Size evolution of Cretaceous calcareous nannofossils
-
Implications for oceanic anoxic events
Dissertation
zur Erlangung des Grades eines Doktors der Naturwissenschaften
an der Fakultät für Geowissenschaften der Ruhr -Universität Bochum
vorgelegt von
Nathalie Lübke
geboren am 7. Januar 1988
in Velbert
Bochum im Januar 2017 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.
Erstgutachter: Prof. Jörg Mutterlose
Zweitgutachter: Prof. Elisabetta Erba
Drittgutachter: Prof. Wolfgang Friederich
Tag der Disputation: 23.5.2017 Eidesstattliche Erklärung
Ich versichere an Eides statt, dass ich die eingereichte Dissertation selbstständig und ohne unzulässige fremde Hilfe verfasst, andere als die in ihr angegebene Literatur nicht benutzt und dass ich alle ganz oder annähernd übernommenen Textstellen sowie verwendete Grafiken, Tabellen und Auswertungsprogramme kenntlich gemacht habe. Außerdem versichere ich, dass die vorgelegte elektronische mit der schriftlichen Version der Dissertation übereinstimmt und die Abhandlung in dies er oder ähnlicher Form noch nicht anderweitig als Promotionsleistung vorgelegt und bewertet wurde.
Nathalie Lübke Table of contents I
Table of contents …bstract V Kurzfassung VIII …cknowledgements .. XI Chapter 1 – Introduction 1 1.1 The Cretaceous.period 1 111.Paleogeography 1 1.1.2 Paleoclimate .. 3 12.The.O…Es.of.the.middle.Cretaceous 5 1.3 Coccolithophores – marine primary producers .. 14 131.Biology.and.taxonomy.of.coccolithophores 16 132.The.fossil.record.of.calcareous.nannofossils 20 133.Issues.in.biometric.studies.on.calcareous.nannofossils 23 1.4 Objectives of the thesis 25 15.Thesis.overview . 26 Chapter 2 – Size variations of coccoliths in Cretaceous oceans – a result of preservation, genetics and ecology?...... 29 …bstract 30 21.Introduction 30 22.Material.and.stratigraphy 33 221.North.Sea.(40°N.paleolatitude) 32 2.2 2.Lower.Saxony.Basin.(39°N.paleolatitude) .. 34 223.Western.Tethys.(20.°N.paleolatitude) .. 36 2.2.4 Mid -Pacific (13 -20°S.paleolatitude) 36 23.Methods 36 2.4 Results .. 38 241.Width.and.length 38 242.Ellipticity .. 41 2.4.3 Central area of W. barnesiae .. 41 244.SEM.images .45 Table of contents II
2.5 Discussion .. 45 2.5.1 Diagenesis 45 2.5.2 Palecology and genetics . 47 2.5.2.1 Sea -surface temperature . 48 2.5.2.2 Light availability and.trophic.load .. 51 253.Ellipticity 51 26.Conclusions .. 52 2.7 …cknowledgements .53 28.…ppendix .54 Chapter 3 – The impact of OAE 1a on marine biota deciphered by size variations of coccoliths . 59 …bstract 59 31.Introduction . 60 32.Material.and.methods .. 61 321.Geological.settings.and.studied.sections 61 3211.North.Sea.(40°N.paleolatitude) 61 3212.Lower.Saxony.Basin.(36°N.paleolatitude) 62 322.Stratigraphic.correlation 63 332.Samples.and.biometry 65 33.Results .. 67 3.3.1 Normal distribution 67 332.Biometry 67 3.3.2.1 North Jens -1 .. 67 3.3.2.2 Adda -2 .. 69 3323.…lstätte.1 . 69 34.Discussion .. 70 341.Diagenesis 70 342.Coccolith.size.and.the.stable.carbon.isotope.record . 71 343.The.global.synchronicity.of.coccolith.size.shifts 73 3.4.4 W. barnesiae .. 79 Table of contents III
3.5 Conclusions .. 80 36.…cknowledgements .81 Chapter 4 – Size changes in calcareous nannofossils documenting the pace of microevolutionary.processes 82 …bstract 82 41.Introduction . 83 42.Material.and.methods .. 85 43.Results .. 88 431.Biometry 88 44.Discussion .. 91 441.Preservation 91 4.4.2 Biostrat igraphy – Numerical ages. . .91 4.4.3 Biostratigraphy - Prediscosphaera .. 92 444.The.two.morphotypes . 92 445.Significance.for.evolutionary.trends.and.timing 93 4.5 Conclusions .. 96 46.…cknowledgements .96 47.…ppendix .97 Chapter 5 – A stable long -term calcareous nannofossil record (lower Aptian – middle/upper Cenomanian).from.the.southern.Indian.Ocean .. 98 …bstract 98 51.Introduction 98 52.Material.and.methods .. 100 53.Results .. 102 531.Biostratigraphy . 103 532.Nannofossil.content .. 104 533.Nannofossil.abundance .. 107 5331.Species .. 107 5332.Families 107 534.Nannofossil.biometry .. 111 Table of contents IV
5.4 Discussion 111 5.4.1 Position of OAE 1d 111 5.4.2 Nannofossil abund ance 112 543.Biometry 114 55.Conclusions 115 56.…cknowledgements 116 Chapter 6 – General.conclusions 117 Chapter 7 – Critical.remarks 121 Taxonomic.index 127 References 130 Curriculum Vitae 152 Abstract V
Abstract
Primary producers have been forming the base of the marine food chain ever since higher life existed on Earth. The composition of this generic group has been shown to vary considerably with time. However, since the Mesozoic, more precisely since the Early Cretaceous roughly 140 Ma ago, the primary producer composition has not changed until today. The three main groups are diatoms, dinoflagellates and coccolithophores. As coccolithophores are Earth’s most important carbonate producers, they are one of the mo st interesting groups in the field of biology, but also in paleontology.
The fundamental obstacle in modern observatory and culture experiments on marine primary producers, as well as for studying and understanding all living organisms, is time. Time, tha t is needed by evolution to change the genome of a species and thus derive new morphological or metabolic characters. Time, that organisms need to adapt to changes in their environment. Time, that it takes for a species to invade and colonize new habitats. Overcoming this obstacle, fossil remains preserved in the geologic record can serve as an analogue testing ground for modern planktonic communities, their evolution and adaptation to environmental changes.
In this PhD thesis, the fossil remains of coccoli thophores, the µm -sized detached calcitic plates (coccoliths), that form the cell cover of each organism, were studied biometrically. The samples are derived from the highly diverse assemblages of the middle Cretaceous (~126.3 Ma to 89.8 Ma). In some cases , carbonate -rich marine sediments from the Cretaceous provide a continuous and undisturbed record of up to tens of millions of years and are therefore ideal to study long -term evolutionary trends or adaptations to persisting environmental change. As the Cr etaceous period is also characterized by various short -term events, lasting few tens of thousands of years up to a million years, momentary and probably reversible changes can be monitored. The main findings of this thesis can be summarized as follows:
1) Three common coccolith species ( Watznaueria barnesiae, Biscutum constans and Zeugrhabdotus erectus) from a synchronous Early Aptian time interval (~126 Ma) have been studied for their size. The material is derived from four locations (proto -North Sea, Lowe r Saxony Basin in northwest Germany, western Tethys in modern -day Italy, and the Mid -Pacific). While the most common species W. barnesiae shows no size variability and the size distribution of Z. erectus is most probably biased by diagenesis, Biscutum cons tans is Abstract VI characterized by a very distinct size pattern. Compared to the two pelagic settings (western Tethys and Mid -Pacific), this species lacks the large -sized coccoliths in the proximal highly turbid proto -North Sea and Lower Saxony Basin. This lack of l arge coccoliths is related to the nonexistence of a clear water niche in the muddy waters of the latter two sites, which the large -coccolith producing organisms preferred. The slight difference in size going along with distinct environmental preferences su ggests, that two cryptic species were present in the Aptian ocean. This species concept honors that slight morphological differences within one traditional species can be accounted to genetic variability rather than to inter -species phenotypic variability, and can result in varying ecological preferences.
2) The lower Aptian has been punctuated by an episode of global carbon cycle perturbation, Oceanic Anoxic Event 1a (OAE 1a). To study the impact of this event on Boreal calcareous nannofossils, three secti ons from the proto -North Sea and the Lower Saxony Basin have been chosen. Three species ( W. barnesiae, B. constans, and Z. erectus ) have been studied biometrically. The results show an abrupt coccolith size decrease of two species ( B. constans, and Z. erec tus ) at the base of OAE 1a followed by a slow size recovery to pre -OAE 1a values. This pattern is in line with previous data from the western Tethys and the Mid -Pacific. The results of the present biometric study reveal that the Tethys, the Pacific and the Boreal Realm have experienced similar paleoenvironmental changes during the OAE 1a.
3) The early evolution of the calcareous nannofossil species Prediscosphaera columnata has been analysed for the middle Cretaceous (~126.3 Ma to 89.8 Ma) by evaluating bio metric data. By assigning absolute ages to several stages in the morphological evolution of this species, an evolutionary lineage has been proposed for an interval of 20 Ma. The data reveal the presence of two morphologically distinct groups present in the sediment: A small form with a subcircular outline and a large one with a circular outline. Both morphotypes can be definitely separated by their size. The large morphotype (> 5.2 µm) evolved from the earlier smaller ancestor within less than 1.1 Ma and ev entually became the dominant P. columnata variety. The recorded time span of roughly 1 Ma gives an estimate of the time needed for new species in the calcareous plankton to evolve.
4) The long -term record of the ODP Site 763B from the southern Indian Ocean , covering almost the entire middle Cretaceous, allows to study the impact of long -term environmental trends, resulting from the formation of the Indian Ocean, as well as the impact of the short - Abstract VII term uppermost Albian Oceanic Anoxic Events 1d (OAE 1d) on th e calcareous plankton community. The raised data set includes a solid biostratigraphic model, assemblage composition data, and biometric data of three common species. The species composition and the species -specific biometric data reveal long -term stabilit y despite major paleogeographic changes in the region and no above background noise response to OAE 1d. Kurzfassung VIII
Kurzfassung
Primärproduzenten bilden die Grundlage der marinen Nahrungskette seitdem es höheres Leben auf der Erde gibt. Die Zusammensetzung innerhalb dieser Gruppe hat sich im Laufe der Zeit erheblich verändert. Seit dem Mesozoikum, genauer seit der Frühen Kreide vor etwa 140 Ma, ist diese Zusammensetzung jedoch gleichgeblieben. Die drei wichtigsten Vertreter dieser Gruppe sind Diatomeen, Dinoflagellaten und Coccolithphoriden. Da Coccolithophoriden die wichtigsten Karbonatbildner auf der Erde sind, wird diese Gruppe i m Feld der Biologie und auch der Paläontologie eingehend untersucht.
Das größte Problem in der Erforschung von marinen Primärproduzenten, seien es Zuchtkulturen oder in -situ -Beobachtungen, wie auch für alle anderen lebenden Organismen, ist der Faktor Zeit. Zeit für die Evolution, damit Veränderungen im Erbgut und folglich auch Veränderungen im Aussehen und Stoffwechsel entstehen können. Zeit, die Organismen brauchen, um sich an veränderte Umweltbedingungen anzupassen. Zeit, die nötig ist, bis eine Art ein n eues Territorium erreichen und erfolgreich bevölkern kann. Um dieses Problem zu lösen, bietet es sich an, Fossilien und die Informationen, die über Jahrmillionen in ihnen erhalten geblieben sind, als Analog moderner Organismen zu betrachten. Dabei können d ie Evolution und die Anpassung planktonischer Gemeinschaften an veränderte Umweltbedingungen untersucht werden.
In dieser Doktorarbeit wurden die fossilen Überreste von Coccolithophoriden, die einzelnen µm -großen Kalzitplättchen (Coccolithen), die die Zell e jedes dieser Organismen bedecken, biometrisch untersucht. Das Probenmaterial stammt aus den hochdiversen fossilen Vergesellschaftungen der mittleren Kreidezeit (~126,3 Ma bis 89,8 Ma). In einigen Fällen können kontinuierliche marine karbonatreiche Ablage rungen aus dieser Zeit Zeitspannen von mehreren Zehnermillionen Jahren umfassen. Aus diesem Grund sind sie ideal, um Langzeittrends der Evolution oder Anpassungen an sich kontinuierlich verändernde Umweltbedingungen zu beobachten. Da sich die Kreidezeit au ch durch mehrere kurzfristige Änderungen der Umweltbedingungen, die oft nur einige zehntausend bis eine Million Jahre andauern, auszeichnet, können auch schlagartige und reversible Veränderungen untersucht werden. Im Folgenden werden die wichtigsten Ergebn isse dieser Doktorarbeit aufgeführt:
1) Drei der häufigsten Coccolithenarten (Watznaueria barnesiae, Biscutum constans und Zeugrhabdotus erectus ) wurden in einem synchronen Intervall des frühen Aptiums (~126 Ma) Kurzfassung IX biometrisch untersucht. Das Material stammt aus vier verschiedenen Lokalitäten (proto - Nordsee, Niedersächsisches Becken in Nordwestdeutschland, West -Tethys im heutigen Italien, und Mittlerer Pazifik). Während die häufigste Art W. barnesiae keinerlei Größenunterschiede zeigt und die Größenverteilunge n von Z. erectus wohl diagenetisch überprägt wurden, weist B. constans eine charakteristische Größenverteilung auf. In den beiden pelagischen Lokalitäten (West -Tethys und Mittlerer Pazifik) sind sowohl große als auch kleine Coccolithen dieser Art vorhanden . Im Gegensatz dazu ist nur die kleine Varietät in der proximalen und küstennahen proto -Nordsee und dem Niedersächsischen Becken zu finden. Das Fehlen der großen Coccolithen deutet auf das Fehlen einer Klarwasser -Nische in den trüben Gewässern der beiden l etztgenannten Lokalitäten hin. Diese Nische wurde von Coccolithophoriden, die große Coccolithen bildeten, bevorzugt. Diese nur leichten Größenunterschiede, die sich aber trotzdem in unterschiedlichen ökologischen Präferenzen widerspiegeln, deuten auf das V orhandensein zweier kryptischer Arten im aptiumzeitlichen Ozean hin. Dieses Artkonzept basiert auf der Beobachtung, dass schon geringe morphologische Unterschiede innerhalb einer traditionellen Art, die ursprünglich auf innerartliche Varianz zurückgeführt wurden, auf genetischer Variabilität beruhen, die zu verschiedenen ökologischen Präferenzen führen kann.
2) Während des frühen Aptiums kam es zur Ablagerung globaler Schwarzschiefer - Horizonte, die durch die Umwälzungen des Kohlenstoffkreislaufes in Folge des Ozeanischen Anoxischen Events 1a (OAE 1a) bedingt wurden. Um den Einfluss des OAE 1a auf die borealen kalkigen Nannofossilien zu untersuchen wurden Proben aus zwei Bohrkernen aus der proto - Nordsee und einem Aufschluss aus dem Niedersächsischen Becken a usgewählt. Die drei Arten W. barnesiae, B. constans, und Z. erectus wurden biometrisch untersucht. Die Ergebnisse zeigen eine abrupte Verkleinerung der Coccolithen zweier Arten ( B. constans und Z. erectus) zu Beginn des OAE 1a und eine anschließende langwi erige aber vollständige Rückentwicklung der Werte. Dieses Muster ist auch in biometrischen Studien aus der West -Tethys und dem Mittleren Pazifik zu finden und deutet darauf hin, dass alle untersuchten Ozeanbecken die gleichen Umweltumwälzungen während OAE 1a erfuhren.
3) Die frühe Entwicklung der Art Prediscosphaera columnata während der mittleren Kreidezeit (~126,3 Ma bis 89,8 Ma) wurde anhand biometrischer Daten analysiert. Dabei wurden verschiedenen Stufen in der Größenentwicklung dieser Art absolute Alt er zugeordnet. Die daraus rekonstruierte Abstammungsgeschichte umfasst etwa 20 Millionen Jahre. Die Kurzfassung X
Daten lassen erkennen, dass zwei unterscheidbare morphologische Arten in den Sedimenten der mittleren Kreide vorhanden sind: eine kleine, seitlich abgeflach te Form und eine größere, runde Form. Die Grenze dieser beiden Morphogruppen liegt bei 5,2 µm. Die größere Art stammt demzufolge von der kleineren ursprünglichen Art ab und entwickelte sich innerhalb von weniger als 1,1 Millionen Jahren, um schlussendlich die dominante Varietät von P. columnata zu werden. Diese Zeitspanne von etwa einer Million Jahren erlaubt es abzuschätzen, wie lange die Entwicklung einer neuen Art dauern kann.
4) Die kontinuierliche Abfolge von ODP Site 763B im südlichen Indischen Ozean, die annähernd die gesamte mittlere Kreidezeit abdeckt, dient als Basis zur Untersuchung langzeitlicher ökologischer Trends innerhalb der planktonischen Organismen. Zusätzlichen konnten die Auswirkungen des Ozeanischen Anoxischen Events 1d (OAE 1d) des spä ten Albiums auf die kalkigen Nannofossilien untersucht werden. Die erhobenen Daten ermöglichen die Aufstellung eines soliden biostratigraphischen Models, die Auswertung der Zusammensetzung der Nannofossil -Vergesellschaftung, sowie die Analyse biometrischer Daten von drei wichtigen Nannofossil -Arten. Sowohl die Artzusammensetzung als auch die biometrischen Daten deuten auf Stabilität innerhalb des Nannoplanktons als auch auf keine durch das OAE 1d -verursachten Veränderungen hin. Ackno wledgements XI
Acknowledgements
I would like to express my deepest gratitude to my supervisor Professor Jörg Mutterlose, his perpetual scientific stimulation and his trust to let me work independently. My thanks are extended to Prof Elisabetta Erba from the University of Milano, who agreed to be the second supervisor of my PhD thesis though we have never met in person before and who took the very long way from Milano to Bochum for my defense talk . I would like to thank Cinzia Bottini for her in troduction into coccolith biometry already during my Master’s thesis.
Special thanks go to my office mates Franziska Häuser, Tobias Püttmann and Lena Wulff, whose presence made me always look forward to come to work every day. Sharing the same experiences and regular professional scientific exchange on the one hand, as well as cheerful laughter and meaningful discussions on the other hand, have made them become as close as family to me.
I would also like to thank my colleagues Ibtisam Beik, André Borneman n, Victor Manual Giraldo Gomez, René Hof fmann, Christian Linnert, Carla Möller, Christoph Schneider and Kevin Stevens for stimulating discussions and their company on various conferences. I would also like to thank Isaline Demangel, Tim Fiedler, Sarah Hass tedt, Ulrich Kaplan, Kirsten Kleefuß, and Mathias Müller
Last but not least, I would like to thank my Mum, my sister Isabelle and my step -father Uwe for their continuous support. Chapter 1 – In troduction 1
1 Introduction
1.1 The Cretaceous period
The Cretaceous period (~145 to 66 Ma; Ogg and Hin nov, 2012) has been named for its unique rock type, the chalk. Though it is lithologically similar to modern deep -sea calcareous oozes, Cretaceous chalks had a much wider geographic distribution. Due to a very high sea - level in the middle Cretaceous (Aptian to Turonian; 126.3 to 89.9 Ma), vast continental areas have been flooded and thus covered by epicon tinental seas, where the deposition of chalky sediments took place. This unprecedented paleoceanographic situation had its origin in the break -up of the supercontinent Pangaea. As the last period of the Mesozoic, the Cretaceous period already represents a world very similar to today. The first flowering plants, the most common plant type in our recent forests, fields and grasslands, are known from the Early Cretaceous, roughly 120 million years ago (Hickey and Doyle, 1977). The continental constellation sta rted to resemble modern geographical outlines (Skelton, 2006).
1.1.1 Paleogeography
This supercontinent Pangaea constituted a continuous landmass from the Carboniferous to the Jurassic (~300 to 150 Ma) combining Laurasia, the northern landmass, and Gondw ana, the southern landmass. No continental masses at the north pole, huge landmasses in the northern and southern mid -latitudes, and the wedge -shaped Tethys Ocean forming a westward -directed indentation deep into the equatorial continental area were the ma jor geographic features of Pangaea. The break -up of this supercontinent was initiated in the late Triassic with the continental rifting between North America and Northwest Africa; the actual separation of both continents and the formation of the southern N orth Atlantic started in the Jurassic. In the early stage of rifting (lower Lias), thick sequences of evaporitic and clastic sediments were deposited in the shallow limnic -marine Atlantic Ocean. Already during the upper Lias, conditions were fully marine, while the first oceanic crust was formed around the Lias/Dogger transition. Pelagic deposition started in the upper Dogger, the Malm was characterized by extended carbonate platforms at the shallow shelf margins of the continents. The southern North Atlant ic had already widened to more than 1500 km in the latest Jurassic and 4000 km in the late Cretaceous (Fig. 1 .1 ). The opening of the South Atlantic started later, in the Cretaceous, however, a connection between both basins was not established before Chapter 1 – In troduction 2 the e nd of the early Cretaceous. By the end of the Cenomanian stage (~95 Ma), Africa and South America were fully separated. On the other side of Pangaea, the former southern continent Gondwana that had already formed
18 Figure 1.1: Summary of Mesozoic and Ceno zoic Era with paleogeography, sea level, CO 2-models,δ O-record.
Maps from Ron Blakey. Sea -level estimates are based on Haq et al. (1987). Three different models of the CO 2- Chapter 1 – In troduction 3 record are shown. These are based on Berner and Kothavala (2001): GEOCARB III; Ber gman et al. (2004): COPSE; Rothman (2001): Rothman. Stable oxygen isotope record is as reported by Veizer et al. (1999). during the Neoproterozoic began to disintegrate in the Jurassic as well. The young Indian Ocean, intruding from the eastern Tethys, beg an to separate western Gondwana, including South America and Africa, and eastern Gondwana, including India, Australia, Antarctica and Madagaskar, in the middle Jurassic. Still during the Jurassic, marine ingressions started to separatetoday’slandmasses ustralia, India/Madagaskar and Antarctica from each other. By the beginning of the Late Cretaceous, Australia, India/Madagaskar and Antarctica were completely separated. In Europe, continental collision of the Eurasian plate and the African plate started t o shrink the western Tethys Ocean (Faupl, 2003; Skelton, 2006).
During this interval of rapid continental break -up, the total volume of mid -ocean ridges increased towards the Cretaceous (Larson, 1991a). The increasing amount of young and light oceanic cru st pushed large portions of the seawater from the basins onto the newly developed shelfs. This resulted in the early Late Cretaceous sea level highstand (Fig. 1 .1 ) and the expansion of epicontinental seas for example in northwestern Russia (Volga Basin), N orth America (Western Interior Seaway), North Africa including the adjacent Arabian Peninsula and great parts of central Europe (Skelton, 2006).
1.1.2 Paleoclimate
The reconstruction of past climatic conditions is based on proxy data from sedimentology, paleontology and geochemistry. All three disciplines can provide information on temperature and the spatial distribution of climate zones.
The geographical distribution of sedimentary rock types can be indicative of certain climatic conditions during the t ime of their deposition. For example, tillites (unsorted glacial deposits) and dropstones or erratic clasts can demonstrate the presence of continental glaciers or sea ice. The presence of glendonites, pseudomorphs after the mineral ikaite, which form in n ear freezing waters, indicate relatively cool waters. On the other hand, coals or evaporates reflect a humid or arid climate, respectively. The presence or absence of climate - sensitive organisms, or more precisely their fossil remains, in the sedimentary s uccessions may as well give insight into the climatic and environmental conditions at that time. Chapter 1 – In troduction 4
More specific, the occurrence of glendonites accumulating in Valanginian (139.4 to 133.9 Ma) and Aptian/Albian (126.3 to 100.5 Ma) sediments, in particular fr om Australia, arctic Canada (Price, 1999) and Spitsbergen (Kemper and Schmitz, 1981) suggest cooler periods with the presence of polar ice. These interpretations are supported by findings of erratic clasts in the same region. Further evidence for these coo l interludes in the otherwise greenhouse climate of the Cretaceous, is given by the analysis of calcareous nannofossil assemblages. During the Aptian/Albian transition, the high latitude species Repagulum parvidentatum increased in abundance in high latitu des (Falkland Plateau, North Sea) and extended its geographical range towards the lower latitudes (off -shore Morocco), while the low latitude Nannoconus group showed a decline. These biogeographical patterns have been attributed to a cooling episode around the Aptian/Albian transition, the Aptian/Albian cold snap (Mutterlose et al., 2009).
Geochemical evidence for the past temperature record can be derived from stable oxygen isotopes from bulk rock samples or pristine fossil carbonate tests, for example fr om those of foraminifera or belemnites, or from biomarker analyses, for example with TEX 86 . Concerning stable oxygen measurements, the ratio of the two common oxygen isotopes 18 O and 16 O in the tested sample material is compared to the same ratio in the us ed standard material, the deviationisreportedin‰.Lowerrelativevaluesreflectanincreaseofthelightisotope,and thus warmer temperatures, compared to the standard and vice versa. This approach of temperature estimation (Fig. 1. 1) has been used in a long -term Early Cretaceous composite record to show climate variability on a global level with cooler episodes in the Valanginian and Aptian (e.g., Veizer et al., 1999; Weissert and Erba, 2004; Bodin et al., 2015). The Cretaceous Thermal Maximum has bee n identified in the Turonian, based on the oxygen isotope signal of glassy foraminifera from a tropic section in the Atlantic (Wilson et al., 2002). The transient 1.7 ‰ negative shift, recorded in the Mid -Pacific, associated with the early Aptian OAE 1a re presents a short -term and reversible 8 °C temperature rise (Ando et al., 2008).
However, the oxygen isotope signal of a geological material may be subjected to diagenetic alteration and thus produce falsified temperature estimates. In addition, for the ca lculation it is necessary to estimate the oxygen isotope signature of the past seawater, which there is no direct evidence of. Therefore, absolute temperature estimates need to be used with caution
(e.g., Killingley, 1983). The TEX 86 biomarker proxy method , that uses archaeal membrane lipids, is less prone to diagenetic alteration (e.g., Huguet et al., 2009) and does not require Chapter 1 – In troduction 5 additional estimates. This method considers , that the growth temperature of the cell membranes of marine unicellular Crenarchaeota governs its chemical structure (Wuchter et al., 2004). This relationship is thermodynamically controlled and thus constant throughout the geological record. The middle Cretaceous temperature maximum, for examples, is also confirmed by the TEX 86 biomarker proxy (Schouten et al., 2003). Mutterlose et al. (2010) used
TEX 86 to estimate the seawater temperature of the Lower Saxony Basin during the Barremian and early Aptian.
Apart from these potential cooler phases (Valanginian, Aptian/Albian), the climate in the Cretaceous appears to have been significantly warmer; also in comparison to recent climate. This state of climatic circumstances is referred to as greenhouse conditions and is characterized by low meridional temperature gradients and ice -free poles. Fo ssil leaf and wood, and associated coal deposition in northern and southern high latitudes in Canada, Greenland, Russia and Antarctica as well as the presence of dinosaur remains indicate a warm and equable climate during the Cretaceous (Skelton, 2006).
1.2 The OAEs of the middle Cretaceous
The Cretaceous system is internationally subdivided into the Lower (145 -100.5 Ma; Berriasian to Albian) and Upper Cretaceous (100.5 -66 Ma; Cenomanian to Maastrichtian). However,theterm“middle”Cretaceous,coveringt he Aptian to Turonian stages (126.3 to 89.8 Ma; Fig. 1. 2), has been established as well and is widely used. This convention is mostly due to the fact, that this time interval is characterized by special climatic conditions, including super -greenhouse or ho thouse conditions as a result of enhanced submarine volcanic activity (Ogg and Hinnov, 2012).
One of the most peculiar features of the middle Cretaceous and a consequence of the super -greenhouse climate are the Oceanic Anoxic Events (OAEs). Associated with that are organic carbon -rich sediments, often deposited in pelagic successions in a variety o f bathymetric positions, including oceanic basins and epicontinental seas. Independent of local basin geometry, these globally occurring black shales were deposited simultaneously and seem to have been caused by global changes in the paleoceanographic stat e of the ocean. The concept of OAEs has first been introduced by Schlanger and Jenkyns (1976), who recognized widespread organic -rich horizons all over the globe in middle Cretaceous sediments. Further Chapter 1 – In troduction 6 studies initiated a more detailed subdivision into T -OAE (Toarcian -OAE), OAE 1a (early Aptian), 1b (earliest Albian), 1c (late Albian), 1d (latest Albian), and 2 (Cenomanian/Turonian boundary). Out of these events, the T -OAE, OAE 1a, b and 2 are regarded as globally significant, while OAE c and d are less dis tinct regarding their spatial distribution (e.g., Jenkyns, 2010). The underlying reasons for these deposits are still under debate, with the two hypotheses of enhanced preservation on the one hand and increased primary productivity on the other hand being widely excepted (Demaison and Moore, 1980; Pederson and Calvert, 1990). Durations of OAEs have been computed based on cyclostratigraphy and can sum up to ~1 Ma in some cases. For example, the entire early Aptian OAE 1a at three different sites has probably had a duration of 1.0 to 1.3 Ma (Li et al., 2008), while the T -OAE carbon isotope excursion lasted 900 kyrs (Suan et al., 2008). Though primarily known from organic -carbon rich deposits in marine sections, OAEs have also been recognized in sections withou t lithological equivalents, for example black shales. In these cases, the identification is established by a significant stable carbon isotope excursion, characteristic for each OAE (e.g., Jenkyns, 2010).
Figure 1.2: Stratigraphic range of studied sections (per chapter) in the Cretaceous period, together with sea -level,
18 CO 2-models, δ O-record (for details, see Fig. 1). Cretaceous OAEs are highlighted. Ages of stage bases are labelled. Data from Ogg and Hinnov (2012). Chapter 1 – In troduction 7
The early Aptian OAE 1a and the latest Albian OAE 1d are subject to this thesis. Paleogeographic maps from 120 Ma (Aptian) and 105 Ma (Albian) illustrate the spatial distribution of sites, where the respective OAEs have been recorded . The record includes sites with distinct lithological features, such as subtle lamination, dark mudstones or well - developed black shales, and/ or geochemical proof of the event from stable carbon isotope data , where lithological expressions in the sediment are missing . Both events exhibit a global distribution, though the early Aptian OAE 1a shows more pronounced black shale deposits compared to the latest Albian. As being younger, the sedimentary record of the uppermost Albian is better than that of the lower Aptian. Therefore, the number of sites recording OAE 1d is greater (Fig. 1.3). The lower Aptian sites, studied in this thesis, were located in the proto - North Sea and the adjacent Lower Saxony Basin in the South, both at a paleolatitude of ~40°N (filled circles in figure 1.3). Two low -latitude sites from modern -day Italy and the Mid -Pacific (half -filled circles in figure 1.3) were used for comparison with the North Sea and the Lower Saxony Basin.
The stable carbon isotope record of OAE 1a is used to identify the event if an indicative lithology is missing. The global nature of the carbon cycle due to fast atmospheric exchange establishes a synchronous equilibration of carbon isotopes in the atmosphere and in the ocean. Disruptions in the global carbon cycle like large -scale releases or withdrawals of certain carbon isotopes cause shifts in the stable carbon isotope ratio. These shifts can be recorded in carbonates precipitated from seawater (biotic or abiotic) and are therefore preserved in the geological and fossil record. The pace and mode of shifts allow a reconstruction of the carbon cycle and associated events. In the case of OAE 1a, the characteristic carbon isotope signal, subdivided in segments C1 to C8 (Fig. 1.4; after Menegatti et al., 1998), can be used for a mondial correlati on of sites.
The stable pre -OAE interval (C1) of the upper Barremian is followed by a release of isotopically -light carbon ( 12 C), causing a slight negative shift of the δ 13 C record. The light carbon isotope is enriched in carbon dioxide of volcanic origin and in methane (Beerling et al.,
12 2002; Jahren et al., 2005; Méhay et al., 2009). At the time of the early Aptian, C-rich CO 2 was released into the atmosphere as a consequence of an early phase of volcanic activity at the Ontong Java Plateau in the Mid -Pacific, a magmatic province in the Pacific Ocean, which has been extremely active in the early Aptian (Larson, 1991a, b; Tejada et al., 2009) . Trace metals, both toxic and biolimiting and thus fertilizing, were delivered to the ocean as well. Methane Chapter 1 – In troduction 8 was r eleased when gas hy drate fields began to destabilize .Theslightshiftinδ 13 C (C2), ~200 kyrs
Figure 1.3: Paleogeographic maps of the middle Cretaceous. All circles mark marine occurrences of OAE 1a and OAE 1d, mainly from deep -sea drillings performed by ODP (data compilation from Friedrich, 2010, Wilson and Norris, 2001). Filled circles represent sites that have been studied in this thesis, half -filled ones are sites, that have been used for detailed data comparison. before the actual OAE 1a, is accomp anied by a relative increase of 188 Os in the sediment (Fig. 1.4; Bottini et al., 2012). The un radiogenic 188 Os is mainly derived from hydrothermal alteration of oceanic crust. An increase in 188 Os thus indicates a plus of oceanic crust produced by submarin e volcanisms exposed for alteration. Following that, the OAE 1a itself is characterizedbyaveryabruptnegativeshiftofδ 13 C (C3) , lasting about 27 to 44 kyrs (Li et al., 2008) , as a consequence of the peak volcanic phase in the Pacific . This event is located at the base or slightly above the base of the lithological expression of OAE 1a (in many cases an organic -rich horizon if well -developed ; LS in figure 1.4 ). This has been observed for example Chapter 1 – In troduction 9 in the Roter Sattel section (Switzerland), the Cismon sec tion (Italy) or DSDP Site 463 (Mid - Pacific Mountains) (Ando et al., 2008; Menegatti et al., 1998). This main volcanic phase caused the release of a huge amount of 12 C-rich carbon dioxide and methane into the atmosphere, causing the negative C3 -shift. These two greenhouse gases dramatically increased the global temperature. Evidence of this temperature increase on the continent is delivered by a documented rise in southern provenance (warmer) pollen in the Alpine Tethys, indicating that entire floral belts w ere shifting their position within few kyrs (Hochuli et al., 1999 ; Keller et al., 2011 ).dropofδ 18 O in the Mid -Pacific (ODP Site 463; ~13 -20°S paleolatitude) indicates a seawatertemperatureriseof8°C(ndoetal.,2008).Similarδ 18 O drops were rec orded in the western Tethys (Cismon, Italy; Roter Sattel, Switzerland; ~20°N paleolatitude), indicating a global temperature rise (Menegatti et al., 1998). Paralleltothisnegativeδ 13 C spike and the temperature rise, the osmium isotope record shows an in crease in the radiogenic 187 Os. This was caused by an intensification of continental weathering due to the more humid climatic conditions on the climax of OAE 1a (C3), also evidenced by calcium isotopes (Blättler et al., 2011).
Figure 1.4: Schematic sta ble carbon and oxygen isotope and osmium isotope record of OAE 1a. C1 to C8 represent the stable carbon isotope segments of Menegatti et al. (1998). LS is the position of the lithological expression of the Livello Selli, the OAE 1a stratotype. BAR=Barremia n. Data according to Menegatti et al. (1998), Ando et al. (2008) and Bottini et al. (2012). Chapter 1 – In troduction 10
This very brief episode was followed by a prolonged positive shift in the stable carbon isotope record (C4 to C6) due to a relative increase in 13 C, incorporated in the marine carbonates. In the lithology, this interval is accompanied by an increased TOC (total organic carbon) content . Preservation of organic matter was probably supported by dysoxic to anoxic conditions of the bottom waters as a consequence of intens e oxidation of the organic matter plus (e.g. Jenkyns, 2010) . The intensified greenhouse climate with its increased continental run -off and thus shedding of nutrients into the ocean and the input of biolimiting metals from submarine volcanism of the Ontong Java Plateau lead to a fertilization of the ocean (Erba et al., 2015). Following, the enhanced primary productivity resulted in an anomalously high burial of organic matter on the seafloor. As the light 12 C-isotope wa s/is preferentially incorporated into t issue and biomin erals , the entire carbon reservoir became depleted with respect to 12 C, resulting in the prolonged positive shift in δ 13 C (Jenkyns, 2010). Global temperatures began to decrease approximating pre -OAE values , evidenced by stable oxygen isotop es (Ando et al., 2008; Fig. 1.4). A cooling in the aftermath (C4 to C6) of the initial major volcanic pulse at Ontong Java Plateau at the base of OAE 1a (C3) is also evidenced by a southwards shift of continental floral belts in the late early Aptian (Hoch uli et al., 1999; Keller et al., 2011). Orbital forcing data by Li et al. (2008) revealed, that this positive carbon isotope shift (C4 to C6) lasted ~1.0 to 1.3 Ma. However, the initial volcanic phase initiating this line of events and negative carbon isot ope peak C3, were comparably brief with 27 to 40 kyrs (Li et al., 2008). Carbon isotope segment C7 represents a positive plateau, just above the black shale horizons of OAE 1a, while C8 shows a slight negative trend (Menegatti et al., 1998). The general co oling trend of the OAE 1a aftermath is continuing in these intervals, however less severely (Ando et al., 2008).
Due to changes in seawater temperature and nutrient levels of the ocean, the planktonic communities of the early Aptian have been affected by O AE 1a as well. The ratio of abundances of calcareous nannofossil species, such as Rhagodiscus asper , with warm water affinities to species with cool water affinities show maximum values at the base of OAE 1a within carbon isotope segment C3 for example in the western Tethys and the M id -Pacific (Bottini et al., 2015). This corresponds to intervals with h igh sea surface temperature data from stable oxygen isotopes (Ando et al., 2008). Also, nutrient sensitive species responded to the early OAE 1a fertilization event (Bottini et al., 2015). The progressive cooling in the aftermath, especially after (C7 and C8) the deposition of black shales, is evident by calcareous Chapter 1 – In troduction 11 nannofossils in western Tethys and Mid -Pacific sites (Bottini et al., 2015). The calcareous nannofossil group of the heavily calcified Nannoconids shows the most dramatic respond to OAE 1a , a bio calcification crisis (Méhay et al., 2009 . After an initial decline in the pre -OAE 1a interval, the Nannoconids were completely absent during OAE 1a its elf, reported from the Atlantic, the Tethys and the Pacific (Erba, 1994; Habermann and Mutterlose, 1999; Rückheim et al., 2006; Bottini and Mutterlose, 2012; Mutterlose and Bottini, 2013; Bottini et al., 2015). Increased continental ruff -off and thus muddy waters and increased fertility during OAE 1a, representing unbearable conditions for this nannofossil gr oup, are the most probable explanation for their synchronous decline. However, this group reappeared in the record after OAE 1a without loss of species (Erba, 1994; Erba and Tremolada, 2004; Bottini and Mutterlose, 2012).
In contrast to higher temperature s, increased fertility and more muddy conditions in the sea surface water affecting coccolithophores, the bottom water conditions during OAE 1a were much different, suggesting effects on for example benthic foraminifera. However, studies on benthic foramin ifera are rare and the few one focus mainly on the Vocontian Basin in France or sections in Italy (Bréhéret, 1997; Premoli -Silva et al., 1999). In most cases, benthic foraminifera are either absent throughout entire studied sections or disappeared during O AE 1a. The few sections with benthic foraminifera present within OAE 1a show impoverished low diversity faunas (Friedrich, 2010). This underrepresentation makes paleoceanographic or paleoecolo gical interpretations difficult.
In contrast to OAE 1a, which is most often represented by isolated black shale occurrences parallel to the characteristic stable carbon isotope excursion in the lithological record, the black shales or any other lithological anomal ies associated with the latest Albian OAE 1d show a stra ti graphically more widespread and inconsistent distribution. In most cases, these lithologies occur below the indicative positive isotope excursion of OAE 1d (Petrizzo et al., 2008). It Is therefore much more useful to implement a chemostratigraphic defini tion of OAE 1dratherthanalithological(Petrizzoetal.,2008).Themostcharacteristicδ 13 C signal of OAE 1d, on the base of which this event should be identified, is a positive shift of ~1.2 to 2.0 ‰ in uppermost Albian sediments within nannofossil zone CC9b and Rotalipora appenninica foraminiferal zone , preceded by a slight negative shift (e.g., Wilson and Norris, 2001; Petrizzo et al., 2008; Watkins et al., 2008) . Single black shale deposits within limestones at ODP Site 1052, Blake Nose in the western Atlantic, occurred already ~1.3 Ma and ~0.35 Ma before the Chapter 1 – In troduction 12 carbon isotope excursion, while an extended black shale deposit starts 0.25 Ma before and ends 0.5 Ma after the actual isotope excursion (Figur e 1.5; Wilson and Norris, 2001).
Figure 1.5: Stable carbon and oxygen isotope record of the upper Albian from ODP Site 1052 based on well - preserved planktonic foraminifera (modified after Wilson and Norris, 2001). The thermocline record is based on Rota lipora . The surface record includes the species Biticinella breggiensis, Costellagerina lybica, Planomalina buxtorfi and Ticinella primula . Paleothermometricdataiscalculatedfromδ 18 O-values.
The most probable explanation for OAE 1d is supported by seawater temperature data from stable oxygen isotope data from pristine foraminifera tests of the tropical western North Atlantic (Wilson and Norris, 2001). Therefore, the data of four planktonic foraminifera species with surface water affinities have been combined and compared to those of a genus supposed to thrive in greater water depth associated with the thermocline for a time span of ~ 2 Ma before OAE 1d and slightly beyond (~101.1 to ~98.9 Ma). The temperature record (Figure 1.5) show s as offset betwe en both signal s of 1 to 6 °C, indicating the intensity of upper water column stratification. As expected, the deeper waters (thermocline signal) show less variability than the surface waters. Ho wever, both records approach each other with a cooling in the s urface waters and a warming in the deeper waters just before the carbon isotope Chapter 1 – In troduction 13 excursion indicating OAE 1d. For the time of OAE 1d and its aftermath, the seawater temperature recorded for both groups is the same, implying a collapse in the upper -ocean the rmal gradient or an increase in the depth of the mixed water layer. The underlying reasons for this line of eve nts, with the co -occurrence of high organic matter burial, a stable carbon isotope excursion and the collapse of water column stratification, all most probably not coincidentally occurring at the same time, appear to be of global nature (Wilson and Norris, 2001). DepositsofOE1datitstypelocalityintheVocontianBasin,the“NiveauBreistroffer”, show o rbitally -paced changes in monsoonal acti vity that have been identified by cyclic variations of terrestrial input and productivity of the surface water in black shale marl alternations (Bornemann et al., 2005).
Though regarded as an OAE with inferior global extension and intensity, OAE 1d has nev ertheless had an impact on macrofaunal assemblages. Radiolarian communities were characterized by an extinction of 28% of all species and a radiation of new species, representing 23 to 28% of the entire community synchronous to OAE 1d (Erbacher and Thurow, 1996; Erbacher et al., 1996). This pattern of balanced extinctions and radiations has also been observed for the earliest Aptian, the early Albian and the Cenomanian/Turonian boundary oceanic anoxic events (OAEs 1a, 1b and 2) , while the middle Albian OAE 1c is devoid of this pattern (Erbacher and Thurow, 1996). The extinctions during OAEs mainly affected deep -dwelling forms as their habitat was lost due to the surface water productivity -driven expansion of the oxygen minimum zone (Erbacher and Thurow, 1996 ). The presence of benthic foraminifera in the few studied sections show no indication of total deep water anoxia during OAE 1d (Nederbragt et al., 2001; Friedrich, 2010) . Communities at the eastern Nort h Atlantic DSDP Site 547 off the Moroccan coast even suggest well -oxygenated bottom water (Nederbragt et al., 2001). Planktonic foraminifera from shallow water show a balance of extinctions and radiation of new species, indicating no major faunal turnover (Nederbragt et al., 2001; Petrizzo et al., 2008). Imp lications from calcareous nannofossils are, however, different and differing among studies. The extinction of some nannofossil species associated with OAE 1d was accompanied by a major radiation in the genus Eiffelithus (Watkins and Bergen, 2003; Watkins et al., 20 05 ). The abundance pattern of the species Biscutum constans , a fertility indicator, in the western North Atlantic (Blake Nose, ODP Site 1052) shows a rise within the 1.5 Ma before OAE 1d culminating in peak values of up to 20% during the event (W atkins et al. , 2005 ). This suggests increased upwelling and instability of the water column, Chapter 1 – In troduction 14 culminating in the total collapse of stratification (Watkins et al. , 2005 ). This fertility pattern has not been observed at eastern North Atlantic DSDP Site 547 of fshore Morocco (Nederbragt et al., 2001). However, this study did not encompass the interval below OAE 1d but recorded only the period of the peak positive excursion. The abundance values of B. constans of more than 20% in this section are, however, compar able to the other side of the Atlantic. The rise in abundance and thus comparable pre -OAE 1d values may have simply been missed.
1.3 Coccolithophores – marine primary producers
The food -chain of the modern ocean system is based on small organisms (up to few hundreds of µm in size), which produce 54 billion metric tons of organic carbon per year (Sverdrup, 2006) and 50 to 80 % of the oxygen in the atmosphere, the phytoplankton. What this group lacks in individual specimens size they make up for in numb ers. One liter of seawater may contain up to 115 million specimens of only one species of phytoplankton (for example a Norwegian fjord bloom in 1955; Berge, 1962). These single -celled primary producers turn sunlight into organic matter and become food for herbivorous zooplankton, thus they serve as the food chain base for small fishes, and squids, and finally for sharks and marine mammals. By definition, the phytoplankton includes all plantlike plankton, inactively floating in the water column only moved by tides, waves and ocean currents, commonly described as algae. If conditions are favorable, blooms of the different phytoplankton groups occur in the surface waters that may cover areas of several hundreds of thousand square kilometers (e.g. 250,000 km 2 in the North Atlantic in 1991, Holligan et al., 1993).
The three main groups of marine algae are diatoms, dinoflagellates and coccolithophores (Fig. 1. 6). Diatoms, sometimes called golden algae due to their chlorophyll -masking yellow - brown pigmentation, are found in cold, nutrient -rich waters at the top of the water column. They dominate the high polar regions of the oceans. Their morphology comprises round and pillbox -shaped radially symmetric specimens and bilaterally elongated ones, with hard and rigid val ves made of silica of some tens to few hundreds of µm in diameter. Representatives of this group may be present in both marine and freshwater environments (Sverdrup, 2006).
Dinoflagellates, at sizes of 5 µm to 2 mm, may be present in two different life -sta ges. The resting stage (cyst) has a robust organic outer wall made from sporopollenin; the membrane of the less resistant vagile phase (theca) is composed of celluloses. These red to green colored Chapter 1 – In troduction 15 organisms possess two flagella, which allow them limited mo tility. Blooms of dinoflagellates are not as extended as those of diatoms and due to their number they are not the most important primary producers (Williams, 1998; Sverdrup, 2006).
Figure 1.6 : The three main primary producers of the recent ocean, (A) diatoms, (B) coccolithophores and (C) dinoflagellates.
The coccolithophores are related to diatoms and limited to the marine realm, with exception of the freshwater species Hymenomonas roseola (Br aarud, 1954). The tests of this group are composed of round to elliptical disc -shaped calcitic plates interlinkingly distributed on the outside of the unicellular organism. They thrive and dominate in stable low -nutrient environments like open -oceanic olig otrophic gyres, and are the most important carbonate producers on Earth. The remains of their cell cover amounts to thick deposits on the ocean floor, where they can represent up to 70 wt% of the sediment (Baumann, 2004; Sverdrup, 2006).
The modern phyto plankton community with its three main groups has its origin in the Mesozoic Era, following the end -Permian mass extinction. Prior to that, starting in the Proterozoic (~2500 to 550 Ma), cyanobacteria were the primary producers of the ocean. Already in the Neoproterozoic (~1000 to 550 Ma) and during most of the Paleozoic (~550 to 360 Ma), prasinophytes (acritarchs) were dominant (Falkowski et al., 2004). The change to modern -day communities came in the Triassic, when dinoflagellates and coccolithophores eme rged in the fossil record. Molecular biomarkers typical for dinoflagellates have been Chapter 1 – In troduction 16 documented in Neoproterozoic sediments. These only became dominant as soon as fossil dinoflagellates occurred in the record, placing their first appearance in the middle Triassic (Moldowan et al., 1996). First reports of diatoms from Jurassic sediments (Rothpletz, 1896) were difficult to reproduce. However, it remains certain that this group had begun to radiate in the Early Cretaceous (Harwood and Nikolaev, 1995). Species richness of dinoflagellates and coccolithophores peaked at the end of the Mesozoic Era and generally declined towards the recent, while diatom diversity shows no such decline (Falkowski et al., 2004).
In the fossil record, the remains of coccolithophores are summarized under the term calcareous nannofossils. This collective term comprises all fossil remains of calcitic mineralogy and with a size smaller than 63 µm. This may per definition include sponge spicules, calcispheres and juvenile foraminifera, as well as nannoliths. The latter group is taxonomically placed close to coccolithophores as both occur in the same environments and show similar trends in diversity and abundance. Due to the overwhelming dominance of coccolithophores, the terms calcareous n annofossils and coccolithophores are often used synonymously (Bown and Young, 1998).
1.3.1 Biology and taxonomy of coccolithophores
Taxonomically, the coccolithophores are assigned to the term haptophytes. This umbrella term includes marine and planktoni c complex unicellular algae, which are either naked, covered with organic scales or covered with calcareous plates. The cell of the latter variant, the coccolithophore cell (Fig. 1. 7), which ranges from 2 to 75 µm in cell diameter, includes several promine nt and important cell organelles and components. Starting from its outer appearance, the cells of coccolithophores are covered with small circular to elliptical calcareous plates, the coccoliths, which are usually the base of species classification. The en tity of these coccoliths is termed the coccosphere. The cell also owns a well -developed to receded haptonema, a filamentary cell structure on the cell surface unique to all haptophytes, which is probably used for food capture or attachment. Another filamen tary structure, the flagellum, is related to limited motility of the cell. As coccolithophores are photoautotrophic algae, the cell contains chloroplasts. The coccoliths, unique to the coccolithophores, are either produced inside the cell or on the cell su rface, thereby creating two distinct types of coccoliths, heterococcoliths and holococcoliths (Jordan, 2009). Chapter 1 – In troduction 17
Fig ure 1.7 : Cell structure of coccolithophores with most important cell organelles and cell cover. Modified after Jordan (2009).
Heterococcolith s (Fig. 1. 8 A und C) are the most common coccolith type and are composed of several different calcite crystals (differing in shape and orientation of the c -axis of the crystal with respect to the coccolith plane), while holococcoliths are constituted of only one unifo rm crystal type, that is rhombohedric. The biomineralisation of the former coccolith type has been studied in culture experiment already in the 1980s, for example on Hymenomonas carterae by van der Wal et al. (1983) and Emiliania huxleyi by Westbroek et al . (1984). The Golgi body produces polysaccharide coccolith vesicles, which serve as complex organic templates for the coccoliths (Westbroek et al., 1984). Primary nucleation of calcite crystals occurs on these templates along a ring -shaped structure, the p roto -coccolith ring, and is followed by three -dimensional growth of the coccolith. Time -lapse video studies on decalcified specimens of Coccolithus pelagicus ssp. braarudii revealed that the process of coccolith secretion may only take 60 to 190 seconds (T aylor et al., 2007). The full -grown coccoliths are expelled from the cell via exocytosis in the flagellum cell region (Young et al., 1999). Holococcoliths (Fig. 1. 8 B) on the other hand, are produced outside the coccolithophore cell (Young et al., 1999). Similar to hetercoccoliths, an organic scale derived from the Golgi body is associated with the holococcoliths. However, calcite growth occurs outside the cell in a controlled manor. Holococcoliths form around the flagellum, opposite of the Golgi body on th e cell exterior, and move upon the cell surface to cover the entire cell (Rowson et al., 1986). Chapter 1 – In troduction 18
Besides the very different modes of biomineralisation (Fig. 1. 8 D) of heterococcoliths and holococcoliths, another prime difference between these two coccolith types is related to the life stages of the coccolithophore cell. It was initially presumed that hetero - and holococcoliths represented just two variants of coc coliths, produced by different species. However, combination coccospheres, with hetero - and holococcoliths present (Parke and Adams, 1960; Manton and Leedale, 1963; Thomsen et al., 1991, 1994; Kleijne, 1993), challenged that theory. A possible explanation are the characteristic life stages of coccolithophores, best known from E. huxleyi (Billard, 1994; Green et al., 1996), with a haploid (genome is present once in a cell) and probably holococcolith -bearing phase and a diploid (genome is present with a dupli cate) phase, which is characterized by the formation of heterococcoliths.
Figure 1.8 : SEM images on (A) one placolith, (B) one holococcolith, (C) two murolith. SEM images from North Jens -1, North Sea. (D) a schematic section through Pleurochrysis carter ae . After Van der Wal et al. (1983).
The morphology of coccoliths has been proven to be an effective tool for species identification, especially in the fossil record, where genetic sequencing is impossible due to the absence of original organic matter. The fundamental components of the more c omplex heterococcoliths (holococcoliths are rather simple in morphology and are not subdivided further, except for species assignments) include a central tube, perpendicular to the coccolithophore cell surface, and two more or less -well developed shields o n either ends of Chapter 1 – In troduction 19 the tube, parallel to the cell surface (Fig. 1. 9). Species with well -developed proximal and distal shields are termed placoliths, while those with very reduced shields are termed muroliths. The shields are composed of multiple radially -arr anged and possibly imbricated crystals of variable number, size, crystallographic orientation and shape. The central tube may be empty or filled, possibly by bars, sieves, plates, and cross -like structures; with or without distal spines. The number of inte rlocking coccoliths in the coccosphere, the entire cell cover, may be less than 20 (e.g., Gephyrocapsa oceanica ) or up to 50 to 200 (e.g., Florisphaera profunda ) (Jordan, 2009).
Figure 1.9 : Schematic representation of structural elements of the two het erococcolith types: placoliths and muroliths in top and side view.
The general distribution of coccolithophores includes the marine realm, with highest diversities of up to 100 species in the photic zone in subtropical to tropical waters and a decrease in diversity towards the poles. In subpolar waters, only approximately 10 species are present. Cosmopolitan species with broad temperature tolerances, like E. huxleyi , can be found in assemblages in most waters, while some other species, like Wigwamma spp., s how restricted occurrences and are limited to certain biogeographic zones, mostly parallel to the equator. However, these zones are limited to the open ocean, as coastal water exhibit special conditions due to upwelling and river outflow (nutrient availabi lity) and thus resulting in the presence of very different coastal communities (Jordan, 2009). Chapter 1 – In troduction 20
1.3.2 The fossil record of calcareous nannofossils
The first fossil occurrence of nannoliths and calcareous dinoflagellates, representing calcareous nannofoss ils, is reported from Carnian (237 to 228.4 Ma; upper Triassic) sediments from the southern Alps in Italy (Janofske, 1992; Bown, 1998). Earliest coccoliths, however, date back to Norian/Rhaetian boundary (~209.5 Ma) deposits (Gardin et a., 2012). Coccolith s of that early age are small (2 -3 µm) muroliths; assemblages show low diversities of 5 species at max (Bown et al., 2004). Within the coccoliths, the earliest holococcolith species, Anfractus youngii , is reported from the Toarcian (182.7 to 174 Ma; lower Jurassic; Young et al., 1999). It is speculated, that this first occurrence is somewhat obscured, as holococcoliths tend to disintegrate easily in the water column and during deposition and may therefore be invisible in the early and thus low diversity and abundance fossil record of coccolithophores. The fragility and the low fossilization potential of holococcoliths may generally explain the low abundance of that group in fossil sample material, while it may be very common in recent samples (Bown et al., 2 004).
Nevertheless, the first occurrences of fossil coccoliths and nannoliths in the sedimentary record only gives a reliable latest first occurrence of these groups, which may actually be older. Firstly, it may very well be probable, that the first hard p arts of these groups were not recorded until now, due to fossilization potential, small sizes, very low abundances or the availability of relevant sediments. Secondly and most probably, the ancestors of the coccolith - and nannoliths -producing algae have ex isted in the pre -Triassic past and were naked, while the first fossil occurrence actually represents an innovation in biomineralisation. It has been proven by molecular clock constructions, that the haptophytes, the umbrella group of the coccolithophores, probably appeared in the Precambrian (~ 850 Ma) (Jordan, 2009).
The very -well studied record of calcareous nannofossils has created a huge data set and allows to trace the evolution of this group along its diversity patterns and to make general conclusions on interactions between this group and climate (Bown et al., 2004). From its first occurrence in the fossil record onwards, coccolithophore and nannoliths diversity (Fig. 1. 10 ) started to increase. By the early Jurassic, the coccolithophores became the do minant nannofossil group and showed a marked size increase on the species level. Starting with the latest Jurassic/earliest Cretaceous, calcareous nannofossils had started to form pelagic carbonates. This enlargement in depositional environment has been in terpreted as the Chapter 1 – In troduction 21 expansion of the broad habitat of this group from the shelfs into the open -ocean (Roth, 1986; Hay, 2008). Maximal diversities of coccoliths and nannoliths are reported in mid -Campanian to mid -Maastrichtian sediments with 149 species, exten sive chalk deposition and significantly large -sized coccoliths. The end -Cretaceous mass extinction, however, had a tremendous impact on calcareous nannofossils with only nine Cretaceous survivor species reported from the lowermost Paleogene. Despite extinc tions already at the Jurassic -Cretaceous boundary, the shift from the Cretaceous to the Cenozoic changed the calcareous nannofossil assemblage and the general morphology of coccoliths and nannoliths more considerably than ever before or after. This makes Cretaceous and Cenozoic assemblages easy to distinguish at first glance (Bown et al., 2004).
Calcareous nannofossils are usually studied in simple object slides, where they can be separately observed after disengaging them from the sediment, using transmit ted light microscopes with crossed nicols. The otherwise transparent calcite of the coccoliths and nannoliths shows low interference colours and appears white to grey, rarely yellowish to orange in more heavily calcified forms, and stand out with respect t o the black background of the slide. Magnifications of up to x2000 are necessary to observe morphological characters to distinguish on the species level. This approach (simple object slides and light microscope) is the easiest and cheapest.
To extract info rmation from the sediment with its in part rock -forming calcareous nannofossils, several approaches are conceivable. First of all, the high amount of morphological characters in these small calcitic platelets and their fast change through geological time a llow a biostratigraphic application of calcareous nannofossils. The presence or absence of special marker species in a sample are noted and this information is interpreted with respect to existing biostratigraphic zonation schemes to derive a relative age of samples (e.g., Alqudah et a., 2014; Bottini and Mutterlose, 2012; Gale et al., 2011; Jeremiah, 2001; Kennedy et al., 2000). An ecological signal from the fossil remains of the primary producers can be gathered from assemblage studies. For that purpose, at least 300 randomly -chosen specimens per sample are classified on species level and counted. From that data set, diversity indices and relative abundance records of individual species can be derived. Single -species abundance data have in many cases been used to characterize for example responses to short - term events like OAEs (e.g, Bornemann et al., 2005; Giraud et al., 2003; Hardas and Mutterlose, 2007; Hardas et al., 2012; Linnert et al., 2010), coastal/open -oceanic assemblages, Chapter 1 – In troduction 22 or assemblages from diff erent latitudes and biogeographic zones (e.g., Lees, 2002; Linnert and Mutterlose, 2011; Roth and Bowdler, 1981).
Figure 1.10 : Mesozoic and Cenozoic diversity record of coccolithophores (light line) and total calcareous nannofossils (bold line) including nannoliths and calcareous dinoflagellates. Species richness are calculated for a 3-million -year intervals. Note, that the firs t occurrence of coccolithophores has been postponed from the Carnian to the Norian/Rhaetian boundary according to Gardin et al. (2012). After Bown et al. (2004).
The greatest obstacle in calcareous nannofossil studies in general is preservation. Calcareous nannofossils, primarily small and less robust species, may be dissolved and thus do not appear in the fossil record, obscuring biostratigraphic and ecological data. For example, Chapter 1 – In troduction 23 the genus Watznaueria has been reported to be one of the most dissolution -res istant species with abundances of more than 40% (Roth and Bowdler, 1981; Thierstein and Roth, 1991) or 70% (Williams and Bralower, 1995) of W. barnesiae indicating altered nannofossil assemblages. On the other hand, overgrowth from intense pore fluid flow can inhibit species recognition. Studying calcareous nannofossils, it is a prerequisite to ascertain that the obtained data represent an actual and pristine ecological signal and no relict from diagenesis.
1.3.3 Issues in biometric studies on calcareous n annofossils
Together with nannofossil biostratigraphy and assemblage studies, biometric studies on calcareous nannofossil are an additional tool to gain information on the past, in this case species -specific. A common approach involves the biometric charac terization of at least 50 single specimens per species per sample using digital images taken with transmitted light microscopy. Basic characters, that are commonly measured, include maximal (length) and minimal (width) dimensions of the round to elliptical outline of the coccolith (e.g., Erba et al., 2010; Linnert and Mutterlose, 2013; Lübke et al., 2015; Lübke and Mutterlose, 2016;), thickness of shields and thus the volume of single coccoliths (e.g., Beaufort, 2005), and specific features such as central tube diameters and structures (e.g., Bornemann and Mutterlose, 2003; Giraud et al., 2006; López -Otálvaro et al., 2012). Figure 1. 11 shows a representation of basic morphological characters of three exemplary species, measured in this PhD thesis. In excepti onally -well preserved material, complete coccospheres can be measured (e.g., Gibbs et al., 2013; Henderiks, 2008) to gain information on the correlation of cell size and coccolith size and the number of coccoliths per coccosphere.
The matters addressed wi th biometric analyses comprise taxonomic issues (e.g., Bornemann and Mutterlose, 2006; Fraguas and Erba, 2010; López -Otálvaro et al., 2012; Mattioli et al., 2004), the estimation of fossil carbonate fluxes (e.g., Bornemann et al., 2003; Erba and Tremolada, 2004; Mattioli et al., 2009), and the responses or adaptations to environmental change (e.g., Bornemann and Mutterlose, 2006; Erba et al., 2010; Fraguas and Young, 2011; Giraud et al., 2006; Linnert and Mutterlose, 2013; Lübke et al., 2015; Lübke and Mutt erlose, 2016; Suchéraz -Marx et al., 2010). For example, López -Otálvaro et al. (2012) used total coccolith diameter as a distinguishing feature to separate the taxonomic species Discorhabdus striatus and Discorhabdus ignotus in Jurassic strata. Chapter 1 – In troduction 24
Figure 1. 11 : Schematic representation of three µm -scaled calcareous nannofossil species, measured in this PhD thesis and the basic biometric characters. Examples include two placoliths species (circular P. columnata and elliptical B. constans ) and one murolith spec ies ( Z. erectus ).
Besides the general problem of preservation, biometric studies may suffer from various obstacles. Low sample resolution, due to availability of material or presence of studied species, can inhibit the recording of significant size shifts. If the sedimentat ion rates in the studied sequence have been too low during time of deposition, size shifts may not even be recorded in the sediment und thus not observable by scientists. Bioturbation of the sediment merges material from different ages and disturbs the chr onological order of the sediment. Comparing data from various sites may suffer from the lack of or the inaccuracy of age models. To prevent this from occurring, detailed and accurate biostratigraphic and chemostratigraphic age models are a prerequisite. Chapter 1 – In troduction 25
1.4 Objectives of the thesis
This thesis comprises the work of four years of PhD study with data from various sites (Fig. 1. 12 ). The goal of this study was to determine the applicability of calcareous nannofossil biometry as indicator for various purp oses. This includes coccolith size responses to short - term and long -term environmental changes, and evolutionary size shifts. Short -term geologic changes represent events with durations of up to several hundreds of thousands to about a million of years are for example oceanic anoxic events (OAEs) or comparable events (e.g., Mid - Cenomanian Event ). The long -term record can comprise several millions of years. Though genetic sequencing is not possible on fossil material and though sample resolution may be not as high as in recent culture batch or sediment traps, the advantage of studies on the geological record is the covered time span. Culture studies on recent coccolithophores, which are, together with diatoms, the most important primary producers of modern -da y oceans, only record very short -term time intervals of days to a few years at the most. Therefore, only the geological record can provide the crucial information on the long -term impact of high CO 2 concentrations on calcium carbonate shelled primary produ cers by looking at the size evolution of the fossil remains of coccolithophores.
In the case of this study, OAE 1a (early Aptian) and 1d (late Albian) were studied as case studies of short -term events; covered in chapters 3 and 5. The results of a long -term record of the lower Aptian to middle/upper Cenomanian based on coccolith biomet ry and assemblage analyses are shown in chapters 4 and 5. The recorded size -shift of calcareous nannofossils, as shown in chapter 4 for the example of the species Prediscosphaera columnata , were interpreted as evolutionary -based. In all cases, generating s ufficiently -accurate age models (biostratigraphic, lithostratigraphic and chemostratigtraphic) has been the base of the studies. Biostratigraphy based on calcareous nannofossils, chemostratigraphy based on stable carbon isotopes, and lithostratigraphy base d on basin -wide equivalent deposits to some extent has been used in chapters 2 and 3 to correlate biometric data from five sites in four different geographical settings (Lübke et al., 2015; Lübke and Mutterlose, 2016). A detailed long -term calcareous nanno fossil biostratigraphic zonation from the southern high -latitudes serves as a framework for biometric and assemblage analyses of the Indian Ocean in chapter 4 and 5. Chapter 1 – In troduction 26
Figure 1.12 : (A) Global map of studied sections with (B) enlargement of central Europe. Modified after Lübke et al. (2015).
1.5 Thesis overview
Apart from the Introduction in chapter 1, this thesis contains six furth er chapters. The chapters 2 to 5 correspond to four manuscripts that have been submitted to and/or published in interna tional peer -reviewed journals, or are rea dy for submission. The chapter 6 represents an integrated summary of results of the previous chapters with regard to the goals and achievements of the PhD project. Supplementa ry data files for chapters 3 and 5 are s tored on the digital version of this PhD thesis , as the data are not included in the chapters themselves .
Chapter2(“SizevariationsofcoccolithsinCretaceousoceans - A result of preservation, genetics and ecology?”) by Nathalie Lübke (NL), Jörg Mutter lose and Cinzia Bottini was publishedinthejournal“MarineMicropaleontology”in2015.Itrepresentsacomparative study of coccolith biometry data from four locations (North Sea: cores North Jens -1, Adda -2; northwest Germany: Alstätte outcrop; Tethys: C ismon core; Pacific: DSDP core 463) derived from a synchronous interval of early Aptian age (~126 Ma). The data of the latter two sites have been taken from the literature (Erba et al., 2010); all other data have been gained in this study using transmitted light microscope images of 50 specimens per sample. This study aimed at comparing absolute coccolith sizes of the species Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae at these sites and to understand the reason for the present discre pancies. Depending on the different morphologies of the species, preservation, Chapter 1 – In troduction 27 genetic variation within one traditional species and purely ecological reasons are discussed. The author of this thesis (NL) conducted the measurements, performed the statistic evaluation of the data and wrote the manuscript. All figures and tables were created by the first author. Jörg Mutterlose and Cinzia Bottini supervised the work and revised the manuscript.
Chapter 3 (“The impact of OE 1a on marine biota deciphered by siz e variations of coccoliths”) by Nathalie Lübke and Jörg Mutterlose, published in the journal “Cretaceous Research” in 2016, is a follow -up study of the first paper of the author. It presents new coccolith biometry data from the North Sea and northwest Germ any and documents the response of the coccolith sizes to the globally -discernable early Aptian OAE 1a in the Boreal Realm for the first time. The author of the thesis (NL) raised all data presented in the study, conducted the statistical evaluations, creat ed all figures and tables and wrote the manuscript. Jörg Mutterlose revised the manuscript.
Chapter 4 (“Size changes in calcareous nannofossils documenting the pace of microevolutionaryprocesses“)byNathalieLübkeandJörgMutterlosehasbeensubmittedt o thejournal“Paleobiology”inNovember2016.Itrepresentsthefirstapproachtodateandto document intra -species size shifts of the calcareous nannofossil species Prediscosphaera columnata during its early evolution in the Albian to Cenomanian stage. The results are used to deduce information on the pace of microevolutionary processes with the calcareous plankton. These time spans cannot be reproduced in genetic studies on living assemblages, therefore the fossil record serves as an analogue testing gr ound. The extended and continuous sedimentary succession of the studied ODP Site 763B offers an ideal opportunity to perform this study. The biometric data presented in this manuscript were gained by Nathalie Lübke. Sample selection, data evaluation, figur e and table creation and manuscript writing were performed by the author of this thesis. Jörg Mutterlose revised the manuscript.
Chapter 5 (“ stable long -term calcareous nannofossil record (lower Aptian – middle/upper Cenomanian) from the southern Indian Ocean“) by Nathalie Lübke and Jörg Mutterloseisreadyforsubmissiontothejournal“MarineMicropaleontology”bytheendof 2016. This study represents the first long -term calcareous nannofossil record (lower Aptian to middle/upper Cenomanian) from the s outhern high -latitude Indian Ocean ODP Site 763B, northwest of Australia. The middle Cretaceous of the southern hemisphere has scarcely been Chapter 1 – In troduction 28 studied at all. However, for a general understanding of this important time interval, it is obligatory to gain data from various sites with a global distribution. This study includes nannofossil assemblage composition and diversity data, an extended biostratigraphic zonation, biometric data as well as a stable carbon isotope record. Paleoceanographic interpretations re fer to the early evolution of the Indian Ocean, taking place in the studied region. The author of the thesis selected the studied section, time interval and samples, obtained all data, created all figures, and wrote the manuscript. Stable carbon isotope da ta were produced at the GeoZentrum Nordbayern by Michael Joachimski. Jörg Mutterlose revised the manuscript.
Chapter 6 summarizes the re sults obtained in chapter 2 to 5 and gives a general summary of the thesis and its results. The manuscripts have been modified to fit the uniform format of the entire thesis . Chapter 7 is a follow -up with critical remarks on theories introduced in specific parts of the thesis, that have not been published with the corresponding papers. Last minor errors have been correcte d. No changes with respect to data, interpretation or conclusions have been made. Reference lists of all chapters have been combined and added at the end of this thesis. Chapter 2 – Size variations of coccoliths in Cretaceous oceans 29
2 SizevariationsofcoccolithsinCretaceousoceans - aresultof preservation,geneticsandecology?
Nathalie,Lübke 1,Jörg,Mutterlose 1,Cinzia,Bottini 2
1, Institute, for, Geology, Mineralogy, and, Geophysics, Ruhr -Universität, Bochum, Universitätsstr, 150, 44801,Bochum,Germany
2, University,of,Milano,via,Mangiagalli,34,20133,Milano,Italy
bstract
Biometric,studies,of,coccoliths,the,remains,of,coccolithophores,offer,the,opportunity,to, survey,single,species,instead,of,entire,assemblages,We,obtained,and, analyzed,size,data,of, three, common, species, ( Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae) in, a, stratigraphically, very, well -defined, interval, of, early, ;ptian, age, (126, Ma, Cretaceous), Material, is, derived, from, four, sites, (Lower, Saxony, B asin, North, Sea, western, Tethys,Mid -Pacific),covering,nearshore,to,open -oceanic,paleosettings,
Length,and,width,measurements,of,1986,specimens,were,evaluated,The,recorded,size, patterns,show,a,larger,data,spread,for, B.constans and, W.barnesiae in,the,we stern,Tethys,and, the,Mid -Pacific,than,in,the,North,Sea,and,the,Lower,Saxony,Basin,The,latter,two,sites,are, dominated,by,small,coccoliths,of, B.constans while,coccoliths,of, W.barnesiae show,similar, sizes,at,all,four,sites,Solely,small,specimens,of, Z. erectus characterize,the,samples,from,the, North,Sea,and,the,Lower,Saxony,Basin,while,only,large,ones,are,present,in,the,samples,of,the, western,Tethys,and,Mid -Pacific
For, explaining, the, recorded, size, patterns, three, theories, are, discussed, in, detail, these, include,(1),preservation,of,nannofossils,(2),genetics,and,(3),palecology,(1),Intense,dissolution, or,overgrowth,of,the,nannofossils,may,have,altered,the,original,coccolith,sizes,particularly, when,biometric,data,from,different,sites,with,potentially,varyin g,states,of,preservation,are, compared, Due, to, its, delicate, morphology, Z. erectus appears, most, prone, to, dissolution, probably,explaining,its,size,pattern,(2),If,the,recorded,size,data,of,the,remaining,two,species, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 30 represent,original,patterns,these,can,be, interpreted,by,new,findings,in,recent,coccolithophore, genetics,It,has,been,shown,that,size,variations,within,a,single,cryptic,species,are,only,minor, Shifts,of,coccolith,sizes,both,in,recent,and,fossil,taxa,can,be,related,to,genotypic,variation, The, var ying, size, ranges, recorded, for, B. constans and, W. barnesiae may, therefore, reflect, diversity,changes,of,cryptic,species,at,the,different,sites,(3),These,cryptic,species,may,have, preferred,different,depth,habitats,depending,on,geographically -controlled,facto rs,such,as,sea - surface,temperatures,light,availability,or,trophic,load
Keywords
Biometry, calcareous, nannofossils, Cretaceous, preservation, sea -surface, temperature, light,availability
2.1 Introduction
The,first, appearance,of,calcareous,nannofossils, i ncluding, remains,of, marine, coccolith - bearing,haptophyte,algae,dates,back,to,the,Late,Triassic,(Janofske,1992,Bown,1998),Since, then,these,unicellular,organisms,have,inhabited,the,photic,zone,and,are,among,the,main, primary,and,carbonate,producers,of,th e,oceans,(eg,Bown,and,Young,1998,Bown,et,al, 2004, de, Vargas, et, al, 2007, Jordan, 2012), Changes, in, coccolithophore, assemblage, composition,may,indicate,and,record,changes,in,the,physical,chemical,and,trophic,conditions, of,the,ocean,due,to,the,envir onmental,sensitivity,of,various,species, Coccolithuspelagicus for, examples,prefers,cold,waters,and,high,nutrient,concentrations,(Roth,1994),The,morphology, of, nannoplankton, and, its, cell, cover, may, indicate, adaptations, to, specific, environmental, conditions The, three, modern, taxa, Florisphaera profunda, Gladiolithus flabellatus and lgirosphaerarobusta are,heavily,calcified,species,with,asymmetrically,arranged,coccospheres, (Young, 1994), They, are, associated, with, the, deep, photic, zone, in, the, low, to, mid -latitudes, (Young,1994),Several,studies,of,fossil,species,suggested,a,link,between,coccolith,size,and, morphology,(eg,Mattioli,et,al,2004,2009,Linnert,and,Mutterlose,2009,2013),
Biometric,studies,can,help,to,better,understand,relationships,between,coccolithophores, and,their,(paleo)environment,The,role,of,coccoliths,and,of,the,entire,coccosphere,is still,under, discussion,but,they,possibly,served,more,than,one,function,The,wide,range,of,coccoliths,and, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 31 coccosphere, morphologies, supports, this, concept, of, a, multi -functional, cell, cover, (Young, 1994),
In,the,fossil,record,Biscutum,constans,represented, by,small -to,medium -sized,elliptical, placoliths,had,its,first,appearance,in,the,early,Bathonian,(168,Ma),and,became,extinct,in,the, late,Maastrichtian,(66,Ma),(Bown,and,Cooper,1998,Burnett,1998), Zeugrhabdotuserectus includes,small,elliptical,loxolith muroliths,and,ranges,from,the,early,Pliensbachian,(191,Ma), to,the,late,Maastrichtian,(66,Ma),(Bown,and,Cooper,1998,Burnett,1998),Both,species,have, been,described,as,high -fertility,indicators,by,Roth,(1981),The,ecological,relationship,between, these, species, is, not, yet, fully, understood, (Lees, et, al, 2005), The, third, studied, species, Watznaueriabarnesiae ,consists,of,broad,elliptical,placoliths,It,has,a,wide,size,range,and,is, abundant,to,dominant,in,sediments,of,early,Bajocian,(170,Ma),to,late,Maastr ichtian,(66, Ma),age,(Bown,and,Cooper,1998,Burnett,1998),It,is,an,ecologically,robust,species,which,is, able,to,occupy,new,and,extreme,biotopes,(eg,Lees,et,al,2004,2005,Mutterlose,et,al,2005, and, references, therein), It, is, considered, as, the, Meso zoic, analogue, of, the, recent, taxon, Emilianiahuxleyi (Lees,et,al,2004)
Biometric,studies,of,B,constans suggest,a,wide,range,of,possible,size -controlling,factors, Bornemann,and,Mutterlose,(2006),reported,that,a,coccolith,size,decrease,of,B,constans,is, most,likely,related,to,a,cooling,trend,during,the,late,;lbian,;,positive,correlation,between,the, coccolith, size, of, Biscutum, spp, and, nutrient, availability, was, proposed, by, Linnert, and, Mutterlose,(2013),Erba,et,al,(2010),recorded,an,abrupt,coccolith,size,r eduction,of, B.constans and, Z.erectus associated,with,Oceanic,;noxic,Event,1a,Conversely,several,studies,on,the,size, of, W, barnesiae, revealed, that, a, stable, mean, coccolith, size, was, retained, with, respect, to, changes,in,temperature,nutrient,concentration,a nd,seawater,chemistry,(eg,Bornemann,and, Mutterlose,2006,Erba,et,al,2010,Linnert,and,Mutterlose,2013),;part,from,these,studies, of,Cretaceous,taxa,biometric,analyses,on,coccoliths,were,also,conducted,for,a,number,of, species,throughout, the,Mesozoic and, Cenozoic, In, Jurassic, sediments,Giraud, et, al, (2006, 2009), described, several, morphotypes, of, Watznaueria britannica , The, smallest -sized, representatives,were,assigned,to,turbulent,unstable,and/or,eutrophic,conditions,Fraguas,and, Young,(2011),observed a,drastic,size,decrease,of,the,coccolith,genus, Lotharingius during,the, early, Toarcian, probably, related, to, unfavorable, paleoenvironmental, conditions, for, biomineralization, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 32
In,the,present,study,we,compare,biometric,measurements,of,the,three,most,common, mi d-Cretaceous,calcareous,nannofossil,species,( B.constans,Z.erectus,W.barnesiae ),from, four,sections,of,early,;ptian,age,The,three,species,account,for,more,than,half,of,the,total, nannofossil,abundance,in,each,sample,The,objective,of,this,study,was,to,d etect,size,variations, in,coccolith,species,at,four,different,settings,from,40°N,to,20°S,paleolatitude,The,results,allow, us,to,evaluate,the,factors,that,might,have,affected,the,size,distribution,of,coccoliths,
2.2 Materialandstratigraphy
2. 2.1NorthSea(40°Npaleolatitude)
The,material,analysed,in,this,study,is,derived,from,two,cores,(North,Jens -1,;dda -2),which, were,drilled,by,Mærsk,Oil,Two,samples,from,North,Jens -1,and,one,sample,from,;dda -2,were, studied,(Table, 2 1),Both,sites,are,loc ated,in,the,center,of,the,Central,Graben,of,the,North,Sea, Situated,about,15,km,apart,the,cores,lie,,200,km,west,of,Denmark,(North,Jens -1,55°50N, 04°33E,;dda -2,55°48N,04°50E,Fig, 2 1),The,encountered,sediments,of,early,;ptian,age, (Fig, 2 2),are mainly,laminated,nannofossil -bearing,claystone,The,studied,samples,which,can, be,classified,as,marls,(Pettijohn,1975),according,to,their,carbonate,content,are,typical,of, pelagic,to,hemipelagic,settings,(Kühnau,and,Michelsen,1994,Mutterlose,and,Bottin i,2013), During, the, Cretaceous, the, North, Sea, formed, the, southern, extension, of, the, Boreal -;rctic, Realm,Temporary,connections,to,the,Tethys,Ocean,in,the,south,allowed,for,floral,and,faunal, exchange,(eg,Mutterlose,1992),The,age,assignment,is,based,on, Mutterlose,and,Bottini, (2013),who,used,calcareous,nannofossils,and,stable,carbon,isotopes,Nannofossil,assemblage, data,were,compiled,by,Bottini,and,Mutterlose,(2012) Chapter 2 – Size variations of coccoliths in Cretaceous oceans 33
Table,21,Details,of,studied,samples,Lithology,according,to carbonate,content,is,based,on,the,classification,by, Pettijohn,(1975),,
Samples Location Age Number of Lithology Lithology Paleolatitude References measured according to according to specimens literature carbonate content
2253.5 m North Sea BC 18 205 laminated limey marl(stone) ~40°N Age, Lithology, North Jens -1 claystone Paleolatitude Mutterlose and 2254.2 m North Sea BC 18 205 laminated marl(stone) Bottini, 2013 North Jens -1 claystone
2370.6 m North Sea BC 18 209 laminated marl(stone) Adda -2 claystone
3.53 m NW -Germany BC 18 205 claystone/ pure clay(stone) ~40°N Age, Lithology, Alstätte 1 mudstone Paleolatitude: Bottini and 3.33 m NW -Germany BC 18 203 claystone/ pure clay(stone) Mutterlose, 2012 Alstätte 1 mudstone
3.15 m NW -Germany BC 18 201 claystone/ pure clay(stone) Alstätte 1 mudstone
24.13 m Italy NC 6 80 carbonaceous marl(stone) ~20°N Age, Lithology, Cismon limestone Paleolatitude: Erba et al., 2010 26.16 m Italy NC 6 110 carbonaceous marl to limestone Cismon limestone
30.08 m Italy NC 6 80 carbonaceous marl to limestone Cismon limestone
marl -to 625.5 m Mid -Pacific NC 6 110 carbonaceous clay(stone) ~13 -20°S Age, Lithology: DSDP Site 463 limestone Erba et al., 2010
626.96 m Mid -Pacific NC 6 110 limestone marl(stone) DSDP Site 463 Paleolatitude: Tarduno et al., 1995 634.98 m Mid -Pacific NC 6 110 carbonaceous marl(stone) Schouten et al., 2003 DSDP Site 463 limestone
marl -to 625.535 m Mid -Pacific NC 6 52 carbonaceous clay(stone) DSDP Site 463 limestone
626.98 m Mid -Pacific NC 6 53 limestone marl(stone) DSDP Site 463
634.995 m Mid -Pacific NC 6 53 carbonaceous marl(stone) DSDP Site 463 limestone
SUM: 1986 Chapter 2 – Size variations of coccoliths in Cretaceous oceans 34
2. 2.2 LowerSaxonyBasin(39°Npaleolatitude)
The,;lstätte,1,outcrop,is,located,in,northwestern,Germany,(52°09N,06°54E,Fig, 2 1),It, exposes, an, alternation, of, mudstones, and, marlstones, of, early, ;ptian, age, (Fig, 2 2), Three, samples, were, studied, from, this,outcrop, (Table, 2 1), The, sediments, were, deposited, at,the, margin,of,the,Lower,Saxony,Basin,(LSB),(Lehmann,et,al,2012),Being,bound,to,the,north,by, the,Pompeckj,Swell,and,to,the,south,by,the,Rhenish,Massif,the,LSB, was,a,geographically - restricted,sub -basin,of,the,North,Sea,during,the,early,Cretaceous,;noxic,conditions,which, prevailed,throughout,most,of,the,late,Barremian,and,early,;ptian,were,replaced,by,a,pelagic, setting,in,the,late,early,;ptian,(eg,Keupp,and, Mutterlose,1994),Bottini,and,Mutterlose, (2012), provided, a, detailed, stratigraphic, framework, for, the, ;lstätte, 1, section, based, on, calcareous,nannofossils,stable,carbon,isotopes,and,lithostratigraphy
Figure,21,(;),Paleogeographical,map,of,the,early,;p tian,(126,Ma),showing,the,studied,locations,(B),Detailed, locations,of,the,North,Sea,and,LSB,sites,Maps,modified,after,Ziegler,(1982/1990),and,Smith,et,al,(1996) Chapter 2 – Size variations of coccoliths in Cretaceous oceans 35 ), Z. n,relative,to, ,common, arbon,isotope, Farhaniavarolii samples,derive,from,a, B.constans to,base, et,al,1998),Carbon,isotope, latitude,sites,O;E,1a,corresponds, - lation,in,different,basi e,low ,2012,Mutterlose,and,Bottini,2013),from, ons,based,solely,on,nannofossils,was,difficult, ,frequent,to,common, Rhagodiscusgallagheri e,O;E,1a,correlates,with,the,local,Fischschiefer,(Larson, (Erba,et,al,2010,Bottini,and,Mutterlose,2012,Mutterlose, W.barnesiae litterarius ),at,Cismon,and,DSDP,Site,463,(Bown,et,al,1998),Further,stratigraphic, C. ofossil,zone,BC,18,(base, Eprolithusfloralis and,the,presence,of, to,base, R.asper 2,and,;lstätte,1,are,from,nann - 2013),for,the,North,Sea,from,Bottini,and,Mutterlose,(2012),for,the,LSB,from,Erba,et,al,(1999),and,Méhay, 1,;dda - Hayesitesirregularis ,common,to,abundant, l,1987,;rthur,et,al,1990,Bralower,et,al,1994,Menegatti,et,al,1998),;ll,samples,studied,here,are,associated,with,c D.ignotus rovided,via,a,nannofossil,event,(nannoconid,decline),for,several,sites,(Erba,et,al,2010,Mutterlose,and,Bottini,2013),;ll ,frequent,to,common, Figure,22,Lithology,age,stratigraphy,and,samples,for,the,five,studied,sections,(Erba,et,al,2010,Bottini,and,Mutterlose four,different,locations,Samples,from,North,Jens This,approximately,corresponds,to,NC6,(base, constraint,is,p lithological,unit,below,the,global,oceanic,anoxic,event,(O;E),1a,In,the,North,Sea,and,northwest,Germany,th et,al,1993,Bralower,et,al,1994,Bischoff,and,Mutterlose,1998,Malkoč,et,al,2010,Bottini,and,Mutterlose,2012),;t,th to,the,Selli,Level,(Coccioni,et,a segment,C2,(after,Menegatti,et,al,1998),and,the,corresponding,segment,;p2,(;ndo,et,al,2008),Correlation,of,secti due,to,the,asynchronous,appearance,of,nannofossil,events,in,the,Boreal,and,Tethys,nannofossil,biostratigraphic,schemes,(Bown, stratigraphy,data,derive,from,Mutterlose,and,Bottini,( et,al,(2009),for,Cismon,and,from,;ndo,et,al,(2008),for,DSDP,Site,463,Carbon,isotope,stratigraphy,is,well,suited,for,corre biostratigraphic,zonation,Nannofossil,assemblages,are,characterized,by,abundant,occurrences,of, erectus and,Bottini,2013,Bottini,at,al,2014) Chapter 2 – Size variations of coccoliths in Cretaceous oceans 36
2. 2.3WesternTethys(20°Npaleolatitude)
The,drill,site,of,the,Cismon,core,is,located,in the,Venetian,Southern,;lps,in,northern,Italy, (46°03N,11°45E,Fig, 2 1),The,core,consists,of,lower,;ptian,limestones,with,radiolarian,beds, and, chert, lenses/beds, (Fig, 2 2), Three, samples, were, chosen, for, this, study, (Table, 2 1), ;ccording, to,their, carbon ate, content, of, 48, to,80, wt%,these, can,be, labelled, as, marls, to, marls/limestones,(Pettijohn,1975),During,the,;ptian,the,site,was,positioned,at,the,southern, margin, of, the, western, Tethys, in, the, Belluno, Basin, (Erba, et, al, 1999), Sedimentation, was, characte rized,by,pelagic,carbonates,(Channell,et,al,1979),;ge,determination,of,the,core,is, based,on,calcareous,nannofossils,magnetostratigraphy,lithostratigraphy,and,stable,carbon, isotopes,(Erba,et,al,1999),
2. 2.4Mid -Pacific(13 -20°Spaleolatitude)
DSDP, Leg,62,targeted,the,central,North,Pacific,Ocean,in,1978,Site,463,is,located,in,the, western,Mid -Pacific,Mountains,(21°21N,174°40E,Fig, 2 1),The,cored,early,;ptian,sediments, (Fig, 2 2),consist,of,pelagic,limestones,marlstones,claystones,and,volcanic, ash,layers,(Thiede, et,al,1981),Three,samples,were,chosen,for,this,study,(Table, 2 1),Their,carbonate,content,of, 20,to,60,wt%,is,typical,of,marls,or,clays,(Pettijohn,1975),The,material,was,dated,based,on, calcareous,nannofossil,and,foraminifera,biostra tigraphy,and,magnetostratigraphy,(Tarduno,et, al,1989,Erba,1994),In,order,to,check,the,coccolith,preservation,and,to,compare,it,with,that, of, the, North, Sea, and, the, LSB, material, three, additional, samples, of, DSDP, Site, 463, were, analysed,The,samples,were,t aken,next,to,the,original,ones,of,Erba,et,al,(2010),their,positions, are,625535,m,62698,m,and,634995,m
2.3 Methods
;,total,of,15,samples,from,four,sites,(see,Fig, 2 1),were,chosen,for,the,biometric,analysis, (see,Table, 2 1),;ll,samples,yield B.constans,Z.erectus and, W.barnesiae (see,Figs, 2 3;,B), For,the,northern,mid -latitude,sections,(North,Sea,and,LSB),new,data,were,generated,For,the, low -latitudes,(western,Tethys,and,Mid -Pacific),we,reprocessed,the,data,collected,by,Erba,et, al, (201 0), Three, additional, samples, from, DSDP, Site, 463, were, used, for, comparing, the, nannofossil,preservation,Sizes,were,measured,on,a,calibrated,transmitted,light,microscope, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 37 mounted,with,a,digital,camera,Species,identification,followed, the,same,criteria,as,in,Er ba,et, al,(2010)
0)
Figure,23,(;),Cross -polarized,transmitted,light,microscope,and,(B),scanning,electron,microscope,(SEM),images, of,the,studied,species,Transmitted,light,microscope,and,SEM,images,of,all,three,specimens,are,derived,from, North,Jens -1, samples,(C),Idealized,elliptical,coccolith,outline,Width,=,minor,axis,length,=,major,axis,of,the, ellipse,Measurements,were,only,performed,on,flat -lying,coccoliths,(D),Light,microscope,image,indicating,length, of,coccolith,and,of,central,area
For,the,d ata,newly,generated,in,this,study,coccoliths,were,investigated,in,settling,slides, with,the,preparation,technique,of,Geisen,et,al,(1999),Coccoliths,were,randomly,chosen,along, a,transect,across,each,slide,under,crossed,polarized,light,using,an,Olympus,B X51,transmitted, light, microscope, ;t, least, 50, specimens,per, sample,of,each,of,the, three, coccolith, species, analyzed,here,were,digitally,captured,with,a,ColorView,II,camera,at,a,magnification,of,x2000, The,digital,images,were,then,used,to,measure,the,maximum length,and,width,of,the,individual, specimen,(see,Fig, 2 3C),using,an,Olympus,semi -automatic,analySIS,software,The,ellipticity, of,the,coccoliths,is,calculated,as,the,ratio,of,coccolith,length,over,coccolith,width,
Both,the,new,and,previously,published,d ata,(Erba,et,al,2010),were,evaluated,statistically, using,P;ST,30,(Hammer,et,al,2001),For,comparison,purposes,the,size,data,of,the,individual, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 38 species,were,summarized,for,each,of,the,four,sites,The,data,sets,were,then,tested,for,normal, distribution, o f, each, parameter, (eg, width, of, W, barnesiae), using, the, Shapiro -Wilks, and, ;nderson -Darling, tests, Each, test, checks, the, null, hypothesis, stating, that, the, sample, was, derived,from,a,population,with,normal,distribution,providing,the,return,value,p,(normal), The, null,hypothesis,is,rejected,when,p,(normal),is,less,than,005,To,test,for,significant,differences, in, the, mean, size, of, the, individual, species, throughout, the, studied, localities, two, tests, can, potentially,be,applied,The,;NOV;,Tukeys,pairwise,post -hoc,t est,is,used,if,the,data,show, normal,distribution,and,have,similar,variances,This,test,indicates,the,probability,(p(equal)), that, the, mean, of, one, parameter, is, equal, at, the, two, localities, which, are, compared, The, Kruskal -Wallis,Mann -Whitney,pairwise,post -hoc test,is,employed,for,the,same,purpose,for, those,samples,that,show,a,non -normal,distribution,;,threshold,value,of,p<0001,was,chosen, to,detect,significant,differences,of,means,Basic,statistical,values,such,as,mean,median,25 th and,75 th percentile,and,t otal,range,of,values,were,calculated,
In,order,to,test,if,diagenetic,alteration,affected,the,reported,size,of,coccoliths,at,least,50, specimens,of, W.barnesiae per,sample,were,measured,in,material,from,the,North,Sea,the,LSB, and, additional, three, Mid -Pacif ic, samples, The, overall, coccolith, width, and, length, and, the, dimensions,of,the,central,area,were,recorded,(see,Fig, 2 3D),The,ratio,of,total,coccolith,length, over,length,of,central,area,was,calculated,(L/L central,area ),SEM,images,of, W.barnesiae in,two, roc k,samples,from,the,North,Sea,(22542,m),and,the,Mid -Pacific,(625535,m),were,studied,to, document,the,state,of,preservation,SEM,images,were,captured,using,a,Zeiss,Leo,Gemini,1530, High,resolution,thermally,aided,Field,Emission,SEM ,Coccoliths,of,W, barnesiae,were,chosen, to,assess,the,state,of,preservation,due,to,their,high,abundance,in,the,studied,samples
2.4 Results
2. 4.1Widthandlength
;ccording,to,the,Shapiro -Wilks,and,;nderson -Darling,tests,for,normality,(see,;ppendix, Table,; 2 1),five,out,of,six,data -sets,do,not,show,a,normal,distribution,The,Kruskal -Wallis, Mann -Whitney, pairwise, post -hoc, test, was, therefore, applied, to, these, five, data -sets, The, results,of,these,tests,are,given,in,Table, 22,and,Figure 2 4,Despite,the,small,amoun t,of,data, points,for,some,of,the,species,or,sites,the,coccolith,size,data,show,prominent,features Chapter 2 – Size variations of coccoliths in Cretaceous oceans 39
Table,22,Probability,p(equal),of,two,compared,data -sets,from,populations,with,equal,means,in,width,and,length,
Width Length Pacific Pacific - - B. constans LSB Tethys western Mid Sea North LSB Tethys western Mid North Sea North
North Sea 1 3,086E -11 3,387E -17 5,684E -24 1 9,659E -14 1,124E -17 1,407E -22
LSB 1 5,956E -23 1,656E -29 1 1,102E -24 2,3E -29
Western Tethys 1 0,01934 1 0,0558
Mid -Pacific 1 1
Z. erectus
North Sea 1 2,559E -21 0,001212 2,635E -21 1 9,853E -23 2,695E -08 6,048E -22
LSB 1 1,147E -12 6,36E -35 1 5,877E -16 1,239E -35
Western Tethys 1 0,00102 1 0,12
Mid -Pacific 1 1
W. barnesiae
North Sea 1 0.07236 1.13E -05 0.9989 1 8.724E -06 5.45E -05 0.9797
LSB 1 0.05278 0.04961 1 0.233 0.001027
Western Tethys 1 9.339E -06 1 0.002165
Mid -Pacific 1 1
Bold numbers are significant differences with p(equal)<0.001.
For, B.constans ,the,data,spread,is,much,wider,in,the,western,Tethys,and,the,Mid -Pacific, than,in,the,North,Sea,and,the,LSB,The,range,of,the,data,for,the,coccolith,width,is,43,µm,in, the, western, Tethys, and, 41, µm, in, the, Mid -Pacific, for, the, length, 48, µm, and, 44, µm, resp ectively,In,the,North,Sea,and,the,LSB,coccolith,width,ranges,are,18,µm,and,20,µm,, length,ranges,are,25,µm,and,24,µm,Very,small,coccoliths,with,dimensions,of,up,to,3,µm,x, 3,µm,(width,x,length),are,present,at,all,sites,while,coccoliths,larger,than, 39,µm,x,52,µm, (width, x, length), are, solely, present, in, the, western, Tethys, and, the, Mid -Pacific, The, mean, coccolith,size,is,lowest,in,the,LSB,(26,µm,x,33,µm),and,the,North,Sea,(29,µm,x,38,µm),and, significantly,higher,in,the,western,Tethys,(40,µm,x,50, µm),and,the,Pacific,(44,µm,x,54,µm), Chapter 2 – Size variations of coccoliths in Cretaceous oceans 40
Figure,24,Scatter,plots,of,width,and,length,of,the,total,coccolith,of,the,three,studied,species,B,constans,Z, erectus,and,W,barnesiae,Numbers,of,measurements,(N),are,given,Mean,values,are,illustrated,by,cro sses,cross, dimensions,indicate,standard,deviations,Data,for,the,North,Sea,the,LSB,and,partially,for,the,Mid -Pacific,were, generated,in,this,study,Data,for,the,western,Tethys,and,the,Mid -Pacific,derive,from,Erba,et,al,(2010)
For, Z.erectus ,the,spread,of,data,is,similar,for,all,four,sites,Small,coccoliths,(smaller,than, 2,µm,x,3,µm),are,exclusively,present,in,the,North,Sea,and,the,LSB,Coccoliths,of, Z.erectus larger,than,34,µm,x,45,µm,(width,x,length),were,only,found,in,the,western,Teth ys,and,the, Mid -Pacific,Mean,sizes,are,lowest,in,the,LSB,(20,µm,x,29,µm),and,the,North,Sea,(23,µm,x, 34,µm),and,larger,in,the,western,Tethys,(26,µm,x,39,µm),and,the,Pacific,(29,µm,x,41,µm)
Sizes,of, W.barnesiae are,more,variable,in,the,western,Teth ys,and,the,Pacific,than,in,the, North,Sea,and,the,LSB,The,range,of,coccolith,width,is,45,µm,in,the,western,Tethys,and,47, µm,in,the,Mid -Pacific,while,length,ranges,are,55,µm,and,56,µm,respectively,In,the,North, Sea,and,the,LSB,samples,coccolith,width ranges,are,31,µm,and,34,µm,length,ranges,are,35, µm, and, 39, µm, The, data, points, show, that, the, smallest, and, the, largest, coccoliths, of, W. Chapter 2 – Size variations of coccoliths in Cretaceous oceans 41 barnesiae are,present,in,the,western,Tethys,and,Mid -Pacific,Mean,sizes,(width,x,length),are, similar,at,all,sites,(N orth,Sea,53,µm,x,64,µm,LSB,52,µm,x,62,µm,Mid -Pacific,53,µm,x,64, µm,western,Tethys,50,µm,x,61,µm)
2. 4.2Ellipticity
;ll,ellipticity,data,(coccolith,length,over,coccolith,width),are,not,normally,distributed,(see, ;ppendix,Table,;, 2 2),The,Kruskal -Wallis,Mann -Whitney,pairwise,post -hoc,test,was,applied, to,check,for,significant,differences,of,means,(see,Table, 2 3),The,ellipticity,data,of,both W. barnesiae and, Z.erectus do,not,show,any,evidence,of,inequality,The,mean,ratio,of,coccoli th, length,over,coccolith,width,of, W.barnesiae ranges,between,119,and,124,(North,Sea,120, LSB,119,western,Tethys,124,Mid -Pacific,120),For, Z.erectus ,the,values,vary,between,144, and,154,(North,Sea,145,LSB,146,western,Tethys,154,Mid -Pacific,144),
The,coccoliths,of, B.constans are,significantly,more,circular,at,the,low -latitude,sites, – in,the, Mid -Pacific,(124),and,the,western,Tethys,(128), - and,more,elliptical,in,the,North,Sea,and,LSB, samples,(132,and,130,respectively),(see,Fig , 2 5,left),The,three,studied,species,show,a,slight, trend, towards, more, circular, shapes, with, larger, sizes, (see, Fig, 2 5, right), The, regression, coefficient,is,low,(R 2=017),as,the,variation,of,values,is,high
2. 4.3Centralareaof W. barnesiae
;,total,of,477,specimens,of, W.barnesiae were,measured,in,nine,samples,from,three,sites, (North,Sea,LSB,Mid -Pacific),The,total,coccolith,length,was,plotted,versus,the,ratio,of,total, coccolith, length, over, length, of, central, area, (L/L central, area ), In, none, of, the, sites, L/L central, area correlates,with,total,coccolith,length,It,shows,constant,values,of,,21,(means,North,Sea, 210,LSB,206,Mid -Pacific,211),for,all,size,classes,(see,Fig, 2 6;,B,and,C),For,all,three,data, sets, the, regression, lines, exhi bit, a, slightly, positive, but, very, low, slope, the, regression, coefficients,are,low,(North,Sea,R 2=00015,LSB,R 2=00012,Mid -Pacific,R 2=00034), Chapter 2 – Size variations of coccoliths in Cretaceous oceans 42
Table,23,Probability,p(equal),of,two,compared,data -sets,from,populations,with,equal,means,in,ellipticity,
Ellipticity Pacific - B. constans North Sea North LSB Tethys western Mid
North Sea 1 0,09817 0,00082 6,194E -08
LSB 1 0,02801 7,894E -06
Western Tethys 1 0,03812
Mid -Pacific 1
Z. erectus
North Sea 1 0,9635 0,02799 0,2895
LSB 1 0,03376 0,2497
Western Tethys 1 0,04677
Mid -Pacific 1
W. barnesiae
North Sea 1 0,1931 0,1178 0,5224
LSB 1 0,02098 0,6184
Western Tethys 1 0,06007
Mid -Pacific 1
Bold numbers are significant differences with p(equal)<0.001. Chapter 2 – Size variations of coccoliths in Cretaceous oceans 43
Figure,25,Box,plots,and,scatter,plot,of,coccolith,ellipticity,versus,coccolith,width,of,the,three,studied,species, Numbers,of,measurements,(N),and,regression,coefficients,are,given,Coccoliths,of,all,three,studied,species,tend, to,be,s haped,more,circular,with,increasing,size Chapter 2 – Size variations of coccoliths in Cretaceous oceans 44
Figure,26,(;),to,(C),Scatter,plots,of,total,coccolith,length,of, W.barnesiae versus,the,ratio,of,total,coccolith,length, over,length,of,central,area,(L/L central,area ),of,the,North,Sea,LSB,and,the,Mid -Pacific, data,(3,samples,per,data,set), Samples,from,the,Mid -Pacific,were,requested,from,the,IODP,Kochi,Core,Center,and,are,located,close,to,the, original,samples,of,Erba,et,al,(2010),(D),Unaltered,coccolith,outline,versus,coccolith,affected,by,overgrowth, Subsequ ent,crystal,growth,of,coccolith,shield,elements,after,its,intracellular,formation,would,have,enlarged,the, total,coccolith,dimensions,and,increased,the,ratio,L/L central,area ,;,stable,ratio,for,all,total,coccolith,sizes,might, indicate,the,same,intensity,or, the,lack,of,overgrowth,on,all,specimens,Similar,mean,ratios,at,different,sites, suggests,a,similar,state,of,preservation Chapter 2 – Size variations of coccoliths in Cretaceous oceans 45
2. 4.4SEMimages
Rock,fragments,from,the,North,Sea,and,the,Mid -Pacific,were,studied,under,the,SEM,to, visualize,the,state,of,nannofossil,preservation,The,SEM,analysis,revealed,that,coccoliths,of, W. barnesiae are, not, perfectly, preserved, but, no, intense, overgrowth, or, dissol ution, was, observed,(Fig, 2 7,;,to,H),In,some,specimens,single,crystal,elements,of,the,shield,may,be, elongated,(eg,Fig, 2 7 B),Small,clay,particles,and,secondarily,grown,crystals,occur,frequently, on,top,of,the,coccolith,shields,in,the,samples,from,the North,Sea,and,the,Mid -Pacific,(eg,Fig, 2 7 ;,C,D,F,and,G),Broken,elements,are,rare,(eg,Fig, 2 7,E),The,representative,collection, of,SEM,images,indicates,similar,states,of,nannofossil,preservation,in,the,North,Sea,and,the, Mid -Pacific,rocks
Fi gure,27,SEM,images,on,rock,fragments,from,(; -D),the,North,Sea,(North,Jens -1,22542,m),and,from,(E -H),the, Mid -Pacific,(DSDP,Site,463,625535,m),captured,by,a,Zeiss,Leo,Gemini,1530,High,resolution,thermally,aided,Field, Emission,SEM,performed,at,Ruhr -Uni versität,Bochum,Images,show,specimens,of, W.barnesiae to,display,the, range, of, preservation, of, nannofossils, in, the, samples, The, images, represent, the, entire, range, of, nannofossil, preservation,states,present,in,the,studied,samples,;rrows,point,to,areas,of,overgrowth,to,clay,particles,or, broken,elements,Scale, bars,=,2,µm,
2.5 Discussion
2. 5.1Diagenesis
Overgrowth,or,dissolution,of,single,coccoliths,may,distort,the,results,of,biometric,studies, This,particularly,applies,when,data,from,sites,with,different,preservation,are,being,compared, In, combination, with, l iterature, data, some, observations, of, this, study, may, be, explained, by, different,site -related,preservation,modes,of,the,nannofossils, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 46
Size,data,of, B.constans show,a,larger,spread,and,a,larger,mean,size,in,the,western,Tethys, and,the,Mid -Pacific,than,in,the,North,Sea,and,the,LSB,The,assemblages,of,the,latter,two,sites, are,characterized,by,high,abundances,of,small,specimens,Partial,dissolution,of,small -sized, sp ecimens,at,the,two,low -latitude,sites,could,have,theoretically,decreased,the,total,and,the, relative, abundance, of, B. constans. This, may, potentially, explain, the, low, abundances, of, B. constans in,the,western,Tethys,(06%),and,the,Mid -Pacific,(3%),in,comparison to,those,of,the, North,Sea,(10%),and,the,LSB,(7%),(see,;ppendix,Table,; 2 3)
The, Z.erectus data,set,shows,high,abundances,of,large,coccoliths,in,the,western,Tethys, and,the,Mid -Pacific,and,high,abundances,of,small,ones,in,the,North,Sea,and,the,LSB,The, de licate, structure, of, Z. erectus may, explain, the, selective, and, total, dissolution, of, small, specimens,(smaller,than,,2,µm,x,3,µm),in,the,two,low -latitude,sites,
Coccoliths,of, W.barnesiae are,of,equal,mean,sizes,at,all,sites,a,larger,spread,of,size,data, was observed,in,the,western,Tethys,and,the,Mid -Pacific,Total,dissolution,of,small,specimens, in,the,North,Sea,and,the,LSB,on,one,hand,and,overgrowth,of,coccoliths,in,the,western,Tethys, and,the,Mid -Pacific,on,the,other,hand,can,be,responsible,for,the,data
Bas ed,on,SEM,images,the,preservation,of, W.barnesiae is,similar,at,the,North,Sea,and,the, Mid -Pacific,significant,dissolution,or,overgrowth,were,not,recorded,Scatter,plots,of,coccolith, length,versus,L/L central,area of, W.barnesiae (Figs, 2 6,;,to,C),reveal,n o,correlation,between,both, parameters,The,ratio,L/L central,area does,not,strongly,change,with,length,which,indicates,that, the,state,of,preservation,is,similar,for,all,coccoliths,from,one,site,Regression,lines,also,suggest, no,differences,in,preservation, (means,North,Sea,210,LSB,206,Mid -Pacific,211),;,detailed, biometric,study,on,3000,specimens,of, W.barnesiae coccoliths,performed,by,Bornemann,and,
Mutterlose,(2006),gave,a,mean,L/L central,area ratio,of,21,This,observation,is,in,good,accordance, wi th,the,current,results,of,our,study,(see,Fig, 2 6),suggesting,that,the,data,obtained,here,were, not,significantly,altered,by,diagenesis,It,is,therefore,reasonable,to,use,our,size,data,from,the, four,different,sites,for,comparison,
The, presence, of, dissolution -prone, coccolith, taxa, ( Biscutum spp, Zeugrhabdotus spp, Discorhabdus spp, Stephanolithiales), in, the, North, Sea, and, LSB, samples, supplies, further, evidence, against, intense, dissolution, The, dissolution, susceptible, species, Rotelapillus crenulatus makes,up,to,13,%,in,the,North,Sea,and,24,%,in,the,LSB,(see,;ppendix,Table,; 2 3), Chapter 2 – Size variations of coccoliths in Cretaceous oceans 47
Nannofossil,species,richness,(see,;ppendix,Table,; 2 3),is,elevated,in,the,North,Sea,with,a, mean,of,22,while,in,the,LSB,the,western,Tethys,and,the,Mid -Pacific,means,are,14 ,15,and,17, species, per, sample, For, the, three, latter, sites, the, balanced, diversities, suggest, that, the, nannofossil,assemblages,were,not,strongly,altered,by,dissolution
Nannofossil,preservation,is,reported,to,be,best,in,marlstones,with,carbonate,contents,o f, ,40,wt%,to,55,wt%,Overgrowth,may,be,strong,in,limestones,dissolution,may,have,biased, the,nannofossil,record,in,sediments,with,a,low,carbonate,content,(eg,Thierstein,and,Roth, 1991, Mattioli, 1997), Based, on, the, carbonate, content, five, samples, studi ed, here, are, considered,marls,(North,Jens -1,22542,m,;dda -2,23706,m,Cismon,2413,m,DSDP,Site,463, 62696,m,and,63498,m),four,samples,show,a,lower,carbonate,content,(three,;lstätte,samples, and,one,from,DSDP,Site,463,6255,m),and,three,samples,have, higher,values,(North,Jens -1,
22535,m,and,Cismon,2616,m,and,3008,m),There,is,no,clear,correlation,of,CaCO 3-content, and,coccolith,size,The,carbonate,content,varies,from,sample,to,sample,at,a,given,site,while, coccolith,sizes,remain,constant
2. 5.2 Palecologyandgenetics
Surveys,of,DN;,variation,have,revealed,that,a,number,of,traditional,species,actually,consist, of,several,“cryptic,species”,(Knowlton,1993, de,Vargas,et,al,2004,Sáez,and,Lozano,2005) , These,cryptic,species,may,belong,to,different evolutionary,lineages,even,if,they,cannot,be, distinguished, morphologically, (Sáez, and, Lozano, 2005), This, phenomenon, is, described, for, many, planktonic, groups, including, coccolithophores, diatoms, and, foraminifera, Cryptic, species,that,can,be,differentiated by,small,morphological,differences,such,as,size,are,called, pseudo -cryptic, species, (Geisen, et, al, 2002, Sáez, and, Lozano, 2005), These, pseudo -cryptic, species,may,occupy,different,habitats,(Sáez,et,al,2003),De,Vargas,et,al,(2004),pointed,out, that,base d,on,recent,genetic,analyses,the,existence,of,traditional,cosmopolitan,unicellular, planktonic,species,with,significant,phenotypic,variability,is,highly,unlikely,if,not,impossible,In, other, words, what, has, traditionally, been, called, an, ecophenotype, is, most, likely, an, actual, species,
;ccording, to, our, data, the, different, sizes, of, coccoliths, may, represent, several, cryptic, species,Due,to,the,lack,of,preserved,organic,tissue,it,is,of,course,impossible,to,prove,this, hypothesis,Size,variations,within,a,single,cry ptic,species,are,only,minor,and,shifts,in,coccolith, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 48 size,in,recent,species,as,well,as,in,the,fossil,record,might,therefore,be,related,to,genotypic, variation,(eg,Thierstein,and,Young,2004,Schmidt,et,al,2006),The,relatively,large,size,range, of, B. cons tans and, W. barnesiae in, the, western, Tethys, and, the, Mid -Pacific, suggests, the, presence,of,a,higher,number,of,cryptic,species,than,in,the,North,Sea,and,the,LSB,This,is,in, accordance, with, latitude -dependent, inter -specific, diversity, patterns, of, recent, coccoli thophores,recorded,by,McIntyre,and,Bé,(1976),in,the,;tlantic,Ocean,(Jordan,2009),
2. 5.2.1Sea -surfacetemperature
;,number,of,studies,demonstrated,that,sea -surface,temperature,may,control,the,size,of, individual,coccoliths,and,the,dimensions,of,the,coc cosphere,Sorrosa,et,al,(2005),described, that, coccolithophore, cells, were, enlarged, and, intracellular, calcification, was, stimulated, in, specimens, of, E. huxleyi and, Gephyrocapsa oceanica when, they, were, exposed, to, lowered, temperatures, Herrmann, and, Thierstein, ( 2012), and, Herrmann, et, al, (2012), analysed, size, patterns,of,all,oval,to,circular,coccoliths,in,Cenozoic,and,Holocene,sediments,from,various, locations,Decreasing,mean,sizes,of,the,nannofossil,assemblage,were,recorded,from,the,early, Cenozoic, (Paleocene, and, E ocene), to, the, Holocene, paralleled, by, the, Cenozoic, cooling, (Herrmann, and, Thierstein, 2012), Highest, size, variability, and, largest, coccolith, sizes, were, observed, in, high -latitude, sediments, This, phenomenon, may, be, explained, by, varying, abundances,of,several,spe cies,with,different,sizes,(Herrmann,and,Thierstein,2012),Coccolith, sizes,of,individual,genera,and,the,annual,mean,sea -surface,temperature,show,no,linear,or, uniform,relationships,for,the,different,genera,(Herrmann,et,al,2012),;,simple,correlation,of, co ccolith,size,and,sea -surface,temperature,is,therefore,not,supported
If, sea -surface, temperature, had, an, impact, on, the, coccolith, sizes, discussed, here, the, dominance,of,small, B.constans specimens,in,the,North,Sea,and,the,LSB,might,suggest,that, the, small -size d, cryptic, species, preferred, ecological, niches, with, cooler, sea -surface, temperatures,The,larger,forms,of,the,low -latitude,settings,might,have,then,preferred,niches, with,warmer,conditions,This,is,in,accordance,with,the,study,of,Bornemann,and,Mutterlose, (200 6),who,reported,a,mean,size,decrease,of, B.constans along,with,a,temperature,decrease, in, the, late, ;lbian, of, the, western, Tethys, Their, findings, also, suggest, a, link, of, higher, temperatures,and,larger, B.constans coccoliths,In,our,case,the,presence,of,small and,large, cryptic, species, at, the, low -latitude, sites, suggests, a, higher, range, of, temperatures, The, Chapter 2 – Size variations of coccoliths in Cretaceous oceans 49 observation,that,solely,small,coccoliths,were,present,in,the,North,Sea,and,the,LSB,suggests,a, smaller, temperature, range, dominated, by, low, temperatures, This, te mperature, distribution, may,be,found,within,the,temperature,gradients,of,the,photic,zone,in,recent,and,past,oceans, In,the,low -latitudes,the,uppermost,part,of,the,photic,zone,is,warm,temperature,decreases, downwards, In, the, mid -latitudes, the, temperature, a lso, decreases, downwards, but, the, uppermost,part,of,the,photic,zone,is,cooler,than,the,corresponding,levels,in,the,low -latitudes, (Pickard,and,Emery,1991),
For,the,stratigraphic,interval,studied,here,(C2 -segment,Fig, 2 2),the,following,sea -surface, temperatures,have,been,reconstructed,The,TEX 86 -data,reported,by,Mutterlose,et,al,(2014), imply,a,temperature,of,28°C,for,the,LSB,Higher,early,;ptian,temperatures,were,suggested, by,TEX 86 -based,findings,of,,34°C,for,the,pre -O;E,1a,interval,from,the,equ atorial,Shatsky,Rise, tropical,Pacific,(Dumitrescu,et,al,2006),These,data,suggest,a,temperature,gradient,of,surface, waters,of,more,than,6°C,over,a,latitude -range,of,40°,(LSB,39°N,to,Shatsky,Rise,equatorial)
Coccoliths,of, Z.erectus follow,the,size, pattern,of, B.constans with,small,specimens,in,the, mid -latitudes,and,smaller,ones,further,south,This,may,indicate,a,preference,of,small,cryptic, species,for,cold,waters,unless,diagenesis,heavily,affected,the,size,variation,of, Z.erectus ,Sizes, of, W.barne siae display,a,larger,spread,of,data,in,the,low -latitude,sites,compared,to,the,North, Sea,and,the,LSB,but,mean,sizes,are,similar,This,may,suggest,that,the,coccolith,size,of, W. barnesiae is,not,sensitive,to,sea -surface,temperature,This,lack,of,correlation between,size,and, sea -surface,temperature,is,supported,by,the,studies,of,Bornemann,and,Mutterlose,(2006),and, Linnert, and, Mutterlose, (2013), Data, from, the, late, ;lbian, revealed, that, coccoliths, of, W. barnesiae remained,stable,in,size,during,a,cooling,event,(B ornemann,and,Mutterlose,2006), ;,size,analysis,of, Watznaueria spp,assemblages,from,the,early,Turonian,which,included,60, to, 70%, W. barnesiae, showed, no, response, to, a, contemporaneous, warming, (Linnert, and, Mutterlose,2013) Chapter 2 – Size variations of coccoliths in Cretaceous oceans 50
Figure,28,Schematic,of, coccolith,size,cryptic,species,diversity,and,ecological,parameters,in,the,western,Tethys, and,the,Mid -Pacific,versus,the,North,Sea,and,the,LSB,for,(; )B.constans and,(B), W.barnesiae ,(1),illustrates,the, schematic,coccolith,width,versus,coccolith,length,p atterns,as,shown,in,figure,4,(2),delineates,the,potential,link, between,coccolith,size,coccolithophore,cryptic,species,diversity,and,the,temperature,of,the,sea -water,while,(3), refers,to,light,availability,and,the,extension,of,the,photic,zone,The,vertica l,arrows,indicate,greater,or,lower, diversities,The,species, Z.erectus is,not,referred,to,here,as,its,coccolith,size,pattern,was,most,likely,severely, affected,by,preservation Chapter 2 – Size variations of coccoliths in Cretaceous oceans 51
2. 5.2.2Lightavailabilityandtrophicload
Light,is,attenuated,with,increasi ng,water -depth,due,to,scattering,and,absorption,by,water, salt,molecules,single -celled,organisms,and,suspended,particles,(Kirk,1977,Sverdrup,et,al, 2006), Clear, waters, show, a, greater, light, penetration, than, “muddy”, waters, and, as, a, consequence, the, for mer, have, a, deeper, photic, zone, The, two, low -latitude, sites, (western, Tethys,and,Mid -Pacific),are,open -oceanic,missing,substantial,clastic,input,sun,light,had,a,deep, penetration,resulting,in,an,extended,photic,zone,The,two,mid -latitude,sites,(North,Sea,and, LSB),were,located,close,to,the,paleocoast,Enhanced,continental,run -off,and,nutrient,input, along, with, enhanced, productivity, resulted, in, a, shallow, photic, zone, With, B. constans coccoliths,being,smaller,in,the,North,Sea,and,the,LSB,it,may,be,hypothesized,that,small -sized, cryptic,species,preferred,attenuated,light,conditions,and/or,elevated,nutrient,concentrations, Large,cryptic,species,appear,to,have,competed,more,successfully,in the,open,ocean,and,its, very,clear,nutrient -depleted,waters,They,outcompeted,the,small,cryptic,species,which,were, forced,to,occupy,the,deeper,and,darker,photic,zone,(Fig, 2 8 ;),The,large,data,spread,of, W. barnesiae in, the, open -oceanic, settings, indicates an, increased, diversity, of, cryptic, species, relatively,to,the,coastal,sites,(Fig, 2 8 B)
2. 5.3Ellipticity
Coccoliths,of, B.constans are,more,circular,and,larger,at,the,low -latitude,sites,and,more, elliptical,and,smaller,in,the,mid -latitudes,This,positive,correlation,of,coccolith,size,and,shape, has,been,observed,in,several,studies,(eg,Mattioli,et,al,2004,Bornemann,and,Mutterlose, 2009),For, W.barnesiae ,the,relation,of,coccolith,size,and,shape,only,becomes,evident,when, the,entire,data,set,is,considered,(Fig, 2 5,right),No,site -related,ellipticity,variation,has,been, observed,Henderiks,(2008),pointed,out,that,larger,placoliths,of Cenozoic,age,are,more,circular, than,smaller,ones,This,may,be,related,to,their,allometric,growth,patterns,width,increases, faster,than,length,during,growth,(Young,1989),Ellipticity,depends,on,the,growth,of,coccoliths, in,the,first,place,and,does,not,seem to,be,directly,influenced,by,the,environment,itself,The, significant,discrepancy,in,coccolith,shape,of, B.constans between,the,sites,therefore,does,not, yield,any,additional,information,The,relation,of,size,and,shape,of, Z.erectus has,been,tested, here, for the, first, time, ;s, the, coccoliths, have, been, severely, affected, by, diagenesis, no, conclusions,can,be,drawn,for, Z.erectus Chapter 2 – Size variations of coccoliths in Cretaceous oceans 52
2. 6Conclusions
Biometric, studies, were, performed, on, a, total, of, 1986, single, coccolith, specimens, of, B. constans,Z.erectus and W.barne siae from,four,sites,of,early,;ptian,age,The,obtained,data, allow,the,following,conclusions,
Coccolith,sizes,of, B.constans and, W.barnesiae show,a,larger,spread,in,the,western, Tethys,and,the,Mid -Pacific,than,in,the,North,Sea,and,the,LSB,Small,coccolith s,of, B.constans dominate,at,the,mid -latitude,sites,No,site -related,dominance,is,recorded,for, W.barnesiae , For, Z.erectus ,the,size,data,show,that,solely,small,coccoliths,are,present,in,the,North,Sea,and, the,LSB,large,forms,are,restricted,to,the,western Tethys,and,the,Mid -Pacific
Dissolution,of,small,coccoliths,or,size -enlarging,overgrowth,may,have,caused,a,shift,of, biometric,data,towards,greater,sizes,and/or,may,have,affected,the,range,of,data,No,evidence, for,extensive,diagenetic,alteration,of, W.barn esiae and, B.constans was,however,found,when, using,SEM,imaging,and,biometric,features,
The,size,pattern,of, Z.erectus ,however,appears,to,have,be en,affected,by,diagenesis, The, absence,of,small,specimens,at,the,low -latitudes,suggests,that,the,delicate,morphology,of, Z.erectus severely,suffered,from,dissolution,
Lithology,does,not,correlate,to,nannofossil,preservation
Shifts, in, coccolith, sizes, are, thought, to, be, related, to, genoty pic, variation, The, data, spread,of, B.constans and, W.barnesiae may,reflect,cryptic,species,diversity,Low,diversities, characterize,the,North,Sea,and,the,LSB,higher,ones,the,western,Tethys,and,the,Mid -Pacific, Small -sized, cryptic, species, of, B. constans pre ferred, or, more, successfully, tolerated, low, temperatures,or,low,light,availability,and,high,nutrient,levels,For, W.barnesiae no,correlation, of,coccolith,size,and,ecological,factors,has,been,observed Chapter 2 – Size variations of coccoliths in Cretaceous oceans 53
2.7 cknowledg ement s
We,would,like,to,thank,Dr,Frans,van,Buchem,(Mærsk,Oil,Copenhagen),and,Dr,Jon,R, Ineson, (Geological, Survey, of, Denmark, and, Greenland, Copenhagen), for, providing, core, material, of,the, wells, North, Jens -1, and, ;dda -2,The,staff,of, the, Kochi, Core, Center, (Japan), supplied, samples, of, DSDP, Site, 463, The, quality, of, the, manuscript, greatly, benefited, from, valuable,comments,by,the,editor,Richard,W,Jordan,and,the,reviewers,Silvia,Gardin,Jackie,;, Lees,and,Jeremy,R,Young,The,authors,appreciate,funding,by,the,DFG,(Mu,667 /46 -1), Chapter 2 – Size variations of coccoliths in Cretaceous oceans 54
2.8ppendix
Table,;,21,Results,of,Shapiro -Wilks,and,;nderson -Darling,tests,for,normal,distribution,of,coccolith,width,and, length,of,the,three,studied,species
Site N Shapiro -Wilks Anderson -Darling p(normal) p(normal)
North Sea 159 0.004427 0.002596 LSB 150 0.001268 0.02926
Width Western Tethys 90 0.05346 0.0722 Mid -Pacific 90 0.01046 0.004327
B.constans North Sea 159 0.1317 0.1163 LSB 150 0.004425 0.002421
Length Western Tethys 90 0.1104 0.1905 Mid -Pacific 90 0.003416 0.009176
North Sea 150 0.2417 0.2515 LSB 150 0.02556 0.1298
Width Western Tethys 30 0.04263 0.1032 Mid -Pacific 90 0.003416 0.1183
Z. erectus North Sea 150 0.6755 0.465 LSB 150 0.000003603 0.00006405
Length Western Tethys 30 0.1164 0.08514 Mid -Pacific 90 0.002262 0.0006176
North Sea 150 0.6109 0.5253 LSB 150 0.3222 0.7391
Width Western Tethys 150 0.5709 0.7992 Mid -Pacific 150 0.3696 0.4543
North Sea 150 0.08431 0.00009952 W. barnesiae LSB 150 0.1972 0.4216
Length Western Tethys 150 0.09216 0.02502 Mid -Pacific 150 0.5343 0.3393
Abbreviations: N=Number of measurements. Bold numbers are significant for non -normal distributed data with p(normal)<0.05. Chapter 2 – Size variations of coccoliths in Cretaceous oceans 55
Table,;,22,Results,of,Shapiro -Wilks,and,;nderson -Darling,tests,for,normal,distribution,of,coccolith,ellipticity,of, the,three,studied,species
Site N Shapiro -Wilks Anderson -Darling p(normal) p(normal)
North Sea 159 0.7543 0.421 LSB 150 0.5595 0.2392 Western Tethys 90 0.0005696 0.003804 B. constans B. Mid -Pacific 90 0.0007255 0.0005372
North Sea 150 0.04591 0.1645 LSB 150 0.00730 0.003476 Western Tethys 30 0.8703 0.8372 Z. erectus Z. Mid -Pacific 90 0.2526 0.2861
North Sea 150 1.87E -12 1.657E -12 LSB 150 0.008735 0.005476 Western Tethys 150 2.095E -09 4.952E -05
W. barnesiae W. Mid -Pacific 150 7.018E -11 1.358E -11
Abbreviations: N=Number of measurements. Bold numbers are significant for non -normal distributed data with p(normal)<0.05. Chapter 2 – Size variations of coccoliths in Cretaceous oceans 56 data: content, 2 sample: 2 - Bottini and Bottini References Bottini, 2013 Bottini, unpubl. Data unpubl. Age, Lithology, Age, and Mutterlose Lithology, Age, Lithology, Age, Lithology, Age, Erba et al., 2010 al., et Erba 1999 al., et Erba 2010 al., et Erba Mutterlose, 2012 Mutterlose, Nannofossil data: Nannofossil data: Nannofossil data: Nannofossil Nannofossil for Adda for Carbonate content, Carbonate content: Carbonate Carbonate content: Carbonate PhD thesis of C. Bottini C. of thesis PhD Bottini C. of thesis PhD 0% 2.0 % 2.0 % 2.4 0.3 % 0.3 % 1.1 % 0.3 Relative abundance R. crenulatus annofossil,counts % 4.6 % 4.6 % 2.0 % 5.7 % 3.3 % 1.0 % 3.4 % 1.0 0.68 % 0.68 % 0.35 % 1.04 6.89 % 7.88 Relative D. ignotus abundance 35.7% 38.3% 43.0% 41.7% 23.5% 48.8 % 48.8 % 67.8 Relative 73.96 % 73.96 % 52.48 % 56.07 % 51.37 % 49.33 abundance W.barnesiae - - erectus 7.1 % 7.1 % 2.0 % 2.7 1.01% 15.2 % 15.2 % 21.8 % 17.9 % 1.39 % 6.89 % 3.77 Relative Z. Z. abundance %),nannofossil,species,richness,(number,of,species,per,sample,based,on,counts, 6.0 % 6.0 % 4.4 8.5 % 8.5 10.5 % 10.5 % 11.7 % 10.3 % 0.34 % 0.35 % 1.04 % 1.64 % 2.74 % 6.71 Relative abundance B.constans 23 20 24 16 13 14 15 18 12 18 17 16 species richness Nannofossil 3 3 3 3 3 3 3 3 3 3 3 3 to clay(stone) to Lithology - marl(stone) marl(stone) marl(stone) marl(stone) marl(stone) according to according 4 wt% CaCO wt% 4 CaCO wt% 3 CaCO wt% 3 71 wt% CaCO wt% 71 CaCO wt% 58 CaCO wt% 43 CaCO wt% 48 CaCO wt% 80 CaCO wt% 80 CaCO wt% 21 CaCO wt% 56 CaCO wt% 46 pure clay(stone) pure clay(stone) pure clay(stone) pure marl to limestone to marl limestone to marl limey marl(stone) limey carbonate content carbonate marl literature Lithology claystone claystone claystone limestone limestone limestone limestone limestone limestone laminated laminated laminated mudstone mudstone mudstone claystone/ claystone/ claystone/ according to according carbonaceous carbonaceous carbonaceous carbonaceous carbonaceous of 80 80 205 205 209 205 203 201 110 110 110 110 measured Number specimens Age NC 6 NC 6 NC NC 6 NC 6 NC 6 NC 6 NC BC 18 BC 18 BC 18 BC 18 BC 18 BC 18 BC 1 1 - - 2 - Pacific Pacific Pacific - - - Italy Italy Italy Germany Germany Germany - - - Adda Cismon Cismon Cismon Location Alstätte 1 Alstätte 1 Alstätte 1 Alstätte North Sea North Sea North Sea North Mid Mid Mid North Jens North Jens North NW NW NW DSDP Site 463 Site DSDP 463 Site DSDP 463 Site DSDP ;,23,;dditional,data,on,the,studied,samples,including,carbonate,content,(in,wt able, T of,300,specimens),and,relative,abundances,of,the,three,studied,species,and,of,other,species,mentioned,in,the,text,based,on,n 3.53 m 3.53 m 3.33 m 3.15 24.13 m 24.13 m 26.16 m 30.08 m 625.5 Samples 2253.5 m 2253.5 m 2254.2 m 2370.6 m 626.96 m 634.98 Chapter 2 – Size variations of coccoliths in Cretaceous oceans 57 Chapter 2 – Size variations of coccoliths in Cretaceous oceans 58
Figure,;,21,Nannofossil,events,at,all,studied,sections,Note,the,absence,and,inconsistency,of,marker,events,in, various,sections,This,is,due,to,the,lack,of,gateways,between,the,Tethys,and,the,North,Sea,and,LSB,during,the, Barremian, and, early, ;ptian, Corre lation, between, sections, based, on, bio -events, therefore, has, a, limited, assignment,of,broad,ages,Detailed,correlation,is,therefore,done,using,C -isotope,stratigraphy,Numerical,ages, assigned,to,bio -events,are,mainly,based,on,Tethys,sections,(see,calibration,re ferences,in,;nthonissen,and,Ogg, (2012), page, 1123), its, applicability, in, the, restricted, North, Sea, and, LSB, is, therefore, limited, Both, chemostratigraphy,and,biostratigraphy,suffers,from,similar,problems,such,as,sedimentation,rates,and,hiatuses, but,the,combin ed,approach,we,used,appears,to,work,best,Numerical,ages,are,based,on,;nthonissen,and,Ogg, (2012),and,Ogg,(2012),Gray,bars,indicate,non -recovered,intervals, Chapter 3 – The impact of OAE 1a on marine biota 59
3 The impact of OAE 1a on marine biota deciphered b y size variations of coccoliths
Nathalie Lübke 1, Jörg Mutterlose 1
1 Institute for Geology, Mineralogy and Geophysics, Ruhr -Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany
Abstract
The early Aptian Oce anic Anoxic Event 1a (OAE 1a; ~ 120 Ma) was caused by a global perturbation of the Early Cretaceous climate . It supposedly affected the composition of the marine biosphere, including the primary producers . This study aims at using the size e volution of three species of coccolithophores (Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae ) for better understanding the impact of the OAE 1a on primary producers. A total of 30 samples derived from three sites , which cover the late Barremian – early Aptian interval, have been analysed from the North Sea and the Lower Saxony Basin . The sections expose near -shore and shallow marine sediments .
The measured data of B. constans and Z. erectus are characterized by a size decrease synchrono us to the negative carbon isotope excursion . This size reduction marks the early phase of the OAE 1a , more precisely the carbon isotope segment C3. Coccolith sizes recover to pre -OAE 1a values in the aftermath of this brief interval . The short termed size reduction is seen as a reaction of specific nannofossil taxa to an increase in humidity during the early phase of OAE 1a. Due to high weathering rates and a substantial run -off, the amount of detrital material transported into the marine system increased s ignificantly. Consequently , light availability diminished in the surface waters, causing habitat changes for the photoautotrophic primary producers. Light -sensitive species like B. constans and Z. erectus adjusted by forming smaller skeletons, thereby redu cing their size. This strategy allowed for dwelling in shallower water depth and thus compensated for the decrease in sun light. The sizes of W. barnesiae in contrast remain stable throughout the entire OAE 1a interval. W. Chapter 3 – The impact of OAE 1a on marine biota 60 barnesiae was not affected by the se environmental shifts and is thus interpreted as being robust with respect to changes of the sun light.
3.1 Introduction
Biometrical studies on calcareous nannoplankton give insight into species -specific and even strain -specific adaptations to the habit at for many recent species (e.g., Batvik et al., 1997; Bollmann and Herrle, 2006; Bollmann, 1997; Green et al., 1998; Knappertsbusch et al., 1997; Langer et al., 2006, 2009; Renaud et al., 2002; Riebesell et al., 2004). These studies are, however, limited by the factor time, that they represent snapshots compared to the long -term trends revealed by the geological record. Biometrical studies on fossil species of calcareous nannofossils include taxonomic approaches, long -term ecological studies on timescales of several million years and responses to dramatic short -term climatic changes (e.g., Bornemann et al., 2003; Bornemann and Mutterlose, 2006; Erba et al., 2010; Fraguas and Erba, 2010; Fraguas and Young, 2011; Giraud et al., 2006, 2009; Herrmann et al., 20 12; Herrmann and Thierstein, 2012; Linnert and Mutterlose, 2009; 2013; Linnert et al., 2014; López -Otálvaro et al., 2012; Lübke, et al., 2015; Mattioli et al., 2004, 2009; Suchéraz -Marx et al., 2010). The Mesozoic oceanic anoxic events (OAEs) of the Aptian to Cenomanian interval represent such short -term climatic and paleoceanographic changes on a supra -regional (OAE 1b, c, d) to global scale (OAE 1a, 2) with widespread deposition of organic -rich sediments ( e.g., Jenkyns, 2010). The early Aptian OAE 1 a (~120 Ma; e.g., Li et al., 2008) is marked by the deposition of fine -grained organic -rich and carbonate -poor sediments at various locations (Bellanca et al., 2002; Dumitrescu and Brassell, 2005; Erba et al., 1999; Menegatti et al., 1998; Renard et al., 2005 ). Geochemically, it is described by a distinctive carbon isotope signature with a sharp negative excursion at its base (carbon isotope segment C3) and an adjacent step -wise shift (carbon isotope segments C4 to C6) to more positive values (Menegatti et al., 1998).
The emplacement of the Ontong Java Plateau in the central Pacific Ocean is seen as the major cause for the OAE 1a. The submarine volcanism released massive amounts of 13 C- depleted carbon possibly via mantle CO 2 outgassing and methane hydrate d estabilization into the Early Aptian atmosphere -ocean system (Méhay et al., 2009; van Breugel et al., 2007). This resulted in a rapid global warming pulse and caused accelerated continental weathering rates in the early phase of OAE 1a ; a climatic trend ev idenced by the Osmium and Lead isotope Chapter 3 – The impact of OAE 1a on marine biota 61 record (Bottini et al., 2012; Bottini et al., 2015; Erba et al., 2015; Kuroda et al., 2011 ; Mutterlose et al., 2014; Tejada et al., 2009). The resulting enhanced primary productivity led to an excessive deposition of organic matter , causing a shift of the carbon isotope curve to positive values (e.g., Jenkyns, 2010).
While the global nature of the OAE 1a is well established by now, it remains unclear to which extend calcareous nannofossils were affected by these envir onmental perturbations. For low latitude sites Erba et al. (2010) for example showed that the sizes of three calcareous nannofossil species sharply decrease as a response to OAE 1a (Cismon core at 20°N, DSDP Site 463 at 13 -20°S). In order to test the impact of the OAE 1a on mid -latitudinal calcareous nannofossil assemblages , we chose two settings in the Boreal Realm . In the Aptian, the North Sea and the Lower Saxony Basin (LSB ) were characterized by hemi -pelagic to coastal positions in a paleogeographi cally rather restricted basin . The sediments expose a characteristic lithological unit associated with the OE 1a, the “Fischschiefer ”. This distinctive marker horizon in combination with bio - and chemostratigraphy allows a detailed correlation with OAE 1a records from other basins and oceans. The biometrical data of three different coccolith species , which are common components of mid -Cretaceous coccolith assemblages ( Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae ), are compared to publ ished biometrical data and proxies for paleo -sea surface temperature and paleo -productivity .
3. 2 Material and methods
3. 2.1 Geological settings and studied sections
3. 2.1.1 North Sea (40°N paleolatitude)
Material for this study is derived from two cores drilled in the North Sea by Maersk Oil. The drill sites of North Jens -1 (55°50'N, 04°33'E) and Adda -2 (55°48'N, 04°50'E) are located in the center of the Central Graben of the North Sea. Situated about 15 k m apart, the two cores are positioned ~ 200 km off Denmark (Fig. 3. 1). The encountered sediments of Barremian to Aptian age, mainly marlstones and chalky limestones, are typical for a pelagic to hemipelagic setting (Kühnau and Michelsen, 1994).
The total thickness of the studied interval of the North Jens -1 core is 35 m (2275.3 m– 2240.3 m). The core, which consists primarily of marlstones, is interrupted by a prominent Chapter 3 – The impact of OAE 1a on marine biota 62 black shale layer (2252.8 m–2250.7 m) with elevated TOC contents of up to 8 wt% and low carbon isotope values (Mutterlose and Bottini, 2013).
The studied interval of the Adda -2 well covers 11 m (2373.2 m–2362.2 m). The lowermost 1.5 m (2373.2 m–2371.6 m) consist of chalky limestones followed by a gap of 0.9 m and 0.3 m of marlstones. A 5.2 m t hick layer of black shales (2370.4 m–2365.2 m) is present thereafter. The uppermost 3 m of the core comprise limestones (2365.2 m–2364.4 m) and marlstones (2364.4 m–2362.2 m).
Figure 3.1. Paleogeography of the mid -Cretaceous of northwest Europe. Sites: 1: North Jens1, 2: Adda -2, 3: Alstätte 1. Abbreviations: LSB: Lower Saxony Basin, PS: Pompeckj Swell. Map after Lübke et al. (2015).
3. 2.1.2 Lower Saxony Basin (36°N paleolatitude)
The Alstätte 1 outcrop is located in northwest Germany (52°09'N, 06°54'E; Fig. 3. 1). During the Early Cretaceous, the Lower Saxony Basin (LSB) which occupied parts of northwest Germany, was connected to the southern North Sea. Sediments deposited in the Alstätte Bay , located at the margin of the LSB (Hoffmann and Mutterlose, 201 1; Lehmann et al., 2012), represent a neritic setting with the coastline probably being 10 to 15 km off to the south . The studied section consists of 16 m of mudstones and marlstones . A 1.8 m thick , finely laminated dark mudstone horizon in the lower part of the succession forms a marker bed, the Fischschiefer ( FS ). This bed, rich in organic matter, is a distinctive horizon which can be followed throughout the entire LSB and the southern North Sea (Bottini and Mutterlose, 2012). Chapter 3 – The impact of OAE 1a on marine biota 63
Being bounded in the north b y the Pompeckj Swell and in the south by the Rhenish Massif the LSB was a geographically restricted sub -basin of the proto North Sea during the Early Cretaceous. Sedimentation patterns and the evolution of biota were therefore controlled by regional factor s causing the radiation of endemic taxa (Bischoff and Mutterlose, 1998; Mutterlose and Böckel, 1998; Bottini and Mutterlose, 2012). An anoxic environment, which prevailed throughout most of the late Barremian and early Aptian , was replaced by well oxygenat ed pelagic conditions in the late early Aptian (Keupp and Mutterlose, 1994).
3. 2.2 Stratigraphic correlation
The dating and the correlation of the three sites is based on chemostratigraphy,
13 lithostratigraphy and biostratigraphy (Fig. 3. 2).Theδ Ccarb record of the North Jens -1 core shows scattered negative values in the lower part of the laminated claystone succession, which can be correlated to the globally occurring characteristic negative stable carbon isotope shift (C3 segment) of OAE 1a (Menegatti et al., 1998). In the Adda -2 core, there are too few
13 δ Ccarb data to supply a reliable curve. Both cores have also been studied for lithostratigraphy and calcareous nannofossil biostratigraphy (Jeremiah, 2001; Mutterlose and Bottini, 2013; Thomsen and Je nsen, 1989). The successions in both cores consist of marlstones which are interrupted by a darker and organic -rich layer. This horizon has lithologically and biostratigraphically been correlated with the FS horizon of the classic German sections (Jeremiah , 2001; Thomsen and Jensen, 1989). The last occurrence (LO) of the Biscutum constans cavum influx below and the first occurrence (FO) of Eprolithus floralis above the organic -rich horizon in both cores indicate their synchronicity. Other important nannofos sil events strengthen the correlation of the North Sea and the LSB organic rich beds. These include the LOs of Nannoconus borealis and Nannoconus abundans and the FO of Chapter 3 – The impact of OAE 1a on marine biota 64 n dark grey. The extension of the of extension The grey. dark n nd lithostratigraphy (Jeremiah, 2001; Mutterlose and Bottini, 2013; e Italiane Selli Level (SL) and the Pacific Selli Level equivalent (SL eq.) are marked in grey. 2: Summary of calcareous nannofossil biostratigraphy, chemostratigraphy, a Figure Figure 3. i marked is interval studied The record. and Pacific Cismon the with compared sites, boreal forthe 1989) andJensen, Thomsen boreal Fischschiefer (FS), th Chapter 3 – The impact of OAE 1a on marine biota 65
Chiastozygus litterarius , which all occur shortly below the FS (Jeremiah, 2001; Mutterlose and Bottini, 2013).
13 Theδ Corg curve of the Alstätte 1 section shows a signature characteristic for the OAE 1a interval, thereby allowing a correlation with the global record (Bottini and Mutterlose, 2012). A distinctive negative shift (C3 segment) is followed by more positive values ( segment C4 to C6). The base of the 1.8 m thick FS coincides with the C3 segment of the global curve, the top of the FS lies within segments C4 to C6. The stratigraphic position of the FS is therefore precisely defined by geochemistry. The chemostratigraphy is supported by biostratigraphic findings. At all three sites the FO of E. floralis is post -dating the laminated FS. This bioevent occurs in segment C4 to C6 of the isotope curve. Throughout the North Sea and the LSB, the FO of E. floralis has been report ed from above the FS (Bischoff and Mutterlose, 1998; Bottini and Mutterlose, 2012; Habermann and Mutterlose, 1999). It is an important datum in Tethyan sections as well, where it has been described from above the FS equivalent beds (Selli Level) from Cismo n and DSDP Site 463. It is an extremely useful tool to correlate the three studied sections to the global record.
3. 2.3 Samples and biometry
A total of 30 samples has been selected from three localities (North Jens -1: 10 samples; Adda -2: 8 samples; Alstä tte 1: 12 samples) for the biometric measurements of B. constans , Z. erectus and W. barnesiae (Fig. 3. 3A , B). An aliquot of the studied core and outcrop material and the settling slides are stored at the Institute for Geology, Mineralogy and Geophysics at the Ruhr -Universität Bochum, Germany. Coccoliths have been investigated in settling slides prepared following the procedure described by Geisen et al. (1999). Coccoliths were measured along random transects across each slide under crossed polarized light, using an Olympus BX51 transmitted light microscope. A total of 50 specimens of each species per sample were digitally captured with a Colorview II camera at a magnification of x2000. A total of 4400 specimens (1500 specimens of B. constans; 1400 specimens of Z. erectus; 1500 specimens of W. barnesiae ) were analyzed for this study. The digital images were used to measure the maximum length and maximum width of the individual specimens (Fig. 3. 3C ), using an Olympus semi -automatic analySIS software. The data were checked for normal distribution using the Shapiro -Wilks test for normal distribution with PAST3 (Hammer et al., 2001). A Chapter 3 – The impact of OAE 1a on marine biota 66 normal distribution (or Gaussian distribution) indicates the presen ce of one size group, which can be very well characterized by its mean size. Other than that, a bimodal or polymodal distribution indicates the presence of two or more groups of coccoliths with different mean sizes. In the latter cases, the mean value of t he entire sample is less meaningful. The results of the normality test are listed in the supplementary data file. With the obtained size data for each sample and species, the mean length, mean width, and the standard deviation were calculated.
Figure 3.3 . (A) Light microscope and (B) SEM images of the three studied species. (C) shows an ideal coccolith outline and the measured parameters length and width. Light microscope images of all three specimens were taken on sample North Jens -1, 2272.6 m; SEM image s were taken on sample North Jens -1, 2274.3 m. Modified after Lübke et al. (2015). Chapter 3 – The impact of OAE 1a on marine biota 67
3. 3 Results
3. 3.1 Normal distribution
All species and samples show normal distribution except for Z. erectus in five Alstätte samples (3.33 m, 3.78 m, 3.88 m, 4.58 m, 5.18 m). The datasets have additionally been tested for polymodal or bimodal distribution using mixture analysis of PAST3. For the length dataset of sample 3.88 m, the analysis failed . The data were not even close to either unimodal or polymodal or bimodal distribution. For the other four data sets, mixture analysis, however, suggested that the best fit was a normal unimodal distribution.
3. 3.2 Biometry
The length and width relations of th e three analysed calcareous nannofossil species are discussed for each section.
3. 3.2.1 North Jens -1
At North Jens -1, the coccoliths of B. constans and Z. erectus show a size decrease associated with the FS interval (Fig. 3. 4). In the lower part of the co re (2274.3 m to 2253.5 m), the mean size of B. constans is constantly ~ 3.7 µm x 2.8 µm (length x width). The mean size abruptly decreases to 3.2 µm x 2.5 µm (2251.7 m) by 15% near the base of the FS and increases thereafter to the top of the studied inter val. The mean size of Z. erectus follows a similar trend with values of ~3.4 µm x 2.4 µm in the lower part followed by a size decrease of 10% to 3.1 µm x 2.1 µm (2251.7 m). After having reached its minimum the mean size increases. The mean size of W. barne siae remains constant, it varies around ~6.4 µm x 5.3 µm throughout the entire studied interval. One sample (2250.7 m) does not follow the trend with dimensions of 7.0 µm x 5.9 µm. Chapter 3 – The impact of OAE 1a on marine biota 68 Chapter 3 – The impact of OAE 1a on marine biota 69
3. 3.2.2 Adda -2
B. constans and Z. erectus show a significant coccolith size excursion associated with the base of the FS (Fig. 3. 4). Below this interval (below 2370.4 m) B. constans has a mean size of ~ 3.8 µm x 2.9 µm (length x width). Smallest coccoliths are present at the base of the FS (2370. 2 m) with 3.0 µm x 2.3 µm , showing a size reduction of 21%. Coccolith size increases up to 3.5 µm x 2.7 µm at the top of the studied interval (2363.3 m). The size of Z. erectus is largest below the FS with a mean of 3.4 µm x 2.3 µm. The smallest coccoliths , 2.8 µm x 1.9 µm, were found at the base of the FS , reflecting a 17% size decrease. The sizes recover after their minimum towards the top of the interval. The mean size of coccoliths of W. barnesiae varies without a visible trend between 6.0 µm x 5.0 µm a nd 6.7 µm x 5.6 µm throughout the studied interval.
3. 3.2.3 Alstätte 1
In the Alstätte 1 section, B. constans and Z. erectus show a size minimum at the base of the FS (Fig. 3. 4). In t he samples below the FS (below 3.6 m), B. constans has a mean size of ~3.3 µm x 2.6 µm. At the base of the FS (3.6 m –5.4 m), coccoliths are smaller by 17% with 2.8 µm x 2.2 µm . The size increases towards the top of the interval and reaches mean dimensions of 3.3 µm x 2.7 µm (10.09 m). Similar , but less pr onounced trends have been observed in the Z. erectus data set. Coccoliths have a mean size of ~2.9 µm x 2.0 µm below the FS. The smallest coccoliths are present at the base of the FS with 2.6 µm x 1.8 µm , decreasing by 9% in size. Sizes increase towards th e top of the studied section. In the lower part of the section , W. barnesiae has a size of ~6.1 µm x 5.1 µm. Between 5.18 m and 6.47 m, the size decreases to 5.7 µm x 4.7 µm. The largest coccoliths occur between 7.93 m and 10.09 m with ~6.4 µm x 5.3 µm.
Figure 3. 4. Length and width data of B. constans , Z. erectus and W. barnesiae . Stable carbon isotope data and chemostratigraphic segments data are from Bottini and Mutterlose (2012 ) and Mutterlose and Bottini (2013). Carbon isotope data of 16 additional sa mples were added to the North Jens -1 record. Phases A to D are based on the coccolith size of B. constans and Z. erectus and the C -segments. The Fischschiefer (FS) and OAE 1a are marked in grey. For North Jens -1, the top of OAE 1a is not fixed and for Adda -2, the stable carbon record does not allow the chemostratigraphic determination of top and base of OAE 1a. Chapter 3 – The impact of OAE 1a on marine biota 70
3. 4 Discussion
3. 4.1 Diagenesis
Different states of nannofossil preservation may have a great impact on the obtained coccolith size data. When coccolith size minima go along with declining nannofossil preservation, the recorded size patterns are most likely altered. The preservation of the coccoliths an alysed here has been tested for all samples in previous studies (Bottini and Mutterlose, 2012; Mutterlose and Bottini, 2013 ). In addition, preservational modes have been checked in this study without observing any poorly preserved specimens.
Apart from vis ual parameters, nannofossil assemblage compositions (diversity and abundance of species) and bulk rock parameters provide information on the state of preservation. Diversity is similar at all three sites with a mean species number of 18 in the two North Se a sites and 16 in the LSB (Bottini and Mutterlose, 2012; Mutterlose and Bottini, 2013 ). Nannofossil diversity and the abundance of dissolution -prone taxa may be affected by preservation. Scatter plots of mean coccolith length versus number of species and a bundance of delicate taxa show almost constant mean lengths with varying diversities or abundances of delicate taxa (Fig. 3. 5). Coccolith sizes therefore appear to be unaltered.
The size decrease of B. constans and Z. erectus associated with the FS is simu ltaneous at all three sites. It is therefore unlikely, that this signal was caused by regional factors like preservation. This synchronous size decrease has also been reported in sections from the Tethys and the Pacific Ocean (Erba et al., 2010), indicatin g therefore a near global phenomenon. Despite the geographical distance of up to several thousands of kilometers between the sites and despite their very different depositional environments and different sediments (ranging from near -shore claystone to pela gic marls and limestone/chalk), all locations show the same characteristic coccolith size excursion. Chapter 3 – The impact of OAE 1a on marine biota 71
Figure 3.5. Scatter plots of mean length data versus (A) diversity of nannofossil assemblage and (B) abundance of delicate taxa for all studied samples and species. The group of delicate taxa includes Rotelapillus crenulatus, Calciosolenia fossilis, Orastrum persicuum and Owenia dispar . This group encompasses all species of the Stephanolithiales and holococcoliths recognized in the studied samples. The di versity data and abundance data of delicate taxa is derived from Bottini and Mutterlose (2012 ) and Mutterlose and Bottini (2013 ).
3. 4.2. Coccolith sizes and the stable carbon isotope record
Based on the biometric results, the size evolution of B. constans and Z. erectus can be divided into four phases (A to D; Fig. 3. 4) for the late Barremian and early Aptian. These four phases correspond to specific segments of the stable carbon isotope record (C1 to C7), which have been proposed by Menegatti et al. (1998 ). This chemostratigraphy allows a relative dating and enables a well -founded correlation of the studied sites. The carbon isotope data of the three sites are derived from Bottini and Mutterlose (2012) and Mutterlose and Bottini (2013). Chapter 3 – The impact of OAE 1a on marine biota 72 - or slightly slightly or d Mutterlose d . in light grey) light in . al. (2008) for DSDP Site 463. The Ap E 1a (FS, SL, SL eq SL SL, (FS, 1a E C record and its carbon isotope segments at theat segments isotope carbon record andits C 13 Selli Level (SL) and the Pacific Selli Level equivalent(SL Level Selli Pacific and(SL)the Level Selli an a global driver. Itali suggests arecomparedtotheδ B. constans B. he synchronicity of the evolution size T segments, introduced by Menegatti et al. (1998) (Ap2 equals C2, e.t.c.) and established for the other sections. other the for established and e.t.c.) C2, equals (Ap2 (1998) al. et Menegatti by introduced segments, - s with carbon isotope segment C3 at all sites. Stable carbon isotope data and chemostratigraphic segments are from Bottini an Bottini from are segments chemostratigraphic and data isotope carbon Stable sites. all at C3 segment isotope carbon with s ometric correlation: Phases A to D based on the mean length of of length mean the on based D to ometricA correlation: Phases Mutterlose and Bottini (2013) for the boreal sites and from Erba et al. (1999) for Cismon and from Ando et ) and Figure 3.6. Bi 3.6. Figure the Fischschiefer borealThe (FS), (2010). al. Erba et by data of transferredthe sites to studied three OA with associated units lithological the of base the at present are line) (bold coccoliths Smallest grey. in marked are eq.) synchronou and below (2012 C the to correspond (2008) al. et Ando of segments extensionThe of OAE 1a itself marked is in dark grey. Chapter 3 – The impact of OAE 1a on marine biota 73
During phase A the coccolith sizes of B. constans and Z. erectus are constant. This phase corresponds to stable carbon isotope segment C2 and possibly to the subjacent segment (C1). It is of late Barremian to early Aptian age and covers the pre -OAE 1a interval.
Phase B is marked by an abrupt size decrease . It encompasses the smallest coccoliths in the lower part of the lithological unit associated with OAE 1a and the minimum in the stable carbon isotope record (segment C3) (Bottini and Mutterlose, 2012; Menegatti et al., 1998; Mutterlose and Bottini, 2013). Due to very low abundances of coccoliths in the North Jens -1 core phase B was not observed at this site.
During phase C co ccoliths show a trend to increasing sizes. This phase covers the stable carbon isotope segments C4 to C6 at Alstätte 1 and the upper part of the FS at Adda -2 and North Jens -1. The stable carbon isotope record shows a step -wise increase of values (C4), a pe riod with stable values (C5) and another step -wise increase (C6). This phase is well documented in all three sites.
Phase D describes the final phase of the size excursion. The transition from the preceding phase C is gradual , boundaries are therefore diff icult to establish. Coccolith sizes have recovered to pre -decrease (phase A) sizes by the end of nannofossil zone BC 18 and the beginning of nannofossil zone BC 19.
3. 4.3 The global synchronicity of coccolith size shifts
Data, raised independently from the current study, show a size decrease of B. constans and Z. erectus in carbon isotope segment C3 and a subsequent recovery of coccolith size for two low -latitudinal sections in the Tethys and the Pacific (Fig. 3. 6; Erba e t al., 2010). The interval with smallest coccoliths of B. constans and Z. erectus , observed at the two low -latitude sites, corresponds stratigraphically to our phase B and thereby with segment C3. The negative carbon isotope excursion C3 is accompanied by a drop in the carbonate content of the sediment and a rise in TEX 86 -derived temperature estimates (Bottini et al., 2015) and a negativeshiftinδ 18 O (Ando et al., 2008) indicating a warming trend (Fig. 3. 7). The size of W. barnesiae at these two sites rem ains unaffected by the environmental perturbations.
The size shift of B. constans and Z. erectus may reflect a primary ecological signal or a short - lived change in ocean water chemistry, which altered the size of the deposited coccoliths. The Chapter 3 – The impact of OAE 1a on marine biota 74 latter facto r appears less likely due to the absence of evidence for diagenetic alteration in the Boreal samples. Bottini and Mutterlose (2012) and Mutterlose and Bottini (2013) reported on the preservation of calcareous nannofossils in the North Jens -1 and Alstätte 1 samples of the Fischschiefer and found neither malformation nor dissolution of coccoliths.
Though B. constans and Z. erectus show parallel trends in their size evolution throughout all five sections considered here, there are differences in the absolute mean sizes and the magnitude of the size decrease associated with OAE 1a. Throughout the entire studied interval, the mean sizes of B. constans and Z. erectus are higher at the low latitudinal sites than those of the three Boreal sections (Fig. 3. 7; Lübke et al., 2015). It is only during phase B, that the mean sizes of B. constans show a less distinctive size gradient between low and higher latitudes. This is due to the large size decrease of B. constans in the C3 segment at the low - latitude sites. At Cismo n and DSDP Site 463, the size decrease is 30% and 34%, while it is only 15% (North Jens -1), 21% (Adda 2) and 17 % (Alstätte 1) in the Boreal Realm. Relative size decreases of Z. erectus are less significant, they amount to 17% (Cismon), 10% (DSDP Site 463) , 10% (North Jens -1), 17% (Adda -2) and 9% (Alstätte 1). The dissimilarity of the relative size decrease of B. constans may be an artifact of sampling. It is possible, that the exact point of maximum size decrease is not covered by the samples from the Nort h Sea and the LSB. Alternatively, it may represent an ecological signal.
The parallel trends of coccolith size evolution, the carbon isotope record and the drop in the carbonate contents suggest a global mechanism behind the changes of the environmental pa rameters. OAE 1a and its driving factors worked on a global scale despite the great distance between the sites. Environmental changes related to these global perturbations may have influenced the composition of calcareous nannofossil assemblages and the si ze of specific species. Regional parameters played a certain role as well and may explain differences in the specific assemblages of the studied sites. Chapter 3 – The impact of OAE 1a on marine biota 75 Chapter 3 – The impact of OAE 1a on marine biota 76
The most prominent features at the Boreal sites is the prominent and abrupt size decrease at the base of the Fischschiefer (phase B) and the subsequent size recovery (phase C ) of B. constans and Z. erectus . The main ecological factors associated with OAE 1a are discussed on a regional and global level , taking into account the results of Erba et al. (2010 ).
Temperature
Our phase B and the corresponding carbon isotope segment C3 were short -lived with a duration of approximately 27 to 44 kyrs (Li et al, 2008). An episode of intensive volcanic activity (Ontong Java Plateau) ( Je nkyns, 2010; Tarduno et al., 1991; 1995; Weissert and Erba,
2004) released significant amounts of CO 2 into the atmosphere (Ando et al., 2008; Jenkyns, 2010). This led to a climate warming recognized in sediments around the globe, both marine and continental (e.g., Bottini and Mutterlose, 2012; Erba, 2004; Hochuli et al., 1999; Jenkyns, 2010; Mutterlose et al., 2010; Keller et al., 2011; van Breugel et al., 2007). Additionally, the destabilization of methane clathydrates and the consequent release of 13 C-depleted carbon further accelerated the climatic warming (Méhay et al., 2009). Sea -surface water tempera tures increased by as much as 8 °C, inferred from a δ18 O Pacific bulk -rock record (Ando et al., 2008 ).
Figure 3.7: Summary of lithology, stable carbon isotope data, coccolith size record of B. constans and Z. erectus , sea -surface temperature estimates based on TEX 86 and nannofossil temperature index (TI) and sea -surface productivity estimates based on nannofossil nutrient index (NI). Relative size decreases of B. constans and Z. erectus associated with the early phase of OAE 1a or phase B in this study are calculated as the percentage drop of coccolith mean length in phase B with respect to the mean coccolith length of the subjacent interval. For North Jens -1, the drop was calculated usi ng the smallest coccolith mean length in phase C (sample at 2251.7 m), as phase B is not present in this section due to very low nannofossil abundance in samples associated with the base of the FS or OAE 1a. Therefore, relative coccolith size decrease esti mates are probably underestimated. The mean length drop of Z. erectus at DSDP Site 463 in brackets indicates the presence of the lowest mean length of this species to be above phase B (sample at 620.51 m), unlike for B. constans (sample at 623.42 m). Pleas e note the reverse way of presentation of the TI at North Jens -1 and the stable oxygen isotope data at DSDP Site 463. Stratigraphic key and extension marks for the FS, SL, SL eq. and OAE 1a are the same as in figures 3.3 and 3.6. Chapter 3 – The impact of OAE 1a on marine biota 77
Based on TEX 86 findings, a n increase by 4 to 5 °C has been postulated for the OAE 1a interval of the LSB (Mutterlose et al., 2014) (Fig. 3. 7). Thereafter a global cooling occurred, evidenced by TEX 86 data from the LSB and geochemical, palynological and nannofossil analyzes from the northern Tethys (e.g., Ando et al., 2008; Bottini et al., 2015; Hochuli et al., 1999; Jenkyns, 2010; Kuhnt et al., 2011; Mutterlose et al., 2014 ). Temperature changes , associated with OAE 1a , have been recognized in the North Sea and the LSB . Mutterlose e t al. (2014) reported a temperature increase in the C3 stage in the Alstätte 1 section based on TEX 86 data. Nannofossil -based temperature estimates for North Jens -1 show a warming in this early phase as well (Mutterlose and Bottini, 2013). Subsequently coc colith sizes fully recovered before pre - OAE 1a temperatures were reached again . Sea -surface temperature may have affected coccolith size in the Boreal Realm in the early phase of OAE 1a, when warming and size decrease went along each other (phase B). Tempe rature does, however, not explain the size variation of phases C and D. A comparison of B. constans and Z. erectus coccolith sizes document that the specimens in the cooler Boreal Realm (North Sea, LSB) are smaller than the warmer low latitudinal ones (Cismon, DSDP Site 463) (Lübke et al., 2015). If the size evolution of these two species was ruled by temperature, the global warming related to the OAE 1a should have enlarged the coccoliths at all sites. Instead, coccolith size decreased during this warming event .
Productivity
Nannofossil -based productivity estimates (NI = nutrient index) have shown, that the productiv ity of surface waters during OAE 1a was variable . In most cases, high values are associated with the interval below the event and its early phase (Fig. 3.7). This coincides with the emplacement of the Ontong Java Plateau. At Cismon productivity is high in the lower part of the Selli Level (segments C3 and C4). At Piobbico productivity is elevated in the interval preceding the Selli Level (segment C2), while the lowermost part of the Selli Level (segments C3 and C4) is missing. At DSDP Site 463, productivity was higher below the Selli Level equivalent and in its lower part (segments C1 to C6) (Bottini et al., 2015 ). In the LSB B. constans and Z. erectus , often seen as productivity indicators (Fig. 3. 7), show similarly high abundances below, during and above t he FS (Bottini and Mutterlose, 2012). In the North Sea the NI is Chapter 3 – The impact of OAE 1a on marine biota 78 elevated just below the FS and drops during the early event . It has , however, been much high before the entire event (Mutterlose and Bottini, 2013).
The global uniformity of the size reducti on of B. constans and Z. erectus argues against productivity as a driver. The reliability of these two species as productivity indicators is arguable (Lees et al., 2005). Apart from productivity B. constans has also been related to temperature as a control ling environmental factor. This latter interpretation is based on its high abundance in higher latitudes in the Late Cretaceous (e.g., Thierstein, 1981; Lees, 2002). The abundance patterns of B. constans and Z. erectus also may have been controlled by other factors. The calculated nutrient indices should therefore be applied with caution. In any case it is problematic to use productivity as a reason for the size variation of B. constans and Z. erectus .
Weathering and light availability
Geometry studies of coccoliths suggest, that size variations may result from variations of light availability in the surface waters (Lübke et al., 2015). A dominance of small specimens thereby indicates a muddy photic zone , caused by inc reased scattering and absorption of light by water, salt molecules, single -celled organisms, and suspended particles (e.g., Kirk, 1977; Sverdrup et al., 2006). Intensified weathering of continental crust associated with the onset of OAE 1a, documented by a positive shift in the 187 Os/ 188 Os -ratio, has been suggested for the early phase of OAE 1a in the Tethys and the Pacific (Bottini et al., 2012; Tejada et al. (2009). A shift to lighter δ7Li values due to a 2.5 times increased silicate weathering rate points to an increase in continental weathering as well (Lechler et al., 2015) . A rise in global temperature and an associated humidity increase caused intensified continental weathering. Higher run - off transported more clastic particles into the sea, whic h diminished the depth of the photic zone in the North Sea and the LSB . A strong decrease in the CaCO 3 content associated with the FS in the North Sea (Fig. 3. 7) indicates a strong clastic input and confirms this hypothesis (Mutterlose and Bottini, 2013). The Alstätte 1 section, positioned only ~10 to 15 km off the former coast line (Lehmann et al., 2012), is too poor in carbonate to display similar changes. Following this “light availability hypothesis”, the Fischschiefer reflects periods of higher fresh water input. Chapter 3 – The impact of OAE 1a on marine biota 79
Independent support for freshwater input into the LSB is supplied by dinoflagellate studies. Below and Kirsch (1997) describe changes of the G/P ratio (G = gonioaulacoid dinoflagellates, P = peridinoid dinoflagellates ) associated with the FS at several LSB sites . Short lived abundance peaks of peridinoid dinoflagellates and the consequent decrease in the G/P ratio suggest a freshwater pulse for these sites.
The recovery of coccolith sizes during phase C in the aftermath of the initial negativ e carbon isotope excursion (C3) corresponds to the time after the weathering pulse. Following the hothouse maximum temperatures of the OAE 1a, run -off diminished, the water became less muddy and consequently coccoliths sizes increased again.
3. 4.4 W. barn esiae
The size stability of W. barnesiae during OAE 1a is in line with various biometric studies suggesting the high ecological tolerance of this species (e.g., Bornemann and Mutterlose, 2006; Linnert and Mutterlose, 2013; Linnert et al., 2014). Neither the initial warming and subsequent cooling nor the changes in surface water productivity level and transparency of the water affected its size.
The prominent shift to smaller coccolith sizes in the middle part of the Alstätte 1 section and the subsequent size increase (between 6.47 m and at 7.93 m ) is most likely related to a sea -level related shift from marginal marine to pelagic marine conditions in the late early Aptian (e.g., Bottini and Mutterlose, 2012; Mutterlose, 1998; Mutterlose and Böckel, 1998; Rawson and Riley, 1982 ; Ruffell, 1991 ). The oceanographic change is evidenced by the immigration of new nannofossil species from the Tethys (Bottini and Mutterlose , 2012 ; Habermann and Mutterlose, 1998; Ke upp and Mutterlose 1994; Mutterlose 1992a; b ). In the Alstätte 1 section, this change of the depositional environment is also reflected by lithology. A prominent rise in the carbonate content and a shift from dark to light grey claystones can be observed i n late early Aptian sediments (Bottini and Mutterlose, 2012). Based on nannofossil assemblage data Bottini and Mutterlose (2012) showed, that surface water conditions and the sedimentation in the LSB were controlled by regional factors following the deposi tion of the FS. Potential explanations for the size variation of W. barnesiae include the immigration of cryptic species and salinity variations. Chapter 3 – The impact of OAE 1a on marine biota 80
A sea -level rise allowed a mass migration of planktonic organisms from the North Sea into the formerly restri cted LSB. This is documented by the first common occurrence of planktonic foraminifera above the Fischschiefer (Rückheim and Mutterlose, 2002). The entire early Aptian succession of the North Sea (North Jens -1, Adda -2) yields large sized specimens of W. ba rnesiae, which became common in the LSB only after the deposition of the FS. It seems therefore plausible, that cryptic species of W. barnesiae , different in size to those common in the restricted LSB, emigrated from the North Sea. These newly arriving North Sea specimens replaced the endemic population.
The late early Aptian transgression established a water mass exchange between the LSB and the North Sea. During the earliest Aptian, when the LSB was isolated, surface water salinity was reduced due to high freshwater input and a reduced mixing with the normal -saline marine water of the North Sea (Below and Kirsch, 1997; Bischoff and Mutterlose, 1998; Habermann and Mutterlose, 1999). W. barnesiae may have been affected by changes in salinity, which favor ed specimens with larger coccoliths.
Biometrical studies of calcareous nannofossils have documented , that some species respond to paleoenvironmental shifts while others show no size related changes (e.g., Bornemann and Mutterlose, 2006; Erba et al., 2010; Linnert and Mutterlose, 2013; Linnert et al., 2014). In the case of this study, B. constans and Z. erectus were globally affected by environmental changes associated with OAE 1a, while W. barnesiae reacted to regional changes in the LSB. The distinct size evolution patterns of the former two species on one hand and W. barnesiae on the other are therefore no contradiction.
3.5 Conclusions
Our biometrical studies of three coccolith species (B. constans, Z. erectus, W. barnesiae ) from the Boreal Realm allow in combination with stable carbon isotope records , temperature and productivity estimates the following conclusions:
• Four phases of size evolution (A -D) were identified for B. constans and Z. erectus across the OAE 1a in the Bor eal Realm . These have also been observed in the western Tethys and the Pacific. Phase A: large coccoliths prior to OAE 1a. Phase B: size decrease during the early OAE 1a. Phase C: size recovery. Phase D: total recovery to pre -OAE 1a sizes. The size variati ons for Chapter 3 – The impact of OAE 1a on marine biota 81 both species are similar at the studied sites. The phases A to D can be correlated to shifts in the carbon isotope record.
• A size decrease of B. constans and Z. erectus is recognized in phase B, which is contemporaneous with the initial climatic wa rming of OAE 1a. The accelerated weathering rates during phase B caused increased clastic input, resulting in muddy waters and a restricted photic zone. As a response, coccolithophores with smaller coccoliths flourished and dominated the assemblage.
• The si ze decrease of B. constans and Z. erectus at the base of OAE 1a and the subsequent recovery has been observed in other oceans as well. A global increase of chemical weathering affected the majority of oceanic basins. Going along with weathering increases, enhanced productivity may have played a role as well .
• The size of W. barnesiae remains constant in most sections (North Jens -1, Adda -2, Cismon and DSDP Site 463). This suggests, that W. barnesiae was an ecologically robust species. The termination of basin restriction in the late early Aptian in the LSB induced a coccolith size increase.
3.6 Acknowledgement s
We would like to thank Dr. Frans van Buchem (Maersk Oil, Copenhagen) and Dr. Jon R. Ineson (Geological Survey of Denmark and Greenland, Copenhagen) fo r providing core material of North Jens -1 and Adda -2. We thank Ron Blakey for detailed paleogeographic maps of the Lower Cretaceous. We thank the editor and the reviewers for their constructive feed - back. The authors appreciate funding by the DFG (Mu 667/4 6-2). Chapter 4 – Size changes in calcareous nannofossils 82
4 Size changes in calcareous nannofossils documenting the pace of microevolutionary processes
Nathalie Lübke 1 , Jörg Mutterlose 1
1 Institut für Geologie, Mineralogie und Geophysik, Ruhr -Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
Abstract
Biometric studies are a powerful tool to define morphotypes in fossil assemblages quantitatively when genetic sequencing is not applicable. They supply evidence for the timing and the course of morphometric changes, thereby shedding light on the pace of microevolutionary processes. Due to their high abundance in sediments, microfossils are ideally suited for biometric studies which try to understand evolutionary trends. In this study, the phylogenetic lineage of the coccolithophorid species Predis cosphaera columnata is analysed with respect to its size evolution in the Indian Ocean. Distinctive size shifts and biometrically -defined first and last occurrences of morphogroups are used as stratigraphic datum levels.
The calcareous nannofossil species P. columnata evolved as a small subcircular placolith (~4 -5µm) in the late Early Cretaceous (~116.3 Ma). Approximately 5 Ma after its first appearance in the latest Aptian to earliest Albian (~111.6 Ma), a larger -sized and circular morphotype appeared. At around 100.6 Ma in the late Albian, the large -sized coccoliths of P. columnata became a permanent component of the nannofossil assemblages. Another ~4.1 Ma later, in the earliest Cenomanian (~96.5 Ma), the large morphotype became the dominant representati ve of Prediscosphaera . Both morphotypes differ in size and coccolith shape, with 5.2 µm being the limit between the small, subcircular morphotype and the larger circular morphotype. A general rise in the mean size of the P. columnata population from the la test Aptian to the earliest Cenomanian is attributed to a size increase of the small morphotype and the rise in abundance of the large morphotype. The diversity -increasing split of the P. columnata lineage is a good example for cladogenesis rather than for anagenesis. Sympatric Chapter 4 – Size changes in calcareous nannofossils 83 speciation of P. columnata resulted in the evolution of the small and the large morphotype. Subsequently, both morphotypes spread into the evolving Indian Ocean, where they co - existed for at least 15 Ma.
Keywords
microevolution, spreading, Cretaceous, coccolithophores
4.1 Introduction
Taxonomic issues concerning the definition of morphological species may be addressed by biometric studies. This has been successfully applied in micropaleontology for radiolarians (e.g., Cortese and Bjørklund, 1998), foraminifera (e.g., Aurahs et al., 2011; Bettenstaedt und Spiegler, 1975) and calcareous nannofossils (e.g., López -Otálvaro et al., 2012; Mattioli et al., 2004). López -Otálvaro et al. (2012) studied the nannofossil genus Discorhabdus in Aalenian to Bajocian (middle Jurassic) sediments from the Lusitanian Basin, Portugal. The two encountered species Discorhabdus ignotus and Discorhabdus striatus were differentiated based on their diameter. In this case, 5 µm were used to define the limit b etween the smaller D. ignotus and the larger D. striatus . Based on biometric data, Mattioli et al. (2004) differentiated the species Biscutum grande , Biscutum dubium , and Biscutum intermedium from other representatives of Biscutum and Similiscutum from early Jurassic sediments of the western Tethys.
The calcareous nannofossil genus Prediscosphaera , which originated in the late Aptian and became extinct at the end of the Maastrichtian (end of Cretaceous; ~66 Ma), is ideally suited for biometric stud ies for the following reasons: The genus is (a) common in most nannofossil assemblages from the Aptian – Maastrichtian and (b) morphologically well -defined and therefore easy to delimit from other genera. The evolutionary lineage of Prediscosphaera columna ta has been described qualitatively by several authors (e.g., Kennedy et al., 2000; Perch -Nielsen, 1979). However, it has not been defined quantitatively with actual biometric data. The current study aims at (a) quantifying the evolutionary size changes by using biometry, (b) dating the pace of size evolution of P. columnata , and (c) interpreting the development of the lineage with regard to microevolution and spreading. Chapter 4 – Size changes in calcareous nannofossils 84
Paleontological background
The calcareous nannofossil genus Prediscosphaera first appea red in the late Aptian (Perch - Nielsen, 1979). It comprises elliptical to circular placoliths with a bicyclic shield, characterized by a broader outer and a narrower inner cycle (Fig. 4. 1). The shield is typically formed of 16 elements, irrespective of the coccolith size. The central area is spanned by cross bars, forming a tall and elongated complex spine with a typical side view. The first species of Prediscosphaera , which evolved in the late Aptian, was Prediscosphaera spinosa , usually small (~3 -4 µm leng th) and elliptical with an axial cross (Barrier, 1977; Perch -Nielsen, 1979; Bralower, 1993; Kennedy et al., 2000). It was followed by small (4 -5 µm) and subcircular forms assigned to P. columnata, originating near the Aptian/Albian boundary. Subsequently i n the early Albian, circular and larger forms of this species appeared and eventually replaced the subcircular type.
Figure 4.1: (A) Light microscope images of the elliptical P. spinosa , the earliest species of Prediscosphaera . (B) Light microscope image of circular to subcircular P. columnata . (C) SEM image of P. columnata in side view, illustrating its prominent spine. Images are from following depths: 467.67 mbsf, 433.37 mbsf, and 408.89 mbsf.
The first occurrence of P. col umnata has been used to approximate the Aptian/Albian boundary in biostratigraphic studies and was suggested as a possible marker for the base of the Albian (Birkelund et al., 1984; Rawson et al., 1996). However, taxonomic inaccuracies in some studies may have compromised this event in some cases. Subcircular forms of P. Chapter 4 – Size changes in calcareous nannofossils 85 columnata might have been attributed to P. spinosa . In some studies, P. spinosa was not the first recorded species of Prediscosphaera (Applegate and Bergen, 1988; Bralower, 1993).
In order to describe and evaluate the early evolutionary trends of early Prediscosphaera (~116 to 96 Ma), 17 samples from a complete sedimentary succession (ODP Site 763B, Exmouth Plateau; ~100 km northwest off Australia) have been studied. In addition to measurem ents of the total diameter, other morphological characteristics of the placoliths, such as number of shield elements or distal shield width, were tested for their usefulness in morphotype differentiation. Numerical ages, assigned to the evolutionary events and described in previous studies, allow an estimation of the pace of evolutionary processes.
Figure 4.2: (A) and (B) show the present day and Albian age location of ODP Site 763B. (C) Lithology, nannofossil zonation and events recorded at ODP Site 763B. Biostratigraphic datums were determined in this study and correlate well with initial biostrati graphy by Bralower and Siesser (1992) and Bralower (1993). An exception is the first occurrence of Rhagodiscus achlyostaurion, which has been recorded further downcore (561.19 mbsf) than recorded by Bralower (1993) (527.27 mbsf). Position of samples for bi ometry of P. columnata are indicated.
4.2 Material and methods
The studied core from ODP Site 763B, Exmouth Plateau, provides a continuous and extended sedimentary record from the upper Aptian to lower Cenomanian (Fig. 4. 2 A and B). The lower 28 m of the studied 230 m section consist of dark grey silty claystone, while the remainder of the sequence is a homogenous light grey nannofossil ooze (Fig. 4. 2 C). Initial Chapter 4 – Size changes in calcareous nannofossils 86 shipboard reports and scientific results mention abundant, e xceptionally well -preserved nannofossil chalks and calcareous claystones of the uppermost Aptian to Cenomanian (Shipboard Scientific Party, 1990; Bralower and Siesser, 1992). Biostratigraphic information (Bralower and Siesser, 1992; Bralower, 1993) is avai lable.
Calcareous nannofossils were studied here using a transmitted polarized light microscope and simple smear slides. A total of 178 samples from the 230 m thick interval (~620 to 390 meters below sea -floor = mbsf) were chosen for biostratigraphy and ab undance estimates. Out of these, 17 samples have been chosen for biometric analyses covering the interval from the first occurrence of P. columnata (528.86 mbsf) to the top of the studied section (408.89 mbsf). Fifty specimens were measured in 8 samples, w hich yield only one morphotype, 100 specimens were analysed from another 8 samples, showing two morphotypes (Tab. 4. 1). One sample provided only 37 specimens (494.74 mbsf) of P .columnata .
In each sample, the individual specimens were digitally captured w ith a digital camera attached to an Olympus BX53 microscope. Image analysis was performed with the semi - automatic image software StreamStart by Olympus. Measured parameters include total diameter (longest axis for the subcircular specimens), shield width a nd inner rim cycle diameter. Statistic evaluation was performed with PAST3 (Hammer et al., 2001) and included the Shapiro -Wilks test for normal distribution and mixture analysis. Normal distribution of the biometric data sets was rejected with a p(normal)< 0.05. Mixture analysis was used to evaluate and quantify the number of size groups and their abundances. Chapter 4 – Size changes in calcareous nannofossils 87 5 6 3 ------56 43 29 10 11 17 Abundance (%) Abundance Large morphotype morphotype Large ------5.85 6.02 5.66 5.45 5.92 5.88 5.52 5.65 5.66 (µm) and theand morphotype. small mean mean diameter, mean diameters of both Mean diameter diameter Mean Large morphotype morphotype Large P. columnata 44 57 71 90 89 95 95 83 97 100 100 100 100 100 100 100 100 P. P. columnata morphotype morphotype Abundance (%) Abundance (normal) estimates, whether the data show normal distribution Small Small p Wilks Wilks - rease recorded is in 4.67 4.39 4.41 4.46 4.28 4.05 4.17 4.69 4.52 4.55 4.25 4.44 4.27 4.20 4.37 4.44 4.60 (µm) Mean diameter diameter Mean Small morphotype morphotype Small Wilks - 0.339 0.398 0.264 0.522 0.826 0.510 0.793 0.0098 0.0590 (normal) 0.000138 p 0.0000151 0.0000469 0.00000367 0.00000552 0.000000925 Shapiro 0.0000000417 0.00000000996 Gaussian distributions. In addition, total - 4.51 4.32 4.05 4.17 5.44 5.20 4.89 4.47 4.60 4.27 4.20 4.37 4.44 4.66 4.67 4.39 4.47 P. columnata P. columnata Total Mean diameter (µm) diameter Mean 50 50 37 50 50 50 50 50 50 100 100 100 100 100 100 100 100 Number of of Number specimens Depth Depth (mbsf) 408.89 414.14 421.89 428.18 434.08 440.92 448.08 453.03 460.03 467.67 475.66 482.56 488.00 494.74 500.44 507.35 525.11 Table Table 4.1: Summary of statistical evaluation of biometric data. Shapiro or not. Bold numbers represent non morphotypes and their abundances are given. A total mean inc size Chapter 4 – Size changes in calcareous nannofossils 88
4.3 Results
4. 3.1 Biometry
From bottom to top of the section, the mean diameter of all P. columnata specimens increases from 4.05 µm (507.35 mbsf) to 5.44 µm (408.89 mbsf; Tab. 4. 1). Based on Shapiro - Wilks p(normal), normal distribution has to be rejected for nine of the 17 studied samples based on p(normal) -values below 0.05 (Tab. 4. 1). Mixture analysis indicates, that these samples are characterized by a bimodal distribution (Tab . 4. 1). Mean size and relative abundance of the two size groups of the best f it model are documented in Table 4. 1. Color - coded frequency plots (Fig. 4. 3 A) display the relative abundance of 0.25 µm -spaced size classes in each sample. Comparing the size cla ss frequencies in unimodally and bimodally distributed samples reveals that the small size group has a similar position (=mean size) in both cases, while the large group is an additional element in the bimodal samples (Fig. 4. 3 B). For those samples with b imodal distribution, the two size groups are shown in Fig. 4. 3 C. In the two lowermost samples, only the small size group is present. The large group was first detected in three consecutive samples from 500.44 mbsf to 488.00 mbsf, where it represents 3 % t o 17 % of all P. columnata specimens. It was not observed in the interval from 482.56 mbsf to 475.66 mbsf and reappears two times in the upper part of the succession (467.67 mbsf and 434.08 mbsf) with low abundances (5 % and 11 %). From 434.08 mbsf onwards , the large morphotype is consistently present in each sample. Towards the top of the studied section, its abundance rises from 11 % to 56 %, eventually becoming the dominant size group of P. columnata . A crossplot of coccolith diameter versus frequency of all measured P. columnata specimens indicates the presence of two size groups. The smaller one shows a preference for subcircular specimens with up to 30 % per size class, while the large -sized group is dominated by circular forms (Fig. 4. 4).
In order to differentiate the two size groups based on additional criteria, the ratio of shield width and total coccolith diameter is plotted against coccolith diameter. With a mean of ~0.21, the shield width remains constant irrespective of total diameter. Both small and large specimens exhibit similar coccolith proportions. SEM images (Fig. 4. 5) of both small and large specimens display the typical 16 shield elements. Chapter 4 – Size changes in calcareous nannofossils 89
Figure 4.3: (A) Size evolution plot of coccolith diameter of all 17 studied samples, distinguishing between the small and the large morphotype. Frequency plots illustrate the abundances of both size groups (B) Exemplary Frequency plots of two studied samples, 440.92 mbsf shows a unimodal distribution, 408.89 mbsf shows a biomodal distribut ion, showing the small and the large morphotype. (C) Size groups revealed by mixture analysis. Chapter 4 – Size changes in calcareous nannofossils 90
Figure 4.4: (A) Frequency plot of all measured P. columnata specimens. Subcircular specimens are commonly associated with the small morphotype. (B) Cross plot of coccolith diameter and the shield width versus coccolith diameter ratio. No trend towards relatively smaller or larger shields with respect to total coccolith size were recorded. N refers to the number of measured specimens.
Figure 4.5: SEM ima ges of the (A) large (~7 µm; 408.89 mbsf) and the (B) small morphotype (~4µm; 434.08 mbsf). Both specimens show the same number of distal shield elements (16). Chapter 4 – Size changes in calcareous nannofossils 91
4.4 Discussion
4. 4.1 Preservation
The understanding of a potential diagenetic overprint is crucial for the interpretation of fossil assemblages. In this case, a diagenetic overprint can be ruled out, because small and large specimens of P. columnata are simultaneously present in many sampl es, while the large ones are missing in some samples. Selective dissolution of the large representative in these samples is unlikely, since smaller specimens are more prone to dissolution than large ones. The absence of diagenesis is supported by the const ant ratio of shield width versus coccolith diameter (~0.21), which is irrespective of coccolith size. Early signs of partial dissolution include a reduced shield width, which would have been detected here. The well -developed bimodality in many samples and the corresponding upper size limit of the small morphotype in the unimodal samples also argues against dissolution or overgrowth.
4. 4.2 Biostratigraphy - Numerical ages
In order to obtain absolute ages for calibrating the pace of thee morphometric change s in the P. columnata lineage, it is necessary to use reliable numerical ages, assigned to various biostratigraphic datum levels. These age assignments are based on a stratigraphic correlation of first and last occurrences of marker species and absolute ag es, derived from volcanic ash layers. Critical datum levels (FO= first occurrence, LO= last occurrence) in mbsf and their numerical ages according to Anthonissen and Ogg (2012) are: FO of Eprolithus floralis (562.70 mbsf; 123.88 Ma), FO of P. columnata (528. 86 mbsf; 112.95 Ma), FO of Hayesites albiensis (521.198; 112.65 Ma); FO of Tranolithus orionatus (489.62 mbsf; 110.73 Ma); FO of Axopodorhabus albianus (488.00 mbsf; 109.94 Ma), FO of Eiffellithus monechiae (464.87 mbsf; 107.59 Ma), FO of Eiffellithus turr iseiffelii (460.03 mbsf; 103.13 Ma), FO of Corollition kennedyi (420.38 mbsf; 100.45 Ma), LO of Gartnerago chiasta (141.14 mbsf; 99.94 Ma) and the FO of Lithraphidites acutus (405.66 mbsf; 96.16 Ma). The resulting sedimentation rate is 0.6 cm/kyrs (Appendi x Fig . 4. 1). Based on these data, numerical ages have been assigned to the morphotype speciation events of P. columnata observed here. Chapter 4 – Size changes in calcareous nannofossils 92
4.4.3 Biostratigraphy - Prediscosphaera
In contrast to many previous studies (see Paleontological background), the FO of Prediscosphaera at ODP Site 763B was recorded with P. columnata at 528.86 mbsf; P. spinosa appears at 525.61 mbsf. The absolute position and the order of appearances are in li ne with initial biostratigraphic results (Bralower, 1993). We observed the first occurrence of P. spinosa ~1.3 m further downcore than previously described, while the FO of P. columnata occurs 11 cm below the depth first reported by Bralower (1993). The ra rity of both species and the very small size (3 - 4 µm) of P. spinosa may explain the unusual sequence of FOs.
4. 4.4 The two morphotypes
Two morphotypes of P. columnata have been identified in the Albian to lower Cenomanian sediments. While coccolith pro portions and number of shield elements are equal, total coccolith size and outer shape are the defining characters of these morphotypes. The small form, which is present in all studied samples has a mean size of 4.05 µm (507.35 mbsf) to 4.69 µm (408.89 mbs f), it is subcircular to mostly circular. The mean size of the large form ranges from 5.45 µm (428.18 mbsf) to 6.02 µm (414.14 mbsf). Its shape is in most cases circular. The overall size increase of P. columnata throughout the section (Tab. 4. 1) is attrib uted to (a) the gradual size increase of the small morphotype from 4.05 µm (507.35 mbsf) to 4.69 µm (408.89 mbsf) and (b) the abundance rise of the large morphotype from 0 % (525.11 mbsf) to 56 % (408.89 mbsf) throughout the section.
Both morphotypes can be differentiated by their size, with the upper limit of the small morphotype being between 5.00 µm and 5.25 µm and vice versa. This delineation, which is clearly visible in the frequency plots and mixture analysis, is also evident in the samples that are characterized by the small morphotype only. In these samples, the maximum diameter of the recorded P. columnata size is usually between 4.5 µm and 5.0 µm, with two exceptions. In sample 475.66 mbsf, five coccoliths are larger than 5.0 µm, with 5.55 µm, 5.1 9 µm, 5.17 µm, 5.12 µm, 5.03 µm. In sample 453.03 mbsf, three coccolith are larger than 5.0 µm (5.11 µm, 5.06 µm, 5.01 µm). The unusually large coccolith (5.55 µm) may very well be a representative of the large morphotype in this sample, which was not reco rded by mixture analysis neglecting Chapter 4 – Size changes in calcareous nannofossils 93 this one outlier. Taking this into account, a conservative approach places the line between both morphotypes at 5.20 µm.
The two morphotypes can be related to previously described types of P. columnata . The small subcir cular to circular morphotype (mean diameter 4.05 to 4.69 µm) characterizes the first P. columnata specimens in the fossil record, occurring in the uppermost Aptian/lowermost Albian at 528.86 mbsf. The first occurrence of P. columnata dates back to 116.3 Ma (Fig. 4. 6). The larger, circular morphotype first occurs in sample 500.44 mbsf in the lower Albian (111.6 Ma), where it is rare in comparison to its small and subcircular counterpart. It is present in two successive samples (500.44 mbsf and 494.74 mbsf), thereafter it disappears. The first appearance and disappearance has been observed between the FOs of H. albiensis and T. orionatus . After transiently reappearing at 467.67 mbsf in the middle Albian close to the FO of E. monechiae , the large morphotype is established as a consistent part of the nannofossil assemblage from 434.08 mbsf onwards (~100.6 Ma). This event follows the FO of G artnerago praeobliquum (442.01 mbsf) in the upper Albian. In the uppermost sample (408.89 mbsf; 96.5 Ma) between the LO of G. chiasta and the FO of L. acutus , the large morphotype exceeds an abundance of 50 %, becoming dominant while the small form is eventually replaced.
4. 4.5 Significance for evolutionary trends and timing
Based on the biometrical analysies, three basic obse rvations can be made with respect to the early evolution of the species P. columnata . (a) Two distinct morphotypes successively occur in the same geographic area. (b) The small morphotype increases in size with time, and (c) the large and younger morphotyp e eventually became the dominant taxon and completely replaced the small form.
To explain these findings, two factors have to be considered: (a) The appearance and disappearance of morphotypes were most likely related to spreading from one or multiple are as of origination into the Indian Ocean. (b) The co -occurrence of the two different morphotypes of P. columnata was probably a result of evolutionary processes.
It is most likely that P. columnata evolved in a geographical area other than that of ODP Site 763B (Indian Ocean). It subsequently spread into the Indian Ocean inter alia, where it has Chapter 4 – Size changes in calcareous nannofossils 94 been observed in the studied core. It is very likely, that the large and younger morphotype developed elsewhere as well and spread into all ocean basins. The evoluti onary trend of the P. columnata lineage described here in detail has been observed at various sites (e.g., Kennedy et al., 2000), indicating a joint distribution of both morphotypes. The fast mixing of sea surface waters and the global sea level highstand (e.g., Haq, 2014) of the middle Cretaceous resulted in widespread pelagic conditions. This allowed for a synchronous and global appearance of P. columnata and its morphotypes in pelagic settings. Jönsson and Watson (2016) used Lagrangian particle tracking and network theory to calculate that it would take less than a decade for a planktonic particle to reach any randomly chosen location in our recent ocean surface waters. Assuming, that ocean circulation during the Cretaceous was much slower (Caldeira and R ampino (1991) suggested a factor of two), these timescales are still extraordinarily short. The resolution of the fossil record, sample spacing and the pace of speciation are much lower than the pace of sea surface circulation and the drift of plankton dur ing the Cretaceous. Therefore, the events recorded at ODP Site 763B and their timing are a good representation of the actual events, irrespective of the place of species formation.
Figure 4.6: Evolutionary history and events of the genus Prediscosphaera during the latest Aptian to the earliest Cenomanian. Numerical ages and biostratigraphic ages were assigned to the individual events. Ap.=Aptian; Cen.=Cenomanian.
The pace of evolution and its timing can be quantified at ODP Site 763 as follows: The small morphotype of P. columnata globally existed for ~4.7 Ma before its larger counterpart evolved around 111.6 Ma; it was recorded in sample 500.44 mbsf. Due to the absence of the large morphotype in the subjacent sample (507.35 mbsf; 112.7 Ma), the speciation process Chapter 4 – Size changes in calcareous nannofossils 95 occurred between 112.7 Ma a nd 111.6 Ma and therefore lasted not longer than 1.1 Ma. Both morphotypes co -occurred for at least 15 Ma. This time span is in line with speciation data for the radiation of the Albian genus Eiffelithus (Watkins and Bergen, 2003). Data show, that in this g enus, new species appear within 0.5 to 1 Ma.
Though this process of speciation has not been observed and suggested for other coccolithophorid species, and especially not for the fossil ones, “sexual reproduction may be quite common in this algal group“ (Jo rdan, 2012). Therefore, the speciation mechanisms for reproductive isolation (Mayr, 1963) are used to explain the evolutionary trends of the coccolithophorid species P. columnata . Both, large and small morphotypes are present in many samples, indicating a co -occurrence during their lifetime, which makes geographic isolation -based allopatry unlikely. In addition, geographic barriers were rare due to the high sea -level of the middle Cretaceous (Haq, 2014). The actual formation site of both morphotypes is stil l unknown so far, too. The obviously overlapping geographic ranges (the extends of which has not been determined yet) of both morphotypes may indicate sympatric speciation based on ecological specialization. This has been observed and suggested in extensiv e biogeographical studies on foraminifera (De Vargas et al., 1999; 2001; 2002) and also for the recent coccolithophorid species Calcidiscus leptoporus and Coccolithus pelagicus (Sáez et al., 2003).
The intermitted absence of the large P. columnata in the ODP 763B record may be due to very low abundances and thus non -recording, which is a sampling artefact. Alternatively, it may represent an actual signal. If the latter was true, the occurrence pattern of the large P. columnata may indicate different ecolog ical preference of the two morphotypes. At least for the time interval of the lower and middle Albian, the small morphotype appears to be better adapted. From the late Albian onwards, the large morphotype is consistently present and eventually replacing th e small one, implying a change to more favorable conditions for the large one. This diversity -increasing split of the P. columnata lineage results in the presence of two sister taxa and is a good example for cladogenesis rather than for anagenesis.
The Indian Ocean data show, that the coccolith size slightly increased in both morphotypes (or morphospecies) separately and strongly increased for the entire P. columnata continuum. The latter observation was also reported outside the Indian Ocean (e.g., Kennedy et al., 2000) and probably has the same reason: The overall size increase in P. columnata is mainly related Chapter 4 – Size changes in calcareous nannofossils 96 to the abundance increase of the large morphotype, rather than to the size increase within the individual morphotypes. So far, the size incr easing trend of the individual morphotypes in other locations than the Indian Ocean may have remained undetected due to the absence of detailed biometric studies.
4.5 Conclusions
Biometric analyses of the coccolith species P. columnata , covering an interva l of ~20 Ma (Aptian/Albian boundary to early Cenomanian) reveal the presence of two distinct morphotypes. Based on frequency distribution, 5.2 µm is the conservative upper limit for the small morphotype and the lower limit of the large morphotype, respecti vely. The small taxon has a mean diameter of 4.05 µm to 4.69 µm and is mostly circular to subcircular, while the large form commonly has mean diameters of 5.45 µm to 6.02 µm and is circular. Both morphotypes have similar coccolith element proportions conce rning shield width versus total coccolith diameter and a fixed number of 16 distal shield elements.
Total diameter of the entire P. columnata group increases towards the uppermost Albian/lowermost Cenomanian due to a size increase within the small morphot ype on the one hand and a rise in abundance of the large morphotype on the other hand. In combination with numerical ages of several nannofossil datums, the biometric findings allow to date the evolutionary events in the P. columnata lineage. Based in the co -occurrence of two discrete morphotypes for more than 15 Ma and the absence of geographic barriers during the speciation, sympatric speciation appears the most likely formation mode.
4.6 Acknowledg ement s
The authors thank IODP for supplying the studied samples. R. Neuser performed the SEM analyses. The authors appreciate funding by the DFG (Mu667/46 -2). Chapter 4 – Size changes in calcareous nannofossils 97
4.7 Appendix
Figure A 4. 1: Time -Depth plot with resulting correlation. Chapter 5 – A stable long -term calcareous nannofossil record 98
5 A stable long -term calcareous nannofossil record (lower Aptian – middle/upper Cenomanian) from the southern Indian Ocean
Nathalie Lübke 1, Jörg Mutterlose 1
1 Fakultät für Geowissenschaften, Ruhr -Universität Bochum, Unive rsitätsstraße 150, 44801 Bochum
Abstract
The break -away of the Indian subcontinent from Australia and the subsequent opening of the Indian Ocean during the Cretaceous changed the paleogeographic and paleoceanographic situation of the southern hemisphere. Together with the global occurre nce of oceanic anoxic events , expressed as back shale deposits in shallow water settings, these events make the southern Indian Ocean an interesting study area. Despite these facts, the Indian Ocean has been scarcely studied and almost neglected by science . We present the first long -term calcareous nannofossil record from this region, located few 100 km NW off the Australian coast on the Exmouth Plateau (ODP Site 763B). The data include nannofossil diversity and assemblage composition data, an extended bios tratigraphic zonation, and biometrical data on several species, as well as a stable carbon isotope record. The results indicate stability of the planktonic community in the course of the more than 20 Ma lasting paleogeographic realignment in the southern h emisphere , despite a minor cooling event in the middle Albian . The position of OAE 1d, located based on stable carbon isotope data, does not show any equivalent lithological expression in the core, as well as no response of the nannofossil community during this episode.
5.1 Introduction
The middle Cretaceous (Aptian to Turonian) period has been studied extensively due to its unique climatic and paleogeographic situation. A eustatic rise in sea -level, associated with high ocean crust productivity, created vast shelf areas and culminated at the end of the middle Chapter 5 – A stable long -term calcareous nannofossil record 99
Cretaceous. The break -up of supercontinent Pangea with the junction of the northern and southern Atlantic Ocean and the separation of modern -day Australia and the Indian subcontinent from Antarctica created a geographic situation ushering the present -day constellation and a modern oceanographic circulation. The intense release of greenhouse gases into the atmosphere induced a n intense greenhouse or hothouse climate with low latitudinal temperature gra dients and ice -free poles. As a result of brief episodes of further enhanced input of such gases, the global oceans experienced widespread anoxia (oceanic anoxic events = OAEs), commonly represented as fine -grained organic -rich deposits (black shales) in s hallow marine environments , for example the early Aptian OAE 1 a. These climatic perturbations are well expressed in the stable carbon isotope record and induced biotic turnover in many fossil groups, including calcareous nannofossils, foraminifera, corals (e.g., Bornemann et al., 2005; Coccioni and Luciani, 2004; Erba, 2004; Erba et al., 2010; Erbacher et al., 1999, Giraud et al., 2003; Hardas and Mutterlose, 2007; Linnert et al., 2010; Philip and Airaud -Crumiere, 1991; Premoli -Silva et al., 1999; Watkins e t al., 2005).
However, not all OAEs showed to have the same characteristics, including possible driving mechanisms. The black shale horizons of latest Albian age associated with OAE 1d , recorded in this study, show a stratigraphically widespread and inconsistent distribution with most black shales deposited long before the positive carbon isotope excursion of OAE 1d , for example in the North Atlantic at ODP Site 1052 . Based on these findings, the definition of this event (latest Albian) should be implemented chemostratigraphically rather than lithostratigraphically (Petrizzo et al., 2008). The organic carbon burial of that time was probably due to pronounced orbitally -driven variability of the thermal structure of the sea surface waters culminating in an upper -ocean stratification collapse (Wilson and Norris, 2001). This stratification disrupted the habitat of depth -specified calcareous nannofossil species, thus inducing their extinction along OAE 1d in the weste rn North Atlantic (Watkins et al., 2005). Orbitally -paced changes in monsoonal activity have been identified by cyclic variations of terrestrial input and productivity of the surface water in black shale marl alternations in southeast France (Bornemann et al., 2005).
The middle Cretaceous period, moreover calcareous nannofossils, has been intensely studied in the past decades almost exclusively at sites from the northern Atlantic Ocean and Chapter 5 – A stable long -term calcareous nannofossil record 100 the Tethys Ocean. Consequently, the southern hemisphere and thus th e Indian Ocean record have been almost neglected. However, to infer a general pattern for understanding the middle Cretaceous climate, a widespread and complete data record is required.
This study represents the first long -term complete calcareous nannofo ssil record from the lower Aptian to middle/late Cenomanian, including nannofossil abundance and diversity patterns and biometrical analyses, from the rarely studied southern hemisphere. The studied section from ODP Site 763 represents a sedimentological r ecord, tracing the early evolution of the Indian Ocean from a juvenile to mature ocean (Haq et al., 1992). Due to the completeness and quality of the fossil record at this site, the results of this study may serve as a reference for a high -latitude setting of the middle Cretaceous as well as for the southern hemisphere. Biostratigraphic data already exist and results were extended downcore. The data obtained in this study were compared to existing nannofossil records from similar part time intervals and to rare Indian Ocean data. Based on stable carbon isotope data, the OAE 1d has been located in the 763B core, though no equivalent lithological expression is present in the core section.
5.2 Material and methods
The studied core from ODP Site 763B, Exmouth P lateau, provides a continuous and extended sedimentary record from the lower Aptian to middle/upper Cenomanian (Fig. 5.1) . The lower 28 m of the studied 230 m section consist of dark grey silty claystone, followed by a homogenous light grey nannofossil ooz e. Initial shipboard reports and scientific results mention abundant, exceptionally well -preserved nannofossil chalks and calcareous claystones of the uppermost Aptian to Cenomanian (Shipboard Scientific Party, 1990; Bralower and Siesser, 1992). Chapter 5 – A stable long -term calcareous nannofossil record 101
Figure 5.1: Present -day and Albian age l ocation of the studied section. Albian map after Ron Blakey.
Calcareous nannofossils were studied here with a transmitted polarized light microscope in 178 simple smear slides. Nannofossil abundance data were compiled on the basis of counts of at least 300 specimens per sample. Diversity indices (Shannon Index and E venness Index) and correlation measures were calculated using PAST3 (Hammer et al., 2001). Preservation and overall abundance of nannofossils were determined (Bown and Young, 1998). Species identification and age determination followed Bown et al. (1998) a nd Burnett (1998). Biometric data of W. barnesiae, B. constans and T. orionatus were obtained in 20 evenly - distributed samples, with 50 specimens per species per sample. The individual specimens were digitally captured with a digital camera attached to an Olympus BX53 microscope. Image analysis was performed using the semi -automatic image software StreamStart by Olympus. Measured parameters include total length (maximal elongation) and width (minimal elongation), as well as length and width of central area if applicable. Ellipticity was calculated as the width by length ratio. Species diversity was evaluated using PAST3 and included the Shannon Index and the Evenness index (Hammer et al., 2001) . Stable isotope analyses (δ 13 C and δ 18 O) were performed on 150 samples at the Geozentrum in Erlangen. Carbonate content data derive from 113 sa mples. Chapter 5 – A stable long -term calcareous nannofossil record 102
5.3 Results
5.3 .1 Biostratigraphy
Within the 230 m of sediment 20 nannofossil events have been recognized (Fig. 5. 2). The lowest event is the first occurrence (FO) of Eprolithus floralis (562.70 mbsf), marking the lower boundary of NC7 and CC7b an d thus an early Aptian age of the lowest deposits. Zones NC7A/B to NC11, and CC7b to CC10 are completely represented in the studied section. The last event is the FO of Lithraphidites acutus (405.66 mbsf), indicating the base of zones NC11, CC10 and UC3 an d thus a middle/upper Cenomanian age of the uppermost section. All recognized events are in normal order, as reported by standard zonation schemes (Burnett, 1998; Bown et al., 1998), except for the last occurrence (LO) of Hayesites albiensis . This event is reported at a depth of 475.67 mbsf, above the FO of Tranolithus orionatus (492.54 mbsf), the FO of Axopodorhabdus albianus (488.00 mbsf) and the LO of Sollasites falklandensis (486.27 mbsf) and below the FOs of Eiffelithus monechiae (464.030 mbsf) and Eiffelithus turriseiffelii (460.03 mbsf). After Burnett (1988), this events is by standard placed above the FO of E. turriseiffelii and it is used to define the CC9a/b -boundary. However, the LO of H. albiensis has already been reported to occur prematurely at high -latitude sites in both hemispheres, namely at the South Atlantic DSDP Site 511 (Wise, 1983) and at the Gault Clay in England (Crux, 1991). The findings at ODP Site 763B confirm the observation, that H. albiensis, or at least its LO, is not a relia ble marker at high latitudes (Crux, 1991).
The encountered nannofossil events in our sample set broadly reflect the depths recorded by Bralower (1993) for ODP Site 763. The positions of most events differed by roughly 0.1 to 2 m from the Bralower (1993) d ata. Three events were placed far deeper than in the previous study: We placed the FO of Corrolithion kennedyi at 420.38 mbsf, in contrast to the 424.29 mbsf of Bralower (1993). The FO of Seribiscutum primitivum (551.92 mbsf) was recorded more than 8 m fur ther below, and the FO of Rhagodiscus achlyostaurion (561.19 mbsf) more than 34 m below the depths of Bralower (1993). The FO of E. floralis has not been recorded at all in the previous study. These findings may be very well due to the extended studied ran ge in our study. Chapter 5 – A stable long -term calcareous nannofossil record 103
Figure 5.2: Biostratigraphic results and time -depth plot for ODP Site 763B. Chapter 5 – A stable long -term calcareous nannofossil record 104
Based on biochronological data (Anthonissen and Ogg, 2012), numerical ages were assigned to ten nannofossil events (FOs of E. floralis, Prediscosphaera columna ta, H. albiensis, T. orionatus, A. albianus, E. monechiae, E. turriseiffelii, C. kennedyi , and L. acutus, and the LO of Gartnerago chiasta). The resulting age depth trendline (Fig. 5. 2) gives a mean sedimentation rate of 0.6 cm/kyrs.
5. 3.2 Nannofossil c ontent
Calcareous nannofossils (Figs. 5.3.1 and 5.3.2) are consistently present in all samples above 561.19 mbsf, while the bulk of samples below is barren of nannofossils (except for 562.70 mbsf, 592.64 mbsf, and 597.86 mbsf). Preservation and general abundance per sample rise from bottom to top of the studie d section, where they are abundant and show good to excellent preservation. The most abundant nannofossil species (> 1%) are in descending order Watznaueria barnesiae, Biscutum constans, T. orionatus, Zeugrhabdotus erectus, Discorhabdus ignotus, Zeugrhabdo tus howei, S. primitivum, Watznaueria fossacincta, E. turriseiffelii, Rhagodiscus asper, Rhagodiscus angustus, Repagulum parvidentatum and E. floralis. The two former species are by the far most abundant with mean abundances of 26 % (W. barnesiae) and 23 % (B. constans).
The number of species ranges from 7 (592.64 mbsf) to 53 (418.73 mbsf) with a mean simple diversity of 40 species per sample. A t otal of 121 species has been encountered throughout the entire studied section. Diversity generally increases f rom bottom to top of the section. The Shannon Index increases from 1.7 at the bottom to 3.1 in the upper ~50 m of the section. Evenness remains at stable values around 0.3. Chapter 5 – A stable long -term calcareous nannofossil record 105
Figure 5.3.1: Light microscope images of important species from ODP Site 763B. Sc ale bar=2µm. Chapter 5 – A stable long -term calcareous nannofossil record 106
Figure 5.3.2: Light microscope images of important species from ODP Site 763B. Scale bar=2µm. Chapter 5 – A stable long -term calcareous nannofossil record 107
5.3.3 Nannofossil abundance
5. 3.3.1 Species
Most species show variability throughout the studied section. However, only few signific ant trends are discernable (Fig s. 5.4 .1 and 5.4.2 ). The abundances of W. barnesiae, W. fossacincta and Watznaueria ovata show marked decreases at the bottom of the section around the Aptian/Albian boundary, which is most prominent for W. fossacincta. R. asper has an abu ndance peak in the upper Aptian (NC7C, CC7b) with 10% (553.14 mbsf). R. achlyostaurion has its maximal abundance in the upper Albian (NC10A, UC0, CC9a/b) with 7%, while its abundance varies around 1% throughout the rest of the section. R. parvidentatum sho ws an acme in the upper Aptian, where it reaches up to 30% compared to its principal absence in Albian and Cenomanian sediments. The species S. primitivum increases in abundance from the middle Albian (NC9A, CC8b) onwards and drops again in the upper Albia n (NC10A, UC0, CC9a/b). The species P. columnata, P. spinosa and T. orionatus generally rise in abundance from their first occurrence onwards. E. floralis is rare at the bottom of the section until it rises in abundance around the Aptian/Albian boundary. H owever, no major floral turnover occurred in the studied section of the upper Aptian to middle/upper Cenomanian.
5. 3.3.2 Families
Discernable trends within the calcareous nannofossil family data include a rise in abundance of the Eiffelithaceae towards th e top of the section and drop in abundance in the Watznaueriaceae at the bottom of the section (Fig. 5.5) . Chapter 5 – A stable long -term calcareous nannofossil record 108
Figure 5.4.1: Calcareous nannofossil species (>0.5 %) relative abundance data from ODP Site 763B from B. constans to S. primitivum . Light grey interval at the bottom indicates barren intervals. Dark grey interval indicates location of OAE 1d. Chapter 5 – A stable long -term calcareous nannofossil record 109
Figure 5.4.2: Calcareous nannofossil species (>0.5 %) relative abundance data from ODP Site 763B from Staurolithites sp. to Z. diplogrammus and diver sity indices. Light grey interval at the bottom indicates barren intervals. Dark grey interval indicates location of OAE 1d. Chapter 5 – A stable long -term calcareous nannofossil record 110
Figure 5.5: Calcareous nannofossil family relative abundance data from ODP Site 763B. Light grey interval at the bottom indicates barren intervals. Dark grey interval indicates location of OAE 1d. Chapter 5 – A stable long -term calcareous nannofossil record 111
5. 3.4 Nannofossil biometry
The mean length of W. barnesiae (Fig. 5.6) varies between 6.28 µm (488.0 µm) and 7.04 µm (460.03 mbsf), with an average value of 6.64 µm. The mean length of this species is 5.70 µm, ranging between 5.42 µm (488.0 µm) and 6.08 µm (460.03 mbsf). Along the entire studied interval , no significant trend was observed. The mean size of B. constans shows a minimum at 500.44 mbsf with 3.33 µm and a maximum at 448.08 m bsf with 4.18 µm. Apart from that, no trend was observable. T. orionatus shows a significant size increase of 0.6 µm from 6.69 µm (434.08 mbsf) to 7.29 µm (428.18 mbsf) in the upper Albian. A maximum is reached at 414.14 mbsf with 7.32 µm.
5.4 Discussion
5. 4.1 Position of OAE 1d
13 The OAE 1d has been recorded at ODP Site 1052 in the δ Ccarb record of pristine foraminifera in the surface water and thermocline record (Wilson and Norris, 2001). Biostratigraphically, this event is located at the CC9a/b boundar y in the latest Albian, parallel to the occurrence of prominent black shale layers. At ODP Site 763B, the CC9a/b boundary cannot be resolved due to the inconsistency of the marker species H. albiensis , more precisely its LO, in the high latitudes. A slight negative peak followed by a positive plateau, a significant negative shift and an adjacent positive peak (from bottom to top of both sections) can be correlated in both records (Fig. 5. 7, shaded interval). The position of OAE 1d can thus be roughly locate d between 440 to 450 mbsf at ODP Site 763B. Deviation in absolute age assignment is due to different methods in age modelling and differing absolute age assumptions, for example for the Albian -Cenomanian boundary. Wilson and Norris (2001) used neutron poro sity down -hole log -based cyclostratigraphy and the present study uses absolute ages based on biostratigraphic events (Anthoni ssen and Ogg, 2012). Chapter 5 – A stable long -term calcareous nannofossil record 112
Figure 5.6: Biometric data of W. barnesiae, B. constans and T. orionatus with data from reference studies for W. barnesiae and B. constans (Bornemann and Mutterlose, 2006; Erba et al., 2010; Linnert et al., 2014; Linnert and Mutterlose, 2013; Lübke et al., 2015). Atlantic data from Linnert et al. (2014) and Linnert and Mutterl ose (2013) include all species of the genus Watznaueria (W. barnesiae makes up 60 to 70% of all specimens) and Biscutum (B. constans makes up more 95% of all specimens).
5. 4.2 Nannofossil abundance
The prominent abundance peaks of W. barnesiae, W. fossa cincta and W. ovata at the bottom of the section are most likely associated with the poor preservation of calcareous nannofossils in this section. The genus Watznaueria has been reported to be one of the most dissolution -resistant species with abundances o f more than 40% (Roth and Bowdler, 1981; Thierstein and Roth, 1991) or 70% (Williams and Bralower, 1995) of W. barnesiae indicating altered nannofossil assemblages. Therefore, the abundance peaks of the three species are a result of diagenesis rather than a primary ecological signal. This is also confirmed by the relative abundance record of delicate species (Fig. 5.4.2). These show a very low abundance in this potential diagenetically influenced section. However, towards the top of the section, Chapter 5 – A stable long -term calcareous nannofossil record 113 this group shows higher abundances, indicating better preservation and a better significance of the single species abundances concerning ecological interpretation.
The species R. parvidentatum shows a significant acme of up to 30 % in the upper Aptian (NC7C, CC7b), which has been previously reported in the Vocontian Basin, northern Germany, the Mazagan Plateau, the Falkland Plateau, and in the Weddell Sea (Mutterlose et al., 2009). This peak has been interpreted as brief cooling event of global nature in the uppermos t Aptian. Additional supporting evidence for this event are oxygen isotope data and the occurrence of glendonites and ice drift deposits (Mutterlose et al., 2009).
The two species S. primitivum and T. orionatus show maximum abundances in the middle Albian (NC9A, CC8b). As both species are associated with high latitudes and thus cooler conditions (Wise and Wind, 1977; Roth and Bowdler, 1981; Thierstein, 1974; Thierstein, 1979; Roth, 1983), the middle Albian may repre sent a slightly cooler interval, which ha s also been reported from the Gault Clay of Munday’s Hill in England (Crux, 1991) This middle Albian cooling may explain the early (compared to low latitude records) disappearance of H. albiensis in this interval (NC9A, CC8b). It has been assumed, that this species is typical for low latitudes and thus warmer conditions (Crux, 1991). Unsuitable conditions in the Indian Ocean have made it impossible for this species to flourish, constraining its pr emature disappearance. The diversity data, Shannon Index, show an increasing trend towards the top of the section. This can be attributed to a rise in basic species number and thus to an equalization of the assemblage. However, the Shannon Index or Evennes s do not show distinct minima or maxima along their basic trends, indicating no change in the general nannofossil assemblage.
The calcareous nannofossil assemblage composition of the southern Indian Ocean represents a boundary position of the temperate and the austral paleobiogeographic Zone (PBZ) of Lees (2002), described for the late Albian. Both PBZs are characterized by abundant W. barnesiae . Common S. primitivum and the intermitted occurrence of R. parvidentatum illustrate an intermediate position betw een both PBZs. Common components of the assemblage, like B. constans, T. orionatus, and E. turriseiffelii, are typical for the austral PBZ. Endemic components of this zone include several species of Gartnerago , as well as Radiolithus hollandicus, Ser ibiscu tum gaultensis , and Octocyclus reinhardtii . This composition does not change dramatically along the section. Changes are only associated with the evolution of new species and not their change in abundance. Calcareous nannofossils show no above Chapter 5 – A stable long -term calcareous nannofossil record 114 background n oise response to OAE 1d. This may on the one hand be a sampling and resolution bias or due to the fact, that the nannofossil community did not actually respond dramatically to this events.
13 13 Figure 57 Chemostratigraphic correlation of δ Ccarb from bulk samples from ODP Site 763B and δ Ccarb from pristine foraminifera from ODP Site 1052 (Wilson and Norris, 2001). The shaded interval has been correlated based on similarities in the carbon isotope record (stippled lines) and roughly represents the location of black shales and thus OAE1d at ODP Site 1052. nfa=nannofossil age correlation (bold lines).
5. 4.3 Biometry
Compared to coccolith length data from various time -equivalent biometric studies, W. barnesiae is generally larger in the middle Cretaceous succession from ODP Site 763B (Fig. 5. 6) by ~0.9 µm ( 13%), while the size of B. constans shows similar sizes in time -equivalent Vocontian Basin and North Atlantic studies. The similarity in size of B. constans ma y indicate a more cosmopolitan distribution of morphotypes, while W. barnesiae is characterized by more endemic populations. For the latter species, mean length data are more similar in the Vocontian Basin and the northern Atlantic compared to the southern Indian Ocean due to the close spatial proximity of the two former sites. T. orionatus has been studied biometrically for the first time in the present study. Except for the size shift of T. orionatus in the upper Albian, Chapter 5 – A stable long -term calcareous nannofossil record 115 the biometric data of all three sp ecies show remarkably steady patterns throughout the studied section, indicating stable environmental conditions. The size shift of T. orionatus cannot be correlated to a change in its abundance or the abundance of other species with a clear paleoecological affinity. Therefore, the reason for this size shift remain s unknown. Additionally, as this has been the first biometric study including T. ori onatus , it may represent typical variation within the species.
5. 5 Conclusions
The calcareous nannofossil record from the southern Indian Ocean shows good to excellent preservation and high diversities throughout the greater part of the section. Constancy of the nannofossil assemblage composition , including species abundances, diversity indices, and the paleobiogeographic zone (PBZ after Lees, 2002) , and o f the biometric record indicate stable oceanographic conditions during the latest Aptian to early Ceno manian. A slight a cooling episode in the middle Albian, recorded by abundance maxima of high - latitude species S. primitivum and T. orionatus , does not alter the overall stability of the nannofossil record. The lower part of the section (Aptian) is charact erized by high abundances of dissolution -resistant species of the genus Watznaueria , indicating a diagenetically - controlled assemblage composition rather than a paleoecological signal . The stability of the record is remarkable despite the fact, that the In dian subcontinent broke away from western Australia and the Indian Ocean started to form. These events had great paleoceanographic impact. However, it did not affect the primary producers in any way.
The position of the OAE 1d in the studied record has been established using calcareous nannofossil biostratigraphy and by correlating stable carbon isotope data to a record from the North Atlantic (ODP Site 1052) . The calcareous nannofossil data do not show an above background noise response to this event. T his may be due to sampling or resolution bias or due to the lack of an actual response of the community. The general stability of this southern Indian Ocean record and the fact, that none of the proxies (species abundance, diversity, biometry) showed any c hange both point to the interpretation, that it may probably be a primary signal of stability along OAE 1d. This may possibly be due to the hemi pelagic to pelagic position of the section, where changes in water stratification played a minor role if at all. Chapter 5 – A stable long -term calcareous nannofossil record 116
5.6 Acknowledgements
The authors thank IODP fo r supplying the studied samples . We thank Ron Blakey for detailed paleogeographic maps of the middle Cretaceous. The authors appreciate funding by the DFG (Mu667/46 -2). Chapter 6 – General conclusions 117
6 General conclusions
This thesis presents calcareous nannofossil data from the middle Cretaceous, including biometric measurements, biostratigraphy, diversity, and species abundance patterns. The studied locations focus on the mid -to high latitudes of both hemispheres, in northwest Germany and the southern Indian Ocean. In total, 8637 individual coccoliths of five different species ( Biscutum constans, Prediscosphaera columnata, Tranolithus orionatus, Watznaueria barnesiae, Zeugrhabdotus erectus ) were di gitally captured under a transmitted -light microscope and biometrically measured. The parameters include total coccolith length and width (for elliptical specimens) or diameter (for the circular P. columnata ), ellipticity, shield width, and central area di mensions. A total of 178 samples from ODP Site 763B were searched for marker species to produce a biozonation for the studied sections. At ODP Site 763B, nannofossil counts were conducted in 113 samples, with a total of 53.060 counted specimens. The main o bjective of this PhD thesis is to determine the applicability of calcareous nannofossil biometry as indicator for various purposes, including responses to short - and long -term changes in the paleoenvironment and evolutionary changes within traditional spec ies. The geological record is the ideal testing ground, as culture studies on recent communities are hindered by the factor time. The main results of the thesis and the implications for the different applications of biometric studies in calcareous nannofos sils are summarized below.
Biometry and the cryptic species of the early Aptian
Coccolith size data of the three species B. constans, Z. erectus and W. barnesiae from four different locations in a synchronous time interval have been compared (Lübke et al. , 2015). These locations include the proto -North Sea and the Lower Saxony Basin in northwest Germany, representing a hemi -pelagic and a coastal setting in the Boreal Realm, the western Tethys and the Mid -Pacific Mountains, both depicting pelagic low latitu de settings. The data from the Boreal Realm have been obtained during this PhD project, while the low latitude data are based on an earlier study by Erba et al. (2010). To ascertain the synchronicity in this comparative study, a chemostratigraphically -defi ned time interval from the early Aptian has been chosen. According to result of recent genetic studies, the coccolithophores most likely show the presence of various cryptic species, explaining minor differences in morphology, that have been interpreted as phenotypic variation prior to that. Chapter 6 – General conclusions 118
Our study shows that at least two cryptic species of the calcareous nannofossil species B. constans were present in the early Aptian ocean, one producing small coccoliths and the other one producing larger ones. In the pelagic sections (Tethys and Pacific), both small and large cryptic species are present. In the more proximal North Sea and Lower Saxony Basin (northwest Germany), only the small one was found. The fundamental difference between these two settings (pelagi c vs. proximal/coastal) is the turbidity of the surface water , introducing the light -attenuation theory . The high surface water turbidity in the proximal settings is unfavorable for the large -coccolith producing cryptic species, which is then not present i n the sediment.
The similar coccolith size signal of W. barnesiae at all sites does not allow to recognize and distinguish possibly -present cryptic species. However, this points to a lack of ecological preference of the cryptic species, that are most proba bly present in all oceanic settings and basins. The data of Z. erectus , the third studied species are probably compromised by diagenesis, as this species is prone to dissolution due to its delicate morphology.
The concept of cryptic species arose in the la st years, as genetic sequencing on recent species revealed the presence of genetically different and albeit morphologically very similar (within the range of variability of the traditional species concept) species. The comparative approach chosen in this s tudy has helped to identify various cryptic species in the early Aptian oceans with the help of biometric studies and confirms the analogy to genetic sequencing studies on recent plankton groups. This analogy includes the biogeographical presence or absenc e of particular cryptic species in oceanic basins as well as that some cryptic species may exhibit ecological preferences while other may not.
Biometry and the response to short -term OAE 1a
The Oceanic Anoxic Event 1a has been associated with a biocalcifi cation crisis, characterized by the intermittent disappearance of the heavily calcified Nannoconus genus and a size reduction of the delicate species B. constans and Z. erectus . However, these results have been obtained in low latitude sections from the Te thys and the Pacific Ocean. Boreal biometric data from the North Sea and the Lower Saxony Basin (northwest Germany) show a coccolith size reduction of B. constans and Z. erectus of up to 21% associated with OAE 1a, confined by quasi -stable sizes before and after OAE 1a. This phenomenon was obviously not Chapter 6 – General conclusions 119 restricted to the low latitude pelagic realm. The same ecological changes and conditions associated with OAE 1a prevailed at a much greater geographical scale. The coccolith size stability recorded for W. ba rnesiae is in accordance with previous studies.
Interpreting the results in the light of the cryptic species concept, the size of B. constans and Z. erectus points to an increase in the turbidity of the surface ocean waters both in the pelagic low latitud es and the more proximal Boreal Realm and thus the loss of the brighter niche for the larger coccoliths. The absence of a coccolith size response to OAE 1a by W. barnesiae confirms that the cryptic species of this species have no ecological preference and are thus not affected by the perturbations of OAE 1a.
Biometry and the long -term record of the southern Indian Ocean
The long -term (upper Aptian to middle/upper Cenomanian) biometric record from the southern Indian Ocean indicates stability. The total co ccolith dimensions of W. barnesiae, B. constans and T. orionatus remain stable throughout this more than 20 Ma lasting interval. The calcareous nannofossil assemblage analysis confirms this interpretation. Diversity and species abundance patterns do not ch ange significantly. The only exception is the Aptian/Albian boundary cooling represented by the R. parvidentatum acme.
The Aptian to Cenomanian record is punctuated by OAE 1d in the uppermost Albian, identified by the stable carbon isotope record. The calc areous nannofossil assemblage composition and diversity, and the single -species biometric analysis do not show a response to this event. Furthermore, the long -term aftermath of this event does not show a change as well. This is in accordance with Bown et a l. (2004) stating, that OAE had no above -background influence on nannofossil diversity. The results for OAE 1d are contrasting those of OAE 1a, indicating, that both OAEs did not have the same impact on primary producers. This concurs with the different dr iving mechanisms of both events, excess atmospheric CO2 for OAE 1a and orbitally -driven water column stratification collapse for OAE 1d (see chapter 1.1.2), obviously having varying impacts on coccolithophores. Chapter 6 – General conclusions 120
Biometry and the early evolutionary lineage of P. columnata
The single -species study on P. columnata has quantitatively confirmed qualitative observations on this species by various authors. The data reveal the presence of two morphogroups within the taxonomic species P. columnata . The small and su bcircular morphogroup appears first in the record (116.3 Ma), while the larger and truly circular morphogroup appears at 111.6 Ma. This larger one becomes a permanent component of the assemblage (100.6 Ma), then dominates over the small (from 96.5 Ma onwar ds) and eventually replaces the small one completely.
Biometric studied here allowed to quantify the early evolutionary lineage of a calcareous nannofossil species. In addition, absolute ages were assigned to evolutionary events. By that, it was possible to estimate the duration of species formation processes in coccolithophores. In this exemplary case, it took less than 1.1 Ma for the large morphogroup to develop. This has been calculated based on the time span between the last sample without the large on e (507.35 mbsf; 112.7 Ma) up until the first sample with the large one present (500.44 mbsf; 111.6 Ma). Chapter 7 - Critical re marks 121
7 Critical remarks
Beyond the discussion in the individual chapters, that have been already published or submitted to a journal, concerning the interpretation of the biometric data raised during this PhD thesis, some remarks are added in the following. This mainly includes a comparative discussion on the light -attenuation theory, introduced in chapter 2 and used to explain the findings concerning OAE 1a in chapter 3, and other possible theories, that may be used to explain the obtained data. This is of special value to this t hesis, as the light attenuation theory introduces a new view into the scientific community.
It is generally agreed upon, that the earliest Aptian oceanic anoxic events 1a represents an extensive perturbation of the global carbon cycle triggered by the inj ection of greenhouse gases into the atmosphere and thus a dramatic climate change due to high submarine volcanic activity in the Mid -Pacific Ocean ( see review in Erba et al., 2015 and chapter 1.2 of this thesis ). The biometric response of some coccolithoph ore species to this event reported from the western Tethys, the Mid -Pacific, the North Sea and the Lower Saxony Basin (Erba et al., 2010; Lübke and Mutterlose, 2016) is therefore most probably a consequence of any possible side - effect in this complex paleo ecological context . The following possible effects and their impact on coccolith size have been vividly discussed in science. Advantages and disadvantages of the theories are pointed out with regard to the data of this PhD thesis.
Ocean acidification
Dis solving CO 2 in the ocean results in a lowered pH, towards more acidic conditions (Fig
7.1) . This is also happening in today’s oceans due to the anthropogenic CO 2 input from intense combustion of fossil fuel resources since the industrial revolution in the 19 th century. Modern - day ocean pH is around 8.1 with a CO 2-concentration in the atmosphere of 400 ppm, pre - industrial values were 8.4 (pH) and 280 ppm (dissolved CO 2). The trend is extrapolated to go on (Caldeira and Wickett, 2003). Several studies suggest that this fast acidification could hamper biocalcification in the ocean . Studying the most important marine carbonate producers on Earth, coccolithophores, Riebesell et al. (2000) found, that the two studied species Emiliania huxleyi and Gephyrocapsa oceanica were effected by more acidic ocean water chemistry. Both showed abundant coccolith malformation and a decrease in carbonate production (Riebesell et al., 2000). Chapter 7 - Critical re marks 122
Increased volcanic activity, whether submarine or continental, introduces excess amounts of CO 2 into the Earth’s system. Based on this, Erba et al. (2010) proposed, that the earliest
Aptian OAE 1a, characterized by intense CO 2 degassing, could also have been a period of ocean acidification, marked by a response of marine calcifying coccolithophores. Indeed, the study revealed a coccolith size decrease of three species ( Biscutum constans, Zeugrhabdotus erectus and Discorhabdus i gnotus ) and malformation in Watznaueria barnesiae associated with the maximum of CO 2 degassing at the base of OAE 1a. This has been taken as proof for ocean acidification during this time (Erba et al., 2010).
However, ocean acidification during OAE 1a is s till not directly proven. Until now, science lacks a proxy for ocean pH. The only proposed method so far, the boron isotope -pH relation in carbonates has been shown to suffer from great obstacles such as the exact understanding of the isotopic relation bet ween borate a nd carbonates, vital effect for example in foraminifera and the low resolution of some million years due to the long residence time of B in seawater of 3 to 20 Ma (see review in Pagani et al., 2005). Therefore, direct proof of ocean acidificat ion during OAE 1a is impossible at the moment. In addition, Erba et al. (2010) had found a delay of acidification in the surface water to deep water of 25 to 30 kyrs, which is, however, much greater than the order of magnitude for ocean mixing of only 1000 yrs (Gibbs et al., 2011). The age -modelling of the sections studied by Erba et al. (2010) is problematic so that the duration of CO 2 releases cannot be quantified (Gibbs et al., 2011). This is of great importance, as only rapid CO 2 releases (<10 kyrs) of huge quantities can cause substantial acidification of the ocean (Ridgwell and Schmidt, 2010). These facts make the presence of ocean acidification during OAE 1a debatable.
Additionally, the relation between pH and coccolith size, lower pH results in smal ler/malformed coccoliths, also lacks evidence. The study by Riebesell et a. (2000) did not include the effect of increased pH on coccolithophores and it only used two species. The response of two other recent species, Coccolithus pelagicus and Calcidiscus leptoporus , to lowered pH was significantly different. In a study of Langer et al. (2006), batch cultures of these two species had been exposed to lowered and increased ambient pH. C. pelagicus showed no change in calcification in either direction, while C. leptoporus showed peak calcification when pH equaled modern -day values. Both lower and higher pH caused a decrease in calcification rate and a rise in number of malformed coccoliths (Langer et al., 2006). Further investigations of the latter species in m aterial from the last glacial maximum Chapter 7 - Critical re marks 123
(25 -18 kyrs B.C.) showed no response to inferred higher pH due to low CO 2 saturation levels (180 ppm). These findings question the direct pH -coccolith size relation.
In this PhD thesis, the same coccolith size decrease reported by Erba et al. (2010) during OAE 1a has been recorded , speaking for similar processes working on a global scale causing this response. However, malformed W. barnesiae specimens, as frequently observed by Erba et al. (2010) during OAE 1a as propos ed to indicate an acidification response, have not been found in the samples from the North Sea and the Lower Saxony Basin (Lübke and Mutterlose, 2016; chapter 3 in this thesis). The results of chapter 2 (Lübke et al., 2015) also show a site related coccol ith size discrepancy in pre -OAE 1a times without ocean acidification, incorporating the data of Erba et al. (2010), that can be interpreted in line with the OAE 1a results.
Still, the lack of proof of ocean acidification is no proof that it never happened . Therefore, ocean acidification may play a role in altering ambient conditions for coccolithophores and provoking a coccolith size response.
Toxic trace metal input
Another consequence of intense submarine volcanism in the earliest Aptian is the release of trace metals into the ocean via magmatic degassing during eruptions and via hydrothermal alteration of new forming oceanic crust (Erba et al, 2015; Fig. 7.1). Of these trace metals, some may act as micronutrients, fertilizing algal communities , for exam ple iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), cobalt (Co), molybdenum (Mo), and nickel (Ni) . At elevated concentrations, however, some (Zn, Cu, Ni) may also be poisonous. Metals such as cadmium (Cd), mercury (Hg), silver (Ag), lead (Pb), tin (Sn), and chromium (Cr) are regarded as toxic (Sunda and Huntsman, 1998). Some modern coccolithophorid species show intolerance towards copper and cadmium (Brand, 1994). Trace metal enrichments associated with OAE 1a have been reported from various sites. In th e Lower Saxony Basin, cadmium, molybdenum, nickel, lead, and selenium are enriched in the Fischschiefer, the local OAE 1a lithology (Hild and Brumsack, 1998). On the Russian platform, Gavrilov et al. (2002) reported similar metal concentrations in black sh ales equivalent to OAE 1a. In the Pacific Ocean, silver, barium (Ba), cadmium, copper, chromium, nickel, lead, scandium (Sc) , selenium, and zinc show abundance peaks in the earliest Aptian before and after OAE 1a. In the western Tethys, mainly zinc, Chapter 7 - Critical re marks 124 copper , vanadium (V) and molybdenum are enriched in the upper Selli level and above (Erba et al., 2015).
Testing a possible correspondence between trace metal enrichments during OAE 1a and the proven calcification response of coccolithophores during this brief t ime interval, Faucher et al. (2017) performed laboratory experiments on recent coccolithophores. These experiments included introducing elevated trace metal concentrations (Ni, Pb, V, Zn) to batch cultures growing C. pelagicus. E. huxley i, G. oceanica, and Pleurochrysis carterae , common recent species. Results indicate species -specific responses suggesting different levels of tolerance towards trace metal poisoning. The former three species show decreased sizes, with G. oceanica being only affected at highe st concentrations . P. carterae showed no response at all.
Trace metal poisoning is a very likely consequence of submarine volcanism during OAE 1a. Elevated trace metal concentrations can directly be measured in the sediment, still today after more ~120 Ma. In the Lower Saxony Basin (Hild and Brumsack, 1998), enrichments of cadmium, molybdenum, nickel, selenium, vanadium are directly correlated to the Fischschiefer horizon. However, the authors interpreted these enrichments as a consequence of absorption to organic matter particles or the formation of stable sulphides. TOC values are as high as 6 wt% in the Fischschiefer (Hild and Brumsack, 1998). Trace metal enrichments in the Pacific Ocean and the western Tethys are less confined to OAE 1a itself (see Erba et al., 2015). There are unfortunately no trace metal data for the studied North Sea and Lower Saxony Basin sections studied in this thesis. It can, however, be hypothesized that values were elevated as the data from the proximal Hoheneggelsen core (Hild a nd Brumsack, 1998) are elevated, too.
Summarizing, trace metal poisoning and associated coccolith diminution cannot be refuted for OAE 1a with the data at hand in this PhD thesis.
Light attenuation due to muddy water
As described in chapters 3, the earlie st Aptian OAE 1a is lithologically represented as a carbonate -depleted often laminated sedimentary unit, with respect to adjacent lithological units. This sedimentary anomaly was observed in the western Tethys, northern Italy (Coccioni et al., 1987, 1989), the Vocontian Basin in southeastern France (Bréhéret, 1997) and the Lower Saxony Basin in northwestern Germany (Mutterlose, 1992 b). Chapter 2 in this thesis is a Chapter 7 - Critical re marks 125 comparative study using coccolith biometry data from the pelagic (wester n Tethys, Mid - Pacific ) and the b oreal coastal realm (North Sea, Lower Saxony Basin) of a synchronous interval unaffected by OAE 1a . It resulted in a light availability -based theory on the coccolith size distribution. Large coccolith -producing coccolithophores of the species B. constans were absent in the coastal setting, but present in the open ocean. Small coccolith -producing ones were present at all sites. This discrepancy can be explained by the existence of two distinct cryptic species, that produce either small or large co ccoliths with different ecological affinities (small coccoliths = deeper in the water column with diminished light; large coccoliths = shallower dwelling needing brighter light). In the open -ocean, the uppermost meters of the water were flooded with light ; deeper in the water, light availability diminished. This setting supported the niches for both cryptic species. The coastal muddy waters, due to continental shedding of clay particles, does only accommodate one cryptic species. These conditions are reflec ted by the sediment deposits at the sites, with higher clay content in the Lower Saxony Basin and the North Sea and low clay content in the western Tethys and the Mid -Pacific.
Additional support for this thesis, is the fact, that the increased continental weathering induced by OAE 1a (Blättler et al., 2001; Bottini et al., 2012 ; Fig. 7.1 ) and reflected in the sedimentary record by a drop in carbonate content, introduced more clay into the water and thus should have caused a drop in coccolith size of B. cons tans – and it did. The light attenuation theory does not only explain the site -related discrepancy of coccolith size but also the response to OAE 1a. The transparency of the water and thus the availability of light for photosynthesis controlling the dwelli ng depth of coccolithophores and the size of coccoliths has also been used by Suchéraz -Marx et al. (2010) to explain the cyclic abundance changes of two size groups of Crepidolithus crassus in lower Pliensbachian sediments. These changes in size were accom panies with changes in lithology, namely the carbonate vs. clay content.
Favored theory for OAE 1a
All three theories, ocean acidification, trace metal poisoning, and light attenuation, could explain the coccolith size signal reported for OAE 1a, as they represent possible side -effects of increased submarine volcanism. However, ocean acidification cannot be dire ctly proven due to a lack of available proxy. Trace metal levels show no direct correlation to the carbon isotope curve at all sites (e.g. Pacific sites). The light availability theory is favored as it explains Chapter 7 - Critical re marks 126 the OAE 1a signal and the site -related size discrepancy. However, none of the theories can be refuted and all are plausible to a certain degree.
Figure 7.1: Schematic of paleocological inter actions during OAE 1a ( based loosely on Jenkyns, 2010) , excluding elements redundant for coccolith size discussion. Red marks those elements, that may be responsible for coccolith size changes during OAE 1a.
Methods
The different approaches in this PhD thesis have shown that calcareous nannofossil biometry can serve different purposes. Biometry is an ideal tool to track single -species evolution and cryptic species occurrences, as it is solely based on quantitative mor phology. Biometric studies may, however, be problematic if preservation of calcareous nannofossils becomes an issue. Small and delicate specimen may be dissolved, while others may be overgrown. This may bias the coccolith size distribution. It is therefore useful to check preservation under the SEM or microscope or rely on other preservation proxies, such as the abundance of delicate and dissolution -prone species. Taxonomic index 127
Taxonomic index Calcareous nannofossils Axopodorhabdus albianus (Black, 1976) Wind and Wise in Wise and Wind, 1977 Biscutaceae Black, 1971 Biscutum Black in Black and Barnes, 1959 Biscutum constans (Górka, 1957) Black in Black and Barnes, 1959 Biscutum constans cavum Jeremiah, 2001 Biscutum dubium (No ël, 1965) Grün in Grün et al., 1974 Biscutum grande Bown, 1987 Biscutum intermedium Bown, 1987 Bukrylithus ambiguus Black, 1971 Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan, 1978 Calciosolenia fossilis (Deflandre in Deflandre and Fert, 1954) Bown in Kennedy et al., 2000 Calculites Prins and Sissingh in Sissingh, 1977 Chiastozygus litterarius (Gorkà, 1957) Manivit, 1971 Coccolithus pelagicus (Wallich, 1877) Schiller, 1930 Coccolithus pelagicus ssp. braarudii (Gaarder, 1962) Geisen et al., 2002 Crepidolithus crassus (Deflandre, 1954) Noël, 1965 Coccolithus Schwarz, 1894 Corollition kennedyi Crux, 1981 Corollithion protosignum (Worsley, 1971) Young and Bown, 2014 Corollithion signum Stradner, 1963 Crucicribrum anglicum Black, 1973 Cyclicargolithus Bukry, 1971 Discorhabdus No ël, 1965 Discorhabdus ignotus (Bukry, 1969) Thierstein, 1973 Discorhabdus striatus Moshkovitz and Ehrlich, 1976 Eiffelithus Reinhardt, 1965 Eiffellithus monechiae Crux, 1991 Eiffellithus parvus Watkins and Bergen, 2003 Eiffellithus turriseiffelii (Deflandre in Deflandre and Fert, 1954) Reinhardt, 1965 Emiliania huxleyi (Lohmann, 1902) Hay and Mohler in Hay et al., 1967 Taxonomic index 128
Eprolithus floralis (Stradner, 1962) Stover, 1966 Florisphaera profunda Okada and Honjo, 1973 Gartnerago chiasta Varol, 1991 Gartnerago nanum Thierstein, 1974 Gartnerago praeobliquum Jakubowski, 1986 Gartnerago theta (Black and Black and Barnes, 1959) Jakubowski, 1986 Gephyr ocapsa oceanica (Kamptner, 1943) Hayesites albiensis Manivit, 1972 Hymenomonas roseola Stein, 1878 Lithraphidites acutus Verbeek and Manivit in Manivit et al., 1977 Nannoconus Kamptner, 1931 Nannoconus abundans Stradner und Grün, 1973 Nannoconus borealis Perch -Nielsen, 1979 Owenia dispar (Varol in Al -Rifaiy et al., 1990) Bown in Kennedy et al., 2000 Orastrum persicuum Varol in Al -Rifaiy et al., 1990 Pleurochrysis carterae (Braarud and Fagerland, 1946) Christensen, 1978 Prediscosphaera Vekshina, 1959 Prediscosphaera columnata (Stover, 1966) Perch -Nielsen, 1984 Prediscosphaera spinosa (Bramlette and Martini, 1964) Gardner 1968 Repagulum parvidentatum (Deflandre and Fert, 1954) Forchheimer, 1972 Reticulofenestra Hay et al., 1966 Rhagodiscus achlyostaurio n (Hill, 1967) Doeven, 1983 Rhagodiscus angustus (Stradner, 1963) Reinhardt, 1971 Rhagodiscus asper (Stradner, 1963) Reinhardt, 1967 Rotelapillus crenulatus (Stover, 1966) Perch -Nielsen, 1984 Seribiscutum primitivum (Thierstein, 1974) Filewicz et al., Wise and Wind, 1977 Similiscutum de Kaenel and Bergen, 1993 Sollasites falklandensis Filewicz et al. In Wise and Wind, 1977 Staurolithites Caratini, 1963 Stoverius achylosus (Stover, 1966) Perch -Nielsen, 1986 Stradnerli thus geometricus (Górka, 1957) Bown and Cooper, 1989 Syracosphaerales (Hay, 1977) Young et al., 2003 Taxonomic index 129
Tegumentum stradneri Thierstein in Roth and Thierstein, 1972 Tetrapodorhabdus decorus (Deflandre in Deflandre and Fert, 1954) Wind and Wise in Wise and Win d, 1977 Tranolithus orionatus (Reinhardt, 1966a) Reinhardt, 1966b Watznaueria Reinhardt, 1964 Watznaueria barnesiae (Black, 1959) Perch -Nielson, 1968 Watznaueria fossacincta (Black, 1971) Bown in Bown and Cooper, 1989 Watznaueria ovata Bukry, 1969 Wigwamma Manton et al., 1977 Zeugrhabdotus Reinhardt, 1965 Zeugrhabdotus acanthus Reinhardt, 1965 Zeugrhabdotus diplogrammus (Deflandre in Deflandre and Fert, 1954) Burnett in Gale et al., 1996 Zeugrhabdotus erectus (Deflandre in Deflandre and Fert, 1954) Reinhardt, 1965 Zeugrhabdotus howei Bown in Kennedy et al., 2000
Foraminifera Biticinella breggiensis Gandolfi, 1942 Costellagerina lybica Barr, 1972 Planomalina buxtorfi Gandolfi, 1942 Rotalipora Brotzen, 1942 Rotalipora appenninica Renz, 1936 Ticinella primula Luterbacher , 1963 Refere nces 130
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Curriculum Vitae
Personal data
Name Nathalie Lübke
Date of birth 7.1.1988
Place of birth Velbert, Germany
Nationality German
Status Unmarried
Education
01/2013 – 06/2017 PhD student and research assistant at Ruhr -Universität Bochum
10/2010 – 09/2012 MSc student of Geosciences at Ruhr -Universität Bochum
MSc thesis: Calcareous Nannofossil of the Lower Cretaceous (Barremian -Aptian) of the southern North Sea
10/2007 – 09/2010 BSc student of Geosciences at Ruhr -Universität Bochum
BSc thesis: Einfl uss von Radionukliden in Mineralen auf Struktur und Eigenschaften
08/1998 – 07/2007 Städtisches Gymnasium Wülfrath