show generally high percentages in the marlstones. Their relative abundances range from 0 to 2.2% (mean: 0.7%) and from 1.3 to 5.6% (mean: 3.7%), respectively (Fig. 4.4). In contrast to the abundance changes of single nannofossil taxa, the NI and TI records suggest a more uniform picture with lower surface water productivity and slightly higher temperatures during black shale formation as compared to marlstones. The NI fluctuates between 39 and 82 (mean: 60; Fig. 4.5), with minima occurring in the two lowermost and the topmost black shale beds. NI maxima are observed in the marlstones. The TI indicates a long-term trend from higher surface water temperatures in the lower part of the succession to cooler conditions in the upper part. Highest values occur during the formation of the second black shale (48.79 m; TI: 13; mean: 29). This temperature maximum is followed by a cooling trend (min. 51.84 m; TI: 49).

4.4.3 Palynomorphs All studied samples contain abundant marine and terrigenous palynomorphs in good preservation. Palynomorph assemblages from the marlstone samples are dominated by marine palynomorphs (mainly dinoflagellate cysts, up to 75%; Fig. 4.5), while there is a high percentage of terrestrial palynomorphs (mainly pollen and spores; up to 50% of the total assemblage) in the black shale samples. Accordingly, TMR values differ strongly between <0.5 in the marlstone samples and 1 to 2.5 in the black shale samples. Absolute spore abundances are less than 4,000 individuals g-1 sediment in the marlstone samples and reach >20,000 indi- 74 Chapter 4: Reconstruction of short-term palaeoceanographic changes ... viduals g-1 sediment in the black shale samples. Non-saccate pollen show a similar distribution pattern, with less than 9,000 individuals g-1 sediment in the marlstones to a maximum of 32,000 individuals g-1 sediment in the black shales (Fig. 4.6).

4.4.4 Planktic foraminifera The relative abundance of hedbergellids varies between 61 and 91% (mean: 83.3%; Fig. 4.5). Thus, hedbergellids are the most common planktic foraminiferal group in the size fraction studied. Highest relative abundances have been observed in marlstones. Rotaliporids are much less abundant and show percentages between 0 to 2.2% (mean: 0.3%). Relative abundances of rotaliporids do peak in the upper three black shale beds. Despite the low percentages these maxima can be considered to be statistically significant as indicated by the error bars in Fig. 4.5.

4.5 Discussion 4.5.1 Diagenesis and microfossil preservation 4.5.1.1 Stable isotopes As the biogenic carbonate fraction of the sediments is dominated by calcitic skeletons of organisms inhabit- ing the uppermost water column (calcareous nannofossils, planktic foraminifera and calcispheres in descending order of abundance), the geochemical composition of the carbonates predominantly reflects a surface water signal. Diagenetic carbonate, i.e. micrite and cements, has been rarely observed during the study of calcareous nannofossils. Thus, its influence on the isotopic signal can be considered to be small. The lack of diagenetic dolomite in sediments from the Marnes Bleues Formation (Weissert and Bréhéret, 1991; Bréhéret, 1997; this study) suggests that alteration caused by bacterial methanogenesis in the black shales (Irwin et al., 1977) has not taken place.

60 (a) (b) 20

15 40 sediment] sediment] -1 -1 g g

3 10 3

5 spores [10 spores [10

marls Black shales: Niveau Paquier black shales Niveau Breistroffer 0 0 0 5 10 15 20 25 30 35 0 20 40 60 80 100 120 140 non-saccate pollen [103 g-1 sediment] non-saccate pollen [103 g-1 sediment] Fig. 4.6. Absolute abundances of non-saccate pollen versus spores for different black shales of mid-Cretaceous age from the Vocontian Basin. (a) Niveau Breistroffer, Col de Palluel section. Absolute spore and pollen abundances are generally higher in black shale samples than in marlstone samples, indicating enhanced humidity during black shale formation. (b) Absolute spore and pollen abundances from two different mid-Cretaceous black shale events (Niveau Paquier, Niveau Breistroffer) in the Vocontian Basin. Absolute abundances of the two palynomorph groups are very low during the formation of the late Albian main Niveau Breistroffer black shales. This is possibly due to a larger distance to the source area during a sea level highstand as compared to times of Niveau Paquier formation. Palynomorph data for the Niveau Paquier are from Herrle et al. (2003a). For stratigraphic position of Niveau Paquier see Fig. 4.2. Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 75

Burial diagenesis can change the isotopic composition of carbonates towards a positive linear correlation between carbon and oxygen isotope values (e.g. Jenkyns and Clayton, 1986; Jenkyns, 1995). Our dataset, however, shows only a slight positive correlation (r = 0.48, N = 50; Fig. 4.7a), suggesting a relatively minor alteration of the isotopic signal. This is in accordance with the findings of Levert and Ferry (1988) and Weissert and Bréhéret (1991) who noted that the Marnes Bleues Formation of SE France had not been subject to deep burial (less than ca. 700 m). Based on the low amounts of diagenetic carbonate in combination with a minor alteration during burial diagenesis, we conclude that the stable isotope data used in this study represent a reliable signal reflecting the original trends.

4.5.1.2 Calcareous nannofossils Nannofossil preservation is a critical factor in the palecological interpretation of nannofossil data. Carbonate dissolution can take place in the water column, at the sediment-water interface and in the sediment (e.g. Honjo, 1976; Steinmetz, 1994). In particular, high TOC values are considered to force carbonate dissolution during early diagenesis (Emerson and Bender, 1981). This should result in a negative correlation between

CaCO3 and TOC contents. As there is no correlation (r = -0.11, N = 50; Fig. 4.7b) between these two parameters for our dataset, the influence of TOC on carbonate dissolution was only minor. In order to characterize preservation, visual criteria for etching (E) and overgrowth (O; Roth and Thierstein, 1972; Roth, 1973) were applied. In all samples, nannofossils are well to moderately preserved and show only slight indications of etching (E1) and/or overgrowth (O1).

-3 3 (a) (b) -3.25 2.5 -3.5 2 -3.75 ] vs. PDB

-4 1.5 [ bulk

-4.25 [wt%] TOC O 1 18 d -4.5 marlstone 0.5 -4.75 black shale marlstone R=0.48 (N=50) black shale R=-0.11 (N=50) -5 0 1 1.25 1.5 1.752 2.25 50 55 60 65 13 d Cbulk [] vs. PDB CaCO3 [wt%]

15 15 (c) (d)

12 12 [%] [%]

9 9

6 6

Watznaueria barnesae Watznaueria 3 barnesae Watznaueria 3

marlstone marlstone black shale R=-0.04 (N=50) black shale R=0.13 (N=50) 0 0 40 45 50 55 60 65 70 1 1.52 2.53 3.5 Species richness Absolute abundance [109 nannofossils g-1 sediment] Fig. 4.7. Scatter plots showing the linear relationships and Pearson correlation coefficient between parameters poten- 13 18 tially indicating diagenetic alteration of the sample material. (a) δ Cbulk [] PDB vs. δ Obulk [] PDB, (b) CaCO3 [wt%] vs. TOC [wt%], (c) species richness vs. Watznaueria barnesae [%] and (d) absolute abundance of nannofossils [109 nannofossils g-1 sediment] vs. W. barnesae [%]. 76 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

A different approach to evaluate nannofossil preservation in Cretaceous sediments is based on the relative abundance of W. barnesae. This species is believed to be relatively resistant to dissolution (e.g. Roth and Krumbach, 1986). According to Roth and Krumbach (1986), assemblages consisting to more than 40% of W. barnesae may have been significantly altered. However, Williams and Bralower (1995) pointed out that high abundances of Watznaueria can also result from special palaeoenvironmental conditions. Therefore they suggested that the abundance pattern of dissolution-resistant species such as W. barnesae should be compared to changes of species richness and absolute abundance. Diagenetically altered assemblages should show low numbers of species and absolute abundance of calcareous nannofossils accompanied by high abundances of W. barnesae. The recorded abundances of W. barnesae (max. 13.3%, Fig. 4.4) are significantly below 40% and delicate forms (holococcoliths, Scapholithus fossilis, small zeugrhabdotids, Biscutaceae) have been observed frequently in all samples studied. Based on these observations and the lack of correlation between the relative abundance of W. barnesae, species richness, absolute abundance, CaCO3 and TOC (Fig. 4.7b-d) we exclude significant changes of the calcareous nannofossil assemblage resulting from diagenetic alteration.

4.5.1.3 Palynomorphs The preservation of palynomorphs depends strongly on the oxygenation of the depositional environment, with reduced oxygenation improving preservation and vice versa (e.g. Traverse, 1988). In order to detect a possible differential degradation of palynomorphs that may have obscured the original signal, the occurrence of foraminiferal test linings in the samples was examined. Such linings are the most oxidation-sensitive elements of palynomorph assemblages (G. Versteegh, 2002, pers. comm.). As well-preserved foraminiferal test linings were found during the quantitative evaluation or during the examination of additional slides from each sample horizon, the palynological assemblages can be considered to represent a primary signal.

4.5.1.4 Planktic foraminifera Planktic foraminiferal tests are moderately preserved and usually filled with secondary calcite or pyrite. SEM studies reveal that the test walls show different forms of diagenetic alteration such as recrystallisation and encrustation. The walls of hedbergellids are often replaced and no pores can be recognized. In contrast, the keeled taxa Rotalipora and Planomalina are generally better preserved. Here, primary pores are still visible, but the originally calcitic walls are also replaced.

4.5.2 Reconstructing palaeoenvironmental and palaeoceanographic changes 4.5.2.1 Surface water productivity Variations of surface water productivity are recorded by the nannofossil based nutrient index (NI) and relative abundances of small hedbergellids. The applicability of the NI in reconstructing surface water productivity and its relation to palynological and foraminiferal signals have been discussed in detail by Herrle et al. (2003a). Increased abundances of hedbergellids during the Cretaceous have often been referred to indicate a rise in surface water productivity (see Premoli Silva and Sliter, 1999 for a detailed discussion). Both the NI and the abundances of Hedbergella spp. show similar trends in the studied succession (Fig. 4.5). Higher surface water productivity can be recognized during the deposition of marlstones, whereas lower productivity prevailed during black shale formation (Fig. 4.5). According to the estimated mean sedimentation rate for the section (~4.4 cm kyr-1; see Section 4.2.2) a black shale/marlstone couplet (~85 cm thick in the lower part) has probably been deposited within ~19.4 kyr or slightly more due to a somewhat lower sedimentation rate Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 77 in this interval. This indicates that productivity changes are possibly controlled by precessional cycles. This is well in accordance with other data from the pale-dark bedding rhythms in the AptianAlbian Marnes Bleues Formation which are generally considered to follow orbitally tuned Milankovitch cycles (Bréhéret, 1994, 1997; Kößler et al., 2001; Herrle et al., 2003a, b). A further attempt to reconstruct changes of surface water productivity have been performed by Giraud et al. (2003). These authors studied the macrofauna (ammonites, bivalves, echinoderms etc.), ichnofossils and calcareous nannofossils over the entire Niveau Breistroffer interval at the Blieux section, which is located in a marginal setting of the Vocontian Basin, on a coarser temporal resolution (80 m thick sequence, 60 samples). Based on their benthic and planktic record they recognized only minor productivity changes.

4.5.2.2 Surface water temperature Both the nannofossil TI and oxygen isotope data indicate increasing surface water temperature with the onset of black shale formation in the lower part of the studied succession. In contrast, the marlstone-dominated succession in the upper part is characterized by a cooling trend. According to our age estimates, these long-term changes must have lasted between ~99 and ~115 kyr, thus suggesting the presence of eccentricity- controlled cycles. Moreover, subordinate short-term temperature fluctuations indicate a warming during the formation of single black shale beds (Figs. 4.5 and 4.8).

4.5.2.3 Humidity Enhanced humidity and terrigenous input into the Vocontian Basin during black shale formation are indicated by high TMR values and maxima in the absolute abundance of spores and pollen (Figs. 4.5 and 4.6a). This interpretation is supported by findings of Bréhéret (1997), who observed increasing amounts of kaolinite in the Niveau Breistroffer black shales from the Col de Palluel section.

4.5.2.4 Surface water stratification In order to assess surface water stratification during black shale formation, two plankton groups that probably inhabited subsurface water masses (Nannoconus, Rotalipora) have been studied. The ecology of Nannoconus spp. has been extensively discussed over the last years and will be briefly summarized in the following. Busson and Noël (1991) interpreted this group as calcareous dinoflagellate cysts which preferred shallow-water environments and oligotrophic conditions with low terrigenous supply. Other authors (e.g. Coccioni et al., 1992; Erba, 1987; Mutterlose, 1996) proposed that the group flourished in warm surface waters impoverished in nutrients. Erba (1994) suggested that Nannoconus spp. inhabited the lower photic zone similar to the recent Florisphaera profunda (Okada and Honjo, 1973; Molfino and McIntyre, 1990; Ahagon et al., 1993). According to Molfino and McIntyre (1990), abundance variations of F. profunda are related to changes in nutricline depth and stability. A shallow nutricline causes a higher nutrient transfer into the upper photic zone leading to blooms of coccoliths and low percentages of F. profunda. During periods of a deep nutricline mesotrophic conditions in the lower photic zone prevailed and led to high percentages of F. profunda. Herrle (2003) presented evidence for similar ecological preferences of Nannoconus and F. profunda. This author observed high abundances of nannoconids and low numbers of coccoliths during periods of enhanced stratification. During times of increased wind stress, in contrast, a rise of the nutricline caused an entrainment of nutrients into the surface waters. This scenario was reflected by low abundances of nannoconids, higher abundances of coccoliths and an increase of the NI. In the OAE 1d black shales, nannoconids occur in higher abundances only in or close to the black shales 78 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

BLAKE NOSE MAIN NIVEAU ODP SITE 1052E BREISTROFFER Col de Palluel (SE France) 18 d Opl. forams vs. PDB [] surface dweller d18O vs. PDB [] subsurface dweller bulk -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -3.0 -3.5 -4.0 -4.5 -5.0

[m]

[m] 500 53.5 53.0 505 52.5 510 52.0

515 51.5 51.0 520 ? 50.5 525 50.0 49.5 530 collapse of 49.0

stratification Eccentricity (~100 kyr) 535 48.5

540 48.0 47.5 545 47.0 550 46.5 50 40 30 20 10 TI Fig. 4.8. Temperature trends during the formation of the Niveau Breistroffer black shales at the Blake Nose Plateau (ODP Site 1052E) and in the Vocontian Basin (Col de Palluel section). In both areas, similar trends in surface water temperature (black arrows) occur. They probably follow eccentricity-controlled cycles (see text for details). Stippled area indicates assumed correlation between Blake Nose and SE France. Thick lines represent smoothed records based on the weighted harmonic mean method.

(Fig. 4.5). Based on our present understanding of Nannoconus palaeoecology, this may indicate phases of a deep nutricline. Higher abundances of Nannoconus correspond to lower NI values and decreasing percentages of Hedbergella, both indicating lower surface water productivity. There is also a trend to higher relative abundances of sub-surface dwelling rotaliporids in the black shales. They are believed to have preferred a stratified upper water column (e.g. Caron and Homewood, 1982; Leckie, 1987; Hart, 1999). The scenario of stratified surface waters during black shale formation is also supported by higher values for the heterogeneity of calcareous nannofossils (McIntyre and Bé, 1967; Brand, 1994). The co-occurrence of the two subsurface-dwelling groups may indicate that both groups proliferated within the lower photic zone and/or under similar oceanographic conditions. This supports the interpretation of Erba (1994) and Herrle (2003) that abundance pattern of nannoconids follow changes between those of more stratified surface waters and phases of enhanced mixing.

4.5.3 Driving mechanisms for OAE 1d formation in SE France As inferred from our age model, temperature changes during OAE 1d deposition (as evidenced by the nannofossil TI and oxygen isotopes) are presumably controlled by eccentricity (Figs. 4.5 and 4.8). The onset of black shale formation is characterized by a warming. At the same time, a subordinate precessional signal has been identified indicating a minor temperature increase during the formation of single Niveau Breistroffer black shale beds. Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 79

Variations of productivity went along with lithological changes from black shales and marlstones (Fig. 4.5). They seem to follow precessional cycles. These observations are supported by a time series analysis of the Early Albian Niveau Paquier in the Vocontian Basin that indicate orbital forcing of the nannofossil-based NI and TI with a dominance of precession for surface water productivity and eccentricity for temperature changes (Herrle et al., 2003a). A modern analogue for the observed climatic variations is found in the Asian/African monsoon system. Palaeoproductivity records from Quaternary sediments in the Arabian Sea are driven by precessionally controlled changes of monsoonal intensity and fluctuations in summer surface water productivity (e.g. Reichart et al., 1997). These changes are associated with strong fluctuations in wind velocities and precipitation rates (e.g. Clemens and Prell, 1990). In analogy to the modern monsoon system, higher precipitation rates during warmer periods may have caused a decrease in evaporation rates during formation of OAE 1d black shales. This led in combination with lower surface water densities to a drastic reduction of deep-water formation in low-latutidinal epicontinental areas when a threshold value was reached (Bice et al., 1997). The shelf areas of the western and eastern Tethys were subject to a strong monsoonal circulation during the latest Albian (100 myr; Oglesby and Park, 1989). Moreover, at least a part of mid-Cretaceous deep-water formation is considered to have taken place in the shelf and epicontinental areas of the western and eastern Tethys driven by the sinking of warm, saline waters due to high evaporation rates (Brass et al., 1982; Barron and Pederson, 1990; Wortmann et al., 1999; Herrle et al., 2003a; Fig. 4.9). In addition to low-latitude deep- water sources, numerical models suggest that the high latitudes may also have played an important role for the global deep-water formation during this period (e.g. Poulsen et al., 2001; Bice and Norris, 2002). Changes in the rates of deep-water formation have been postulated for the deposition of the Niveau Paquier black shale (OAE 1b) in the Vocontian Basin (Herrle et al., 2003a, b). For this black shale, high TMR values and spore/pollen ratios indicate that the formation occurred under extremely humid conditions. The extreme humidity has probably reduced evaporation in the low latitudes (i.e. the western Tethys) and thus slowed down deep-water formation. This had a widespread impact on the bottom water ventilation and the preservation potential of organic matter (Herrle et al., 2003a). Similar scenarios for the preservation of organic matter have been proposed for the formation of the OAE 1b in the Atlantic Ocean (Erbacher et al., 2001) and the Vocontian Basin (Tribovillard and Gorin, 1991). The above interpretation of our data is also well compatible with a model for the formation of Mediter- ranean sapropels. There, higher rates of monsoonal fluvial discharge led to sapropel formation (Rossignol- Strick, 1985; Rossignol-Strick et al., 1998). Elevated runoff was responsible for lower surface water salinities and stratification (Rohling and Gieskes, 1989), allowing the transport of nutrient-rich intermediate waters into the lower photic zone and leading to high abundances of the subsurface-dwelling coccolithophorid F. profunda in the sapropels (Castradori, 1993). In analogy, low abundances of subsurface-dwelling plankton (nannoconids, rotaliporids) in the Late Albian black shales may indicate a lower influence of fluvial discharge compared to the sapropel formation in the Mediterranean Sea. Numerous authors (e.g. Weissert et al., 1998; Föllmi et al., 1994; Hochuli et al., 1999) have linked black shale formation to intensified weathering and elevated runoff. This is believed to have caused enhanced surface water productivity. However, during the formation of Quaternary sapropels increasing fluvial supply did not necessarily lead to elevated productivity as pointed out by Rossignol-Strick and Paterne (1999). These authors argued that a runoff increase lead to elevated productivity, which was restricted to the river mouths where nutrient fixation mainly takes place. Further offshore only a water lens with lower salinity and 80 Chapter 4: Reconstruction of short-term palaeoceanographic changes ...

60° 30°0° 30° 60°

60° 60° ASIA NORTH AMERICA L strong monsoon 30° VB 30° BN* * restricted H deep-water formation weak NE trades 0° 0° AFRICA ITCZ SOUTH AMERICA L warm-humid 30° 30°

30° 0°30° 60° 60° 30°0° 30° 60°

60° 60° ASIA NORTH AMERICA L weak monsoon 30° VB 30° BN* * enhanced H deep-water formation moderate NE trades

0° 0° AFRICA ITCZ SOUTH AMERICA L cool-dry 30° 30°

30° 0°30° 60°

presumed landmass atmospheric circulation area of restricted deep-water formation

area of enhanced deep-water formation

Fig. 4.9. Model for deep-water formation during periods of black shale respectively marlstone deposition of the main Niveau Breistroffer with schematic mean annual atmospheric circulation pattern for the low latitudes at insolation maximum (adopted from Herrle et al., 2003a). Principal elements of the mid-Cretaceous climate in the low-latitude region as depicted by climate models (e.g. Oglesby and Park, 1989; Barron and Pederson, 1990; Poulsen et al., 1998). Palaeogeography is adopted from Hay et al. (1999) for the late Albian (100 myr). During warm and humid climate conditions and a strong monsoonal circulation (as proposed for the formation of the Niveau Breistroffer black shales), deep-water formation was restricted in the northern and eastern Tethyan area. During marlstone deposition, no restriction of deep-water formation occurred, leading to a better oxygenation of the bottom water in the Vocontian Basin. Abbreviations: H, high-pressure system; L, low-pressure system; ITCZ, intertropical convergence zone; BN, Blake Nose; VB, Vocontian Basin. Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 81 low nutrient content would develop. Such conditions are supported by findings of Sachs and Repeta (1999), who observed high abundances of algae in the sapropels, which prefer oligotrophic, stratified environments with reduced salinities in the surface water. For the Niveau Breistroffer black shales, our data suggest a deep nutricline and a low primary production in the upper photic zone. Based on the above discussion, we consider the Niveau Breistroffer black shale periods to have been characterized by a strong monsoon and warm-humid climate presumably accompanied by elevated runoff. This should have increased the density contrast within the water column. Consequently, a more stable stratification prevented the mixing of nutrient-rich intermediate waters with oligotrophic surface water masses. Increasing humidity and runoff additionally diminished the rates of deep-water formation in the eastern and western Tethys leading to poor benthic oxygenation. We conclude that increased preservation of organic matter at the sea floor was more important for the formation of the Niveau Breistroffer black shales in the Vocontian Basin than enhanced production of organic matter in the upper water column. Drier and cooler conditions during marlstone deposition were probably characterized by increased mixing and well oxygenated bottom waters caused by enhanced deep-water formation. This interpretation is also supported by obvious bioturbation in the marlstones.

4.5.4 Supraregional palaeoenvironmental signals for OAE 1d formation The OAE 1d black shale event occurs mainly in the northern Atlantic and western Tethys, but sporadic occurrences are also reported from the South Atlantic, the Pacific Ocean, the Antarctic Sea and the Western Interior Seaway (Wilson and Norris, 2001). To date, only few localities have been studied in detail. These include the Mazagan Plateau (DSDP Site 547; Nederbragt et al., 2001), the Blake Nose Plateau (ODP Site 1052E; Wilson and Norris, 2001) and the Vocontian Basin (Bréhéret, 1988, 1994, 1997; Giraud et al., 2003). At all three localities the late Albian is characterized by a positive carbon isotope excursion with an amplitude between 0.5 (Mazagan Plateau; Nederbragt et al., 2001) and 1.5 (Blake Nose - Wilson and Norris, 2001; Vocontian Basin - this study). At the Mazagan Plateau (eastern Atlantic, DSDP Site 547) the micropalaeontological study of calcareous nannofossils, planktic and benthic foraminifera by Nederbragt et al. (2001) did not yield major productivity changes. Only high turnover rates within the planktic foraminifera mark the OAE 1d interval. Accord- ing to Nederbragt et al. (2001) these changes are linked to an oxygenation decrease of the subsurface waters coupled with the expansion of the oxygen minimum zone, which affected the habitat of some of the disappearing planktic foraminiferal species. For the OAE 1d formation on the Blake Nose Plateau (western Atlantic, ODP Site 1052E) Wilson and Norris (2001) proposed a collapse of the thermocline. Based on oxygen isotope data of subsurface-dwelling planktic foraminifera and those of surface-dwelling species they observed a decrease in the temperature gradient (Fig. 4.8), suggesting enhanced mixing and thereby elevated surface water productivity. As shown in Fig. 4.8 presumably eccentricity controlled temperature changes can be observed both on the Blake Nose Plateau and in the Vocontian Basin. According to these data, the onset of black shale formation went along with a temperature increase of ~2°C. These observations suggest that the same driving mechanisms influenced the OAE 1d formation. We assume that eccentricity-controlled monsoonal climate, characterized by increasing temperature, humidity and windstress is very well coupled with the collapse of the thermocline in the NW Atlantic Ocean. Both models for the OAE 1d formation can be brought into line, when considering the different palaeogeographic and -oceanographic settings. Due to the epicontinental position of the Vocontian Basin 82 Chapter 4: Reconstruction of short-term palaeoceanographic changes ... water temperatures may have been higher than in the western Atlantic Ocean. In addition, increased humidity must have had a greater influence due to the proximal situation. As shown in Fig. 4.9, phases of black shale formation are characterized by warmer climate accompanied by monsoonal activity, enhanced seasonality and a strengthening of the Westerlies. We believe that stronger wind stress presumably forced a cooling of the surface water masses and led to increasing mixing rates in the open-oceanic Atlantic Ocean. Enhanced wind stress may also account for the observed long-term surface water cooling, which predates the OAE 1d formation on the Blake Nose (see data Wilson and Norris, 2001) and may have contributed to the collapse of stratification.

4.5.5 Comparison between OAE 1d and OAE 1b formation in SE France In comparison to black shales formed during the latest Aptian and earliest Albian (Herrle et al., 2003a, b), the boundary conditions for the formation of the late Albian main Niveau Breistroffer black shales were significantly different. According to Haq et al. (1987) the sea-level was nearly 100 m higher than in the earliest Albian, causing a more distal position of the studied succession and smaller continent masses in the west and the north (see palaeogeography of Hay et al., 1999; Fig. 4.9). We assume that the density of vegetation in the hinterland did not significantly change from the lowermost to the upper Albian. Thus, the small difference in the sedimentation rate (Niveau Paquier: ~3.7 cm kyr-1; Herrle et al. (2003a, b); Niveau Breistroffer: ~4.4 cm kyr-1, this study) and the fact that the absolute numbers of spores and pollen are up to six times lower than in the Niveau Paquier (Herrle et al., 2003a; Fig. 4.6) support our assumption that the Niveau Breistroffer was deposited in a larger distance to the hinterland. A more distal position would also explain the oligotrophic surface waters during black shale formation. In contrast, the onset of the Early Albian Niveau Paquier formation as observed by Herrle et al. (2003a, b) went along with increasing humidity and elevated surface water productivity.

4.6 Conclusions The high-resolution quantitative analysis of different microfossil groups and stable isotope data provide insights into palaeoclimatic and -oceanographic conditions prevailing during the formation of the main Niveau Breistroffer black shales in the Vocontian Basin: (1) The main Niveau Breistroffer has formed under short-term cool-dry/warm-humid cycles probably controlled by intensity changes of monsoonal activity, with black shales forming under relatively warm and humid conditions. (2) As inferred from the terrigenous/marine ratio of palynomorphs, periods of black shale formation were characterized by high terrigenous input and thus increased runoff. (3) The more warm-humid phases were accompanied by higher abundances of subsurface dwelling plankton (nannoconids, rotaliporids). The co-occurrence of higher abundances of nannoconids and rotaliporids supports the hypothesis that nannoconids flourished within the lower photic zone and proliferated under stratified, stable conditions with low nutrient levels in the surface water. (4) The application of nannofossil indices to record changes in surface water nutrient conditions and temperature correspond well to abundance changes of higher nutrient levels indicating planktic foraminifera (small hedbergellids) and variations of oxygen isotope data. (5) We assume that increasing humidity during black shale formation led to a decrease in deep-water formation and probably an increase in surface water stratification. Both mechanisms led to oxygen con- Chapter 4: Reconstruction of short-term palaeoceanographic changes ... 83 sumption in the bottom water which in turn increased the preservation potential of organic matter. Therefore the accumulation of organic matter in the main Niveau Breistroffer black shales was controlled by preservation rather than by increased productivity in the photic zone. (6) Based on carbon isotope data, the Niveau Breistroffer can be correlated with the OAE 1d observed at the Blake Nose Plateau (ODP Site 1052E). The supraregional distribution of the OAE 1d within the Atlantic Ocean is explained by enhanced mixing during periods of enhanced monsoonal activity.

Acknowledgements This study was funded by the German Research Foundation (DFG) within the Collaborative Research Center 275 (project A5) of the University of Tübingen and through Grant He 697/34 and He 697/41. Dr. P.A. Wilson kindly provided the isotope data from ODP Site 1052E. Thoughtful reviews by xxx and yyy are gratefully acknowledged. Dr. J. Lehmann is thanked for field assistance. 84 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

5 Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae from the Late Albian 'Niveau Breistroffer' (SE France): taxonomic and palaeoecological implications

André Bornemann and Jörg Mutterlose

Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany (submitted for publication to Geobios)

Abstract The size variability of the coccolith species Biscutum constans and Watznaueria barnesae has been studied in 50 samples of the Late Albian Niveau Breistroffer black shales (Col de Palluel section, SE France). For each species length and width of the total coccolith and the central unit have been measured in 60 specimens per sample. In addition, ellipticity and central unit length/total coccolith length ratio have been calculated. This study aims to improve our understanding of the taxonomic concepts of B. constans and W. barnesae and to test whether the size changes correspond to palaeoceanographic changes interpreted from other proxies. Two morphotypes were recognized and differentiated for each of the two taxa studied. B. constans includes morphotypes with a narrow (B. constans var. constans) and a wide central unit (B. constans var. ellipticum). Between these two morphotypes no significant size differentiation has been observed. They seem to represent end members of a size continuum, concerning both the total coccolith- and central unit size distribution. The two morphotypes of W. barnesae differ only by the absence (W. barnesae var. barnesae) and the presence of a narrow central opening (W. barnesae var. fossacincta). No significant size differences have been observed between these two morphotypes. Both show similar distributions of the measured characteristics. We recommend that the studied morphotypes of both B. constans and W. barnesae should not be assigned to different morphospecies. Throughout the studied black shale succession the measured parameters show mostly statistically insignificant, short-term fluctuations. Significant long-term trends have been observed for B. constans. These show a trend to smaller forms with a narrower central unit in the upper part of the succession. This change coincides with a cooling trend, as indicated by a nannofossil based temperature index. Therefore the two morphotypes of B. constans are interpreted to represent ecophenotypic varieties rather than two different morphospecies. No clear relationship has been recognized between the size of W. barnesae and the palaeoenvironmental conditions.

Keywords: calcareous nannofossils; morphometry; numerical taxonomy; palaeoceanography; Cretaceous; black shales

5.1 Introduction Mesozoic coccoliths are widely used as biostratigraphic zonal markers (e.g. Sissingh, 1977; Bown et al., 1998) and as palaeoceanographic proxies (e.g. Roth and Krumbach, 1986; Erba, 1994; Mutterlose, 1996; Herrle, 2003). The taxonomy of Mesozoic coccoliths and calcareous nannofossils in general is exclusively based on morphological criteria (morphospecies), which in turn are believed to be at least partly controlled by environmental parameters (e.g. Young, 1990; Bollmann, 1997; Colmenero-Hidalgo et al., 2002). There- fore morphometric studies are essential to improve our taxonomic concepts and to understand the controls of morphological variability of coccolith species. Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 85

During the last decade size variations of coccoliths were the subject of numerous studies. Many of them are based on culture material, sediment traps or surface sediments (e.g. Bollmann, 1997; Knappertsbusch et al., 1997; Riebesell et al., 2000; Baumann and Sprengel, 2001; Renaud et al., 2002), where environmental parameters, such as temperature, salinity and nutrients have been directly measured from seawater. These studies aim at gaining more precise taxonomic concepts of the studied taxa or to reveal a presumed palaeoenvironmental control on coccolith morphology or calcification. These investigations cover short time intervals from days to years, whereas the impact of global climatic and oceanographic changes on the coccolith size can only be understood on geological timescales (thousands to millions of years). For the fossil record many morphometric studies have been performed on Cenozoic coccoliths (e.g. Samtleben, 1980; Backman and Hermelin, 1986; Young, 1990; Beaufort, 1992; Bralower and Parrow, 1996; Knappertsbusch, 2000; Colmenero-Hidalgo et al., 2002). Detailed studies are rather rare for the Mesozoic (Bornemann et al., 2003; Mattioli et al., subm.). More attention has been paid to obvious size and abundance changes of Mesozoic nannoliths throughout intervals of major palaeoceanographic perturbations. These intervals include the Posidonia shale (Toarcian - Schizosphaerella spp.; Mattioli and Pittet, 2002), the Juras- sic-Cretaceous boundary (Conusphaera mexicana, Polycostella beckmannii, Nannoconus spp.; Bornemann et al., 2003) and the Oceanic Anoxic Event (OAE) 1a (Aptian - Assipetra infracretacea, Rucinolithus terebrodentarius; Tremolada and Erba, 2002). The observed size changes of Mesozoic nannofossils occur on a longer timescale between several 100 kyr and more than 1 myr, and may reflect evolutionary responses to the long-term changes of the palaeoceanographic conditions. So far, however, no efforts have been made for the Mesozoic to study size and morphological changes of nannofossils on shorter time-scales. This paper presents data from the Late Albian Niveau Breistroffer black shale succession (Col de Palluel section, SE France), which represents the regional equivalent of the widespread distributed OAE 1d in SE France (Lehmann, 2000; Wilson and Norris, 2001; Bornemann et al., subm.). We choose the species Biscutum constans and Watznaueria barnesae for this study, because both taxa occur in high abundances throughout the investigated interval and are believed to respond to palaeoenvironmental changes. High numbers of B. constans have often been linked to higher surface water nutrient conditions (e.g. Roth and Krumbach, 1986; Premoli Silva et al., 1989; Erba et al., 1992; Eshet and Almogi-Labin, 1996). In contrast, W. barnesae is considered to prefer more oligotrophic conditions (e.g. Roth and Krumbach, 1986; Erba et al., 1992; Williams and Bralower, 1995; Herrle et al., 2003a, b). Two morphotypes (varieties) of each species have been observed and distinguished for this study. B. constans includes forms with a narrow central unit (B. constans var. constans) and those with a larger central unit (B. constans var. ellipticum). W. barnesae is represented by morphotypes consisting of a closed central area (W. barnesae var. barnesae) and those consisting of a narrow central opening (W. barnesae var. fossacincta). Previously these morphotypes have been assigned to different species, but in many ecological studies they have been lumped together (e.g. see taxonomic comments to B. ellipticum and W. barnesae in Grün and Allemann, 1975). The formation of the Niveau Breistroffer black shales is characterized by short-term palaeoceanographic changes. These have been studied and reconstructed by Bornemann et al. (subm.) on a high-resolution scale with a temporal resolution of ~3.1 kyr, using the same sample material which has been analyzed for this study. The palaeoenvironmental reconstruction allows us to test (1) whether morphometric changes are environmentally controlled and (2) whether the different morphotypes represent ecophenotypic varieties of the studied taxa. In addition, the taxonomic definition of the investigated morphospecies and -types will be refined. 86 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

5.2 Palaeogeography During the early Cretaceous the Vocontian Basin (SE France) was located at a palaeo-latitude of 25-30°N (Savostin et al., 1986; Hay et al., 1999) and was part of the European continental margin of the Ligurian Tethys (Lemoine et al., 1986). The basin was surrounded by the Massif Central landmass in the northwest and by carbonate platforms in the north and south (Urgonian platform carbonates, Arnaud-Vanneau and Arnaud, 1990). To the eas t it was open toward the Tethys (Fig. 5.1).

5.3 Palaeoceanography of the Late Albian OAE 1d in SE France The mid-Cretaceous has often been characterized as a period of greenhouse conditions. These were accompanied by a sea level rise (Haq et al., 1987), high rates of ocean crust production, and submarine volcanism (Larson, 1991a, b). During this long-term global warmth a number of regionally to supraregionally distributed black shales, named Oceanic Anoxic Events (OAEs; Schlanger and Jenkyns, 1976), were deposited. Huge amounts of organic carbon accumulated on the sea-floor during OAE formation and led to major perturbations of the global carbon cycle. This is recorded in the carbon isotope signature of marine carbonates and organic matter (e.g. Jenkyns et al., 1994; Weissert et al., 1998). The OAE intervals correspond to positive carbon isotope excursions with amplitudes larger than 1.5 and high turnover rates of marine biota (e.g. Premoli Silva et al., 1999; Leckie et al., 2002). The Late Albian OAE 1d black shale event occurs mainly in the northern Atlantic and western Tethys, but sporadic occurrences are also reported from the South Atlantic, the Pacific Ocean, the Southern Ocean and the Western Interior Seaway (Wilson and Norris, 2001). To date, only few localities have been studied in

Palaeogeography

eroded Albian

Grenoble

Massif Central Valence Vercors Die Internal zones of the Alps

Col de Gap Palluel

Mt. VENTOUX Digne

Blieux

Nice eroded

Fig. 5.1. Palaeogeographic reconstruction of SE France for the Albian (modified after Arnaud and Lemoine, 1993). The locations of the Col de Palluel and the Blieux section are shown. Black box marks the geographic area of Fig. 5.2. Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 87

5°10 5°20 5°30 5°40 5°50 6° 6°10

Die N 44°40 44°40

Gap

44°30 44°30

Serres

Col de Palluel 44°20 44°20

Sisteron

5°10 5°20 5°30 5°40 5°50 6° 6°10 Fig. 5.2. Location map of the Col de Palluel section in SE France (after Herrle, 2003). detail. These include the Mazagan Plateau (DSDP Site 547, Nederbragt et al., 2001), the Blake Nose Plateau (ODP Site 1052E, Wilson and Norris, 2001) and the Vocontian Basin (Bréhéret, 1994, 1997; Giraud et al., 2003; Bornemann et al., subm.). Late Albian Niveau Breistroffer black shales from the Vocontian Basin have been successfully correlated to the widespread distributed OAE 1d in the Atlantic Ocean (Wilson and Norris, 2001; Bornemann et al., subm.). Two detailed studies (Giraud et al., 2003; Bornemann et al., subm.) have yet been performed for the OAE 1d sediments in the Vocontian Basin. Giraud et al. (2003) studied the Blieux section, which is located in a marginal position of the basin (Fig. 5.1). Based on the macrofauna, ichnofossils and calcareous nannofossils only minor changes of surface water productivity have been interpreted. Bornemann et al. (subm.) studied the Col de Palluel section, which is located close to the center of the basin (Fig. 5.1). To reconstruct the palaeoceanography during black shale formation calcareous nannofossil, planktic foraminifera, palynomorph and stable isotope data were used. The results suggest that black shale formation was controlled by orbitally- induced changes in the monsoonal activity leading to increased humidity, stratified surface waters, and slightly elevated temperatures compared to the marlstone facies. Periods of black shale formation were characterized by lower surface water productivity, while higher fertility prevailed during marlstone deposition. The humidity increase, as indicated by the palynomorphs, during black shale formation may have led to a decrease in low-latitude deep-water formation. This combined with more stratified surface waters, caused an increase in the preservation of organic matter.

5.4 Material and methods 5.4.1 Location, lithology and stratigraphy of the Col de Palluel section For this study the Niveau Breistroffer was analyzed from the Col de Palluel section along the road D994 c. 5 km east of Rosans (SE France; Fig. 5.2), Département Drôme (TK 25 Rosans, Nr. 3239 Ouest, Série 88 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

Bleues, Lambert III coordinates x: 853 750, y: 3238 425). The succession is situated on the north-eastern slope of the Mt. Risou south of the D994. The sequence of Aptian-Albian sediments is about 750 m thick and consists of cyclically bedded marlstones and limestones, which were deposited in the center of the Vocontian Basin (Marnes Bleues Forma- tion; Flandrin 1963). Black shales, limestones, and glauconite bearing turbidites are intercalated into this sequence. They serve as key beds for lithostratigraphic correlation (Bréhéret, 1997; Fig. 5.3). The investigated sequence is 6.28 m thick and comprises carbonate-rich grayish marlstones (47-67 wt% CaCO ) and 3 laminated black shales with up to 2.5 wt% TOC (Bornemann et al., subm.). Pale-dark bedding rhythms are indicated by slight differences in colour which reflect variations in carbonate and organic carbon content. The intercalated black shale beds are between 0.2 to 0.7 m thick and show different degrees of lamination. The studied interval is named main Niveau Breistroffer (Bornemann et al., subm.; Fig. 5.3). According to Gale et al. (1996) the main Niveau Breistroffer can be assigned to the Upper Albian Stoliczkaia dispar ammonite zone and NC10A (Bralower et al., 1993) nannofossil subzone. The last appearance datum of the planktic foraminifer Planomalina buxtorfi occurs 17 m above the top of the main Niveau Breistroffer supporting the assignment to the P. buxtorfi/Rotalipora appenninica planktic foraminiferal subzone (Reichelt et al., subm.). Based on the correlation of the carbon isotope record from the Col de Palluel section (Reichelt et al., subm.) to that from the Blake Nose Plateau (ODP Site 1052E; Wilson and Norris, 2001), Bornemann et al. (subm.) proposed that the entire main Niveau Breistroffer comprises ~159 kyr.

MARNES BLEUES FORMATION Vocontian Basin MAIN NIVEAU s BREISTROFFER a u il NIVEAU r o s ite tic ife re s BREISTROFFER Col de Palluel e n k a fo s g o n n in n lc o n d y la m a n e g ta m tio a tio n tio b lo Col de Palluel y s s m a P r a C a a o g b A n fo n n n y lo le u o o o e ith y ] o p S z z z K L g h m lo [m it a o L S Rotalipora [m] ith Mantelliceras globotrunc- L mantelli anoides

L. Cen. 70 53 Rotalipora CC appeninica 9B

65 Stoliczkaia Niveau dispar R. appeninica + Breistroffer P. buxtorfi (OAE 1d) 52 NC10

Rotalipora 60 appeninica Upper Albian Rotalipora CC ticinensis 9A 51 Mortoniceras 55 inflatum T. subticinensis Petite Ticinella Verole praeticinensis

Euhoplites NC9 CC Ticinella 8B 50 lautus 50 primula Hoplites NC8C

Mid.-Alb. dentatus

Douvilleiceras Niveau mammillatum Leenhardt 45 Hedbergella CC NC8B Niveau planispira 8A L. tardefurcata Paquier 49 (OAE 1b) Lower Albian Niveau Hypacant- Kilian 40 hoplites

NC8A Niveau jacobi Jacob Faisceau Fromaget A. nolani Ticinella 48 beajouensis Niveau Nolan 35 P. nutfieldensis

CC 7B Hedbergella 30

Upper Aptian trocoidea 47

E. G. algerianus martinoides Niveau Globigerinel- Fallot loides ferreolensis marlstone marlstone (dark) calcareous nodules

NC7ANiveau NC7B/C D. deshayesi Leupoldina cabri CC Goguel marlstone (pale) black shale turbidites

L. Apt. 7A (OAE 1a) NC6 Fig. 5.3. Lithologic and stratigraphic framework for the Marnes Bleues Formation and the Niveau Breistroffer (Col de Palluel section, SE France) based on ammonites (Bréhéret, 1997), planktic foraminifera (Moullade, 1966; Reichelt et al., subm.), calcareous nannofossils (Gale et al., 1996; Herrle and Mutterlose, 2003) and lithostratigraphy (Bréhéret, 1997). Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 89

5.4.2 Methods 5.4.2.1 Morphometry A total of 50 samples has been investigated. In order to obtain a statistical distribution of particles the random settling technique (Williams and Bralower, 1995; Geisen et al., 1999) was applied for slide preparation. Morphometric analyses were performed with an image analysis system consisting of a CCD camera mounted on an OLYMPUS BH-2 light-microscope and an Apple PowerPC with a SCION LG-3 framegrabber card. In addition, the software package ScionImage 1.62 and modified macro routines of J. Young (NHM, London) and M. Geisen (AWI-Bremerhaven) were used. The accuracy of size measurements yields 0.056 µm per pixel at a microscope magnification of 1250x. The studied coccolith taxa (B. constans, W. barnesae) are placoliths, which are composed of a proximal and a distal shield, connected by a central tube. The two species show fundamental differences in their ultrastructure. B. constans consists of a proximal shield and a central tube, which are made up by an R-unit (radially oriented c-axes), and a V-unit (vertically oriented c-axes) forming the distal shield (Young et al., 1992, 1999; Fig. 5.4a). Coccoliths of the genus Watznaueria consist of large R-units, which form the inner tube element, the distal and proximal shield elements. Only the peg-shaped mid-tube elements are formed by a V-unit (Young et al., 1992). Under polarizing light the peg-like V-unit is darker than the surrounding R- unit, thus these units can be easily distinguished (Fig. 5.4b). Phase contrast was used for B. constans to increase the contrast between the outer rim and the background. W. barnesae has been measured under cross-polarized light as described by Bornemann et al. (2003). For each of the two taxa, length and width of the total coccolith and the central unit have been measured for 60 specimens per sample (Fig. 5.4). This resulted in a total of 24,000 size measurements. In addition ratios have been calculated, including central unit length/total length, length/width central unit (= ellipticity central unit), length/width total coccolith (= ellipticity total coccolith) and the central unit area/total coccolith area. In addition, the width of the outer rim has been calculated. According to Bornemann et al. (2003) the measurement of at least 50 specimens for each given species per sample provides a sufficient reproducibility for the mean and the 95% confidence limit.

(a) Biscutum constans (b) Watznaueria barnesae

R-unit (radial c-axis) R-unit (radial c-axis) V-unit (vertical c-axis) V-unit (vertical c-axis) V-unit nucleation site V-unit nucleation site

Biscutum constans var. Biscutum constans var. Watznaueria barnesae var. Watznaueria barnesae var. ellipticum constans barnesae fossacincta

W W w centra al V l ntr -u w ce it nit -un R

l central V-unit l c R e - n u t r n a it l L

Fig. 5.4. Ultrastructure, light-microscope micrographs and the measured size parameters for Biscutum constans var. constans/B. constans var. ellipticum (a, phase contrast) and Watznaueria barnesae var. barnesae/W. barnesae var. fossacincta (b, XPL). Abbreviations: l, central unit length; L, total coccolith length; w, width central unit; W, width total coccolith. 90 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

Varimax-rotated principle component analyses (PCA) were performed to determine possible relationships between the morphological parameters. For this processing each morphotype has been studied separately in order to recognize possible differences between them. The results have been used to select representative parameters for the calculated components.

5.4.2.2 Tools for palaeoenvironemental reconstruction (calcareous nannofossil nutrient and temperature indices, oxygen isotopes) Temporal and spatial changes in the distribution and the abundances of calcareous nannofossils are controlled by climatic and oceanographic conditions. Such changes allow the use of calcareous nannofossils to reconstruct palaeoenvironmental conditions in the Mesozoic (e.g. Erba et al., 1992; Mutterlose, 1996; Street and Bown, 2000). Among other factors, surface water temperature and nutrient availability are thought to be important parameters controlling the distribution and composition of recent calcareous nannoplankton (e.g. Brand, 1994; Winter et al., 1994). In this study short-term changes in surface-water nutrient content and temperature have been assessed using nannofossil-based nutrient and temperature indices (NI, TI). These have been calculated from 300 counted nannofossil specimens per sample (for details see Bornemann et al., subm.). The NI has been adapted from Herrle et al. (2003a, b; high nutrient indicators: Discorhabdus ignotus, Zeugrhabdotus erectus; low nutrient indicator: W. barnesae). The TI is based on observations taken from the literature (warm water taxa: Rhagodiscus spp. = Rhagodiscus asper, Rhagodiscus achlyostaurion; cool water taxa: Crucibiscutum salebrosum, Tranolithus orionatus, Repagulum parvidentatum). For further details see Herrle et al. (2003 a, b) and Bornemann et al. (subm.). These authors tested both indices and compared the signals to other micropalaeontological and geochemical proxies. D.ignotus + Z.erectus (1) NI = ⋅100 D.ignotus + Z.erectus +W.barnesae C. salebrosum + T.orionatus + R. parvidentatum (2) TI = ⋅100 C. salebrosum + T.orionatus + R. parvidentatum + Rhagodiscus spp. Oxygen isotopes have been measured from bulk-rock material using an off-line preparation technique, measurements were performed using a Finnigan MAT Delta S mass spectrometer at the Ruhr-University Bochum. The precision of isotope measurements is better than ±0.12. The results have been discussed by Bornemann et al. (subm.; see Section 4.5.1.1 of this thesis) and they are here used to support the temperature signal obtained from the TI.

5.5 Results 5.5.1 Size variability of Biscutum constans Total length of the species Biscutum constans ranges from 1.58 to 5.85 µm (mean 3.43 µm, standard deviation (std. dev.) 0.67; see Fig. 5.5 and Table 5.1). For the two morphotypes the mean lengths are 2.90 µm (std. dev. 0.49; range 1.58-5.05 µm) for B. constans var. constans and 3.64 µm (std. dev. 0.61; range 2.08-5.85 µm) for B. constans var. ellipticum. The length of the central unit of B. constans varies between 0.5 and 2.66 µm (width 0.27-1.74 µm). Table 5.1 gives an overview of the measured and calculated size parameters of the studied species and morphotypes. Coccoliths having a wide central unit are generally larger than those consisting of a narrow one, but the frequency distribution shows a significant overlap at least between the 90%-percentile of the smaller morphotype with the 25%-percentile of the larger one (Figs. 5.5 and 5.6). Both length and width of the Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 91 measured units are unimodally distributed. Frequency histograms (Fig. 5.5) and box plots (Fig. 5.6) exhibit a positive skewness of the total coccolith and the central unit length. The scatter plot in Fig. 5.5 exhibits for both morphotypes that the total coccolith length is linearly correlated with the total width. They show similar regression functions and high Pearson correlation coeffi- cients (r = 0.92 and 0.93 respectively). A slight difference in the slope of the linear regression function and a lower correlation coefficient (r = 0.74 and 0.72 respectively) suggest a weaker relationship of the central unit width/length ratio compared to the total coccolith length/width ratio. The linear regression function of the total coccolith length/width is significantly steeper (0.85 and 0.86) than that of the central unit (0.53 and 0.48, respectively). Size variations through time are shown in Fig. 5.7. The size of Biscutum constans changes significantly above the second black shale bed (~48.70 m). The mean length of the central R-unit decreases from 1.5 µm at the base to 1.25 µm at the top (Fig. 5.7). The course of the total length is much less consistent than that of the central unit, but a decrease of size is also obvious (3.6 to 3.3 µm; Fig. 5.7). As a consequence of a smaller size coccolith ellipticity increases, on the other hand, total length/central unit length decreases. All these shifts are statistically significant at the 95% confidence interval. In contrast, short-term fluctuations in the frequency of lithological changes occur, but no consistent pattern with respect to lithology has been observed.

600 Biscutum constans 400 var. ellipticum

200 N=2160 N=2160 central R-unit 0 300 total coccolith 200 Biscutum constans N=840 Frequency (length) 100 var. constans Frequency (width) N=840 0 01000200 400 0 500 5 central B. constans var. constans (N=840) Biscutum Biscutum R-unit B. constans var. ellipticum (N=2160) constans constans var. constans var. ellipticum total B. constans var. constans (N=840) 4 coccolith B. constans var. ellipticum (N=2160)

B. constans var. ellipticum (total coccolith) N=840 N=2160 3 B. constans var. constans y=-0.02+0.858x (total coccolith) r=0.93, N=2160 y=-0.013+0.845x r=0.92, N=840

B. constans var. ellipticum (central R-unit) 1 y=0.229+0.479x r=0.72, N=2160

Width (central R-unit, total coccolith) [µm] (central R-unit, total Width N=2160 B. constans var. constans (central R-unit) N=840 y=0.048+0.534x, r=0.74, N=840 0 0 1 2 3 4 5 6 Length (central R-unit, total coccolith) [µm] Fig. 5.5. Scatter plots and frequency histograms of length and width of the central R-unit and the total coccolith of the two morphospecies of Biscutum constans. In addition, the Pearson correlation coefficient (r), the number of measurements (N) and the linear regression function is given. Histogram class sizes are 0.2 µm. 92 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

[µm] 0 1 2 3 4 5 6 7 8 9

B. constans var. constans (N=840) 25%-percentilemedian 75%-percentile B. constans var. ellipticum R-unit central Length (N=2160) Max. Min.

mean B. constans var. constans (N=840) 10%-percentile 90%-percentile

B. constans var. ellipticum (N=2160) Width R-unit central

B. constans var. constans (N=840)

total B. constans var. ellipticum

Length (N=2160) coccolith Biscutum constans B. constans var. constans (N=840)

total B. constans var. ellipticum (N=2160) Width coccolith

W. barnesae var. barnesae (N=2391)

W. barnesae var. fossacincta (N=709) V-unit central Length

W. barnesae var. barnesae (N=2391)

W. barnesae var. fossacincta (N=709) Width V-unit central

W. barnesae var. barnesae (N=2391)

total W. barnesae var. Length

coccolith fossacincta (N=709)

W. barnesae var. barnesae Watznaueria barnesae Watznaueria (N=2391) W. barnesae var. fossacincta total Width (N=709) coccolith

0123456789 [µm] Fig. 5.6. Box plots of the measured parameters of the two pairs of morphotypes of Watznaueria barnesae and Biscutum constans.

According to the Varimax-rotated PCA the coccolith morphology can be characterized by four principal components (Table 5.2). The first component shows highest loadings for the directly measured parameters, i.e. length and width (total coccolith, central unit). The second is controlled by the central unit length/total length ratio, the third and fourth by the ellipticities of the central unit and the total coccolith, respectively. Similar patterns have been observed for both morphotypes, with a slight difference that central unit length and width is more dominant in the first PC for B. constans var. constans.

5.5.2 Size variability of Watznaueria barnesae The maximum coccolith length of W. barnesae varies between 2.88 and 8.83 µm (mean 5.75 µm, std. dev. 0.90; see Fig. 5.8 and Table 5.1). The two morphotypes exhibit a mean length of 5.70 µm (std. dev. 0.89, range 2.88-8.83 µm) for W. barnesae var. barnesae and 5.90 µm (std. dev. 0.91; range 2.91-8.35 µm) for W. barnesae var. fossacincta. The size ranges and the frequency distribution of the two morphotypes show a similar pattern (Fig. 5.8). For both the linear regression functions of the length/width scatter plot of the total coccolith and of the V-unit are nearly identical. As observed for B. constans a very high bivariate correlation exists, r varies between 0.88 and 0.93 (Fig. 5.8). The slope of the regression functions of the total coccolith length (0.85 and 0.86) is steeper compared to the V-unit (0.72 and 0.71, respectively). In contrast to Biscutum, the Watznaueria morphotypes exhibit a nearly normal distribution. Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 93

Table 5.1. Overview of simple statistical parameters for the studied species and morphotypes. Abbreviations: max., maximum; min., minimum; mean; N, number of measurements; std.dev., standard deviation.

Table 5.2. Varimax rotated principal component analyses (PCA) of the measured and calculated parameters. The studied species and morphotypes have been analyzed separately. For each group studied four components have been extracted (eigenvalue > 1). Loadings with values >0.5 have been typed in bold. Abbreviations: l, length central unit; w, width central unit; L, length total coccolith; W, width total coccolith; l/L, ratio length central unit/length total coccolith; rim, outer coccolith rim; area l, area central unit; area L, area total coccolith; l/w, ellipticity central unit; L/W, ellipticity total coccolith; area l/L, ratio area central unit/area total coccolith. 94 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

It is apparent that no differentiation between the two taxa W. barnesae var. barnesae and W. barnesae var. fossacincta can be observed based on the performed morphometric measurements (Figs. 5.6 and 5.8). The linear correlation function does not differ between these two species neither for the total length of the coccolith nor for the V-unit. The species W. barnesae reveals only minor size changes through time which are statistically significant at the 95% confidence limit (Fig. 5.7). Fluctuations occur on the scale of lithological changes, but they do 1.25 1.20 placolith Ellipticity placolith Ellipticity 1.15 1.20 1.15 1.6 1.9 1.8 1.5 1.7 Ellipticity Ellipticity 1.6 central V-unit central R-unit 1.4 1.5 0.44 0.42 0.40 total length ratio total 0.38 total length ratio total 0.46 0.48 0.50 Length central R-unit/ Length central V-unit/ 0.36 3.8 3.6 m] m] m [ m [ 3.4 3.2 Total coccolith length Total Total coccolith length Total 3.0 1.6 m] m] m 1.5 m 1.4 Length Length 1.3 central R-unit [ central V-unit [ central V-unit 1.2 S I Z E Biscutum constans 2.5 2.6 2.7 2.8 2.9 3 5.2 5.4 5.6 5.8 6.0 6.2 S I Z E barnesae Watznaueria 1.1 1 -5 -5 0.1 bulk bulk T+ T+ O O 0.2 18 18 d d vs. vs. bulk bulk -4 -4 0.3 O O PDB [] 18 PDB [] 18 d d 0.4 Temperature-index Temperature-index (calc. nannofossils) (calc. nannofossils) TI TI TEMPERATURE TEMPERATURE - - SURFACE WATER SURFACE SURFACE WATER SURFACE T T -3 -3 0.5 0.5 0.4 0.3 0.2 0. 0.8 N+ N+ 0.7 0.6 Nutrient-index Nutrient-index NUTRIENTS 0.5 NUTRIENTS - - (calc. nannofossils) (calc. nannofossils) N N SURFACE WATER SURFACE SURFACE WATER SURFACE 0.4

0.4 0.5 0.6 0.7 0.8

Lithology

[m] [m]

53.5 53.0 52.5 52.0 51.5 51.0 50.5 50.0 49.5 49.0 48.5 48.0 47.5 47.0 46.5 53.5 53.0 52.5 52.0 51.5 51.0 50.5 50.0 49.5 49.0 48.5 48.0 47.5 47.0 46.5

C a lc . N F

z o n e C a lc .

N F

z o n e (CC 9b) (pars) 9b) (CC turriseiffelii Eiffellithus (CC 9b) (pars) 9b) (CC turriseiffelii Eiffellithus

Upper Albian (pars) Albian Upper Upper Albian (pars) Albian Upper S u b s ta g e S u b s ta g e

Fig. 5.7. Size changes of the two studied nannofossil species during the formation of the Niveau Breistroffer. They are compared to nannofossil based nutrient/temperature indices and oxygen isotope data (for details see Bornemann et al., subm.). Error bars indicate the 95% confidence limit. For lithological explanations see Fig. 5.3. Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 95

100

50 Watznaueria barnesae var. fossacincta N=709 central N=709 V-unit 0 400 total coccolith 200 Watznaueria barnesae N=2291 var. barnesae Frequency (width) Frequency (length) N=2291 0 0 200 400 0 50 100 8 central W. barnesae var. barnesae (N=2291) Watznaueria Watznaueria V-unit W. barnesae var. fossacincta (N=709) barnesae var. barnesae var. 7 barnesae fossacincta total W. barnesae var. barnesae (N=2291) coccolith W. barnesae var. fossacincta (N=709) 6 N=2291 N=709

W. barnesae 5 var. fossacincta W. barnesae (total coccolith) var. barnesae y=0.046+0.859x (total coccolith) 4 r=0.93, N=709 y=0.088+0.849x r=0.93, N=2391

W. barnesae var. 2 fossacincta (central V-unit) y=-0.031+0.714x r=0.88, N=709 1 N=709

Width (central V-unit, total coccolith) [µm] total (central V-unit, Width N=2291 W. barnesae var. barnesae (central V-unit) y=-0.112+0.723x, r=0.94, N=2391 0 1 2 3 4 5 6 7 8 9 Length (central V-unit, total coccolith) [µm] Fig. 5.8. Scatter plots and frequency histograms of length and width of the central V-unit and the total coccolith of the two morphotypes of Watznaueria barnesae. In addition, the Pearson correlation coefficient (r), the number of measurements (N) and the linear regression function is given. Histogram class sizes are 0.2 µm. not follow the lithology, the nutrient or temperature index. However, only the ellipticity shows a consistent shift towards more elliptical coccoliths in the upper part of the studied succession. The Varimax-rotated PCA revealed four principal components (Table 5.2), which are dominated by the same parameters as observed for B. constans and its morphotypes.

5.6 Discussion 5.6.1 Nannofossil preservation Based on visual criteria for etching (E) and overgrowth (O; Roth and Thierstein, 1972; Roth, 1973) the nannofossils in the samples investigated are well to moderately preserved and show only slight indications of etching (E1) and/or overgrowth (O1). This alteration has not significantly effected measurements. A major influence of dissolution on the nannofossils has also been excluded by Bornemann et al. (subm.), due to the presence of a highly diverse nannoflora and high abundances of delicate forms (holococcoliths, Scapholithus fossilis, small Zeugrhabdotus spp., Biscutum spp.). This is supported by a lack of correlation between the relative abundance of the dissolution-resistant taxon W. barnesae and species richness, as well as between nannofossil absolute abundance, CaCO and TOC (see Section 4.5.1 of this thesis; Bornemann et 3 al., subm.). 96 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ...

5.6.2 Coccolith growth pattern Recent coccolithophores develop two types of calcitic base scale plates (holococcoliths and heterococcoliths) as a cell wall covering. Both plate-types can be produced by the same organism and are thought to represent phase changes in a presumably haplo-diplontic life cycle (e.g. Billard, 1994; Geisen et al., 2002). The calcification of heterococcoliths, which have been studied here, takes place in intracellular vesicles (Pienaar, 1994). The calcification process starts with the nucleation of a proto-coccolith ring (PCR) of small crystals around the rim of a precursor base scale plate. Subsequently the crystals of the PCR grow equally in various directions to form crystal units (Young et al., 1992, 1999). This usually causes a growth approaching a parallel ellipse (Young, 1989; Young et al., 1996) leading to less elliptical forms with increasing size. This pattern is supported by two observations in this study. (1) According to Young et al. (1992) the measured central units of both studied placolith species are approximating the original PCR. We used these units as a reference system to calculate the outer rim width, which we believe to represent the grown unit, independent from the given PCR geometry. Figure 5.9 shows for both studied taxa that forms consisting of a larger outer rim tend to develop less elliptical specimens. (2) The linear regression of the total coccolith length/width is significantly steeper (0.85 and 0.86 for the two morphotypes B. constans, 0.85 and 0.86 for those of W. barnesae) than that for the central unit (0.48 and 0.53 for the two morphotypes B. constans, 0.71 and 0.72 for those of W. barnesae), describing a growth strategy to a less elliptical outline in comparison to the central unit. The presented morphometric data of the two studied placolith species reveal similarities and dissimilari- ties, apart from the differences in the ultrastructure (see Section 5.4.2.1). The calculated PCAs suggest similar geometric relationships for both groups. In particular, the nearly identical slope of the linear regression functions of the total coccolith length/width scatter (Figs. 5.5 and 5.8) may indicate that the growth of the total coccolith dimensions is (1) independent from the given initial system of the central unit and (2) is similar for both groups and perhaps also for other placolith taxa. The strong linear correlations of coccolith length and width suggests an isometric growth of the outer rim. A wide scattering and irregularities of the data clusters (Figs. 5.5, 5.8 and 5.9) presumably indicate that a strict geometric growth of coccoliths in plane view is hindered by their three-dimensional morphological plasticity (i.e. curvature of the shields).

Biscutum constans Watznaueria barnesae 600 600 400 Frequency 400 Frequency 400 800 200 200 400 800 0 0 0 0 Frequency Frequency 1.6 1.6 (a) N=3000 (b) N=3000 1.5 1.5

1.4 1.4

1.3 1.3

1.2 1.2

Coccolith ellipticity 1.1 Coccolith ellipticity 1.1

1.0 1.0 0.5 1.0 1.5 1.01.5 2.0 Outer rim width [µm] Outer rim width [µm] Fig. 5.9. Scatter plots and frequency histograms of ellipticity versus outer rim width of the species Biscutum constans (a) and Watznaueria barnesae (b). Histogram class sizes are 0.1 µm for the outer rim width and 0.05 for the ellipticity. The outer rim is considered to represent the grown distance away from the initial central unit. Both taxa show a trend towards less elliptical forms with increasing outer rim width. Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 97

5.6.3 Taxonomic implications The observed size ranges for the studied taxa are in good agreement with literature data, thus no major revision for the species and morphotypes is necessary. The holotype of Biscutum testudinarium, which has been considered to be synonymous to both B. constans (Perch-Nielsen, 1968; Bukry, 1969) and B. ellipticum (Grün and Allemann, 1975), has a length of 3.7 µm according to Black in Black and Barnes (1959). Bukry (1969) documented a maximum length of 6.6 µm for B. testudinarium. In the current study size measurements of B. constans var. constans range from 1.58 to 5.05 µm with a mean of 2.9 µm. For B. ellipticum Górka (1957) documented a maximum dimension of 7 µm, Grün and Allemann (1975) observed length measurements between 2 and 10 µm (this study B. constans var. ellipticum: 2.08-5.85 µm; mean: 3.64 µm). For W. barnesae the following values were reported: 4.5-8.5 µm (Gartner, 1968), 2.5-8 µm (Grün and Allemann, 1975), a maximum length of 7 µm by Bukry (1969) and for the holotype 5.5 µm (Black in Black and Barnes, 1959; this study: 2.88-8.83 µm, mean: 5.7 µm). For W. fossacincta Black (1971) reported a size range between 6 and 8.5 µm (this study W. barnesae var. fossacincta: 2.91-8.35, mean: 5.9 µm). Separation of different taxa or morphotypes based on simple morphometric measurements is rather difficult. There is no general rule to separate morphotypes and -species from each other by their dimensions. Usually, as in this study, the separation is hampered by a continuous transition between different morphotypes. For the morphotypes of the two studied species the similar growth patterns and length relationships (Figs. 5.5-5.8; Tables 5.1 and 5.2) suggest that the sets of morphotypes cannot be differentiated from the performed size measurements alone. Based on the results it is more likely that both sets of morphotypes are closely related to each other. For the two morphotypes of B. constans the results reveal no significant differences between the dimensions of the central unit and the total coccolith length. This conclusion is also supported by the performed PCA (Table 5.2). Both morphotypes, the small-sized B. constans var. constans and the larger-sized B. constans var. ellipticum, seem to represent end members of a continuum in terms of the coccolith and central unit size, which show a significant overlap in their size distributions (Figs. 5.5 and 5.6). Due to the fact that species definitions of fossil taxa are exclusively based on coccolith morphology, no separation into different morphospecies is recommended in this case. The significance of the central opening as a qualitative taxonomic criterion of Watznaueria species has been discussed by Young and Bown (1991). According to these authors, forms with a large central opening, usually assigned to Watznaueria ovata, are interpreted as an early ontogenetic stage of W. fossacincta. They also presumed that forms consisting of an entirely closed central area (W. barnesae) represent an evolutionary development. This is in contradiction to the stratigraphic range of the two morphotypes, both have been documented from the base of the Bajocian to the top of the Maastrichtian (Bown and Young, 1997). Due to the similar morphometric pattern, the general variability of the central opening dimensions and the similar stratigraphic range, we consider the late Albian W. barnesae var. fossacincta as a variant of W. barnesae, possibly a distinctive ontogenetic stage. Therefore W. barnesae var. fossacincta should be not distinguished from W. barnesae var. barnesae at the species-level.

5.6.4 Palaeoecological implications Control of coccolith size, morphology and calcification by palaeoenvironmental factors is still under discussion. The most prominent parameters which are believed to control the calcification and/or the size of coccoliths are the temperature and the trophic conditions (e.g. McIntyre et al., 1970; Winter et al., 1994). Interpreta- tions of a palaeoenvironmental control of coccolith size are sometimes contradictory and vary from species 98 Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... to species. Young (1990), for instance, observed large forms of the genus Reticulofenestra during cooler periods, and larger morphotypes of Emiliania huxleyi have been reported by Colmenero-Hidalgo (2002) and others during glacial times. A correlation of the morphology of Gephyrocapsa and seawater temperature data has been observed by Bollmann (1997). Larger morphotypes with a high bridge angle are more adapted to higher temperatures, while smaller forms with low bridge angles are more common under moderate temperatures. Renaud et al. (2002) observed a large morphotype of the recent coccolithophore Calcidiscus leptoporus during spring blooms in the North Atlantic Ocean and Arabian Sea. These settings are characterized by nutrient-rich conditions and low surface water temperatures. The large morphotype of C. leptoporus also seems to be common at low latitudes in a range of intermediate to high temperatures (Knappertsbusch et al., 1997; Renaud and Klaas, 2001; Renaud et al., 2002). Culture experiments of Quinn et al. (2003) suggest that the morphotypes of C. leptoporus do not represent temperature dependent ecophenotypes as suggested by Knappertsbusch et al. (1997). Recently Sáez et al. (2003) demonstrated that the C. leptoporus morphotypes show continuous distribution in their size, but based on genetic divergences the authors were able to differentiated between the intermediate and large morphotype on the species level. The results of recent or Neo- gene coccolith taxa show that no general rule for temperature dependent change in calcification or size exists. Furthermore, it becomes apparent that morphological varieties, which correspond to environmental changes, reflect genotypic rather than ecophenotypic variations (Young and Westbroek, 1991; Knappertsbusch et al., 1997; Baumann and Sprengel, 2001; Sáez et al., 2003). For the formation of the Niveau Breistroffer black shales at the Col de Palluel section palaeoceanographic changes have been reconstructed (Bornemann et al., subm.). Important changes in nutrient availability and temperature are believed to be reflected by the nannofossil-based nutrient and temperature indices (NI, TI, Fig. 5.7). The results have been compared to planktic foraminifera percentages and oxygen isotope data (Fig. 5.7; Bornemann et al., subm.), suggesting that they reflect surface water conditions. According to this dataset and the presumed age model the coccolith sizes of the studied groups show more or less significant size changes which vary at the frequency of Milankovitch cycles. So far, however, only few variations seem to correspond to the recorded changes of the surface water conditions (Fig. 5.7). The data presented suggest that for B. constans the size of both the central unit and the total coccolith shows a similar long-term trend as presumed for the surface water temperature. Our observations suggest that forms consisting of a larger central unit were formed during intervals with presumed warmer surface waters according to the TI and oxygen isotopes in the lower part of the studied succession. Smaller coccoliths with a narrow central unit on the other hand are related to a cooler interval in the upper part of the succession. No obvious correlation between the coccolith size and surface water nutrient variations has been observed. If a palaeoenvironmental control on the measured parameters exists apart from the long-term temperature trend, this is not recorded in the nannofossil based indices. Palaeoenvironmental preferences have been proposed for forms consisting of a wide central unit, here referred to as B. constans var. ellipticum. Giraud et al. (2003) linked higher abundances of these forms to higher surface-water productivity for sediments from the same interval of the Vocontian Basin. Mattioli et al. (subm.) studied early Jurassic Biscutaceae from the western Tethys and the Mazagan Plateau (eastern Atlan- tic). The results of these authors support both assumptions made for B. ellipticum of Giraud et al. (2003) and for B. constans var. ellipticum in this study. Mattioli et al. (subm.) observed large forms in different settings under presumably warmer surface-water conditions or high nutrient levels. Generally, they presume that the morphometric variability is mainly controlled by local/regional palaeoenvironmental conditions or is related Chapter 5: Size analyses of the coccolith taxa Biscutum constans and Watznaueria barnesae ... 99 to evolutionary changes during the Pliensbachian-Toarcian interval. A contradicting interpretation is suggested by Herrle et al. (2003a). In this study B. aff. ellipticum is viewed as a cool-water taxon based on a statistical evaluation of a large dataset covering the Aptian-Albian boundary interval. Unfortunately, the morphotypes of B. constans have been combined in many studies, thus datasets revealing their ecological preferences are rather rare. In order to confirm or reject the hypothesis that temperature controlled the phenotypic variations of B. constans, the two morphotypes must be differentiated in further ecological studies. Compared to B. constans a less uniform trend has been recorded for W. barnesae. None of the measured characteristics shows a trend following the environmental changes as suggested by the nannofossil indices (Fig. 5.7). Only the calculated shape parameter may have shifted from more elliptical coccoliths in the lower part of the studied interval to rounder forms in the upper part. Size changes of the genus Watznaueria have been studied by Bornemann et al. (2003) across the Jurassic-Cretaceous boundary on the timescales of several million years. According to this study larger morphospecies were linked to presumed oligotrophic surface water conditions. This assumption has not been confirmed by our results at the species level.

5.7 Conclusions The morphometric study of the Mesozoic coccolith taxa B. constans and W. barnesae yields the following conclusions: (1) For each taxon two morphotypes were recognized. B. constans includes morphotypes with a narrow (B. constans var. constans) and a wide central unit (B. constans var. ellipticum). Between these two morphotypes, no statistically significant differentiation has been observed based on the size of the central unit. They seem to represent end members of a size continuum of both the coccolith and central unit size distribution. The two morphotypes of W. barnesae differ only by the absence (W. barnesae var. barnesae) and the presence of a narrow central opening (W. barnesae var. fossacincta). No significant size differences have been observed between the two W. barnesae morphotypes. Both show similar patterns for the measured characteristics. We recommend that the studied morphotypes of both B. constans and W. barnesae should not be assigned to different morphospecies. (2) The measured parameters show mostly insignificant, short-term fluctuations through time. Signifi- cant long-term trends have been observed for the length (central unit, total coccolith) of B. constans. These show a shift to smaller forms with a narrower central unit in the upper part of the succession. This change coincide with a cooling trend, as indicated by the nannofossil based temperature index and oxygen isotopes. The two morphotypes of B. constans are assumed to represent ecophenotypic varieties rather than two different morphospecies. No obvious relationship between the size of W. barnesae and the palaeoenvironmental conditions has been observed.

Acknowledgements This study was funded by the Deutsche Forschungsgemeinschaft (Mu 667/20-1). Comments on the manuscripts by T. Bralower, H. Legge and J. Steffahn are highly appreciated. J.O. Herrle and J. Lehmann kindly provided sample material. Thoughtful reviews by xxx and yyy are gratefully acknowledged. 100 Chapter 6: Summary and perspectives

6 Summary and perspectives This chapter summarizes the results of the studies presented in the Chapters 3 to 5 with respect to palaeoecology, calcareous nannofossil morphology and carbonate accumulation. Furthermore, an outlook to future empha- ses on nannofossil research is given.

6.1 Palaeoecology A study of DSDP sites from the Atlantic Ocean during the JurassicCretaceous boundary interval revealed long-term shifts on the scale of several million years in the nannofossil assemblage composition. A low- diverse coccolith assemblage dominates the early Tithonian, whilst in the mid- to late Tithonian mass occurrences of strongly calcified coccoliths and nannoliths occur. These include forms which may indicate more oligotrophic surface water conditions. Environmental factors which may have favoured this assemblage are low nutrient levels of the surface water, a cool-arid climate or a sea-level lowstand. A change to a highly diverse coccolith assemblage takes place in the earliest Cretaceous. The observed nannofossil assemblage comprises numerous taxa in relatively high abundances, which may have preferred higher nutrient levels than the nannoflora of the mid- to late Tithonian interval. The opening of a deep Atlantic-Pacific seaway in the earliest Berriasian may have caused changes in the palaeoceanography and thereby the shift in the nannofossil assemblage. Generally, the observed shifts are interpreted on the one hand to have been caused by palaeoceanographic and -environmental changes, or on the hand they presumably reflect evolutionary changes in the group of calcareous nannofossils. The similarity of trends in the composition of calcareous nannofossils (abundance, diversity) in palaeogeographically different DSDP sites from the Central Atlantic suggests that the observed variations reflect a primary signal on at least a regional scale. The quantitative analysis of different microfossil groups and a stable isotope record provides detailed insights into palaeoclimatic and -oceanographic conditions during the formation of the Late Albian Niveau Breistroffer black shales (Vocontian Basin, SE France). The application of nannofossil indices to record changes in surface water nutrient and temperature conditions correspond well to abundance changes of higher nutrient levels indicating planktic foraminifera (small hedbergellids) and variations of oxygen isotope data, respectively. Thus, these indices represent a reliable signal of the surface water conditions. The co- occurrence of higher abundances of two subsurface dwelling species (nannoconids, rotaliporids) has confirmed the hypothesis that nannoconids may have flourished within the lower photic zone and proliferated under stratified, stable conditions with low nutrient levels in the upper photic zone. The gained dataset from the Niveau Breistroffer allowed the reconstruction of the palaeoceanographic situation. Surface water productivity was reduced during black shale formation and increased during marlstone deposition. Relative temperature changes recorded by the nannofossil temperature index and oxygen isotope data indicate warmer surface waters during black shale deposition and cooler conditions for marlstones. It is suggested that an orbitally induced increase in monsoonal activity led to enhanced humidity, as indicated by the palynomorphs, during periods of black shale formation. The humidity increase, in turn, caused a decrease in low latitude deep- water formation and probably an increase in surface water stratification. The combination of both mechanisms caused oxygen consumption in the bottom water that increased the preservation potential of organic matter.

6.2 Calcareous nannofossil morphology The results of a morphometric study of the genus Watznaueria during the Tithonian and Berriasian interval, and that of common nannolith taxa from DSDP Site 105 show significant variations in size. Most studied Chapter 6: Summary and perspectives 101 groups show large forms in the mid- to late Tithonian interval. The increase in size of the studied taxa may have been caused by various factors. A sea-level lowstand, oligotrophic surface water conditions or lower pCO2 levels may have favoured the evolution and higher abundances of larger, strongly-calcified nannofossils. The observation in the size trend of Watznaueria and the co-occurrence of high abundances of nannolith taxa in the mid- to late Tithonian in all DSDP sites studied suggest that this increase in size and nannofossil calcification occurred at least in the entire Atlantic area. This event has been named Nannofossil Calcifica- tion Event (NCE) due to the co-occurrence of numerous strongly calcified taxa, which exhibit a size-increase during this particular interval. The subsequent decline in the size of Watznaueria and decrease in abundance of strongly calcified nannoliths have been linked to major changes in the oceanic current system caused by the opening of a deep Atlantic-Pacific seaway. Short-term variations on the scale of Milankovitch cycles of the coccolith size of the species Biscutum constans and Watznaueria barnesae have been studied during the Niveau Breistroffer event in SE France. For each taxon two morphotypes were recognized. B. constans includes morphotypes with a narrow and a wide central unit. Between these two morphotypes no statistically significant size differentiation has been observed depending on the size of the central unit. They seem to represent end members of a size continuum concerning both the coccolith and central unit size distribution. The two morphotypes of W. barnesae differ only by the absence and the presence of a narrow central opening from each other. No significant size differences have been observed between the two W. barnesae morphotypes. Both show very similar pattern for the measured characteristics. We recommend that the studied morphotypes of both B. constans and W. barnesae should not be assigned to different morphospecies. During the studied black shale succession the measured parameters show statistically insignificant, short-term fluctuations. Significant long-term trends have been observed for the length (central unit, total coccolith) of B. constans. These show a shift to smaller forms with a narrower central unit in the upper part of the succession. These changes coincide with a cooling trend, as indicated by a nannofossil based temperature index. Therefore the two morphotypes of B. constans are here interpreted to represent ecophenotypic varieties rather than two different morphospecies. For the size of W. barnesae no clear relationship with respect to the palaeoenvironmental conditions has been recognized.

6.3 Carbonate accumulation The quantification of the nannofossil carbonate at DSDP Site 105 revealed that on average only 27% of the total carbonate can be explained by calcareous nannofossils. The reminder of the carbonate is most likely attributed by high amounts of unidentifiable micrite and fragments of calcareous nannofossils. Other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations. Due to the observed high amounts of nannofossil fragments in addition to the calculated nannofossil carbonate and the lack of other important carbonate producers, it can be stated that nannofossils are the most important pelagic carbonate producer during the studied interval. The record of nannofossil carbonate accumulation at DSDP Site 105 (JurassicCretaceous boundary) is punctuated in the mid-Tithonian NCE. During the Berriasian (nannofossil) carbonate accumulation rates increased, possibly reflecting a general shift to higher carbonate productivity triggered by high abundances of small-scaled coccoliths rather than by relatively lower abundances of strongly calcified forms. The observed carbonate accumulation rates are within the same range as those of todays oceans. 102 Chapter 6: Summary and perspectives

6.4 Perspectives Ecological interpretations of Mesozoic nannofossil taxa are hampered by the absence of direct recent relatives. Therefore interpretations are sometimes rather speculative and are often only based on qualitative and semi-quantitative data. Another critical factor in nannofossil studies is the influence of diagenesis (i.e. dissolution) on nannofossil preservation (Honjo, 1976; Steinmetz, 1994). This may alter the palaeoecological interpretation of calcareous nannofossils. The usual procedure, applying qualitative criteria such as different degrees of etching and overgrowth (e.g. Roth and Thierstein, 1972) is not sufficient, often rather subjective or requires enhanced experience. Additional valuable information is given by absolute abundances obtained from modern quantitative techniques (e.g. settling preparation, filtration), species richness and abundances of dissolution-susceptible or -resistant species (see Sections 3.7.1.1 and 4.5.1.2). Quantitative techniques, e.g. settling or filtration, have to be applied carefully and it is recommended to test the results by comparing the nannofossil preservation with that of unprocessed smear slides. As inferred from the investigation of the Niveau Breistroffer (Chapter 4), the application of a multi- proxy approach is a successful way to reconstruct the palaeoceanographic conditions. Furthermore, multi- dimensional datasets allow the comparison of signals received from geochemistry and different microfossil groups with each other. This gives way to more accurate information about the ecological affinities of different species and groups. The results of this study turned out that ratios of different nannofossil species should be used rather than the abundances of single taxa during high-resolution studies, if the influence of evolutionary changes within the assemblages is limited. Since no hard data about past oceanographic conditions (e.g. seawater temperature, nutrients, pH/alkalinity etc.) are available, the parallel use of other micropalaeontological and geochemical proxies is essential to confirm the proposed ecological preferences of calcareous nannofossil taxa. This provides also an additional control on samples, which may have been diagenetically altered. The driving mechanisms controlling coccolith size and calcification of recent and fossil taxa are only barely understood. Therefore it is advisable to spent more efforts on culture and plankton studies, where the environmental parameters can be directly measured or controlled, rather than on fossil material. These should include rates of genotypic variations versus environmental control leading to the evolution of simple ecophenotypes. Furthermore, culture experiments may help to evaluate whether calcareous nannofossils and recent coccolithophores act as a source or sink of CO2 on different timescales. For fossil material there is a need for more quantitative data about the presumable ecological preferences of different morphotypes, thus it would be useful to differentiate between them in upcoming palaeoecological studies (see Chapter 5). A detailed, global database of carbonate contents combined with a refined Mesozoic stratigraphy, would help to improve our knowledge of the influence of the deep-sea carbonate sink on the global carbon cycle. Moreover, this would help to understand changes within the Mesozoic carbon budget. More accurate data on the composition of carbonates are necessary to evaluate the importance of the different Mesozoic carbonate producers in both the pelagic and neritic system, and their contribution to the total CaCO3 budget. References 103

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