The Onset of Planktic Foraminifera in the Mid-Cretaceous of the Boreal Realm

The onset of planktic foraminifera in the mid-Cretaceous of the Boreal Realm

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

vorgelegt von Sylvia Rückheim aus Essen

Januar 2005

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.

Referent: Prof. Dr. J. Mutterlose Korreferent: Prof. Dr. D. Michalzik 3. Gutachter: Prof. Dr. H. Gies

Tag der Disputation: 09.05.2005

Chapter 1: Introduction 1

1. Introduction

1.1. Palaeoceanography and climate in the mid-Cretaceous The mid-Cretaceous (Aptian-Turonian; ~121 to 89 Ma; Gradstein et al., 1994) was a period of major changes in the oceanic environment. These changes were caused by increased tectonic activity which was linked to the opening of the Atlantic ocean. The onset of elevated oceanic crust production, coupled with enhanced volcanism (e.g., Larson, 1991a,b) led from cool conditions in the early Cretaceous to a greenhouse world in Aptian times. These warm and humid conditions were accompanied by a long-term sea-level rise (Haq et al., 1987; Fig. 1.1), elevated average temperatures (Barron et al., 1995) and low latitudinal temperature gradients (e.g., Huber et al., 1995). The warming trend reached its maximum during the Turonian and persisted through the early Campanian (Clarke and Jenkyns, 1999; Wilson et al., 2002). These specific palaeoceanographic conditions in the mid-Cretaceous favoured the deposition and burial of organic, carbon-rich sediments subsequently preserved as black shales (e.g., Schlanger and Jenkyns, 1976; Arthur and Premoli Silva, 1982). Black shales have a regionally and supraregionally distribution. These sediments, which were deposited in all major oceans, are commonly referred as Oceanic Anoxic Events (OAEs; e.g., Schlanger and Jenkyns, 1976; Arthur et al., 1990).

Cretaceous climate changes Plankton evolutionary events

Sea level Humidity - Calcareous Planktic Stage high low aridity nannofossils foraminifera Radiolaria

65 System 200 100 0 m Maastrichtian 6% speciation 49% 71 9% 6% 20% OAE 2 58% 22% extinction Campanian OAE? 5% 23% 26%

83 Santonian 23% 85 26% 30% OAE 1d Coniacian 28% 89 27% 20% Turonian OAE 1c 93 5% 20% Cenomanian 98 27% 5%

Age (Ma) Albian 36% 29% 29% 42% CRETACEOUS 112 OAE 1b

Aptian 69% 23%

121 31% Barremian OAE? 127 Aptian Albian Cenomanian Turon. Stage Hauterivian 7% 30% 20% 132 22% 7% 27% OAE 1a Valanginian 7% 22% 41% 137 7% 26% Berriasian Humid Alter- nating Arid 144 Barrem.

Fig. 1.1: Cretaceous climate changes (sea-level changes after Hardenbol et al., 1998; humid-arid cycles exemplarily for western Europe) and mid-Cretaceous plankton evolutionary trends showing percentages of species first appearances (speciation) or disappearances (extinction; modified after Leckie et al., 2002). 2 Chapter 1: Introduction

The mid-Cretaceous was also a time of rapid radiation of marine biota and turnover of marine planktic organisms like calcareous nannofossils, planktic foraminifera and radiolaria (e.g., Lipps, 1970; Tappan and Loeblich, 1973; Erbacher and Thurow, 1997; Larson and Erba, 1999; Leckie et al., 2002). The opening of new niches as a result of changed oceanic circulation permitted the occupation and partition of new habitats. Diversity and abundance of marine organisms increased dramatically during this period of reorganisation (Premoli Silva and Sliter, 1999; Leckie et al., 2002). A causal relationship between the evolution of planktic organisms and environmental changes seems to exist due to the fact that radiolaria show high rates of evolutionary turnover at or near the mid-Cretaceous OAEs (Erbacher et al., 1996; Erbacher and Thurow, 1997). Calcareous nannofossils and planktic foraminifera were also influenced to varying degrees by the OAEs (e.g., Leckie, 1987; Bralower, 1988; Erba, 1994; Hart, 1999; Premoli Silva et al., 1999; Leckie et al., 2002).

1.2. Early Cretaceous palaeogeographical setting in NW Europe During the Early Cretaceous the NW European area was composed of a number of basins, which formed the southernmost extension of the Boreal-Arctic Sea further to the north. There also existed marine sea-ways towards the Tethys in the south (Mutterlose, 1992a). Due to the palaeogeographical position of NW Europe between the Boreal and Tethyan Realms, changes in nannofloral and faunal assemblages become more obvious in this area than elsewhere. Furthermore, the mesozoic history of the North Sea area is closely related to the opening of the Atlantic (Ziegler, 1978). As a result of the tectonical movements the North Sea area was separated into several sub-basins by barriers and islands (Ziegler, 1981). The Barremian is characterised by an overall regressive nature (Rawson and Riley, 1982; Ruffel, 1991). The Carpathian sea-way was closed in the Early Barremian-Early Aptian, with the North Sea and the adjacent basins becoming restricted marginal seas without any direct connection to the Tethys (Mutterlose, 1992a). This palaeogeographical configuration led to the deposition of several finely laminated beds enriched in organic matter (Hauptblätterton/Munk Marl Bed). They occur throughout the North Sea area and the NW German Basin (e.g., Rawson and Mutterlose, 1983; Mutterlose and Harding, 1987; Thomsen, 1987). Since there was no direct sea-way to the Tethys, endemic species evolved simultane- ously within the nannofloras and faunas and became quite abundant (Mutterlose and Böckel, 1998). Tethyan derived planktic foraminifera may have migrated into the NW European basins via an open sea-way extending west of England and north of Scotland. Kemper (1995a) suggested the existence of a direct connection between the Boreal and Tethyan Realms. Based on similarities of ammonite assemblages from the North Sea area and the Kimmerian-Cauca- sian seas he supposed a sea-way via S Poland, but no evidence was supplied due to a lack of Aptian ammonites on the northern Russian Platform and S Poland. During the Aptian several transgressions, with intervals of shallowing, enlarged the Chapter 1: Introduction 3 depositional areas in NW Europe (Ruffell, 1991). The early Aptian transgression caused significant palaeoceanographic and palaeogeographical changes. New sea-ways between the Tethys and the Boreal Realm opened via the English Proto-Channel area and the Western Approaches Trough (Mutterlose, 1992a). The palaeoceanographic shifts are also reflected in the composition of the mid-Cretaceous marine floras and faunas, which show a rapid evolution and radiation of planktic organisms (e.g., Erbacher and Thurow, 1997; Larson and Erba, 1999; Leckie et al., 2002). Endemic taxa hitherto restricted to the Boreal and Tethyan Realms disappeared and were replaced by more cosmopolitan organisms (e.g., Mutterlose, 1998; Mutterlose and Böckel, 1998). For the first time planktic foraminifera experienced a significant radiation (e.g., Hart, 1999; Premoli Silva and Sliter, 1999). Tethyan derived foraminiferal taxa were observed in the NW European basins (Weiss, 1995). Calcareous nannofossils, ammonites and belemintes experienced the extinction of Boreal species while new cosmopolitan taxa evolved (Mutterlose, 1992a). The Early Albian is marked by another transgression which further flooded the NW European area. Marine Albian sediments were observed in SE and NE England, the Nether- lands, Germany, parts of Scandinavia and the Russian Platform (Schott et al., 1969; Ziegler, 1990). A direct connection between the Boreal and the Tethyan Realm via the Proto-Channel and the Angelo-Parisan Basin opened in theEarly Albian (e.g., Destombes et al., 1973; Kemper, 1982). The early Albian microfaunas and nannofloras of the European Basins are strongly impoverished (Price, 1977).

1.3. Planktic foraminifera Foraminifera are single-celled marine organisms belonging to the rizhopod protozoa (Pr- otista). They possess an elaborate, mineralised, intra-ectoplasmic skeleton (shell or test; Bignot, 1985). According to their way of life, two major groups of foraminifera can be distinguished: 1. Benthic forms, which are known from the earliest Cambrian onwards, occupy a large variety of ecological niches. They occur in and on different substrates (endobenthic and epibenthic) and at various depths in the marine realm. Furthermore they are found in brackish estuaries or salt marshes. Their distribution is mainly controlled by the oxygen content of the bottom water and nutrient availability (e.g., Van der Zwaan et al., 1999) 2. Planktic species occur for the first time in the mid-Jurassic (Bajocian or Bathonian). They inhabit in general the open ocean and float freely in the upper part (photic zone) of the water column. Although planktic foraminifera are most common in tropical and subtropical waters, they also occur in all latitudinal provinces including the Arctic and Antarctic ice (Hemleben et al., 1989). Living foraminifera have their maximum abundance in eutrophic near-surface waters between 10 and 50 m depth (Arnold and Parker, 1999). They can also be found at several hundred meters of water depth. According to Bé (1965) the spinose species Hastigerinella digitata was observed below 1000 m. The small-sized tests (max. 600 µm in 4 Chapter 1: Introduction diameter; BouDagher Fadel et al., 1997a) of planktic foraminifera are glass-like transparent (hyaline). The perforate and lamellar shells consist mostly of low-Mg calcite, which is radially structured (e.g., Towe and Cifelli, 1967; Hansen, 1968). Only the Jurassic and Early Creta- ceous Favusellaceae had an aragonitic test (Sen Gupta, 1999).

1.3.1. Evolution of planktic foraminifera The origin of planktic foraminifera is still uncertain. But it is generally agreed that this group probably evolved from a benthic ancestor via a meroplanktic (partially planktic) stage into a holoplanktic (totally planktic) mode of life. Fuchs (1967) described a group of ‘Trias- Globigerinen’ and postulated that this was the first group with a planktic mode of life. Ac- cording to Fuchs’ (1975) scheme, the Triassic genus Oberhauserella was the ancestor of the Jurassic planktic genera Conoglobigerina and Praegubkinella and gave rise to the evolution and diversification of the planktic foraminifera. Re-examinations of the poorly preserved Triassic specimens classify the oberhauserellids as trochospiral benthic taxa with calcareous- hyaline tests (Simmons et al., 1997; Hart et al., 2002). Nevertheless, Hart et al. (2003) con- firm that the ancestral stock of planktic foraminifera is identified as Oberhauserella quadrilobata s.s. (Fig. 1.2). Simmons et al. (1997) postulate the meroplanktic conoglobigerinids (C. avariformis, C. balakhmatovae, C. dagestanica and (?) C. avarica) of Bajocian age to be the oldest known planktic species, which are only known from eastern Europe (central and northern Tethys). All these species possess an aragonitic, microperforate and pseudomuricate test with an umbilical aperture and subglobular chambers (Simmons et al., 1997). Their ancestor must be sought under a benthic species with subglobular aragonitic chambers, which underwent a series of morphological changes in pre-Bajocian age (e.g., Hart, 1980; Caron, 1983). According to Hemleben et al. (1989) sea surface temperature is one of the main con- trolling factors for evolutionary trends. During cold periods, the diversity of species dimin- ished markedly and the morphology of the tests reverted to more simple trochospiral forms. Subsequent warm periods were accompanied by species radiation, vacated niches were in- vaded and exploited by newly evolving species. Throughout the Mesozoic the evolution and diversification of planktic foraminifera appears to coincide with the repeated development of anoxic water masses in the world oceans and a subsequent sea-level rise. Hart et al. (2003) suggested the Early Toarcian OAE (Oce- anic Anoxic Event) in combination with a sea-level highstand as the triggering mechanism for a change from a benthic to a planktic mode of life. In the Early Toarcian sudden dissociations of sub-sea gas hydrates may have led to an oceanic perturbation (Hesselbo et al., 2000) and opened up new ecological niches. As a probable consequence, foraminifera merged into a meroplanktic mode of life (Hart et al., 2003). The Bajocian sea-level highstand (Haq et al., 1987) enforced the evolution and distribution of the early planktic foraminifera. Chapter 1: Introduction 5

Stage Zonation

65 System Abathomphalus mayaroensis ? Maastrichtian C. contusa - R. fructicosa 70 Gansserina 71.3 ± 0.5

gansseri Abathomphalus Gublerina Plummerita

Globotruncana aegyptica Globotruncana kassabiana Planoglobulina Globotruncanella 75 Globotruncanella havanensis Rugotruncana Radotruncana calcarata Rugoglobigerina ? Campanian Globotruncana Hedbergella ventricosa Globotruncanita Heterohelix 80 Racemiguembelina

Globotruncanita Pseudotextularia

elevata Marginotruncana ? Pseudoguembelina 83.5 ± 0.5 Hastigerinoides Ventilabrella

Dicarinella asymetrica Guembelitria 85 Santonian Laeviheterohelix 85.8 ± 0.5 ? Sigalia OAE 3 Coniacian Dicarinella concavata 89.0 ± 0.5 Archaeoglobigerina ? Falsotruncana 90 Marginotruncana sigali Globigerinelloides Dicarinella Helvetoglobotruncana Whiteinella Turonian Helvetoglobotruncana Praeglobotruncana helvetica ? Lunatriella 93.5 ± 0.2 Whiteinella archaeocretacea ? OAE 2 95 Rotalipora cushmani ? Cenomanian R. reicheli

Rotalipora brotzeni Schackoina 98.9 ± 0.6 ? 100 Rotalipora appenninica OAE 1d Rotalipora ticinensis Rotalipora

Rotalipora subticinensis Anaticinella ? OAE 1c Ticinella praeticinensis ? 105 Albian Ticinella primula Biticinella Ticinella 110

Hedbergella Blefuscuiana Planomalina Alanlordella planispira Ascoliella OAE 1b 112.2 ± 1.1 Ticinella bejaouaensis 115 Hedbergella trocoidea G. algerianus Aptian Leupoldina G. ferreolensis Claviblowiella Leupoldina cabri OAE 1a 120 Favusella ? 121.0 ± 1.1 Globigerinelloides

blowi Lilliputianella Praehedbergella Blowiella Barremian 125 H. similis-H. kuznetsovae

127.0 ± 1.4 Gorbachikella Hedbergella sigali 130 Hauterivian - Hedbergella delrioensis 132.0 ± 1.9

135 Valanginian

137.0 ± 2.2 Favusella hoterivica

140 Berriasian Conoglobigerina ?

145 144.2 ± 2.6 ? Tithonian 150 150.7 ± 3.0 ? Kimmeridgian globuligerinids 154.1 ± 3.2 155 and Oxfordian conoglobigerinids Compactogerina Haeuslerina 159.4 ± 3.6

160 Globuligerina ? Callovian ?

165 164.4 ± 3.8 Bathonian

170 169.2 ± 4.0

Bajocian 175

JURASSIC176.5 ± 4.0 CRETACEOUS

Aalenian 180 180.1 ± 4.0

? ? 185 Toacian

OAE 190 189.6 ± 4.0

Pliensbachian

195 195.3 ± 3.9

Sinemurian 200 Oberhauserella Praegubkinella 201.9 ± 3.9

Hettangian 205 205.7 ± 4.0

Fig. 1.2: The Jurassic origin and the Cretaceous evolution of planktic foraminifera (modified after Hart, 1999 and Hart et al., 2003), plotted against the biostratigraphical zonation scheme of Sliter (1989) and Premoli Silva and Sliter (1999). The study interval is marked by a light grey bar. 6 Chapter 1: Introduction

Major changes in the composition of planktic foraminifera are also related to mid-Cretaceous OAEs. The Early Aptian OAE 1a goes along with a first major diversification of planktic foraminifera (Fig. 1.2), which became abundant in the world oceans for the first time (e.g., Leckie et al, 2002). The first occurrence of the aberant species Leupoldina cabri coincides with the onset of the OAE 1a (Leckie et al., 2002). This goes along with the first occurrence of ornamentated forms and an increase in size of the tests (Premoli Silva and Sliter, 1999). Another significant turnover within the planktic assemblage at the Aptian/Albian boundary goes along with the OAE 1b. According to Leckie et al. (2002) planktic foraminifera suffered their greatest rates of extinction (up to 69%) and speciation (max. 23%) within this interval. The development of a single-keeled periphery is paralleled by the Late Albian OAE 1c and the following seal-level rise (e.g., Hart et al. 2002; Leckie et al., 2002). This evolutionary step allowed the colonisation of deeper habitats in the water column (Caron and Homewood, 1983; Hart, 1999). The single-keeled morphotypes became extinct at the base of the OAE 2 (Cenomanian-Turonian boundary interval). The elimination of niches of deeper water dwellers was triggered by the breakdown of the thermocline during the OAE 2. This may have been caused by an expansion of the oxygen minimum zone into deeper waters (Hart, 1980; Won- ders, 1980; Leckie, 1985, 1987) or by an abrupt warming of deeper waters (Huber et al., 1999). After the OAE 2 these niches were occupied by the newly evolved double-keeled taxa.

1.3.2. Applications of planktic foraminifera Planktic foraminifera are a very useful tool for biostratigraphical purposes and palaeoenvironmental reconstructions (see Hemleben et al., 1989 and references therein). In the late fifties and early sixties of the twentieth century the oil industry detected the practical value of planktic foraminifera for oil exploration and supported further research on this group (e.g., Hiltermann and Koch, 1962). Planktic foraminifera serve as an excellent stratigraphical tool for marine Cretaceous and Cenozoic sediments due to their great abundance, their short living- range, their rapid morphological evolution and their good fossilisation potential (see Bolli et al., 1985 and references therein). During the Cretaceous planktic foraminifera occur in high abundances and diversities in the southern latitudes. Therefore detailed zonation schemes based on planktic foraminifera were developed for the Tethyan Realm (e.g., Sigal, 1977; Premoli Silva and Sliter, 1995). In the Boreal Realm planktic foraminifera are less important for biostratigraphical purposes due to their low abundances and/or the small size of this group in the northern latitudes. Thus only few approaches for a correlation with Tethyan schemes exist (Hart et al., 1989; Weiss, 1995). Zonation schemes restricted to the North Sea area were published by Burnhill and Ramsey (1981) and Banner et al. (1993). The horizontal and vertical distribution of planktic foraminifera in the world oceans is mainly controlled by abiotic factors like temperature, salinity, nutrient supply and oxygen (e.g., Hemleben et al., 1989). Furthermore, the distribution is affected by biological factors Chapter 1: Introduction 7 like productivity, food supply or symbiosis (Bé, 1977). Studies on palaeobiogeographical distribution patterns show a distinctive depth stratification of planktic foraminifera (e.g., Hart and Bailey, 1979; Hart, 1999; Price and Hart, 2002). Globular forms, such as the genus Hedbergella seem to have inhabited near-surface waters, while flattened, keeled morphotypes represent deeper habitats (e.g., Caron and Homewood, 1983). According to Leckie (1987) depth stratification of planktic foraminifera seem to be partially a function of water density, especially in low latitudes. The ratio of keeled specimens within a planktic assemblage can be regarded as a proxy for water depth with increasing ratios indicating deeper waters (e.g., Leckie, 1987). Moreover, the ratio of planktic to benthic specimens of a foraminiferal assemblage (P/B ratio) gives an indication of open-oceanic versus near-shore conditions (e.g., Murray, 1976), with enhanced planktic ratios pointing to a more marine environment. This proxy can therefore be used for palaeoceanographic reconstructions (e.g., Gräfe, 1999). Another important application of planktic (and benthic) foraminifera is the analysis of stable isotopes and trace elements in their tests in order to reconstruct and understand the palaeoceanography. The ratio of the stable oxygen isotopes 18O /16O is used for the estimation of water temperature (e.g., Vincent and Berger, 1981; Anderson and Arthur, 1983). Variations of δ13C isotopes are a useful tool for the reconstruction of water mass movement and palaeoproductivity (e.g., Shackelton, 1977; Ganssen and Kroon, 2000). The δ11B composition of foraminiferal shells displays changes of sea-water pH (e.g., Sanyal et al., 1995). Ca/Mg ratios can be used to reconstruct fluctuations in sea surface temperatures (e.g., Hastings et al., 1998). The Sr/Ca ratio serves as a proxy for the estimation of sea-level fluctuations (e.g., Graham et al., 1982). Stable isotope and trace element analysis provide good results for recent foraminifera, but various factors like diagenetic alteration and dissolution of the tests or the depth of calcification can hamper the application on fossil specimens Further on, the calcareous tests of planktic foraminifera produce a significant amount of carbonate in the world oceans. In todays oceans planktic foraminifera are one of the most important contributors of pelagic carbonates in addition to calcareous nannofossils and pteropods. According to Schiebel (2002) they contribute 32-80% of the total deep marine

CaCO3 budget. Although hardly any supporting data exist for the Mesozoic, it is reasonable to assume that planktic foraminifera have been one of the most important carbonate producers since their radiation in the Early Aptian.

1.4. Objectives The PhD thesis focuses on late Early Cretaceous (Barremian-Albian) planktic foraminifera from marine sediments of NW Europe. While planktic foraminifera are well studied in low latitudinal sections (e.g., Bellanca et al., 2002; Leckie et al., 2002; Verga and Premoli Silva 2002, 2003a,b), only few data exist from Boreal sections (e.g., Gradstein et al, 1999, Ainsworth et al., 2000). Due to their small size (in general <200 µm) and the often low abundances they 8 Chapter 1: Introduction may have been overlooked by previous workers. In the past only little attention has been payed to the stratigraphical value of Early Cretaceous planktic foraminifera in NW Europe (Banner et al., 1993, Weiss, 1995). A NW German on-shore section and a BGS borehole, located in the Central North Sea Basin, were studied with respect to the palaeoceanographic and palaeogeographical implications of planktic foraminifera. The investigated interval covers the first major phase of adap- tive radiation of this group in the Boreal Realm. Apart from reconstructing the Barremian- Albian palaeoenvironment, planktic foraminifera were used for establishing a biostratigraphical zonation scheme. Six Boreal Planktic Foraminiferal zones encompassing the Barremian to Albian interval were identified and allow for a comparison with Tethyan sections.

1.5. Thesis overview In addition to the introduction (Chapter 1) this thesis consists of four chapters. Chapters 2 to 4 contain three manuscripts, which either have been published or have been submitted for publication in international peer-reviewed journals. Chapter 2 (“The Early Aptian migration of planktic foraminifera to NW Europe: the onset of the mid-Cretaceous plankton revolution in the Boreal Realm” by S. Rückheim and J. Mutterlose; published 2002 in Cretaceous Research 23, 49-63) presents a study of Early Aptian planktic and benthic foraminifera from NW Germany. The palaeoecological reaction of the studied microfossils is described with respect to the significant palaeogeographical changes during this period. The findings suggest an evolution of the Lower Saxony Basin from a restricted marginal sea in Barremian-earliest Aptian times to a hemipelagic ocean in the late Early Aptian. In addition, a first correlation with the Tethyan planktic foraminiferal zonation scheme was established. S. Rückheim has gained the foraminiferal data and has written the paper. J. Mutterlose supervised the work on this paper and revised the text. In Chapter 3 (“Integrated stratigraphy of an Early Cretaceous (Barremian - Early Albian) North Sea borehole (BGS 81/40)” by S. Rückheim, A. Bornemann and J. Mutterlose; accepted for publication in Cretaceous Research) a stratigraphical scheme of a North Sea borehole is presented. Based on calcareous nannofossils, planktic foraminifera and stable carbon isotopes a hiatus covering the early Aptian is documented for the Barremian to Albian succession. This is in contrast to previous assumptions of a complete sedimentary record for this borehole. Furthermore, a new Boreal planktic foraminiferal zonation scheme was developed. S. Rückheim has studied the foraminifera and δ13C signature. She has written this research paper and tied together the different types of data. A. Bornemann has contributed the calcareous nannofossil counts and wrote the nannofossil section. J. Mutterlose advised the work on this paper and emended the text. In the study presented in Chapter 4 (“Planktic foraminifera from the mid-Cretaceous (Barremian - Early Albian) of the North Sea Basin: Palaeoecological and palaeoceanographic Chapter 1: Introduction 9 implications” by S. Rückheim, A. Bornemann and J. Mutterlose; submitted for publication to Marine Micropaleontology) the ecological and palaeoceanographic implications of planktic and benthic foraminifera and calcareous nannofossils of BGS Borehole 81/40 are discussed. The findings indicate a restricted environment with enhanced stratification and/or warm, oligotrophic conditions of the surface water in the Barremian and earliest Aptian. Reduced oxygenation of the bottom water prevailed in this period. A palaeoceanographic change towards an open-oceanic setting with presumably cool, more eutrophic and aerobic surface waters and aerobic to dysaerobic bottom-water conditions is described for the Late Aptian. This article was written by S. Rückheim who was in charge of the foraminiferal data and connected them with the calcareous nannofossil data. A. Bornemann contributed the nannofossil dataset and wrote this section. J. Mutterlose supervised the work on this paper and revised the text. Chapter 5 summarises the results of the thesis. The manuscripts (Chapters 2, 3, 4) have been modified to achieve a uniform format of the PhD thesis, misspellings and formal errors have been corrected. The taxonomy of planktic foraminifera follows BouDagher Fadel et al., (1997b and references therein), Premoli Silva and Sliter (2002 and references therein) and Verga and Premoli Silva (2002, 2003a, b). Sam- ple material, smear slides and residues are housed at the Ruhr-Universität Bochum. 10 Chapter 1: Introduction Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 11

2. The Early Aptian migration of planktic foraminifera to NW Europe: the onset of the mid-Cretaceous plankton revolution in the Boreal Realm

Sylvia Rückheim and Jörg Mutterlose

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany (published 2002 in Cretaceous Research 23, 49-63)

Abstract Occurrences of planktic foraminifera in marine sediments of Early Aptian age (Early Creta- ceous) are described from NW Germany. The distribution patterns are discussed with respect to their abundance and diversity. The data reflect the palaeoceanographic change from a restricted marginal sea with a stratified water column in earliest Aptian times to a hemipelagic open oceanic, well-oxygenated water setting in the late Early Aptian. Small hedbergellids appear in the Early Aptian Fischschiefer. They are rare within this and the overlaying Dark Clays and common within the Hedbergella Marl. Some horizons within the Fischschiefer and Dark Clays yield the planktic foraminiferal genera Claviblowiella, Blowiella and Globige- rinelloides. These three genera indicate the first short-term Tethyan influxes. Occurrences of Leupoldina and Lilliputianella, typical Tethyan genera, within the Hedbergella Marl support further warm-water influxes. The first appearance of Leupoldina cabri allows for a correlation with the planktic foraminiferal zonation of the Tethys. Within the Rethmar section the Blowiella blowi Zone includes the Fischschiefer, the Dark Clays and the base of the Hedbergella Marl. The subsequent L. cabri Zone includes the upper part of the Hedbergella Marl.

Keywords: Aptian; Foraminifera; Lower Saxony Basin; Palaeoecology; Biostratigraphical zonation; Migration.

2.1. Introduction In Early Cretaceous times the Lower Saxony Basin (LSB) formed a link between the Boreal- Arctic Realm in the north and the Tethys in the south. This intermediate palaeogeographical position allowed preservation of both Boreal- and Tethyan-derived marine floras and faunas which is more apparent in this part of NW Europe than elsewhere. Widespread palaeoceanographic and palaeogeographical changes that can be linked to a long-term sea-level rise and the onset of greenhouse conditions (e.g., Schlanger et al., 1981; Larson, 1991a,b) occurred close to the Barremian/Aptian transition. While neritic conditions prevailed throughout Berriasian-Barremian times in NW Europe, the Early Aptian was marked by a shift towards 12 Chapter 2: The Early Aptian migration of planknic foraminifera to NW Europe... more hemipelagic settings in the LSB. These palaeoceanographic changes can be linked to a worldwide shift in the composition of marine taxa: endemic taxa, restricted to the Boreal or the Tethyan Regions, became extinct and were replaced by more cosmopolitan organisms. Since the beginning of the last century the Aptian sediments of the LSB have been studied with respect to their faunal and floral content. Based on previous studies (e.g., v. Koenen, 1902; Stolley, 1908), high resolution biostratigraphical zonations have been suggested for ammonites (Kemper, 1995a), belemnites (Mutterlose, 1990) and calcareous nannofossils (Bischoff and Mutterlose, 1998). Earlier studies of foraminifera (e.g., Eichenberg, 1933; Hecht, 1938) were synthesised by Bartenstein and Bettenstaedt (1962) and Meyn and Vespermann (1994); these authors mainly dealt with benthic foraminifera. Apart from Weiss (1995), who concentrated on biostratigraphical aspects, hardly any attention has been paid to the Early Aptian planktic foraminifera in the LSB. The aim of this paper is to describe the early radiation of planktic foraminifera that have been recovered from the Lower Aptian of the LSB. This study focuses in particular on the onset of planktic foraminifera in the Early Aptian black shales (Fischschiefer). These shales are considered to be a product of a global anoxic event (OAE 1a; e.g., Larson 1991a,b; Bralower et al., 1994; Mutterlose and Böckel, 1998). The composition of the foraminiferal assemblages is used to understand better the properties of surface and bottom waters (O2-content, salinity, temperature). Finally, an improved biostratigraphical correlation of the Lower Aptian succession of the LSB with the standard low-latitude biostratigraphical schemes (e.g., Caron, 1985; Sliter, 1989, 1992; Robaszynsky and Caron, 1995; Premoli-Silva and Sliter, 1999) is achieved.

2.2. Geological and palaeontological background 2.2.1. The Aptian setting In NW Europe and elsewhere the Barremian/Aptian boundary interval is marked by profound palaeoceanographical changes. Increased sea-floor-spreading rates linked to the opening of the Atlantic (Ziegler, 1989) and the Pacific superplume (Larson, 1991a,b) added to increased production of CO2, which in turn caused the acceleration of the mid-Cretaceous Greenhouse climate. These conditions, combined with a high eustatic sea-level (Hardenbol et al., 1998), led to global warmth with, presumably, a lack of climatic extremes. Ultimately linked to these processes were shifts in the composition of marine floras and faunas, as well as rapid evolution and radiation of planktic organisms. Indeed the Aptian-Albian period may be viewed as a major period of turnover during which new planktic organisms appeared. The Barremian and earliest Aptian successions of the LSB and the North Sea may be described as representing a restricted marginal sea with a stable, well-stratified water column. Owing to increased run-off during more humid periods, the salinity of surface waters was slightly reduced. This caused a stable surface-water stratification and a stable thermo- and pycnocline; as a result, suboxic to anoxic conditions prevailed in bottom waters in an epi- Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 13 continental shelf setting (e.g., Mutterlose and Böckel, 1998). These conditions led to the deposition of finely laminated black shales, the Fischschiefer. Biostratigraphically the Early Aptian Fischschiefer can be correlated with black shales in the Mediterranean region, e.g., the Livello Selli in Italy and the Niveau Goguel in SE France. All of these black shales are assigned to the Oceanic Anoxic Event 1a (OAE 1a), which has been observed throughout the Atlantic Ocean. In northwest Europe, calcareous mudstones (Hedbergella Marl), which reflect a return to well-oxygenated bottom-water conditions, overlie the black shales of the Fischschiefer. Detailed data for this part of the sequence of the LSB have been published recently by Littke et al. (1998: organic geochemistry), Hild and Brumsack (1998: inorganic geochemistry), Benesch et al. (1998: mineralogy), Bischoff and Mutterlose (1998: nannofossils), and Below and Kirsch (1997: palynology). A comprehensive review of the succession under discussion is given by Mutterlose and Böckel (1998).

2.2.2. Early evolution of planktic foraminifera The earliest fossil planktic foraminifera (genus Conoglobigerina) appear in the Bajocian of the central and northwest Tethys (Simmons et al., 1997). Meanwhile, older forms, eg. Triassic planktic species from Austria described by Fuchs (e.g., 1967, 1975), are pressumed to be recrystallised benthic foraminifera (Simmons et al., 1997). Throughout Bajocian-Berriasian deposits planktic foraminifera are characterised by low abundance and low diversity assemblages (reflecting stasis), as only conoglobigerinids are known from this interval (Premoli Silva and Sliter 1999). The Cretaceous shows three periods of diversification and stasis of planktic foraminifera. These are Valanginian-Aptian, Albian, and Cenomanian-Maastrichtian. Throughout the Valanginian-late Aptian the planktic foraminifera of the low latitudes were characterised by an increase in diversity, size and morphological complexity (Premoli Silva and Sliter, 1999). Species abundance was low in Valanginian and Hauterivian times (e.g., Gorbachikella kugleri, Praehedbergella handousi); it gradually increased in the Barremian (e.g., Blefuscuiana aptiana, B. kuznetsovae, Blowiella blowi, Lilliputianella globulifera, Praehedbergella tuschepsensis). A first maximum was reached in the late Early Aptian (Leupoldina cabri and Globigerinelloides ferreolensis zones) of the Tethys (e.g., Blefuscuiana excelsa, B. infracretacea, B. gorbachikae, B. occulta, Blowiella blowi, B. duboisi, B. gottisi, Globigerinelloides ferreolensis, Guembelitria cretacea, Leupoldina cabri L. protuberans, Praehedbergella sigali). The Late Aptian was characterised by a decrease in abundance (e.g., Blefuscuiana delrioensis, Globigerinelloides algerianus, Hedbergella trocoidea, Ticinella bejaouaensis). According to Premoli Silva and Sliter (1999) the Early Aptian Selli Level of Italy indicates a major turnover (in number of originations and extinctions) related to oceano-graphic changes in the upper water column. While small r-selected opportunists dominated throughout Berriasian-Valanginian times, the Early Aptian saw a radiation of clavate k-selected forms. 14 Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe...

Records of planktic foraminifera from pre-Aptian sediments of the Boreal Realm are extremely rare. A few small hedbergellids have been reported from the latest Hauterivian of the LSB (Michael, 1967). Gradstein et al. (1999) mentioned planktic assemblages that are composed of blefuscuids from presumably Lower Barremian sediments of the seaway between Norway and Greenland. These forms have either been overlooked in previous studies because of their small size and scarcity, or the stratigraphical assignment of the strata is inac- curate. Weiss (1995) was the first to use planktic foraminifera for a biostratigraphical zonation of the Lower Aptian of the LSB.

2.3. Material and methods Thirty-eight samples of Early Aptian age were studied with respect to their planktic foraminiferal content. The samples were disaggregated using 10% hydrogen peroxide and washed through 63 µm- and 250 µm mesh sieves. After drying the residue the samples were divided into five fractions: <100 µm, 100-200 µm, 200-315 µm, 315-630 µm and >630 µm. Apart from the <100 µm fraction all residues were investigated qualitatively and quanti- tatively under a microscope. Planktic foraminifera only occur in the <315 µm fractions, the 100-200 µm fraction yielded the highest abundance. The residues were mechanically divided until each split yielded about 300 specimens. From these data the total number of specimens in the residues has been calculated. This allows absolute abundances (number of specimens/g sediment) to be calculated. The overall good preservation of the tests allows the use of the most recent taxonomical classification (Banner and Desai, 1988; BouDagher-Fadel et al., 1997b), which is based on wall structure (perforation, morphology). The CaCO3 content of the samples was measured on an Atomic Absorption-Spectrometer (AAS) and the total organic carbon content (TOC) on a coulometer. Samples and residues are stored in the Institut für Geologie, Mineralogie und Geophysik, Bochum.

2.4. Location and lithology The Rethmar section investigated is located in the central part of NW Germany, approximately 20 km southeast of Hannover (52°18.37’ N, 10°01.21’ E; Fig. 2.1). A detailed description of the lithology and biostratigraphy was given by Mutterlose and Wiedenroth (1995). The 197-m-thick succession is of Barremian-Early Aptian age (Fig. 2.2). The lower 144 m have been assigned a Barremian age, whilst the upper 53 meters are of Early Aptian age (Mutterlose and Wiedenroth, 1995). Lithologically the Aptian sediments can be subdivided from bottom to top into three units: (1) the Early Aptian Fischschiefer, 2.65 m thick; (2) the overlying Dark Clay unit of middle Early Aptian age, 4.90 m thick; (3) the Hedbergella Marl of late Early Aptian age, at least 42 m thick. The Fischschiefer consists of grey, occasionally calcareous clays, with thin clay and Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 15 calcareous layers forming mm-thick laminations. These sediments are relatively soft and not bioturbated, and only a few layers contain fossils. The abundance of pyrite as well as a high organic carbon (Corg) content up to 9.3% is typical for the Fischschiefer. Frequently the tests of foraminifera and ostracods are filled with pyrite. Pyrite rhomboids, which may represent casts of radiolarians, were observed in most samples. In addition fishbones, scales and teeth are common. The overlying Dark Clay unit is dark grey in colour and rarely bioturbated; calcareous horizons and concretions occur occasionally. The Dark Clays show a Corg content up to

5.5%. The uppermost member, the Hedbergella Marl is composed of calcareous, bioturbated clays and clay-calcareous rhythms containing sideritic or calcite concretions. These sediments are grey to yellow, and occasionally red or green. The Corg content shows average values of

1%. The residues of the Hedbergella Marl contain ostracods; fragments of shells (including bivalves and brachiopods), and echinoid spines occur sporadically. The sediments studied biostratigraphically cover the Deshayesites deshayesi and the Tropaeum bowerbanki ammonite zones, and parts of the Neohibolites ewaldi (belemnite) Zone, and are thus attributed to the Early Aptian (Fig. 2.3). They were deposited in the eastern part of the LSB, which was subsiding in Aptian times although water-depths were only moderate. The palaeogeography was influenced locally by the Lehrter saltdome.

Fenno - Aptian

Scandia Presumed land

OSLO Tethyan influx, migration of planktonic foraminifera

0 100 200 300km

EDINBURGH COPENHAGEN

Anglia WARSAW

BERLIN HANNOVER

AMSTERDAM Rethmar LONDON Rheno - Bohemia

PARIS

Armorica MUNICH Tethys

Fig. 2.1: Palaeogeographical map showing the location of the Rethmar section (modified after Mutterlose, 1992a). 16 Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... Colour [m] Samples Bed No. Stage Belemnite Zone Lithology

451 449

443 45

433 432 Legend

429 Colours

425 40 light grey 423 medium grey 421 dark grey 418 dark grey to black

35 411 Lithologies

Marl 408 Clay

406 Black shales 30 layers of sideritic or calcareous concretions 401 (pars)

bioturbation 397 belemnites

25 bivalves 394 sideritic 392 concretions 390 pyrite 386

385 20 384

L o w e r A p t i a n (pars) 379

N e o h i b l t s w a d 377

15 374

373

10 372

371

369 367

5 365

363

361 359

356 Fig. 2.2: Lithologic log of the section in- 0 vestigated showing the samples studied.

2.5. Foraminiferal data 2.5.1. Benthic foraminifera In the lower and middle parts of the section studied (Fischschiefer, Dark Clays) benthic foraminifera are more common than planktic foraminifera. The benthic associations are taxonomically impoverished in all samples. Primitive arenaceous genera (Ammobaculites, Ammodiscus, Glomospira, Trochammina) are the most common. Calcareous benthic foraminifera (Dentalina, Lenticulina, Nodosaria) were observed in only a few samples. These associations clearly reflect restricted, suboxic conditions of bottom waters, which led to ‘black shale’ sedimentation. A significant shift in taxonomic composition within the benthic assemblages is indicated by the onset of the Hedbergella Marl in the upper part of the section. Diversity increases for both arenaceous and calcareous foraminifera and remains at a fairly high level up to the top of the section. Calcareous foraminifera show a distinctive increase Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 17 from the base to the top of the Hedbergella Marl; they are the most common group in the upper third of the section investigated. The number of arenaceous foraminifera increases in the uppermost part. The most common genera (A, arenaceous; C, calcareous) in the Hedbergella Marl are Ammodiscus (A), Glomospira (A), Textularia (A), Trochammina (A), Gavelinella (C), Lenticulina (C) and Dentalina (C). The assemblages of the Hedbergella Marl indicate a much better oxygenated, more open-oceanic setting.

STAGE Belemnite Bio- and Ammonite zones Lithology Range of the zones lithostrati- NW Rethmar graphic units Germany section Germany England

H. jacobi Neohibolites jacobi nolani Hypacanthoplites wollemanni jacobi Clay Nolaniceras nolani

N. inflexus inflexus M. Parahoplites nutfieldiensis

Neohibolites Upper E. tschernyschewi clava M. Epicheloniceras clava Marl martinoides Tropaeum drewi

APTIAN ewaldi Neohibolites Marl Tropaeum bowerbanki ewaldi Hedbergella Deshayesites D. deshayesi Fischschiefer deshayesi D. forbesi Lower P. tenuicostatus bodei Clay Prodeshayesites P. bodei fissicostatus Oxyteuthis depressa Parancyloceras bidentatum depressa Clay Simancyloceras stolleyi

Oxyteuthis Ancyloceras innexum germanica Oxyteuthis Upper Paracrioceras denckmanni Beds Oxyteuthis brunsvicensis Paracrioceras elegans Legend A. absolutiformis A. compressa Aulacoteuthis

BARREMIAN Hoplocrioceras fissicostatum Beds clay A. speetonensis marl Lower Praeoxyteuthis Chondrites Hoplocrioceras rarocinctum pugio Beds shale

Fig. 2.3: Lithology and biostratigraphy of the Barremian /Aptian interval showing the range of the Rethmar section studied.

2.5.2. Planktic foraminifera Rare and poorly preserved specimens of Blefuscuiana first appear in the Fischschiefer. In addition to Blefuscuiana (B. aptiana, B. gorbachikae, B. occulta and B. praetrocoidea), specimens of Blowiella (B. duboisi; also referred to as Globigerinelloides), Claviblowiella, Gorbachikella (G. kugleri) and Favusella (F. hoterivica) were observed (Figs. 2.4, 2.5). Blefuscuiana (B. gorbachikae, B. infracretacea, B. paetrocoidea) is most common in the Dark Clays. However specimens of Blowiella, Claviblowiella and Praehedbergella (P. sigali) have also been observed at the base of the Dark Clays. Specimens of Blefuscuiana (in the past often assigned to Hedbergella) are dominant in the Hedbergella Marl: B. excelsa, B. infracretacea, B. gorbachikae and B. praetrocoidea are common. In addition Praehedbergella (e.g., P. sigali) occurs in relatively low numbers in all samples. A few samples from the lower third of the Hedbergella Marl contain specimens of Blowiella, Guembelitria, Leupoldina and Lilliputianella; these are interpreted as Tethyan genera, reflecting a short-lived influx of warm 18 Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe...

Tethyan surface waters (Figs. 2.6, 2.7). The small, low trochospiral and opportunistic blefuscuids (hedbergellids) are interpreted as shallow-water dwellers indicative of rather eutrophic conditions following r-mode selection (e.g., Premoli Silva and Sliter, 1999). The genus Blowiella, with its planispiral morphology occupied the same niche, although it was slightly more tolerant towards oligotrophication (Premoli Silva and Sliter, 1999). This implies r-selection for both groups throughout the Early Aptian, which is in agreement with the occurrence of Tethyan taxa. The

a b c d

e f g h

i j k l

Fig. 2.4: Scanning electronic microscope micrographs (Scale-bar = 50 µm). (a) Blefuscuiana infracretacea occidentalis(right-coiling, dorsal view), Sample 398/1; (b) Blefuscuiana infracretacea occidentalis (right-coiling, ventral view), Sample 398/1; (c) Blefuscuiana infracretacea occidentalis (right-coiling, peripheral view), Sample 398/1; (d) Hedbergella infracretacea occidentalis (left-coiling, dorsal view), Sample 398/1; (e) Hedbergella delrioensis (dorsal view), Sample 373/7; (f) Hedbergella delrioensis (ventral view), Sample 373/7; (g) Blefuscuiana occulta (dorsal view), Sample 372/2; (h) Blefuscuiana gorbachikae (dorsal view), Sample 397/1; (i) Blefuscuiana gorbachikae (ventral view), Sample 397/1; (j) Blefuscuiana praetrocoidea (dorsal view), Sample 393/1; (k) Blefuscuiana praetrocoidea (ventral view), Sample 393/1; (l) Praehedbergella ruka ( dorsal view), Sample 373/5. Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 19 presence of slightly larger, thick-walled H. trocoidea suggests a shift from r-selected to r/k- intermediate forms. Clavate forms include Leupoldina and Claviblowiella, and are thought to have inhabited both the lower, fully oxygenated surface waters as well as deeper waters with less oxygen (BouDagher-Fadel et al., 1997c). Diversification of planktic species began when the deep chlorophyll maximum (DCM) moved into the upper part of the photic zone. Surface waters thus became more suitable for planktic foraminifera owing to a sufficient supply of nutrients (Premoli Silva and Sliter, 1999). A mixed layer developed within the upper part of

a b c d

e f g h

i j k

Fig.2.5: Scanning electronic microscope micrographs (Scale-bar = 50 µm). (a) Blefuscuiana excelsa sensu stricto (dorsal view), Sample, 410/1; (b) Praehedbergella sigali (dorsal view), Sample 397/3; (c) Praehedbergella sigali (ventral view), Sample 379/3; (d) Blowiella blowi, Sample 372/2; (e) Blowiella gottisi, Sample 373/7; (f) Blowiella blowi, Sample 373/5; (g) Guembelitria cretacea, Sam- ple 433/1; (h) Claviblowiella saundersi, Sample 363/1; (i) Leupoldina pustulans, Sample 373/5; (j) Leupoldina cabri, Sample 373/5; (k) Fish teeth, Sample 356/3. 20 Chapter 2: The Early Aptian migration of planknic foraminifera to NW Europe... the water column above the thermocline (Hemleben et al., 1989), and a change from instable, eutrophic conditions towards stable and more oligotrophic waters gave rise to an increase of different environmental niches. An increase in diversity and size of planktic foraminifera is associated with this niche-partitioning (Premoli Silva and Sliter, 1999). Species with subglobular chambers (e.g., Blefuscuiana, Blowiella, Praehedbergella) are thought to have lived both in surface-near waters and slightly deeper waters, where dissolved oxygen was gained from algal photosynthesis (BouDagher-Fadel et al., 1997c). The highest abundance occured at a depth of approximately 85 m in the water column. Planktic foraminifera with elongate chambers (Claviblowiella, Leupoldina, Lilliputianella) are thought to have inhabited shallow, oxygenated waters and also deeper waters with less oxygen (BouDagher-Fadel et al. 1997c). Boersma et al. (1987) postulated that these clavate genera may have inhabited oxygen-depleted surface to near-surface waters in common with the Eocene hantkeninids. The vertical distribution of planktic foraminifera within the water column is described by Hart and Ball (1986), Leckie (1987, 1989), Hart (1999), and others.

2.6. Biostratigraphy The well-established planktic foraminiferal zonation of the Tethys (e.g., Caron, 1985; Sliter, 1989; Premoli Silva and Sliter, 1999) places the late Barremian-early Aptian interval in the Blowiella blowi Zone. The overlying Leupoldina cabri Zone corresponds to the late Early Aptian (e.g., Robaszynski and Caron, 1995). The base of the B. blowi Zone is marked by the first occurrence (FO) of B. blowi, its top by the FO of L. cabri and B. maridalensis. The overlying L. cabri Zone is a total range zone. It is defined by the FO and last occurrence (LO) of L. cabri. The Upper Aptian is subdivided into the Globigerinelloides ferreolensis, Globigerinelloides algerianus, Hedbergella trocoidea and Ticinella bejaouaensis zones. Aguado et al. (1999) proved the validity of this zonation for SE Spain (Almandich Formation) and Cobianchi et al. (1997) for southern Italy (Gargano Promontory). Weiss (1995) was the first to apply the Tethyan zonation scheme above described to sediments of northwest Europe by identifying the B. blowi and the L. cabri Zones in northwest German sections. According to Weiss (1995) the B. blowi Zone begins directly above the Fischschiefer and the L. cabri Zone marks the base of Boreal upper Lower Aptian (equal to the Middle Aptian sensu Kemper, 1995a). The first appearance of L. cabri in the section investigated allows for a correlation to the planktic foraminiferal zonation scheme of the Tethys (Fig. 2.8). In the Rethmar section the B. blowi Zone includes the Fischschiefer, the Dark Clays and the base of the Hedbergella Marl. The Italian Selli Level, which has been proposed to be the same age as the Fischschiefer (Bischoff and Mutterlose, 1998) has been assigned to the B. blowi Zone (Cobianchi et al., 1999). Characteristic foraminifera of this zone in the Rethmar section are B. aptiana, B. infracretacea, B. blowi, H. planispira as well as P. sigali and sporadic Favusella hoterivica. The L. cabri Zone includes the upper part of Hedbergella Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 21

Marl and contains a fauna including B. infracretacea and B. gottisi, subordinate P. sigali and B. blowi.

Total amount Plankton of Foraminifera Plankton Benthos TOC [%] P/B - content [%] C/A - content [%] diversity S/g sediment S/g sediment S/g sediment

0100 100 0 100 0 25 0 3500 0 3500 0 2000

Zone Lithology [m] Stage Bed No.

451 449

443 45

433 432

429

425 40 423

421

35 411

) 408 )

406

pars 30 ( pars Hedbergella Marl P C 397

25 394 392 390 386 B A

385 20 Neohibolites ewaldi

L o w e r A p t i a n ( 384

379

377

15 374

373

10 372

371

369 367 Dark Clay 5 365

363

361 359 Fischschiefer 356

Fig. 2.6: Lithology, TOC content and various micropalaeontological parameters of the Rethmar section investigated. P/B, ratio of planktic/benthic foraminifera; C/A, ratio of calcareous/arenaceous foraminifera; S/g sediment, specimens/gram sediment.

2.7. Palaeoecology and palaeoceanography The onset of pelagic conditions within the Boreal LSB in Early Aptian time was caused by the opening of new sea-ways towards the Tethys via N France and S England (Mutterlose and Böckel, 1998). Planktic foraminifera migrated into the Proto-North Sea and adjoining areas giving rise to the continuous presence of this group in the Boreal Realm. The associations are dominated by specimens of Blefuscuiana, particularly B. infracretacea. Specific horizons contain planispiral specimens of Blowiella, Claviblowiella and Leupoldina verifying the oc- casional influx of Tethyan waters (Fig. 2.8). The coiling direction of planktic foraminifera has been used by various authors (e.g., Bolli, 1971) as a palaeoceanographical proxy. In general a dominance of right-coiled specimens indicates warm surface waters, left-coiled specimens are believed to prefer cooler surface waters (e.g., Carter and Hart, 1977). Coiling patterns of recent foraminifera are well studied (e.g., Norris and Nishi, 2001), while coiling trends for the Early Cretaceous planktic foraminifera are often derived from recent data. Within the 22 Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe...

Blefuscfuiana spp. B. infracretacea Hedbergella spp. Praehedbergella spp. Blowiella spp. Claviblowiella spp. Globigerinelloides spp. Lilliputianella spp. Leupoldina spp. S/g sediment S/g sediment S/g sediment S/g sediment S/g sediment S/g sediment S/g sediment S/g sediment S/g sediment 0 3000 0 2500 0500 100 0100 0.07 0 0.07 02001.5

Zone Lithology [m] Stage Bed No.

451 449

443 45

433 432

429

425 40 423

421

35 411

) 408 )

406

pars 30 ( pars Hedbergella Marl

397

25 394 392 390

386

385 20

Neohibolites ewaldi 384 L o w e r A p t i a n (

379

377

15 374

373

10 372

371

369 367 Dark Clay 5 365

363

361 359 Fischschiefer 356

Fig. 2.7: Lithology and distribution patterns of planktonic foraminifera throughout the Rethmar section investigated. S/g sediment, specimens/gram sediment.

Fischschiefer the coiling direction of B. infracretacea shows clear fluctuations, possibly reflecting unstable conditions for the earliest Aptian with only sporadic warm surface water (Fig. 2.8). A remarkable dominance of right-coiled B. infracretacea near the transition from the Fischschiefer to the overlaying Dark Clays may indicate a longer period of warm surface- water conditions. The presence of foraminifera with subglobular chambers (e.g., Blefuscuiana, Blowiella, Praehedbergella) as well as species with elongated chambers (Claviblowiella, Leupoldina, Lilliputianella) may indicate habitats at different water depths and the evolution of a thermocline and a mixed layer. The anomalous abundance of benthic foraminifera (up to 80%) in these sediments con- tradicts Kemper and Zimmerle (1978) who postulated anoxic conditions in the bottom waters during this period. The presence of primitive arenaceous taxa (e.g., Ammodiscus, Glomospira, Trochammina) indicates at least suboxic conditions, supporting the idea of a change of bottom water conditions from suboxic to anoxic. In late Early Aptian times the palaeogeographic and palaeoceanographical conditions changed considerably. The hitherto (i.e., Neocomian) prevailing neritic conditions were replaced by a hemipelagic environment. This is marked lithologically by the deposition of col- oured marls. An abrupt mass appearance of planktic foraminifera linked to the onset of marl sedimentation has frequently been described (e.g., Kemper, 1995a). Within the Rethmar section, however, the mass of planktic foraminifera appear almost immediately above the base of Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 23 the Hedbergella Marl. Hence the changes of sedimentological patterns predate this shift in composition of the planktic foraminiferal assemblage (Fig. 2.8). The onset of the Early Aptian transgression allowed various Tethyan genera such as Blowiella, Guembelitria, Lilliputianella and Leupoldina to migrate into the LSB. One migration wave occurred at the transition from the Fischschiefer to the Dark Clays and another is indicated directly above the base of the Hedbergella Marl. Specimens of these taxa occur only in few samples, which implies that environmental conditions were not suitable for them for long periods. Blefuscuiana (B. infracretacea, B. gorbachikae) is common in all associations within the succession investigated. The coiling direction of B. infracretacea shows a dominance of right-coiled specimens 1.5 m above the base of the Hedbergella Marl, possibly indicating yet another influx of warm surface waters from the Tethys. The relationship between right- and left-coiled specimens remains constant, with a slight dominance of right-coiled specimens up to the upper part of the Hedbergella Marl. These findings suggest relatively warm, uniform surface-water conditions in the late Early Aptian reflecting a stable hemipelagic setting (Fig. 2.8). By contrast the composition of the benthic foraminiferal assemblages changed rapidly in

coiling direction interpreted Blefuscuiana temperature Foraminiferal infracretacea [%] of surface Lithology Foraminiferal distribution zonation 0 100 waters O Content Water Depth Zone Stage [m] 2 cooler surface waters

well 30 oxygenated ) Hedbergella upper slope

) warm Marl

pars surface ( R pars waters

25 Planktic Foraminifera Calcareous Benthic Foraminifera Agglutinated Benthic Foraminifera Benthic Foraminifera Zone

20 Neohibolites ewaldi L. cabri L o w e r A p t i a n (

FO 10 L. cabri

barren suboxic warm water influx well oxygenated Dark Clay 5 Zone cool surface anoxic neritic waters to suboxic B. blowi barren Fischschiefer

Fig. 2.8: Turnover and fluctuations of planktonic foraminifera in the Rethmar section during the Early Aptian; interpretation of the environmental changes. 24 Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... response to environmental changes. An increase in the number of genera and the abundance of calcareous benthic foraminifera becomes apparent directly below the base of Hedbergella Marl. In the lower third of the Hedbergella Marl calcareous foraminifera are dominant, arenaceous species being subordinate, indicating well-oxygenated shallow waters. Arenaceous foraminifera steadily increase in numbers towards the top of the Hedbergella Marl, possibly reflecting an increase in water depth.

2.8. Conclusions The foraminiferal data gathered from the Early Aptian of the LSB clearly reflect palaeogeographical changes from a restricted marginal sea in earliest Aptian times to a hemipelagic open-oceanic setting in the late Early Aptian. In earliest Aptian times restricted conditions led to a stratified water column and suboxic to oxic bottom waters (Mutterlose and Böckel, 1998). Significant oceanographic changes in the late Early Aptian gave rise to well-oxygenated shallow-water settings. The benthic assemblages changed from an arenaceous-dominated association in the earliest Aptian towards a calcareous-dominated association during the Early Aptian. This indicates changes in bottom-water conditions from a restricted environment towards a better oxygenated, more open-oceanic setting. Small hedbergellids, which are thought to have been opportunistic taxa in shallow, rather eutrophic water, appear in the Fischschiefer. They are the most common forms within the Fischschiefer, the Dark Clays and the Hedbergella Marl. Some horizons within the Fischschiefer and Dark Clays yield specimens of Blowiella and Claviblowiella. The clavate morphotype of Claviblowiella may have inhabited oxygen-depleted near-surface waters. The small planispiral genus Blowiella was an opportunistic form like the hedbergellids, but slightly more tolerant towards oligotrophication. These taxa indicate first short-lived Tethyan influxes. Records of Leupoldina and Lilliputianella, typical Tethyan genera, within the Hedbergella Marl may verify further warm-water influxes. The first appearance of L. cabri allows for a correlation of the planktic foraminiferal zonation of the Tethys and the Boreal Rethmar section. Within the Rethmar section the G. blowi Zone includes the Fischschiefer, the Dark Clays and the base of the Hedbergella Marl, the L. cabri Zone includes the upper part of Hedbergella Marl.

Acknowledgements We acknowledge financial support by the Deutsche Forschungsgemeinschaft (Mu 667/18-1, 18-2). H. Bartenstein (Celle) made many valuable comments and helped improve the manuscript. A. Ruffell (Belfast) and D. Batten (Aberystwyth) helped to improve the English. We thank I. Premoli Silva (Milano) and M. Hart (Plymouth) for critically reviewing the manuscript. Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe... 25

Appendix List of species cited in the text and the figures. In alphabetical order by generic epithets.

Arenaceous benthic foraminifera Ammobaculites Cushman, 1910 Ammodiscus Reuss, 1861 Glomospira Rzehak, 1885 Textularia Defrance, 1824 Trochammina Parker & Jones, 1859

Calcareous benthic foraminifera Dentalina Orbigny, 1839 Gavelinella Brotzen, 1942 Lenticulina Lamarck, 1804 Nodosaria Lamarck, 1812

Planktic foraminifera Blefuscuiana Banner & Desai, 1988 Blefuscuiana aptiana (Bartenstein, 1965) Blefuscuiana excelsa (Longoria, 1974) Blefuscuiana gorbachikae (Longoria, 1974) Blefuscuiana infracretacea (Glaessner, 1937) Blefuscuiana kuznetsovae (Banner & Desai, 1988) Blefuscuiana occulta (Longoria, 1974) Blefuscuiana praetrocoidea (Kretchmar & Gorbachik, 1986) Blefuscuiana occulta Longoria, 1974 Blowiella Kretchmar & Gorbachik, 1971 Blowiella blowi (Bolli, 1959) Blowiella duboisi (Chevalier, 1961) Blowiella gottisi (Chevalier, 1961) Blowiella maridalensis (Bolli, 1959) Claviblowiella BouDagher, Banner & Whittaker, 1997 Favusella Michael, 1971 Favusella hoterivica (Subbotina, 1953) Globigerinelloides Cushman & ten Dam, 1948 Globigerinelloides algerianus Cushman & Ten Dam, 1948 Globigerinelloides ferreolensis (Moullade, 1961) Globotruncana Cushman, 1927 Gorbachikella Banner & Desai, 1988 Gorbachikella kugleri (Bolli, 1959) Guembelitria Cushman, 1933 Guembelitria cretacea Cushman, 1933 Hedbergella Brönnimann & Brown, 1958 Hedbergella delrioensis (Carsey, 1926) Hedbergella planispira (Tappan, 1940) Hedbergella trocoidea (Gandolfi, 1942) Leupoldina Bolli, 1957 Leupoldina protuberans Bolli, 1957 Leupoldina pustulans (Bolli, 1957) Lilliputianella Banner & Desai, 1988 Lilliputianella globulifera (Kretchmar & Gorbachik, 1971) 26 Chapter 2: The Early Aptian migration of planktic foraminifera to NW Europe...

Praehedbergella Gorbachik & Moullade, 1973 Praehedbergella handousi (Salaj, 1984) Praehedbergella sigali (Moullade, 1966) Praehedbergella tuschepsensis (Antonova, 1964) Schackoina Thalmann, 1932 Schackoina cabri Sigal, 1952 Ticinella Reichel, 1950 Ticinella bejaouaensis Sigal, 1966

Ammonites Deshayesites deshayesites (Legmerie in d’Orbigny, 1841) Tropaeum bowerbanki Sowerby, 1837

Belemnites Neohibolites ewaldi (Strombeck, 1861) Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 27

3. Integrated stratigraphy of an Early Cretaceous (Barremian - Early Albian) North Sea borehole (BGS 81/40)

Sylvia Rückheim , André Bornemann and Jörg Mutterlose

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany (accepted for publication in Cretaceous Research)

Abstract The Early Cretaceous (Barremian-Albian) sedimentary succession of a cored borehole (BGS 81/40, Central North Sea Basin) has been examined. The 41.30-m-thick sequence has been studied for calcareous nannofossils, planktic foraminifera and bulk rock carbon isotopes. The core was dated by applying the well established calcareous nannofossil zonation schemes of the Boreal Realm. Planktic foraminifera were observed throughout the late Barremian-early Albian interval. The fragmentary and inadequate planktic foraminiferal zonation of the Boreal Realm has therefore been replaced by a new zonation scheme (Boreal Planktic Foraminiferal Zones 1-6). This new zonation scheme allows for a comparison with the established Tethyan planktic foraminiferal zonation schemes. Furthermore, carbon isotopes were correlated with Tethyan records. The resulting integrated stratigraphical framework of the core suggests a Barremian to Albian age, with hiati for parts of the Early Aptian and the Aptian/Albian boundary interval.

Keywords: Stratigraphy; Barremian; Aptian; Albian; Central North Sea; Calcareous nannofossils; Planktic foraminifera; Stable carbon isotopes.

3.1. Introduction The biostratigraphy of Early Cretaceous sediments in the North Sea was first described by Taylor (1982) and Jakubowski (1987) for calcareous nannofossils and in some detail by Crittenden (1984) for foraminifera. Subsequent studies refined the biostratigraphical framework by using calcareous nannofossils, palynomorphs, foraminifera and ostracods (e.g., Lott et al., 1985; Crittenden, 1987; Hart et al., 1989; King et al., 1989; Bralower, 1991; Riley et al., 1992; Banner et al., 1993; Crittenden and Kirk, 1997; Ainsworth, 2000; Jeremiah, 2001). Despite various efforts no consistent stratigraphical scheme was developed for the entire North Sea area, mainly due to the tectonically forced evolution of independent sub-basins (Ziegler, 1981). Useful biostratigraphical schemes for the Boreal based on calcareous nannofossils have been successfully established by Jakubowski (1987), Mutterlose (1992b), Bown et al. (1998) 28 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... and Jeremiah (2001). Early Cretaceous planktic foraminifera are quite common in low latitudinal sections, where they have served as an excellent stratigraphical tool (e.g., Coccioni and Premoli Silva, 1994; Premoli Silva and Sliter, 1995; Erba et al., 1999). In the Boreal Realm they are, however, absent in pre-Barremian sediments and occur only sporadically in the Barremian-Aptian succession. The Tethyan derived planktic foraminiferal biozonation turned out to be difficult to apply to the Early Cretaceous successions of the North Sea. In 1981 a research borehole (BGS 81/40) of the British Geological Survey (BGS) recovered 94 m of Cretaceous sediments (late Barremian-Turonian) from the central North Sea. The lithology and faunal content (benthic foraminifera, ostracods) of this succession were first investigated by Lott et al. (1985). According to these observations the borehole seemed to have recovered a complete Barremian-Albian succession and therefore offered an excellent opportunity to study this interval. Earlier planktic foraminferal biozonation schemes of the North Sea are restricted to this area and are therefore not suitable for a global correlation. It is the main goal of this study to develop a new Boreal planktic foraminiferal biozonation scheme, which allows for a comparison with the well established Tethyan zonation schemes. BGS Borehole 81/40 has therefore been re-examined with respect to its content in calcareous nannofossils and planktic foraminifera. In order to apply an independent stratigraphical tool bulk-rock carbon isotopes have been measured and correlated to the isotope signature of Tethyan successions.

-5º 0º 5º 10º 60º 60º

Hondra

Viking Graben Viking Platform East Shetland Platform

Moray Firth Basin

Central Graben

Forth Approaches Norwegian Danish Basin Embayment

igh BGS 81/40 bing Fyn H Edinburgh Ringko Mid North Sea High 55º 55º Newcastle

Anglo-Duch Basin

Amsterdam Hannover

London Brussels

0 200km 400 50º 50º -5º 0º 5º 10º

Fig. 3.1: Location of BGS Borehole 81/40, Central North Sea Basin. Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 29

3.2. Material and methods 3.2.1. Locality BGS Borehole 81/40 was drilled in the central North Sea (56°08.03’N, 0°43.60’W). The site is situated approximately 140 km northeast of Newcastle on the southern margin of the Forth Approaches Embayment (Fig. 3.1). The core is in the custody of the BGS (Edinburgh).

3.2.2. Lithology and material The Ryazanian to Albian sediments of the North Sea Basin were assigned to the Cromer Knoll Group (Johnson and Lott, 1993 and references therein), which can be subdivided from bottom to top into the Valhall Formation (Ryazanian-Aptian), the Carrack Formation (Late Aptian-Early Albian) and the Rødby Formation (Early-Late Albian). According to Lott et al. (1985) the 94-m-thick sequence of BGS Borehole 81/40 includes sediments of the Barremian- Aptian (112.60-65.70 m), Albian (65.70-58.45 m), Cenomanian (58.45-30.00 m) and Turonian (30.00-18.00 m). The succession has been attributed to six lithological units (from old to young: Unit I - greenish-grey mudstones (112.6-82.98 m); Unit II - red mudstones (82.98- 71.77 m); Unit III - grey-brown mudstones (71.77-68.15 m); Unit IV - variegated beds (68.15- 65.50 m); Unit V - red chalk (65.50-58.45 m); Unit VI - white chalk (58.45-18.00 m). One hundred and eight samples were collected from the Barremian-earliest Albian interval, covering Units I to V (Fig. 3.2). The studied section can therefore be assigned to the Valhall and Carrack formations (Tables 3.1, 3.2).

Lithostratigraphy Calcareous nannofossil Planktic foraminifera Carbon isotope stratigraphy stratigraphy stratigraphy integrated (stratigraphic segments after Menegatti et al., 1998, (after Lott stratigraphy (after Bown et al., 1998) Bralower et al., 1999) et al., 1985) this study d13C [] vs. PDB Stage Lithology Zones Zones 05 Depth [m]

UNIT V 65 B. viriosa/ T. roberti S. primitivum BPF 6 Alb. Alb. UNIT IV BC22 Albian Lower M.

Acme H. praetrocoidea R. parvidentatum UNIT III 70 F. varolii H. excelsa L. houghtonii/ L. moray-firthensis C8 H. gorbachikae

BC20 BC21 L. moray-firthensis 75 Upper H. occulta L. pustulans G. ferreolensis/G. aptiense/ UNIT II G. maridalensis/G. saundersi

Upper F. varolii (uncertain) H. similis L. pustulans Upper Aptian Upper Upper Aptian Upper Guembelitria

G. blowi Aptian Upper L. houghtonii C7 80 G. gottisi BPF 4 BPF 5

E. apertior/E. floralis G. ferreolensis C6 B. hockwoldensis BC18B. africana/F. BC19 oblongus G. saundersi R. angustus H. gorbachikae/H. planispira

L. H. trocoidea L. A. L. Aptian L. 85 N. abundans G. graysonensis

L. Aptian L. H. excelsa/H. praetrocoidea BC17 N. borealis N. abundans (consistent) 90 ?

? BC16

Lower N. abundans (abundant) b a r e n

Acme O. dispar 95 C1 Barremian

BarremianUNIT I AptianZ. scutula (abundant) Albian Upper Barremian BPF 2 H. kuznetsovae G. duboisi/G. maridalensis 100 H. similis/H. occulta/G. aptiense G. blowi B. galloisii (uncertain) H. kuznetsovae Legend BC15 105 Marlstone Barremian Upper Barremian Aptian Albian BPF 1 Mudstone 110

Fig. 3.2: Lithology and stratigraphical framework of the BGS 81/40 borehole. 30 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole...

3.2.3. Methods For the study of calcareous nannofossils, simple-smear slides of all 108 samples were made, following Bown and Young (1998). The samples were examined using an OLYMPUS BH-2 polarising light microscope at a magnification of x1250. For each sample, two transverses of the slide were studied (>100 fields of view = FOV) to identify the content of calcareous nannofossils. The preservation of calcareous nannofossils has been characterised by using visual criteria concerning the degree of etching (E1-E3) and overgrowth (O1-O3; see also Bown and Young, 1998; Table 3.1). Semi-quantitative abundance estimates were applied, using the following categories: very rare (VR): <1 specimen/20 FOV rare (R): >1 specimen/20 FOV - 5 specimens/10 FOV few (F): >5 specimens/10 FOV - 1 specimen/FOV common (C): 2-5 specimens/FOV abundant (A): >5 specimens/FOV, major component of the assemblage Taxonomy of calcareous nannofossils follows standard literature (Bown, 1998 and references therein; Bown in Kennedy et al., 2000 and references therein). In order to analyse the foraminifera, 108 samples were disaggregated using 10%-hydrogen peroxide and washed through 63-µm and 250-µm mesh sieves. After drying the residue, the samples were divided into five fractions: 63-100 µm, 100-200 µm, 200-315 µm, 315-630 µm and >630 µm. For this qualitative study all residues were investigated under an OLYMPUS SZX12 binocular microscope at a maximum magnification of x90. Taxonomy of the planktic foraminifera follows Premoli Silva and Sliter (2002 and references therein). Stable isotope ananlysis (δ13C) of 107 samples (bulk rock) was carried out using a Finnigan MAT 251 mass spectrometer, coupled to the Carbo Kiel device at the Leibniz-Labor for Radiome-tric Dating and Stable Isotope Research, Kiel, Germany. These data are given in δ notation with respect to the V-PDB standard. Reproducibility of replicate analyses of stan- dards for δ13C was generally better than 0.1‰. Sample material, slides and foraminifera are housed at the Institut für Geologie, Mineralogie and Geophysik, Ruhr-Universität Bochum.

3.3. Results and discussion 3.3.1. Nannofossil biostratigraphy The nannofossil data are presented in Table 3.1 a-d. Preservation of the calcareous nannofossils varies strongly throughout the studied interval. Between 106.10 m and 84.40 m nannofloras are well to excellently preserved. From 84.80 m to 69.70 m they are often moderately preserved with indications of overgrowth. By contrast, calcareous nannofossils from the interval 69.70-65.50 m are strongly etched or are even completely missing. Only the two uppermost samples of the section show moderate preservation. Nannofossil biostratigraphical zonation schemes of the Barremian to Albian interval Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 31 mainly involve Tethyan localities and taxa (e.g., Manivit, 1971; Thierstein, 1971, 1973; Sissingh, 1977; Perch-Nielsen, 1979; Bralower et al., 1995). In the Boreal region, however, Tethyan marker-species are often rare, absent, or have different age ranges. Recent nannofossil schemes of the Boreal have been proposed by Jakubowski (1987), Mutterlose (1992b), Bischoff and Mutterlose (1998), Bown et al. (1998), Ainsworth et al. (2000) and Jeremiah (2001). In the following, we have applied the latter three schemes, which are, to a large extent based on North Sea sites, to our data (Table 3.1). According to these schemes the studied sediments cover the Barremian to earliest Albian interval. Our data suggest at least two hiati, presumably encompassing the Barremian/Aptian and the Aptian/Albian stage boundaries. Figure 3.3 shows micrographs of some stratigraphically important nannofossils. Based on nannofossil observations, Lithological Unit I can be assigned to the Barremian. The lowermost 9 m of the studied interval (106.10-97.00 m) show three biostratigraphical events (from bottom to top). The first occurrence (= FO) of Broinsonia galloisii (101.50 m) and the last occurrence (= LO) of frequent Zeugrhabdotus scutula (97.30 m) both suggest a Barremian age (Fig. 3.3). Another useful late Barremian datum is described by an acme of the holococcolith species Isocrystallithus dispar from 96.41-95.17 m. This acme was formerly described by Jeremiah (2001) from the lower Upper Barremian of the North Sea Basin (Nannofossil Biozone BC15 of Bown et al., 1998), supporting the age assignment given above. A major decline in abundance of nannoconids (mainly Nannoconus abundans and Nannoconus borealis) above 92.25 m has been formerly linked to the early Aptian nannoconid crisis (Erba, 1994; Bischoff and Mutterlose, 1998). Due to the fact that these two nannoconid species (N. abundans, N. borealis) did not survive into the Aptian (Jakubowski, 1987; Mutterlose, 1991; Bown et al., 1998), this interval has to be assigned a Barremian age. The decline of nannnoconids therefore predates the “nannoconid crisis” of Erba (1994) and others. The LO of N. abundans (84.47 m) approximates the Barremian-Aptian boundary (Jakubowski, 1987; Mutterlose, 1991, 1992b; Bown et al., 1998). The base of unit II is characterised by numerous, densely spaced FOs, from 83.66- 82.33 m (Rhagodiscus angustus, Braarudosphaera africana, Flabellites oblongus, Braarudo- sphaera hockwoldensis, Eprolithus apertior, Eprolithus floralis). These events correspond to Nannofossil Biozones BC18-BC20 of Bown et al. (1998). The close occurrence of all six FOs in about a metre strongly suggests a major sedimentary gap, with the earliest Aptian missing. The hiatus also includes the time interval of the globally distributed Oceanic Anoxic Event (OAE) 1a. The FO of Lithraphidites houghtonii (79.95 m) and the co-occurrence of Lithraphidites moray-firthensis and Lithraphidites houghtonii up to a core depth of 71.73 m, supports a late Early Aptian (Bown et al., 1998) or even Late Aptian age (Jeremiah, 2001) for this interval. The same level is characterised by the onset of an acme of Repagulum parvidentatum (73.95-68.70 m) extending into Unit III. Unit III has a Late Aptian to Early Albian age, according to the schemes of Jakubowski 32 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole...

(1987) and Herrle and Mutterlose (2003). The high abundances of R. parvidentatum either indicate a mid- to late Aptian age (Nannofossil Biozone BC19-21 of Bown et al., 1998) or even an earliest Albian age (Jakubowski, 1987; Herrle and Mutterlose, 2003; Bornemann and Mutterlose in Mutterlose et al., 2003). Unit IV did not yield any age-diagnostic nannofossils. Unit V (red chalk; 65.50-58.45 m) is marked by the FOs of Broinsonia viriosa (>8 µm) and Seribiscutum primitivum (64.92 m), indicating at least a latest Aptian age (Bown et al., 1998; Bown in Kennedy et al., 2000). According to Bornemann and Mutterlose in Mutterlose et al. (2003) the FO of S. primitivum occurs in the lowermost Albian of NW Germany. A zonation scheme from the North Sea Basin (Jeremiah, 2001) also suggests an Albian age. No specimens of the Aptian-Albian

Fig. 3.3: Calcareous nannofossil micrographs (SEM = scanning electron microscope; XPL = cross- polarised light; scale-bar = 2 µm). (a) Braarudosphaera africana Stradner, XPL, Sample 82.83 m; (b) Braarudosphaera hockwoldensis Black, XPL, Sample 82.83 m; (c) Broinsonia galloisii (Black) Bown in Kennedy et al., XPL, Sample 98.31 m; (d) Farhania varolii (Jakubowski) Varol, XPL, Sample 71.55 m; (e) Lithraphidites houghtonii Jeremiah, XPL, Sample 71.55 m; (f) Nannoconus abundans Stradner and Grün, XPL (side view), sample 95.17 m; (g) N. abundans, SEM (side view), Sample 95.17 m; (h) Orastrum cf. O. dispar (Varol in Al-Rifaiy et al.) Bown in Kennedy et al., XPL, Sample 95.17 m; (i) O. cf. O. dispar, SEM (distal view), Sample 95.17 m; (j) O. cf. O. dispar, XPL (side view), Sample 95.17 m; (k) O. cf. dispar (side view), SEM, Sample 95.42 m; (l) Zeugrhabdotus scutula (Bergen) Rutledge and Bown, XPL, Sample 98.31 m. Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 33 boundary marker, Prediscosphaera columnata, have been observed in the studied samples. Due to the poor preservation of nannofossils in unit IV, and due to uncertainties in the stratigraphical occurrences of B. viriosa, S. primitivum and the R. parvidentatum acme, the position of the Aptian-Albian boundary remains uncertain here.

3.3.2. Planktic foraminiferal biostratigraphy The planktic foraminiferal data and a summary of the biostratigraphy are presented in Table 3.2 a-b. Preservation of the planktic foraminifera varies from moderate to poor through the studied interval. The tests show indications of crystal overgrowth and primary pores are poorly preserved. Between 106.10 m and 82.83 m, the specimens show a moderate preservation, in the interval 106.10-95.42 m, pyrite steinkerns of planktic foraminiferal tests were observed. The microfauna of the interval 82.83-70.00 m is slightly better preserved, primary pores being observed in single specimens from 88.50-70.00 m. From 70.00-64.92 m, the foraminifera show a strong diagenetic overprint and are therefore difficult to determine at the species- level. Figure 3.4 shows SEM micrographs of stratigraphically important planktic foraminifera. Most of the Early Cretaceous biostratigraphical zonation schemes for planktic foraminifera are based on material from Tethyan sections, similarily to those of nannofossils (e.g., Bolli, 1959; Moullade, 1966; Longoria, 1974; Sigal, 1977; Caron, 1985). Despite intensive study since the 1960s, planktic foraminifera have only rarely been examined from the Boreal Realm due to their small size and low abundance (e.g., Bartenstein and Bettenstaedt, 1962; Lutze, 1968; Kemper, 1995b; Ainsworth et al., 2000). Early foraminiferal biozonation schemes for the North Sea area are based on both benthic and planktic key-species (King et al., 1989). Moreover, it seems difficult to develop a biozonation scheme for the entire North Sea Basin due to the often geographically and environmentally restricted distribution of benthic foraminifera. Banner et al. (1993) published the first scheme purely based on planktic foraminifera. They subdivide the Barremian-Aptian strata into five zones, which were not related to international zonation schemes and thus are not useful for a correlation with other sections outside the North Sea Basin. More recent studies of material from the Boreal Realm (Weiss, 1995; Rückheim and Mutterlose, 2002) have demonstrated the presence of two Early Aptian planktic foraminiferal biozones in NW Germany: the Globigerinelloides blowi and the Leupoldina cabri Zones. Nevertheless, a correlation of Boreal and Tethyan sections is difficult because of the absence of marker species or their different ranges in the Boreal region. Based on the observations of BGS Borehole 81/40, we suggest a new planktic foraminiferal zonation scheme for the Boreal. This is based on the established Tethyan biozonation scheme of Premoli Silva and Sliter (1999) and allows for a more detailed comparison of Boreal and Tethyan sections. A discrepancy between the Boreal and Tethyan biozonation schemes can be attributed to an incomplete Aptian sequence in BGS Borehole 81/40 and the absence of Tethyan foraminiferal index-species in the North Sea Basin. 34 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole...

In combination with studies of NW German sections (Weiss, 1995; Rückheim and Mutterlose, 2002), we differentiate six Boreal Planktic Foraminiferal Zones (BPF 1-6). The Late Barremian to earliest Albian succession of BGS Borehole 81/40 encompasses five Boreal Zones (BPF 1-2 and 4-6) and reveals a hiatus near the Barremian-Aptian boundary interval

a b c d

e f g h

i j k l

m n o

Fig. 3.4: SEM micrographs of planktic foraminifera of stratigraphical importance (Scale-bar = 20 µm). (a, b) Globigerinelloides blowi, Sample 92.99 m; (c) Globigerinelloides duboisi, Sample 82.96 m; (d) Globigerinelloides maridalensis, Sample 82.83 m; (e) Globigerinelloides aptiense, Sample 78.99 m; (f) Hedbergella kuznetsovae, Sample 100.10 m; (g) Hedbergella kuznetsovae, Sample 99.30 m; (h) Hedbergella similis, Sample 82.96 m; (i) Hedbergella infracretacea, Sample 88.39 m; (j) Hedbergella trocoidea, Sample 71.00 m; (k) Hedbergella planispira, Sample 83.20 m; (l) Hedbergella planispira, Sample 64.28 m; (m) Globigerinelloides ferreolensis, Sample 82.33 m; (n, o) Ticinella roberti, Sample 64.92 m. Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 35

(Fig. 3.2, Table 3.2). The observed planktic foraminiferal zones are discussed, from bottom to top, as follows. 1. BPF 1 (106.10-100.98 m). The planktic foraminiferal assemblage of this interval is mainly composed of globigerinelloids (Globigerinelloides aptiense, G. blowi, G. duboisi, G. gottisi), Gubkinella graysonensis and hedbergellids (Hedbergella. aptiana, H. infracretacea, H. kuznetsovae, H. sigali). The top of this zone is marked by the FO of G. blowi. BPF 1 has an Eearly to early Late Barremian age and corresponds to the lower part of Unit I. It approximates the Tethyan Hedbergella similis-Hedbergella kuznetsovae Zone. 2. BPF 2 (100.98-?82.33 m). The base of this zone is characterised by the FO of G. blowi, the top is marked by the FO of Leupoldina cabri. The assemblage of this interval consists mainly of small-sized globigerinelloids (G. aptiense, G. blowi, G. “cepedai”, G. duboisi, G. gottisi, G. maridalensis, G. saundersi) and hedbergellids (H. aptiana, H. infracretacea, H. excelsa, H. gorbachikae, H. kuznetsovae, H. occulta, H. praetrocoidea, H. sigali, H. similis, H. trocoidea). The interval 95.50-88.50 m is barren of planktic foraminifera and therefore cannot be dated. The overlying six metres (88.50-82.33 m) contained no marker- species. BPF 2 encompasses the upper part of the Unit I and the base of Unit II and has a Late Barremian to Early Aptian age. It corresponds to the Globigerinelloides blowi Zone in Tethyan sections. 3. BPF 3 (absent in BGS Borehole 81/40). This zone was described in the Lower Saxony Basin (NW Germany) by Weiss (1995) and Rückheim and Mutterlose (2002). The absence of this zone in BGS Borehole 81/40 indicates a hiatus. The base of BPF 3 is marked by the FO of L. cabri, the top by the FO of Globigerinelloides ferreolensis. This zone is dated as Early Aptian and is equivalent to the Leupoldina cabri Zone in the Tethys. 4. BPF 4 (82.33-76.40 m). This zone is the full range zone of G. ferreolensis. The assemblage is dominated by globigerinelloids (G. aptiense, G. blowi, G. duboisi, G. gottisi, G. maridalensis, G. saundersi) and hedbergellids (H. aptiana, H. excelsa, H. gorbachikae, H. infracretacea, H. occulta, H. planispira, H. praetrocoidea, H. sigali, H. similis, H. solida, H. trocoidea). Secondary Clavihedbergella sigali, Guembelitria cretacea and Leupoldina pustulans occur. This zone encompasses the lower and middle part of Unit II and has a late Aptian age. BPF 4 can be correlated to the Globigerinelloides ferreolensis Zone of Tethyan sections. 5. BPF 5 (76.40-65.25 m). This zone covers the interval between the LO of G. ferreolensis and the FO of the genus Ticinella. It is dominated by hedbergellids (H. aptiana, H. excelsa, H. gorbachikae, H. infracretacea, H. occulta, H. planispira, H. praetrocoidea, H. sigali, H. trocoidea), furthermore C. sigali, Favusella hotrivica, G. benthonensis, G. bolli, G. cretacea and L. pustulans were observed. This zone has a Late Aptian age and encompasses the top of Unit II, Unit III and the base of Unit IV. BPF 5 cannot be exactly correlated to the Tethyan zonation scheme, but it seems to correspond most likely to the Hedbergella trocoidea 36 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... East C7 C5 C2 C1 C8 ) vs. PDB 345 ( C6 carb C C4 13 d Italy C3

stratigraphy of the

Stage Upper Barremian U. Aptian U. Aptian L. Barremian Upper Cismon Apticore Barrem. L. carb (Erba et al., 1999) C

U. Alb. L. Alb. 13

d Zones

G. blowi L. cabri L. blowi G. - Foram T. pr. T. B. br. H. sim. G. ferr. H. kuts. 60 40 20 0 Black shales of OAEs 1a and 1b C8 C7 C5 3.2 3.6 4.0 4.4 ) C1 C2 C6 ( 2.8 carb C 13 d C4 1.6 2.0 2.4 stratigraphy of the Switzerland carb 1.2 Roter Sattel section C3 C (Menegatti et al., 1998)

Stage Albian Aptian Upper Aptian Lower

Barr.

Zones

T. primulaT. prae. primulaT. T. L. cabri G. ferreolensis G. cabri L. G. blowi G. Foram T. bej. T. G. alg. H. sigali 30 20 10 0 C8 C9 C13 C10 C11 4.0 C12 ) vs. PDB 3.0 ( carb C 13 d France C7 C5-6 stratigraphy of the Vocontian Basin Vocontian 1.0 2.0 carb

(Herrle et al., 2004)

Stage Upper Aptian Upper Aptian L. 13 Albian Lower

G. alg. G. Zones

Ticinella bejaouaensis Ticinella G. ferreolensis G. cabri L. Hedbergella planispira Hedbergella

Foram H. trocoidea H. 0 50 300 250 200 150 100 C8 C7 C1 ) vs. PDB ( C6 carb C 13 d stratigraphy of 12345 (this study) carb C Central North Sea

13 BGS Borehole 81/40

d Upper Aptian Upper Alb. Barremian Stage

L. A.

Zones

BPF 4 BPF BPF 1 BPF BPF 2 BPF 5 BPF 2 BPF Foram ? BPF 6 40 20 0 C7 1a C10 OAE C11 C13 C1 C6 C14 ) C2 ( C4 C15 org C 13 C8 C12 C9 d Mexico C3 stratigraphy of the 1b org -27 -26 -25 -24 -23 OAE

(Bralower et al., 1999) Stage Aptian 13 Albian

Barr. Santa Rosa Canyon section

T. bejaouaens. T. Zones

B. breg. B. primula T. blowi G.

H. planispira - planispira H. Foram G. ferr. L. cab. G. alg. 200 150 100 50 0 West Fig. 3.5: Combined carbon istope stratigraphy of the Barremian-Albian interval from Tethyan sections (Mexico, France, Switzerland, Italy) and the Boreal BGS 81/40 borehole. C-Stratigraphy after Menegatti et al. (1998) and Bralower et al. (1999). Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 37

Zone. 6. BPF 6 (65.25-64.92 m). The base of this zone is marked by the FO of Ticinella spp. Due to the rather bad preservation of microfossils, the species richness of planktic foraminifera is impoverished within this interval. Apart from Ticinella primula and T. roberti, H. aptiana, H. infracretacea, H. planispira, H. sigali and H. trocoidea were observed. BPF 6 encompasses the top of Unit IV and Unit V. It has ans Early Albian age and can be correlated to the Hedbergella planispira Zone in the Tethys.

3.3.3. Carbon isotope stratigraphy The use of carbon isotopes (δ13C) has turned out to be an effective tool for a global stratigraphical correlation of Cretaceous sediments (e.g., Scholle and Arthur, 1980; Arthur et al., 1985; Weissert, 1989; Kuhnt et al., 1998; Herrle et al., 2004). It has, however, to be kept in mind that the stable isotopes only record trends, without giving a proper age assignment. The δ13C curve for the Early Cretaceous (Barremian-Albian) was split into several segments (C1-C15) by Menegatti et al. (1998) and Bralower et al. (1999). The Barremian is characterised by overall constant values (C1) extending into the lowermost Aptian (C2). A distinctive decrease in the δ13C values (C3) signifies the base of OAE 1a and is followed by a marked increase in values through the lowermost part of the OAE 1a black shales (C4; e.g., Weissert et al., 1985; Menegatti et al., 1998). Up to the top of the black shales, the values remain stable (C5), with a further increase of values near the top of OAE 1a (C6). This positive excursion is followed by an interval of relatively constant values (C7). Another decrease (C8) is followed by a sharp increase (C9). An interval with overall constant values characterises the Late Aptian (C10). The Early Albian shows another drop of the δ13C values (C11). The carbon isotope curve of BGS Borehole 81/40 (Figs. 3.2, 3.5) suggests a hiatus incorporating probably most of the Lower Aptian. From 106.10-84.47 m, values vary from 1.45-3.59‰ and can be correlated with Segment C1. Segments C2 to C5 are missing. The increase from 1.78‰ at 83.20 m to 4.04‰ at 82.72 m may correspond to C6. The sharp lithological change, from greenish grey mudstones of Unit I to red mudstones of Unit II, is interpreted as the base of the Segment C7 (82.33-74.96 m), which includes a positive trend between 80.25-75.86 m. The following decrease >2‰ between 74.94-70.40 m was identified as Segment C8. The uppermost part of the BGS 81/40 borehole studied (70.00-64.92 m) shows very high fluctuations of δ13C values (min. -5.70‰; max. 2.61‰). Due to the fact that carbonate preservation of calcareous nannofossils and planktic foraminifera is rather poor during this interval, diagenetic alteration may have also affected the carbon isotope values. Therefore this interval is not suited for the purpose of a stratigraphical correlation. 38 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole...

3.4. Conclusions A new Boreal planktic foraminiferal biozonation scheme, based on the established Tethyan biozonation schemes, is suggested. We differentiate six zones (BPF 1-6), encompassing the Barremian- Albian interval. This period can now be compared in much more detail with Early Cretaceous successions of the Tethyan Realm. Furthermore, a stratigraphical framework of Barremian to Albian sediments of the North Sea BGS Borehole 81/40 has been established. All three stratigraphical tools (calcareous nannofossils, planktic foraminifera, stable carbon isotopes) indicate a hiatus of Early Aptian age. A combination of the three methods resulted in a useful multi-stratigraphical scheme, refining the duration of the hiatus. The base of the studied section lies directly above the Munk Marl bed (Hauptblätterton). The interval can be assigned to the Valhall and Carrack formations of the North Sea lithostratigraphical schemes. It covers Units V3-V7 of the Valhall Formation and parts of the Carrack Formation of the Johnson and Lott (1993) scheme. This interval corresponds to the Units V3B-V4 and S1 of the Ainsworth et al. (2000) scheme. Using the last occurrence of Nannoconus abundans in combination with the stable carbon isotope record (top of Segment C1), we placed the Barremian/Aptian boundary at 84.47 m. The first occurrence of Globigerinelloides ferreolensis at 82.33 m indicates a defi- nitely Late Aptian age. This first occurrence coincides with a distinctive lithological shift at 82.98 m. Therefore, we placed the Early/Late Aptian boundary in the interval 82.33-82.98 m. The stable carbon isotope Segments C7 and C8 indicate a Late Aptian age for the interval 82.33-70.40 m. The Aptian/Albian boundary is located within an interval of poorly preserved sediments and microfossils and cannot be identified accurately. We placed this boundary between 68.70 m and 65.27 m, based on the last occurrence of the Aptian species Hedbergella praetrocoidea and the first occurrence of Broinsonia viriosa, Seribiscutum primitivum and Ticinella roberti. These latter three taxa are indicative for the Albian. The Aptian/Albian boundary definition is under discussion, because the biozonation schemes based on ammonites (e.g., Kemper, 1975; Casey, 1999; Owen, 2002) have failed to correlate Boreal and Tethyan sections successfully (Kennedy et al., 2000; Mutterlose et al., 2003). Further stratigraphical markers including microfossils and geochemical proxies may help to define the position of this boundary in the future. Due to the fact that neither calcareous nannofossils, planktic foraminifera nor stable carbon isotopes are suitable for a precise age assignment of the Aptian/Albian boundary in BGS Borehole 81/40, we recommend the use of palynology to improve the biostratigraphy of this interval (e.g., Ainsworth et al., 2000).

Acknowlegdements We acknowledge financial support by the Deutsche Forschungsgemeinschaft (Mu 667/18-1, 18-2). We are indebted to R. Knox (BGS, Keyworth) and G.J. Tulloch (BGS, Edinburgh) who enabled the sampling of BGS Borehole 81/40. J. Eggenstein, J. Onneken and D. Riechel- Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 39 mann are thanked for sample preparation. Isotope analyses were carried out by Dr. H. Erlen- keuser at the Leibniz-Labor in Kiel. H.W. Bailey and M. Hampton (both Network Stratigraphic Consulting, Ltd., Herfordshire) are thanked for critically reviewing the manuscript.

40 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole...

Nannoconus truitti frequens truitti Nannoconus Nannoconus truitti Nannoconus

Nannoconus steinmannii Nannoconus

Nannoconus minutus Nannoconus

Nannoconus inornatus Nannoconus

Nannoconus globulus Nannoconus

(large) elongatus Nannoconus

Nannoconus elongatus Nannoconus

Nannoconus douvllicera Nannoconus

Nannoconus circularis Nannoconus

Nannoconus borealis Nannoconus

Nannoconus bonetii Nannoconus

Nannoconus abundans Nannoconus

F R F F/C F/C

Micrantholithus stellatus Micrantholithus

Micrantholithus obtusus Micrantholithus

RCF R F

Micrantholithus hochulzii Micrantholithus

Manivitella pemmatoidea Manivitella

Lithraphidites moray-firthensis Lithraphidites

Lithraphidites houghtonii Lithraphidites Lithraphidites carniolensis Lithraphidites

R R R R RRVRFR/FFF R R F/C F C F

R/F

sp. ?Lapideacassis

Kokia borealis Kokia Isorystallithus dispar Isorystallithus

A R R F R VR R/F F

VR R VR VR VR

Hemipodorhabdus gorkae Hemipodorhabdus

Helicolithus trabeculatus Helicolithus

trabeculatus cf. Helicolithus

Helenea chiastia Helenea

Haquis circumradiatus Haquis

sp. Hayesites Grantarhabdus coronadventis Grantarhabdus

VR VR VR VR VR VR VR VR

Flabellites oblongus Flabellites

Farhalia varolii Farhalia

Ethmorhabdus hauterivianus Ethmorhabdus

Eprolithus floralis Eprolithus

Eprolithus apertior Eprolithus

Eiffellithus hancockii Eiffellithus Discorhabdus rotatorius Discorhabdus

Diloma placinum Diloma

Diazomatolithus lehmanii Diazomatolithus

Dekapodorhabdus typicus Dekapodorhabdus

Cyclagelosphaera margerelii Cyclagelosphaera

Crucibiscutum salebrosum Crucibiscutum

Crucibiscutum hayi Crucibiscutum

Crucibiscutum bosunensis Crucibiscutum

Cretarhabdus madingleyensis Cretarhabdus

Cretarhabdus inaequalis Cretarhabdus Cretarhabdus conicus Cretarhabdus

R RR R R VR R R R VR F R VR F/C R R/F R F F VR R RR VR VR R VR VR VR VR R/F

VR R VR VR VR F VR VR F R R R Corollithion geometricum Corollithion

VR R VR

Conusphaera rothii Conusphaera

Chiastozygus litterarius Chiastozygus

sp. Calculites

Calcicalathina alta Calcicalathina

Bukrylithus ambiguus Bukrylithus

Broinsonia viriosa Broinsonia

Broinsonia matalosa Broinsonia

Broinsonia galloisii Broinsonia

Braloweria boletiformis Braloweria

sp. Braarudosphaera

Braarudosphaera regularis Braarudosphaera

Braarudosphaera hockwoldensis Braarudosphaera

Brrarudosphaera africana Brrarudosphaera

Biscutum constans Biscutum

Axopodorhabdus dietzmannii Axopodorhabdus

(large) infracretacea Assipetra

(small) infracretacea Assipetra Amphizygus brooksii Amphizygus

B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N

Simple diversity Simple Preservation [m]

99,6899,77 E1-2 E1 32 26 R C C/A R VR VR VR R VR R VR VR VR VR F/C R R R 85,2585,39 X/E185,84 33 E186,27 E1 40 X/E1 28 28 VR A89,13 C VR C/A R E1 A 29 VR VR A VR VR VR VR VR R/F R/F R R VR R R VR R VR R/F R R R VR R VR VR R R VR R VR VR VR VR VR VR R R R VR VR 87,6287,83 E188,00 E188,39 36 X/E188,44 18 44 E188,82 E1-2 23 E1 2889,41 R89,81 39 E190,25 VR E3 R90,46 A 30 A C/A VR VR E190,83 VR E191,15 A 37 VR E191,48 C 36 A E3 91,89 VR 28 E191,93 E192,25 26 E3 A92,39 E1-2/O1 40 VR 38 R92,77 F/C VR E3 VR C92,99 VR E1 VR93,47 C/A E193,77 R 37 E1-294,15 VR R 40 R E3 34 F R A 94,50 C E194,75 VR E1 VR95,17 40 E3 VR95,42 40 VR E1 VR VR E1 VR C VR 36 F/C F 29 VR VR R VR VR R VR VR R R R VR VR C VR VR VR R R VR C VR VR R VR VR VR VR R R C VR R F/C R VR VR VR VR VR R VR R VR VR VR VR R99,10 VR VR R VR VR99,30 VR E1-2 R R R R R R VR 32 E2 VR R VR 21 VR VR VR R VR R VR VR VR VR VR VR R VR VR C VR R VR R F VR VR R VR VR VR VR R VR VR VR R VR VR VR R VR VR VR VR R R VR VR VR VR VR VR VR VR VR VR VR VR VR R VR VR VR VR R VR VR R VR VR VR VR R VR R VR VR R VR VR R R R VR VR VR VR VR R R VR VR VR VR R VR VR R R VR VR VR VR R VR R R VR F A VR VR R VR R VR VR R VR R R/F F F R R VR VR R F R VR F F VR R R VR VR R R VR R R R VR F R R R 95,6596,00 E3 E1 3497,15 VR97,30 E3 97,58 E1/O197,85 33 E3 F/C 98,31 E1-298,72 R/F E1 34 E1-2 39 VR 32 C R F VR R C C F/C VR VR VR R R R R R R VR VR VR VR VR R VR VR VR R VR VR VR VR R VR VR VR VR VR VR R R VR VR R/F F R R R VR R/F VR F 96,0396,35 E3 96,41 E196,77 E1 34 E1 34 17 R VR C C F VR VR VR R R R VR R VR R VR R VR VR VR R VR VR VR C VR R VR R/F C VR F/C VR R VR F VR VR R R 100,05100,10 E1100,44 E1100,98 28 E1101,50 32 E1103,60 30 E1 28 R E1 37 R 35 F C F/C R/F C R C F/C C VR VR R VR R VR VR R R VR VR R VR VR VR VR R R VR VR VR VR R VR R VR R R R VR VR VR VR VR VR VR VR R R R R VR F R F F/C R R/F VR VR R R F R R R R/F R Bown et al. (1998) al. et Bown

BC17 BC16 BC15 (cont.)

(2000) Ainsworth et al. et Ainsworth BN9 BN8 BN13 105,60 E1 37 R C VR R R VR R VR VR VR R VR F VR F R R BN10 BN12 BN11 (cont.)

stratigraphy Jeremiah (2001) Jeremiah 15 19 - LK

14B - LK16 LK17 (cont.) North Sea nannofossil LK18

(2000) Ainsworth et al. et Ainsworth V3C

V4 V3 V3B (1993)

(cont.)

North Sea Johnson and Lott and Johnson

lithostratigraphy (cont.) Formation Valhall

(after Lott et al., 1985) al., et Lott (after Lithologyical units Lithologyical UNIT I (cont.)

mudstones

greenish-grey

Upper (cont.) Upper Stage (cont.) BARREMIAN

Table 3.1a: Range chart of calcareous nannofossils (Barremian, Part 1).

Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 41

Zeugrhabdotus xenotus Zeugrhabdotus

Zeugrhabdotus trivectis Zeugrhabdotus

Zeugrhabdotus streetiae Zeugrhabdotus

Zeugrhabdotus scutula Zeugrhabdotus

Zeugrhabdotus noeliae Zeugrhabdotus

Zeugrhabdotus embergeri Zeugrhabdotus

Zeugrhabdotus erectus Zeugrhabdotus

Zeugrhabdotus diplogrammus Zeugrhabdotus

Zeugrhabdotus bicrescenticus Zeugrhabdotus

Watznaueria ovata Watznaueria

manivitae cf. Watznaueria

Watznaueria fossacincta Watznaueria

Watznaueria communis Watznaueria

Watznaueria britannica Watznaueria

Watznaueria biporta Watznaueria

Watznaueria barnesae Watznaueria

CFRVRRRR C F VR F

Tubodiscus burnettiae Tubodiscus

sp. Thoracosphaera

Triquetrarhabdulus shetlandensis Triquetrarhabdulus

Tranolithus gabalus Tranolithus

Tetrapodorhabdus decorus Tetrapodorhabdus

Tegumentum striatum Tegumentum

Tegumentum octiformis Tegumentum

sp. Stoverius

sp. Staurolithites

Staurolithites palmula Staurolithites

Staurolithites mitcheneri Staurolithites

Staurolithites glabra Staurolithites

glabra cf. Staurolithites

dorfii cf. Staurolithites

(subcircular) crux Staurolithites

Staurolithites crux Staurolithites

mutterlosei cf. Staurolithites

Staurolithites angustus Staurolithites

Sollasites horticus Sollasites

Seribiscutum primitvum Seribiscutum

Seribiscutum gaultensis Seribiscutum

Seribiscutum bijugum Seribiscutum

Scapholithus fossilis Scapholithus

Rucinolithus terebrodentarius Rucinolithus

Rotelapillus laffitei Rotelapillus

Rhombolithion rhombicum Rhombolithion

Rhagodiscus swinnertonii Rhagodiscus

Rhagodiscus splendens Rhagodiscus

Rhagodiscus gallagheri Rhagodiscus

Rhagodiscus pseudoangustus Rhagodiscus

Rhagodiscus infinitus Rhagodiscus

(small) asper Rhagodiscus

(large) asper Rhagodiscus

F F/CF FF FF R F RF CF R F R R VR R R R R R R R R/F R R F F VR R/F R R R R/F R/F R R VR R R R R R/F VR R VR VR R R C R F/C VR R VR F/C F F VR R C R C R VR C R F F VR C F/C F R/F F C F R VR F R F R VR F R R/F R VR R R R VR F F VR VR R VR VR R VR VR VR C F VR VR R/F VR VR VR

VR C R VR R/F F R R R VR VR VR F/C VR F R/F R R R/F F R R VR R R/F R R C F/C R/F R R/F R VR

Rhagodiscus achlyostaurion Rhagodiscus

Rhagodiscus angustus Rhagodiscus Retecapsa troniccki Retecapsa

VR C F VR VR VR VR VR VR VR VR R VR VR F/C R/F VR R/F R R Retecapsa surirella Retecapsa

R R/F R VR R VR R VR R/F VR R R R C C VR VR R/F R R/F VR VR F F R VR R/F R VR VR VR R C F/C R F VR F

Retecapsa crenulata Retecapsa

Retecapsa angustiforata Retecapsa

Repagulum parvidentatum Repagulum

Radiolithus hollandicus Radiolithus

Perissocyclus taylori Perissocyclus

Percivalia fenestrata Percivalia

sp. Orastrum

Orastrum perspicuum Orastrum

Octopodorhabdus magnus Octopodorhabdus

Octocyclus decussatus Octocyclus

Nannoconus wasallii Nannoconus

Nannoconus vocontiensis Nannoconus Nannoconus truitti rectangularis truitti Nannoconus

B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N

Simple diversity Simple Preservation [m]

99,68 E1-2 32 VR 98,3198,72 E199,10 E1-2 3999,30 E1-2 32 R F 32 E299,77 R 21 E1 R VR 26 VR VR R R F/C F/C F F R R R VR R F VR R R VR VR R R C R F/C F R F R C F F R R 85,2585,39 X/E185,84 33 E186,27 E1 4087,62 X/E1 28 87,83 28 E1 E1 36 VR VR 18 VR VR VR VR VR R R R R VR F F/C VR R VR F R/F R F/C R/F R VR VR R VR R VR VR R VR R97,58 VR97,85 VR E3 VR E1-2 34 F R F R VR VR R VR R R R VR R F VR F VR VR VR R R R VR R R R VR C C VR F/C F/C F R C F VR R/F R/F R F/C F F/C VR VR R F VR R R F F/C R R/F 88,0088,39 X/E188,44 44 E188,82 E1-2 2389,13 28 E189,41 E1 3989,81 E1 2990,25 R E3 3090,46 VR E190,83 E1 3791,15 E1 3691,48 VR E3 VR 2891,89 VR E1 VR91,93 VR E1 2692,25 E3 VR E1-2/O1 4092,39 3892,77 VR E3 92,99 R E1 VR93,47 E1 R 3793,77 E1-2 VR R 40 R94,15 VR VR 34 VR E3 94,50 E194,75 R E1 VR 4095,17 VR VR E3 VR VR R/F 4095,42 E1 VR R95,65 E1 3696,00 VR VR R VR E3 R/F F 2996,03 E1 VR VR96,35 VR E3 R R 3496,41 F R R/F VR R R/F E1 R VR96,77 R R E1 34 R97,15 VR E1 VR R VR VR 3497,30 VR VR R/F E3 VR R 17 VR E1/O1 R VR R VR VR R VR 33 C VR F F VR VR VR VR F/C VR VR VR R R F R F R F VR VR F R VR VR R VR R R VR R R R VR R R R F F VR VR R VR R VR R VR R VR R R R R/F R VR VR R VR F VR R VR F R VR VR R VR VR R R R/F VR F VR R R R R R/F F R R R VR F VR VR VR VR VR R VR VR VR F VR VR VR F/C VR VR VR VR R VR VR VR VR R R R/F R R R F R R R VR F VR VR VR R R R VR VR VR VR R VR R R VR VR VR VR VR R R R/F R R VR VR VR R R VR VR VR VR F VR VR R R VR R R R F R/F R/F R R VR R VR R VR F R F VR R/F VR R VR VR VR C VR VR C R C R F/C VR VR R R/F VR R R R R R R R/F R VR R F/C F F/C R F/C F F R C R/F R R VR VR R R C VR R VR C VR R R VR R VR VR VR R VR C R VR R R R R VR VR R R F/C R/F C/A F C VR F/C VR VR R VR VR R VR R R R R R VR F R A VR C F R VR F/C VR F R/F C R R R VR C VR R R F VR R/F VR R R VR VR VR R VR F/C R R VR R F F F/C VR C F R R R R C VR R C R C R R R F R/F R F VR VR R R C F R F C VR R R VR VR VR R VR VR F/C F R R R R VR R/F VR R VR R R/F R F 100,05100,10 E1100,44 E1 28100,98 E1 32101,50 E1 30 103,60 E1 28 E1 37 R 35 VR VR VR VR R/F F R F R R VR R R/F VR VR R R R/F R/F VR VR R VR VR R C F/C F/C F/C F VR R/F F R F/C F R F VR Bown et al. (1998) al. et Bown

BC16 BC15 BC17 (cont.) Ainsworth et al. (2000) al. et Ainsworth BN8 BN9 BN10 BN11 BN12 BN13 105,60 E1 37 R R F F R R R R VR VR VR VR VR R F/C C R/F R R R (cont.)

stratigraphy Jeremiah (2001) Jeremiah 19 - 15 LK

LK17 LK16 14B - LK18 (cont.) North Sea nannofossil Ainsworth et al. (2000) al. et Ainsworth V3C

V3 V3B V4 (1993)

(cont.)

North Sea Johnson and Lott and Johnson

lithostratigraphy (cont.) Formation Valhall

(after Lott et al., 1985) al., et Lott (after Lithologyical units Lithologyical UNIT I greenish-grey

mudstones (cont)

Upper (cont.) Upper Stage (cont.) BARREMIAN

Table 3.1b: Range chart of calcareous nannofossils (Barremian, Part 2).

42 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... Nannoconus truitti frequens truitti Nannoconus

VR Nannoconus truitti Nannoconus

Nannoconus steinmannii Nannoconus Nannoconus minutus Nannoconus

Nannoconus inornatus Nannoconus Nannoconus globulus Nannoconus

RVR VR VR VR VR

(large) elongatus Nannoconus Nannoconus elongatus Nannoconus

VR VR

Nannoconus douvllicera Nannoconus

Nannoconus circularis Nannoconus

Nannoconus borealis Nannoconus

Nannoconus bonetii Nannoconus

Nannoconus abundans Nannoconus

Micrantholithus stellatus Micrantholithus

Micrantholithus obtusus Micrantholithus

Micrantholithus hochulzii Micrantholithus

Manivitella pemmatoidea Manivitella

Lithraphidites moray-firthensis Lithraphidites

Lithraphidites houghtonii Lithraphidites Lithraphidites carniolensis Lithraphidites

R R VR VR

sp. ?Lapideacassis

Kokia borealis Kokia

Isorystallithus dispar Isorystallithus

Hemipodorhabdus gorkae Hemipodorhabdus

Helicolithus trabeculatus Helicolithus

trabeculatus cf. Helicolithus Helenea chiastia Helenea

VR R VR

Haquis circumradiatus Haquis

sp. Hayesites

Grantarhabdus coronadventis Grantarhabdus

Flabellites oblongus Flabellites

Farhalia varolii Farhalia

Ethmorhabdus hauterivianus Ethmorhabdus

Eprolithus floralis Eprolithus

Eprolithus apertior Eprolithus

Eiffellithus hancockii Eiffellithus

Discorhabdus rotatorius Discorhabdus

Diloma placinum Diloma

Diazomatolithus lehmanii Diazomatolithus

Dekapodorhabdus typicus Dekapodorhabdus

Cyclagelosphaera margerelii Cyclagelosphaera

Crucibiscutum salebrosum Crucibiscutum

Crucibiscutum hayi Crucibiscutum

Crucibiscutum bosunensis Crucibiscutum

F F F R R F VR VR VR

Cretarhabdus madingleyensis Cretarhabdus

Cretarhabdus inaequalis Cretarhabdus

RR Cretarhabdus conicus Cretarhabdus

R VRVRVRVR VRR

Corollithion geometricum Corollithion

Conusphaera rothii Conusphaera

Chiastozygus litterarius Chiastozygus

sp. Calculites

Calcicalathina alta Calcicalathina

Bukrylithus ambiguus Bukrylithus

Broinsonia viriosa Broinsonia

Broinsonia matalosa Broinsonia

Broinsonia galloisii Broinsonia

Braloweria boletiformis Braloweria

sp. Braarudosphaera

Braarudosphaera regularis Braarudosphaera

Braarudosphaera hockwoldensis Braarudosphaera

Brrarudosphaera africana Brrarudosphaera

Biscutum constans Biscutum

Axopodorhabdus dietzmannii Axopodorhabdus

(large) infracretacea Assipetra

(small) infracretacea Assipetra Amphizygus brooksii Amphizygus

B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N

Simple diversity Simple Preservation

[m]

74,96 E2/O1 35 VR C VR VR VR VR R VR R R F VR VR VR 72,25 E1-2/O172,55 38 E1-2/O172,65 R 43 E1-2/O173,10 VR 35 E1-2/O173,45 35 E1-2/O173,95 3574,30 VR E1-2 E1/2 O1 C 41 36 C VR75,48 VR E2/O175,86 VR C R/F E1-2/O1 VR 2476,24 VR 35 VR R E1-2/O1 A76,40 C 4176,90 VR E1-2/O1 R R77,40 VR VR 29 F VR E1-2/O177,55 35 F E2/O177,94 VR F R R VR F 3178,12 E1-2 R E1-2/O1 VR78,55 34 32 VR C R E1/2 O1 VR VR R78,99 VR VR VR 39 VR E1/2 O1 F/C F VR79,30 VR 40 VR79,49 E1 VR79,95 VR E1/2 R E1/2 O1 VR 37 R/F F80,25 VR VR 49 R 43 VR F VR82,33 E3 VR F/C82,72 E1-2 VR VR C82,83 R VR 52 E3 VR VR VR VR VR VR82,96 E1-2 C R VR F R VR R VR VR83,20 E1-2 46 VR R F R VR 37 E2 R VR VR R R/F VR R F 20 VR R R R/F VR VR R/F VR84,47 R R84,60 R/F R E1 F/C VR VR F R R R84,81 VR E1-2 R VR VR VR R/F 32 F X/E1 R 34 R VR VR R VR F VR F 36 VR VR R R R F/C F VR R/F R VR R VR VR R VR R VR VR VR R VR C VR C R R VR VR F/C VR VR VR VR R A VR VR VR VR R VR VR R F R/F VR R R VR VR VR F VR VR R R VR VR VR R VR R VR R VR R/F VR VR VR VR R VR VR R R R VR VR VR VR R R VR VR VR R/F VR VR VR VR R R R R VR R VR VR R R R F VR VR VR VR VR VR VR F F VR VR VR R VR R VR VR R R VR F R C VR VR VR VR R/F VR R VR VR VR VR R VR R VR R VR VR VR R VR VR R VR VR R/F VR R VR R VR R/F R VR R VR VR R R R VR F VR R VR R ?VR VR VR R R VR VR R VR VR R VR VR VR R R VR VR VR R R VR VR R VR VR VR VR R VR VR VR VR R VR R VR VR R R R/F R VR R VR R R/F VR VR R R R/F VR VR VR R VR VR VR VR VR R/F VR VR R VR VR VR VR VR VR R VR VR VR R R VR VR VR VR VR VR VR R R VR VR R VR VR VR R R R VR R VR VR VR F R R R VR R VR VR VR VR VR VR R R 65,27 E2-E365,70 1066,10 E3 68,70 E3 68,75 E268,90 E1/E2 21 2269,30 E3 69,60 E3 VR69,65 E1/E2 3269,98 E3 70,00 X/E170,40 35 E1 C70,48 A F E1 3270,80 E1 VR 30 E1-E271,00 27 2471,31 A R E1 F R71,55 E1 F/C 3271,73 E1 F 3371,90 C E1-2 R E1-2/O1 35 F/C 27 28 R C/A R VR R C R A C C VR VR C R C F VR C F R C VR R VR F R VR VR F F VR R VR R F F/C R R F R R F VR R VR VR F F VR F VR F/C VR C VR F/C R VR F VR F/C F/C VR C C/F F/C F R R VR R R VR R C R VR F F F VR VR F/C R F R C VR83,66 F/C83,95 R VR E1 VR84,26 E1-2 VR 31 VR 36 E1 R R VR R 34 R R VR R C/A A VR F/C VR R VR VR VR VR R VR VR VR VR VR F R R R R R VR VR R VR R VR R R R R VR VR VR VR R R VR VR R VR VR VR VR VR VR VR VR R F Bown et al. (1998) al. et Bown

BC22 BC21 BC20 BC19 BC18 BC17 (cont.) Ainsworth et al. (2000) al. et Ainsworth - - 3 D - 7 BN8 BN5 ?BN1 BN4A BN4C (cont.)

stratigraphy Jeremiah (2001) Jeremiah - - LK 11 15 LK9 14A LK8 64,92 E1-E2 28 R F/C VR F R VR VR R R

14B

LK12- (cont.) North Sea nannofossil LK 10B LK 10A Ainsworth et al. (2000) al. et Ainsworth V4

V3D V3C

?S1-3 (cont.) NOT DEFINED NOT

V4 (1993) V 7 V 6

North Sea (cont.) Johnson and Lott and Johnson

Lithostratigraphy Carrack Formation Carrack (cont.) Formation Valhall

(after Lottet al., 1985) al., Lottet (after Lithologyical units Lithologyical (cont.) UNIT I UNIT II UNIT III UNIT IV mudstones mudstones grey-brown

greenish-grey UNIT V red chalk varigated beds red mudstones

Upper Lower U. Lower

APTIAN BARR. Stage ALBIAN

Table 3.1c: Range chart of calcareous nannofossils (Barremian-Albian interval, Part 1).

Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... 43

Zeugrhabdotus xenotus Zeugrhabdotus

Zeugrhabdotus trivectis Zeugrhabdotus

Zeugrhabdotus streetiae Zeugrhabdotus

Zeugrhabdotus scutula Zeugrhabdotus

Zeugrhabdotus noeliae Zeugrhabdotus

Zeugrhabdotus embergeri Zeugrhabdotus

Zeugrhabdotus erectus Zeugrhabdotus

Zeugrhabdotus diplogrammus Zeugrhabdotus

Zeugrhabdotus bicrescenticus Zeugrhabdotus

Watznaueria ovata Watznaueria

manivitae cf. Watznaueria

Watznaueria fossacincta Watznaueria

Watznaueria communis Watznaueria

Watznaueria britannica Watznaueria

Watznaueria biporta Watznaueria Watznaueria barnesae Watznaueria

C VR C VR R VR R/F R

R/F C VR VR R Tubodiscus burnettiae Tubodiscus

VR A A R VR VR

sp. Thoracosphaera

Triquetrarhabdulus shetlandensis Triquetrarhabdulus

Tranolithus gabalus Tranolithus

Tetrapodorhabdus decorus Tetrapodorhabdus

Tegumentum striatum Tegumentum

Tegumentum octiformis Tegumentum

sp. Stoverius

sp. Staurolithites

Staurolithites palmula Staurolithites

RCVRCRCRR

Staurolithites mitcheneri Staurolithites

Staurolithites glabra Staurolithites

glabra cf. Staurolithites

dorfii cf. Staurolithites

(subcircular) crux Staurolithites Staurolithites crux Staurolithites

VR VR VR R VR C F R F F/C

mutterlosei cf. Staurolithites

Staurolithites angustus Staurolithites

Sollasites horticus Sollasites

Seribiscutum primitvum Seribiscutum

Seribiscutum gaultensis Seribiscutum

Seribiscutum bijugum Seribiscutum

Scapholithus fossilis Scapholithus

Rucinolithus terebrodentarius Rucinolithus

Rotelapillus laffitei Rotelapillus

Rhombolithion rhombicum Rhombolithion

Rhagodiscus swinnertonii Rhagodiscus

Rhagodiscus splendens Rhagodiscus

Rhagodiscus gallagheri Rhagodiscus

Rhagodiscus pseudoangustus Rhagodiscus

Rhagodiscus infinitus Rhagodiscus

(small) asper Rhagodiscus

(large) asper Rhagodiscus

R F/C R R R VR VR VR C VR F R R VR R

Rhagodiscus achlyostaurion Rhagodiscus

Rhagodiscus angustus Rhagodiscus

Retecapsa troniccki Retecapsa Retecapsa surirella Retecapsa

R R R F F/C F R VR VR R R F VR C F/C F R/F F/C R R RF VVRA CRVRRRRVR VRR VRRFF RVRVRR VR R R VR VR R

Retecapsa crenulata Retecapsa

Retecapsa angustiforata Retecapsa

Repagulum parvidentatum Repagulum

Radiolithus hollandicus Radiolithus

Perissocyclus taylori Perissocyclus

Percivalia fenestrata Percivalia

sp. Orastrum

Orastrum perspicuum Orastrum

Octopodorhabdus magnus Octopodorhabdus

Octocyclus decussatus Octocyclus

Nannoconus wasallii Nannoconus

Nannoconus vocontiensis Nannoconus Nannoconus truitti rectangularis truitti Nannoconus

B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N B A R E N

Simple diversity Simple Preservation

[m]

65,7066,10 E3 68,70 E3 68,75 E2 E1/E2 21 22 VR F/C F/C VR VR VR F/C R F/C 83,9584,26 E1-284,47 36 E184,60 E1 3484,81 E1-2 32 X/E1 34 36 VR VR VR R VR VR VR VR VR R R VR VR VR R/F R R VR VR R F R F F F F VR VR VR R VR R VR VR R VR R VR VR VR VR R R VR VR VR VR R VR VR VR VR R/F R VR R R R VR VR R R VR R R R VR VR VR VR VR C R R R F/C C VR VR F/C C VR R C R/F VR F VR F VR R C F R VR VR VR R R C VR R R F R F R VR VR R 65,27 E2-E3 10 68,9069,30 E3 69,60 E3 E1/E269,65 3269,98 E3 70,00 X/E170,40 35 E170,48 E1 3270,80 E1 VR 30 E1-E271,00 VR 27 2471,31 E171,55 E1 3271,73 E1 VR 3371,90 E1-2 VR E1-2/O1 3572,25 27 28 E1-2/O1 R72,55 38 E1-2/O1 R72,65E1-2/O135 43 C F/C73,10 E1-2/O173,45 35 VR E1-2/O173,95 35 R VR R74,30 FF/C C/A E1-2 E1/2 O174,96 F R R 41 36 F/C VR E2/O175,48 R VR 35 E2/O175,86 R F F E1-2/O1 2476,24 R F 35 R VR E1-2/O1 R VR76,40 C VR VR 41 VR VR R R A VR VR76,90 VR VR A VR F/C R E1-2/O1 VR VR77,40 F F/C 29 R C R R E1-2/O1 C F/C VR77,55E2/O131 35 VR VR R F F/C77,94 VR VR R C F R VR F R78,12 VR E1-2 R VR E1-2/O1 R F VR78,55 R 34 F 32 E1/2 O1 VR F F/C78,99 VR R VR R F/C 39 F/C R E1/2 O1 R79,30 C F/C F 40 R VR R VR79,49 F R VR E1 R VR VR RVRVR R/F C F VR79,95 R E1/2 F/C RF/CVR VR R R/F R E1/2 O1 37 C VR80,25 F F/C 49 R VR 43 VRVR R VR F/C R F R VR VR82,33 VR VR R E3 VR R R F82,72 VR R/F E1-2 R VR VR82,83 F R 52 E3 VR F/C82,96E1-237 E1-2 R R VR R VR R83,20 VR F/C 46 R R VR VR R R F/C83,66 E2 R F VR VR F E1 F VR 20 R VR VR VR RRRR VR VR VR 31 R R R/F R/F VR R R F VRRVR VR R R F VR F R RVR VR VR R VR VR F C VRR/F F R VR R C VR VR R R VR VR F VR VR VR R R R/F R VR R VR R VR VR VR R VR R VR F R R VR F/C F/CVRVRVRFVR VR R R/F R VR VR F/C R R R R F R F/C VR VR VR R VR R VR R R F/C VR R/F VR VR R R F VR R VR VR VR R R VR R VR VR F VR VR R VR R R VR VR VR C VR VR R VR F VRVR VR VR VR R VR VR F VR VR R F R R A VR R R VR R VR F/C VR R VR R R R R VR F VR VR VR F VR R VR F R VR C R VR F VR VR F VR VR R R RRRR VR R VR VR VR R VR VR VR VR VR VR VR VR VR R/F VR VR VR R/F F R R VR F R VR C R R VR R F/C R VR VR VR VR VR R R/F VR R F F/C C VR VR VR R F/C CR R VR R C R/F F/C R R F F/C F/C R F VR R/F VR R C F/C F/C F/C VR VR R VR VR VR VR VR F/C RRRRVR C F/C R VR F/C VR VR VR VR C VR VR VR F/C R VR VR F/C R F F R F/C VR R R VR VR R R F C F VR F R VR VR R VR VR VR VR F R VR R R C F/C R R F C R R R F VR R F/C VR R VR VR R F/C F R F/C VR VR F VR R R VR R C VR VR R R/F R R R C VR VR VR VR VR F/C VR F/C VR R R VR R/F VR C VR VR VR VR R R VR VR VR A VR R R VR R C VR R F R VR VR R/F VR VR VR VR VR R/F VR VR VR VR VR R V F/C VR F/C VR VR VR C VR C F R F R VR R VR C R A VR VR F VR F R F/C VR C F R C R C C/A R R F R/F VR C VR R R VR R VR R R F F R F F VR C R R F R R VR R VR VR R R R R R R R VR VR R VR R F R/F R F VR F R/F VR R VR VR R R/F R VR R Bown et al. (1998) al. et Bown

BC22 BC21 BC20 BC17 BC19 BC18 (cont.) Ainsworth et al. (2000) al. et Ainsworth - - 3 - D 7 BN5 BN8 ?BN1 BN4A BN4C (cont.)

stratigraphy Jeremiah (2001) Jeremiah - - LK 15 11 LK9 LK8 64,92 E1-E2 28 14A

14B

(cont.) LK12- North Sea nannofossil LK 10A LK 10B Ainsworth et al. (2000) al. et Ainsworth V4

V3D V3C

?S1-3 (cont.) NOT DEFINED NOT

(1993) V 7 V 6

North Sea (cont.) Johnson and Lott and Johnson

Lithostratigraphy Valhall Formation (cont.) Formation Valhall Carrack Formation Carrack

(after Lott et al., 1985) al., et Lott (after Lithologyical units Lithologyical (cont.) UNIT I UNIT II UNIT III UNIT IV mudstones mudstones grey-brown

greenish-grey varigated beds UNIT V red chalk red mudstones

U. Upper Lower Lower

BARR. APTIAN Stage ALBIAN

Table 3.1d: Range chart of calcareous nannofossils (Barremian-Albian interval, Part 2).

44 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole...

Ticinella roberti Ticinella

Ticinella primula Ticinella

Praetrocoidea tuschepsensis Praetrocoidea

Leupoldina pustulans Leupoldina

Hedbergella trocoidea Hedbergella

Hedbergella solida Hedbergella

Hedbergella similis Hedbergella

Hedbergella sigali Hedbergella

Hedbergella praetrocoidea Hedbergella

Hedbergella planispira Hedbergella

Hedbergella occulta Hedbergella

Hedbergella kuznetsovae Hedbergella

Hedbergella infracretacea Hedbergella

Hedbergella gorbachikae Hedbergella

Hedbergella excelsa excelsa Hedbergella

Hedbergella aptiana Hedbergella

Guembelitria creatcea Guembelitria

Gubkinella graysonensis Gubkinella

Globigerinelloides saundersi Globigerinelloides

Globigerinelloides maridalensis Globigerinelloides

Globigerinelloides gottisi Globigerinelloides

Globigerinelloides duboisi Globigerinelloides

Globigerinelloides ferreolensis Globigerinelloides

Globigerinelloides "cepedai" Globigerinelloides

Globigerinelloides bolli Globigerinelloides

Globigerinelloides blowi Globigerinelloides

Globigerinelloides benthonensis Globigerinelloides

Globigerinelloides aptiense Globigerinelloides

Favusella hoterivica Favusella

Clavihedbergella sigali Clavihedbergella Blefuscuiana speetonensis Blefuscuiana BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN BARREN [m]

85,2585,3985,84 x86,27 x87,6287,83 88,00 88,39 88,44 88,82 x89,13 89,41 x89,81 90,25 xxxxxx 90,46 90,83 91,15 91,48 91,89 91,93 92,25 x92,39 x92,77 x x92,99 x x93,47 93,77 x 94,15 x x94,50 x94,75 95,17 95,42 x 95,65 x 96,00 x 96,03 x 96,3596,4196,7797,1597,30 x97,5897,85 x98,3198,72xxxxx x99,10xxxxxxxxxx x99,30 x99,68xxxxxxxxx x x99,77xxxxxxx x x x x x x x x x x x x x x x x x xxx x x xx x x x x x x x 100,05100,10100,44 100,98101,50103,60 x105,60 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Foraminiferal Zonation Zonation Foraminiferal BPF 1 BPF

BPF 2 BPF 2 (cont.)

(2000) Ainsworth et al. al. et Ainsworth

V3B V3C V3C

(cont.) (1993) V4 V3

North Sea Sea North (cont.) Johnson and Lott Lott and Johnson

Lithostratigraphy Valhall Formation (cont.) Formation Valhall

(after Lott et al., 1985) al., et Lott (after Lithologyical units units Lithologyical (cont.) UNIT I UNIT

mudstones mudstones

greenish-grey greenish-grey

Upper (cont.) Upper BARREMIAN (cont.) BARREMIAN Stage

Table 3.2a: Range chart of planktic foraminifera (Barremian).

Range chart of planktic foraminifera (Barremian-Albian interval). (Barremian-Albian foraminifera planktic of chart Range 3.2b: Table

BARR. APTIAN ALBIAN Stage

U. Lower Upper Lower red mudstones UNIT V red chalk red V UNIT varigated beds greenish-grey grey-brown mudstones mudstones UNIT IV UNIT III UNIT III UNIT II UNIT II UNIT I UNIT I (cont.) Lithologyical units (after Lott et al., 1985)

Valhall Formation (cont.) Carrack Formation Lithostratigraphy Johnson and Lott (cont.) North Sea V 6 V 7 V4 (1993) NOT DEFINED (cont.) ?S1-3 V3C V3D V4 Ainsworth et al. (2000) BPF 2 (cont.) BPF 6 BPF 4 BPF5 Foraminiferal Zonation 48 x x x x x x x x x x x x x x x x x x x x xxx x x x x xx xxxx x x xx xx x xx x x xxxx x xx xx xx xxxx xx x xxxxxx x xx 84.81 xxxx xx xx xxx 84.60 xx x x x x xxxx 84.47 xx x xxx xx xx xx xx 84.26 xxx x xxxxx xx 83.95 x x xxx xx xx x xxxx 83.66 xx x x xx x 83.20x xx x x x x 82.96 xx x x xxx x xxxx 82.83 xx xx xx xxx xxx x 82.72 x x xxx xxx x x xxx x xxx xxxxx x 82.33 x x xxx x 80.25 x x xx x x x xxxx xxx xxx 79.95 xx xxx xxx x xxxxx xxxx x xxx 79.49 x xx xxxxxxxx x x xxx 79.30 xxx xx x x xx x 78.99 x x xxxx x x x xxxx xxx 78.55 x xxxxx xx xx xx x 78.12 x x x xxx 77.94 x x 77.55 x xx x x x 77.40 xx x 76.90 x x x 76.40 x x 76.24 x x x x 75.86 xxx x 75.48 x x 74.96 x 74.30 x x x 73.9573.45 xxx x 73.10 72.6572.55 x 72.25 x 71.90 71.73 71.55 71.31 71.00 70.80 70.4870.40 70.00 69.98 69.65 xxxx69.60 xxxxx69.30 68.90 68.75 68.70 xxxxx x 66.10 65.70 65.27 64.92 [m] BARREN BARREN BARREN Blefuscuiana speetonensis Clavihedbergella sigali Favusella hoterivica Globigerinelloides aptiense Globigerinelloides benthonensis Globigerinelloides blowi Globigerinelloides bolli Globigerinelloides "cepedai" Globigerinelloides ferreolensis Globigerinelloides duboisi Globigerinelloides gottisi Globigerinelloides maridalensis Globigerinelloides saundersi Gubkinella graysonensis Guembelitria creatcea Hedbergella aptiana Hedbergella excelsa Hedbergella gorbachikae

xx x xx xxxxxxHedbergella infracretacea Hedbergella kuznetsovae Hedbergella occulta Hedbergella planispira Hedbergella praetrocoidea Hedbergella sigali Hedbergella similis Hedbergella solida Hedbergella trocoidea Leupoldina pustulans Praetrocoidea tuschepsensis Ticinella primula

Ticinella roberti hpe :Itgae tairpyo nEryCeaeu ot e oeoe. 45 borehole... Sea North Cretaceous Early an of stratigraphy Integrated 3: Chapter 46 Chapter 3: Integrated stratigraphy of an Early Cretaceous North Sea borehole... Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 47

4. Palaeoecological and palaeoceanographic implications of planktic foraminifera in the Early Cretaceous (Barremian - Early Albian) of the North Sea Basin

Sylvia Rückheim, André Bornemann and Jörg Mutterlose

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany (submitted for publication to Marine Micropaleontology)

Abstract A marine Cretaceous succession (Barremian-Albian) of a cored borehole (BGS 81/40), located in the Central North Sea Basin, has been examined with respect to its planktic and benthic foraminiferal content, as well as for calcareous nannofossils. Based on the distribution patterns of foraminifera and calcareous nannofossils the palaeoecologic and palaeoceanographic conditions are discussed. The observed patterns allow for a two fold division of the investigated interval. 1. The Barremian-earliest Aptian interval, which reflects a marine, temporary restricted setting. This is indicated by sporadic occurrences of planktic foraminifera with very rare planispiral forms suggesting short-termed connections of the Boreal and Tethyan Realms. The benthic foraminiferal assemblages indicate aerobic, sometimes dysaerobic bottom-water conditions. High abundances of nannoconids in the Barremian suggest enhanced stratification and/or warm, oligotrophic conditions of the surface water. 2. The late Aptian- early Albian interval, which was characterised by open-oceanic conditions with cool and aerobic surface water. Planktic foraminifera are more abundant and diverse than in the lower interval. Trochospiral hedbergellids dominate the assemblages. The episodic occurrences of planispiral, clavate and trochospiral-flattened planktic morphotypes proof a sea-way between the Boreal and the Tethyan Realms. Aerobic to dysaerobic bottom-water conditions are suggested by the composition of the benthic foraminiferal assemblages. High abundances of cool- water taxa within the calcareous nannofossil assemblages indicate a cooling trend.

Keywords: Early Cretaceous; Foraminifera; Calcareous nannofossils; Central North Sea Ba- sin; Palaeoecology; Palaeoceanography.

4.1. Introduction The Barremian-Aptian boundary interval is characterised by significant palaeoceanographic changes caused by increased seafloor-spreading rates linked to the opening of the Atlantic

(Ziegler, 1989; Larson, 1991a, b). Submarine volcanism (Larson, 1991a, b) increased the CO2 production, which in turn is believed to have caused the onset of the mid-Cretaceous green- 48 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... house climate (e.g., Arthur et al., 1985; Larson and Erba, 1999). These conditions, combined with a high eustatic sea-level (Haq et al., 1987), led to a greenhouse-world with increasing temperatures throughout the Aptian-Turonian (e.g., Abreu et al., 1998; Clarke and Jenkyns, 1999, Wilson et al., 2002). The palaeoceanographic changes favoured the deposition of organic-rich black shales during the Oceanic Anoxic Events (OAEs). These shifts are also reflected in the composition of the mid-Cretaceous marine floras and faunas which show a rapid evolution and radiation of planktic organisms (e.g., Erbacher and Thurow, 1997; Larson and Erba, 1999; Leckie et al., 2002). Endemic taxa hitherto restricted to the Boreal and Tethyan Realms disappeared and were replaced by more cosmopolitan organisms (e.g., Mutterlose, 1998; Mutterlose and Böckel, 1998). For the first time planktic foraminifera experienced a significant radiation in the Aptian (e.g., Hart, 1999; Premoli Silva and Sliter, 1999). Tethyan derived taxa like Leupoldina or Clavihedbergella migrated into the NW European Basins (Rückheim and Mutterlose, 2002). Calcareous nannofossils experienced the extinction of Boreal species like Nannoconus abundans and Nannoconus borealis, while new species like Rhagodiscus angustus, Eprolithus varolii, Eprolithus floralis, and others evolved. Previous studies (e.g., Leckie, 1989; Winter et al., 1994; Mutterlose, 1996; Premoli Silva and Sliter, 1999; Herrle et al., 2003) show that marine microorganisms like foraminifera and calcareous nannofossils are sensible to climatic and oceanographic variations. Palaeoecological affinities of Creatceous taxa were reconstructed by comparing these with the morphology, feeding habitat and environment of modern taxa (e.g., Leckie, 1987; Nagy, 1992; Erba, 1994; Premoli Silva and Sliter, 1999). In this paper we present foraminiferal and calcareous nannofossil data from the Barremian-Albian interval of a North Sea borehole (BGS 81/40; Fig. 4.1). Variations in the composition of the foraminiferal and calcareous nannofossil assemblages have been used to assess changes in the palaeoceanographic conditions. Morphogroup analyses of planktic and benthic foraminifera were carried out in order to reconstruct surface and bottom water conditions.

4.2. Mesozoic Evolution of the North Sea Basin The Mesozoic history of the North Sea Basin is closely related to the opening of the Atlantic. The late Kimmerian tectonic phase strongly effected the entire North Sea area (Ziegler, 1978). This major rifting pulse coincided with a distinctive drop of the eustatic sea level (Vail et al., 1977) and a break of sedimentation. Tensional tectonic caused the down-faulting of the Viking and Central grabens accompanied by temporary uplift and emergence of the rift flanks (Ziegler, 1978). During the Cretaceous the rifting movements slowed down and the uplifted flanks started to subside (Ziegler, 1975). For the Barremian, a tectonically quite phase, an overall regressive nature for the North Sea Basin is often quoted (e.g., Rawson and Riley, 1982; Ruffell, 1991). The Carpathian sea-way was closed in Early Barremian-Early Aptian times, Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 49

-5º 0º 5º 10º 60º 60º

Hondra

Viking G Viking Platform East Shetland

Platform raben

Moray Firth Basin

Central Graben

Forth Approaches Norwegian Danish Basin Embayment

BGS 81/40 Edinburgh Ringkobing Fyn High Mid North Sea High 55º 55º Newcastle

Anglo-Duch Basin

Amsterdam Hannover

London Brussels

0 200km 400 50º 50º -5º 0º 5º 10º

Fig. 4.1: Location of the BGS Borehole 81/40 in the Central North Sea Basin.

with the North Sea and adjacent basins becoming restricted marginal seas without any direct connections to the Tethys (Mutterlose, 1992a; Fig. 4.2). This palaeogeographical configuration led to the deposition of several finely laminated beds, which are enriched in organic matter and occur throughout the North Sea Basin (e.g., Rawson and Mutterlose, 1983; Mutterlose and Harding, 1987; Thomsen, 1987). Several transgressions, interrupted by intervals of regression and shallowing, enlarged the depositional areas in NW Europe throughout the Aptian and Albian 4.2). In the Early Aptian another rifting pulse, the Austrian phase, affected the morphology of the North Sea Basin (Ziegler, 1978).This went along with another eustatic sea-level drop (Vail et al., 1977). These tensional movements were accompanied by a renewed sharp accentuation of the marginal troughs, followed by a short-lived emergence of the rift-flanks (Ziegler, 1978). The North Sea area was separated into several sub-basins by barriers and islands (Ziegler, 1981), which led to the development of hiati within the Cretaceous successions (Rückheim et al., subm.). The Early Albian is marked by another transgression (e.g. Destombes et al., 1973; Kemper, 1982). 50 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ...

Early Cretaceous sediments in the North Sea Basin are characterised by thick clay sequences. The Ryazanian to Aptian sediments are generally assigned to the Valhall Formation of the Cromer Knoll Group (e.g., Deegan and Scull, 1977; Johnson and Lott, 1993). The thickness of the greyish mudstones varies strongly throughout the Central North Sea. The thick- est sequences overlay the Mesozoic graben areas and attain thicknesses of about 1800 m (Gatcliff et al., 1994). The Valhall Formation is thought to be deposited under predominantly aeorbic marine conditions (Johnson and Lott, 1993). The overlaying Carrack Formation (Up- per Aptian-Lower Albian) consists of essentially non-calcareous, carbonaceous, pyritic, mi- caceous mudstones and siltstones with localised mass-flow sandstones (Johnson and Lott, 1993). This unit, which reaches a maximum thickness of 100 m, is thought to have been deposited under restricted, oxygen-depleted bottom-water conditions (Johnson and Lott, 1993).

a Fenno - Barremian Scandia Non-marine Wealden

Presumed land OSLO Tethyan influx, migration of planktonic foraminifera 0 100 200 300km

BGS 81/40

EDINBURGH COPENHAGEN

Anglia WARSAW

BERLIN

LONDON AMSTERDAM

Rheno - Bohemia

PARIS Armorica MUNICH Tethys

b Fenno - Aptian

Scandia Presumed land

OSLO Tethyan influx, migration of planktonic foraminifera

0 100 200 300km

BGS 81/40

COPENHAGEN EDINBURGH

Anglia WARSAW

BERLIN

AMSTERDAM LONDON Rheno - Bohemia

PARIS

Armorica MUNICH Tethys

Fig. 4.2: Palaeogeographical situation of NW Europe in the (a) Barremian and (b) Aptian (modified after Mutterlose, 1992a). Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 51

4.3. Locality and material Late Barremian-Early Albian sediments of the BGS Borehole 81/40, drilled in the central North Sea (56°08.03’N; 0°43.60’W), have been studied. The site is situated approximately 140 km NE of Newcastle on the southern margin of the Forth Approaches Embayment in the western part of the Central North Sea Basin (Fig. 4.1). The 94.60-m-thick succession of Cretaceous age covers a complete sequence of late Barremian-Turonian sediments (Lott et al., 1985). According to Rückheim et al. (subm.) the stratigraphy for the Barremian-Albian interval has been revised and new age assignments have been proposed. Unit I : Middle-Late Barremian, possibly earliest Aptian. Unit II: Late Aptian. Unit III: Late Aptian. Unit IV: Late Aptian-Early Albian. Unit V: Early Albian (Fig. 4.3). A total of one hundred and eight samples were collected from core depth 106.10-64.80 m, covering the Barremian-earliest Albian interval. The biostratigraphical zonation schemes shown in Figure 4.3 are based on those of Rückheim et al. (subm.) for planktic foraminifera, Bown et al. (1998) and Jeremiah (2001) for calcareous nannofossils, Menegatti et al. (1998) and Bralower et al. (1999) for carbon isotopes.

Lithostratigraphy Calcareous nannofossil Planktic foraminifera Carbon isotope stratigraphy stratigraphy stratigraphy d13C [] vs. PDB (after Lott (Rückheim et al., 1985) et al., subm.) Stage Lithology 05 Depth [m]

UNIT V 65 B. viriosa/ T. roberti S. primitivum BPF 6 UNIT IV BC22 Lower M.

Acme H. praetrocoidea R. parvidentatum UNIT III 70 F. varolii H. excelsa L. houghtonii/ L. moray-firthensis C8 H. gorbachikae

BC20 BC21 L. moray-firthensis 75 Upper H. occulta L. pustulans G. ferreolensis/G. aptiense/ UNIT II G. maridalensis/G. saundersi

Upper F. varolii (uncertain) H. similis L. pustulans Guembelitria G. blowi L. houghtonii C7 80 G. gottisi BPF 4 BPF 5

E. apertior/E. floralis G. ferreolensis C6 B. hockwoldensis BC18B. africana/F. BC19 oblongus G. saundersi R. angustus H. gorbachikae/H. planispira

L. H. trocoidea 85 N. abundans G. graysonensis H. excelsa/H. praetrocoidea BC17 N. borealis N. abundans (consistent) 90 ? BC16

Lower N. abundans (abundant) b a r e n

Acme O. dispar 95 C1

BarremianUNIT I AptianZ. scutula (abundant) Albian

BPF 2 H. kuznetsovae G. duboisi/G. maridalensis 100 H. similis/H. occulta/G. aptiense G. blowi B. galloisii (uncertain) H. kuznetsovae Legend BC15 105 Marlstone Upper Barremian Aptian Albian BPF 1 Mudstone 110

Fig. 4.3: Lithological and stratigraphical framework of the BGS Borehole 81/40 (after Rückheim et al., subm.). The carbon isotope stratigraphy with C-segmentation follows Menegatti et al. (1998) and Bralower et al. (1999). 52 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ...

4. 4. Methods 4.4.1. Foraminifera In order to study foraminifera the samples were disaggregated using 10%-hydrogen peroxide and subsequently washed through 63 µm and 250 µm mesh sieves. After drying the residue the samples were divided into five fractions: 63-100 µm, 100-200 µm, 200-315 µm, 315-630 µm and >630 µm. For a qualitative study all residues of the 108 samples were investigated under an OLYMPUS SZX12 binocular microscope at a maximum magnification of x90. For a quantitative study the residues >100 µm of 61 samples were analysed. Planktic foraminifera, which were identified on the species level, only occur in the <315 µm fractions, the 100-200 µm fraction yields the highest abundances. Benthic foraminifera were identified on the generic level. The residues were mechanically splitted until each split contained at least 300 specimens. From these data the total abundances (number of specimen/g sediment = s/gS) have been calculated. Morphogroup analyses for the planktic and benthic assemblages have been performed. According to Leckie (1987) five planktic morphotypes (PM1-PM5) can be classified. Previ- ous studies of benthic morphological groups show a strong relationship of environmental conditions and morphotypes (e.g., Koutsoukos and Hart, 1990; Nagy, 1992; Tyska, 1994). Based on morphology, life position, feeding habitat and environment six calcareous morphogroups (CM1-CM6) and four arenaceous morphogroups (AM1-AM4) can be recognised. Table 4.1 shows the planktic and benthic morphotypes tabulated with their inferred life position, feeding habitat and environment.

4.4.2. Calcareous nannofossils For the study of calcareous nannofossils simple smear slides of 108 samples were processed. The samples were examined by using an OLYMPUS BH-2 polarising light microscope at a magnification of x1250. From each sample two transverses of the slide have been scanned (>100 fields of view = FOV) to identify nannofossil index markers for biostratigraphical age assignments. Furthermore, semiquantitative abundance estimates of nannofossils have been made. In this study we focused on two nannofossil groups, which are believed to react sensi- tive to palaeoenvironmental changes, in order to assess the palaeoceanographic conditions: (1) nannoconids (Erba, 1994; Herrle, 2003; Bornemann et al., in press) and (2) typical high- latitudinal calcareous nannofossil taxa (Repagulum parvidentatum, Crucibiscutum spp.; Mutter- lose and Kessels, 2000; Street and Bown, 2000; Mutterlose & Bornemann, subm.). For the semi-quantitative study the following categories were used: very rare (VR): <1 specimen/20 FOV rare (R): >1 specimen/20 FOV-5 specimens/10 FOV frequent (F): >5 specimens/10 FOV-1 specimen/FOV common (C): 2-5 specimens/FOV Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 53 Guembelitria Guembelitria Hedbergella Leupoldina Clavihedbergella, Globigerinelloides Ticinella Rhabdammina Rhizammina, Glomospira Ammodiscus, Recurvoides Haplophragmoides, Reophax, Gaudryina, Ammobaculites, Verneulinoides Triplasia, Textulariopsis, Conorotalites, Epistomina Ramulina Gyroidinoides Gavelinella, Dentalina, Citharina, Astacolus, Lagena, Globulina, Frondicularia, Nodosaria, Marginulinopsis, Lingulina, Saracenaria, Psilocitharella, Planularia, Vaginulinopsis Tristix, Pseudonodosaria Levidentalina, Lenticulina pelagic, epicontinental sea, aerobic, aerobic, sea, epicontinental pelagic, eutrophic highly shallow sea, epicontinental pelagic, eutrophic aerobic, water, shallow sea, epicontinental pelagic, aerobic/dysaerobic, water, mesotrophic/oligotrohic aerobic, water, shallow pelagic, mesotrohic aerobic, water, shallow pelagic, oligotrophic and abyssal, dysaerobic/quasi- bathyal eutrophic/mesotrophic/ anaerobic, matter organic low oligotrophic, and neritic & estuary, lagoon energy high dysaerobic/quasi-anaerobic, bathyal, eutrophic/mesotrophic/oligotrophic bathyal, to upper shelf inner aerobic/dysaerobic; mesotrophic/eutorphic bathyal, to upper shelf inner dysaerobic/quasi-anaerobic, organic increased mesotrophic/eutrophic, matter middle neritic to upper bathyal, aerobic, aerobic, bathyal, to upper neritic middle mesotrophic/eutrophic slope; upper to shelf middle-outer eutrophic/mesotrohic/oligtrophic aerobic, bathyal, to upper neritic mesotrphic/eutrophic aerobic/dysaerobic, bathyal, to upper neritic aerobic/dysaerobic, mesotrophic/eutrophic bathyal, to upper sublithoral aerobic/dysaerobic, eutrophic/mesotrophic/oligotrophic suspension feeder feeder suspension feeder suspension feeder suspension feeder suspension feeder suspension suspension- primary feeder passive and active feeder deposit feeder deposit active deposit feeder feeder deposit feeder deposit feeder deposit active floating in the water the water in floating column the water in floating column the water in floating column the water in floating column the water in floating column epifaunal, surficial, phytal to deep shallow infaunal epifaunal deposit feeder feeder deposit epifaunal epifaunal to deep shallow feeder deposit active infaunal to deep shallow infaunal deep to epifaunal infaunal trochospiral, trochospiral, flattened planispiral flattened; trochospiral and elongate; subcylindrical and tapered biconvex tochospiral tochospiral biconvex to planoconvex trochospiral to planispiral trochospiral with elongated periphery straight PM1 elongate, triserial triserial elongate, PM1 trochospiral PM2 PM3 planispiral PM4 elongate and clavate PM5 epifaunal tubular AM1 erect AM2 AM3 planispiral rounded infaunal shallow AM4 feeder deposit active CM1 CM1 CM2 CM3 flattened meandrine irregular epifaunal elongated CM4 feeder deposit aerobic bathyal, to upper neritic CM5 biconvex CM6 Morphogroup Test shape Life position Feeding habitat Environment Taxa

Table 4.1: Planktic and benthic foraminiferal morphogroups tabulated with their inferred life position, feeding habitat and environment. PM = planktic morphogroup; CM = calcareous morphogroup; AM = arenaceous morphogroup. (After Leckie, 1987; Koutsoukos and Hart, 1990; Nagy, 1992; Tyska, 1994; Galeotti, 1998; Hart, 1999). 54 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... abundant (A): >5 specimens/FOV, major component of the assemblage Taxonomy of calcareous nannofossils follows standard literature (Bown, 1998 and references therein; Bown in Kennedy et al., 2000 and references therein). Sample material, slides and foraminifera are housed at the Institut für Geologie, Mineralogie and Geophysik, Ruhr-Universität Bochum.

4.5. Results 4.5.1. Planktic foraminifera The preservation of planktic foraminifera of BGS Borehole 81/40 varies from moderate to poor throughout the studied interval. The tests show indication of crystal overgrowth, primary pores are poorly preserved. In the lower part of the Barremian (101.10-95.65 m) isolated steinkerns of planktic foraminifera occur. The subsequent upper part of the Barremian (95.60- 87.83 m) is barren of planktic foraminifera. The Aptian (87.83-65.27 m) yields a slightly better preserved fauna, in its lower and middle part (88.50-70.00 m) primary pores can even be observed in individual samples. Foraminifera from the uppermost Aptian and Albian (70.00- 64.92 m), however, show a strong diagenetic overprint and are therefore difficult to identify on the species level. The planktic assemblages are dominated by small sized trochospiral hedbergellids, in particular by Hedbergella infracretacea. The latter shows total abundances up to 5600 specimen/g sediment (s/gS). Planktic foraminiferal distribution patterns are shown in the Figures 4.4, 4.5 and 4.6. The total abundance of planktic foraminifera is low in the Barremian and earliest Aptian (max. 24 s/gS; 86.27 m). Abundance increases in the middle part of the Aptian and reaches a first maximum with 2088 s/gS (79.49 m); the interval 73.45-72.25 m yields the highest abundances within the studied section with max. 5638 s/gS (72.65 m). Towards the Aptian/Albian boundary interval the total abundance shows a distinctive decrease with a minimum of 2 s/gS (69.98 m). In the Albian another increase up to 2029 s/gS (64.92 m) can be observed. In the lower part of the Barremian (101.00-95.65 m) the ratio of planktic foraminifera reaches max. 1%. Eleven planktic species, belonging to two genera (Globigerinelloides, Hedbergella) were observed: G. aptiense, G. blowi, G. duboisi, G. gottisi, G. maridalensis, H. aptiana, H. infracretacea, H. kuznetsovae, H. occulta, H. sigali, H. similis. The upper part of the Barremian (95.65-87.83 m) is barren of planktic foraminifera. The Aptian is characterised by a distinctive increase of both abundance and diversity of planktic foraminifera. Only in the lowermost part (87.83-83.20 m) planktics are absent in individual samples. The ratio of planktic forminifera varies between 0-17% of the foraminiferal assemblage (0-24 s/ gS). In the middle part of the Aptian (83.20-71.90 m) the ratio increases abruptly to 93% (83.20 m) and persists from there on relatively stable values >60%. A decline to values <40%, with three exceptions of 49% (71.90 m), 69% (70.80 m) and 76% (70.40 m) respec- Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 55

Foraminifera Planktic Number of benthic Calcareous (specimens / C / A - P/ B - foraminifera foraminiferal nannofossils g sediment) ratio (%) ratio (%) species richness genera species richness Stage Lithology Depth [m] 04000 8000 0 20 40 60 80 100 0 20 40 60 80 100 0204 8 12 160204 8 12 16 020 40

UNIT V 65 A A

Albian UNIT IV

UNIT III 70

B P P

Upper UNIT II Aptian

80 C C L. 85

Upper 90

I 95 UNIT I

Barremian I 100

105

110

Fig. 4.4: Vertical distribution patterns of abundance of planktic and benthic foraminera, calcareous (C) / arenaceous (A) ratio, planktic (P) / benthic (B) ratio, species richness of planktic foraminifera, number of benthic genera and species richness of calcareous nannofossils, (I = foraminifera indet.). tively, was observed for the upper part of the Aptian (71.90-65.27 m). With 0-6 species the species richness is low in the lower part of the Aptian but increases in the middle part and peaks with 18 species (82.96 m). In the upper part of the Aptian the number declines to two species (69.98 m) and shows again a slight increase towards the Aptian/Albian boundary. We observed nine genera (Clavihedbergella, Favusella, Globigerinelloides, Guembelitria, Hedbergella, Leupoldina, Lilliputianella, Preahedbergella, Ticinella) and 29 species, most of them are hedbergellids. In the Albian the planktic ratio increases from 9% (65.27 m) to 58% (64.92 m). Three genera (Globigerinelloides, Hedbergella, Ticinella) and seven species were observed. A morphogroup analysis of the mid-Cretaceous planktic assemblages has been performed. Five morphotypes (PM1-PM5) can be classified (Table 4.1). The Barremian and earliest Aptian assemblages are composed of trochospiral hedbergellids exclusively of morphogroup PM2 (Fig. 4.7). Specimens of morphogroup PM2 are still dominating the assemblages of the middle and upper part of the Aptian, but we observe single peaks of elongate triserial (PM1), clavate elongate (PM3) and planispiral (PM4) morphotypes. The latter reach max. 35 % of the planktic assemblage in the middle part of the Aptian. In the Albian the hedbergellids (PM2) are still dominant. Single occurrences with low frequencies of trochospiral-flattened ticinellids (PM5) were observed in the middle part of the Aptian and in the Albian (Fig. 4.6), but their percentage of the planktic assemblage is too low to be plotted in Fig. 4.7. 56 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ...

Hedbergella H. H. H. spp. H. aptiana gorbachikae infracretacea H. planispira praetrocoidea H. sigali H. trocoidea (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) Stage Lithology Depth [m] 03000 6000 0300 600 08020 40 60 03000 6000 0100 200 300 400 050 100 150 200 0100 200 300 400 06015 30 45

UNIT V 65

Albian UNIT IV

UNIT III 70

Upper UNIT II Aptian

80 L. 85

Upper 90

95 UNIT I Barremian

100

105

110

Fig 4.5: Vertical distribution patterns of selected hedbergellids.

4.5.2. Benthic foraminifera Benthic foraminifera were identified on the generic level. The preservation of the specimens varies from good to bad throughout the studied interval. The Barremian (106.10-87.83m) yields a moderate fauna, most of the Aptian (87.83-70.00 m) shows moderate to well preserved faunas. The specimens of the uppermost Aptian and Albian (70.00-64.92 m) show a diagenetic overprint. Within the Barremian (106.10-87.83 m) the foraminiferal assemblage is dominated by benthic specimens, which reach ratios up to 89-100% of the association. The ratio of foraminifera which cannot be identified due to poor preservation (foraminifera indet.) varies from 0 to 11%. Calcareous specimens prevail within the benthic assemblage of this interval with 81- 99% (Fig. 4.4). We observed 20 genera of calcareous and nine genera of arenaceous benthic foraminifera. Gavelinella, Globorotalites, Gyroidinoides, Lenticulina and Pseudonubeculina are the most frequent calcareous genera, Glomospira spp. dominate the arenaceous taxa. The benthic assemblage of the lower and middle part of the Aptian is still dominated by calcareous specimens which reach up to 100% (84.26 m, 82.96 m, 79.30 m). A gradual decline to 48% towards the upper part of the Aptian can be observed. Within the upper part the ratio of calcareous specimens varies from 42% to 91%, the mean value is 66%. 23 calcareous and 12 arenaceous genera were observed. Gavelinella, Globorotalites, Gyroidinoides, Laevidentalina and Lenticulina are the most common calcareous benthics. The arenaceous assemblage is dominated by Ammodiscus, Glomospira and Triplasia. Within the Albian part the calcareous ratio shows a decrease to 42% (65.27 m), followed by an increase to 78% (64.92 m). We observed seven calcareous and ten arenaceous genera, but due to the poor preservation of Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 57

Globigerinelloides Guembelitria Clavihedbergella Favusella Praehedbergella Lilliputianella Leupoldina Ticinella spp. G. blowi G. duboisi spp. spp. hoterivica spp. spp. pustulans spp. (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) (s/g S) Stage Lithology Depth [m] 06015 30 45 051234 03015 08020 40 60 0105 06015 30 45 03015 031.5 0102468 08246 UNIT V 65

Albian UNIT IV

UNIT III 70

Upper UNIT II Aptian

80 L. 85

Upper 90

95 UNIT I Barremian

100

105

110

Fig. 4.6: Vertical distribution patterns of selected planktic foraminiferal taxa and genera. foraminifera in this interval, we assume that the original benthic assemblage showed a higher abundance and diversity. Gavelinella, Gyroidinoides, Lenticulina and Glomospira are the dominant genera. A morphogroup analysis of th ebenthic foraminiferal assemblage has been performed. Based on morphology, life position, feeding habitat and environment six calcareous morphogroups (CM1-CM6) and four arenaceous morphogroups (AM1-AM4) can be distinguished (Table 4.1, Fig. 4.7).

4.5.3. Calcareous nannofossils The preservation of calcareous nannofossils varies strongly throughout the studied interval. Most of the Barremian (106.10-84.40 m) contains a well to excellently preserved nannoflora, from 84.80 to 69.70 m nannofossils are generally moderately preserved with indications of overgrowth. In contrast, calcareous nannofossils from the Aptian-Albian boundary interval (69.70-65.50 m) are strongly etched or even completely dissolved. Only the two topmost samples show a moderate preservation. The samples studied contain a highly diverse nannoflora with a species richness (S) of up to 51 taxa per sample. S ranges in most samples from 30 to 40 species and shows maxima in the Aptian part of the succession (Fig. 4.4). Abundance changes of the studied taxa reveal two distinctive phases (Fig. 4.7). The first phase is characterised by common to abundant occurrences of nannoconids (mainly N. abundans, N. borealis) in the mid- to late Barremian (105.6-91.89 m). During the late Barremian nannoconids decline in abundance, and occur only sporadically in the lower part of the Aptian. They re-appear in the late Aptian where they are dominated by N. globulus and N. truitti. They do not occur during the second phase, which covers the Aptian-Albian boundary interval. 58 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ...

This phase is characterised by coinciding abundance increases (common to abundant) of Cruci- biscutum spp. (C. bosunensis, C. hayi, C. salebrosum) and Repagulum parvidentatum.

4.6. Discussion 4.6.1. Ecology and palaeoceanography of planktic foraminifera While the ecology of Quaternary and Recent planktic foraminifera is well understood (e.g., Kucera & Darling, 2002; de Vargas et al., 2002; Rohling et al., 2004; Schiebel and Hemleben, in press), little is known about the palaeoecology of Cretaceous planktic foraminifera. First assumptions concerning their depth habitats can be given with the development of keels in the late Albian. These enabled the specimens to occupy deeper parts of the water column (e.g., Hart, 1980; Caron & Homewood, 1983; Bornemann et al., in press). For the unkeeled Jurassic to early Albian forms the following, sometimes contradictory, ecological affinities have been suggested: The low trochospiral, smooth-walled hedbergellids are thought to be the most tolerant taxa of the trochospiral planktic foraminifera with several species described from the high latitudes (e.g., Herb, 1974; Leckie, 1990). Gasinski (1997) described them as typical opportunists of the Boreal bioprovinces and Sliter (1972) specified them as epipelagic specimens. According to Herb (1974) and Leckie (1987) Hedbergella is a typical shallow-water and epicontinental sea dweller. Our data support the view of an opportunistic taxon, which occurs in restricted environments as well as in open-oceanic settings. The life habitat of the planispiral globigerinelloids is still under debate. Coccioni et al. (1992) postulated that Globigerinelloides is a mesotrophic taxon. This statement was con- firmed by Galeotti (1998), who describes a preference of mesotrophic conditions due to the absence of globigerinelloids in eutrophic environments. Friedrich et al. (2003) suggested that G. ferreolensis and G. blowi may have been adapted to restricted environments marked by mesotrophic conditions. According to Koutsoukos (2002) globigerinelloids are deep-water dwellers, while Hart (1980) postulated them to be a group living in intermediate surface waters. Price at al. (2003) suggested that G. algerianus, G. aptiense and G. ferreolensis lived within or near the thermocline. We assume that globigerinelloids lived in the upper part of the water column, due to the fact that water depth of the North Sea Basin during the Barremian to early Albian did not exceed 200 m (see Chapter 4.6.2.). The palaeoecology and palaeogeography of species with elongated chambers like Clavihedbergella and Leupoldina is still ambiguous. According to BouDagher et al. (1997c) leupoldinids and clavihedergellids lived in warm and well-oxygenated surface waters. On the other hand they are thought to be adapted to less oxygenated and deeper waters since they have a higher surface area. Magniez-Jannin (1998) suggested that radial elongation of chambers is an adaption to poor oxygen content in the upper water column. Most recently chamber elongation in planktic foraminifera has been interpreted as an adaptation to low oxygen levels in the Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 59 upper water column, and Leupoldina is thought to have inhabited the most poorly-oxygenated waters (Verga and Premoli Silva, 2003c). Our data are not strong enough to support any of these theories. According to Hart and Bailey (1979) and Hart (1980) the triserial and elongate specimens of Guembelitria are thought to have inhabitated the near-surface waters and to have had wider ecological tolerances than other mid-Cretaceous genera. Koutsoukos (1994) suggested this taxon to be a typical surface-water dweller, which was not restricted to deposits of shallower neritic environments. These findings are not contradicted by our data. Specimens of the genus Ticinella, which are characterised by a trochospiral and flattened test, are thought to be deep-water dwellers (Koutsoukos, 2002), not occurring north of 35°N palaeolatitude (Hart, 1976). According to Leckie (1987) this taxon may have lived at greater depths than most species of Hedbergella or Globigerinelloides. Premoli Silva and Sliter (1999) suggested the ticinellids to be the first large-sized, thick-walled forms with a r/K intermediate life strategy. Ticinella is too rare in our samples to be used ecologically. Based on the suggested ecological affinities and the morphogroup analysis of the planktic foraminifera, we suggest the following relative complex scenario. Marine conditions prevailed throughout the Barremian, Aptian and Albian. The overall dominance of trochospiral hedbergellids (morphotype PM2) indicates aerobic and eutrophic conditions of the surface water (Fig. 4.7). These conditions are punctuated by short-termed intercalations of the clavate clavihedbergellids and leupoldinids (morphotype PM3) in the Upper Aptian. The occurrence of this morphogroup hints toward intervals of more dysaeorbic conditions in the upper water column. In addition, several peaks of the planispiral globigerinellids (morphogroup PM4) in the Aptian suggest temporary mesotrophic conditions of the surface water. The occurrence of the trochospiral-flattened ticinellids (morphotype PM5) in the uppermost Aptian and lowermost Albian hints to a slight deepening of the North Sea Basin and to the development of more oligotrophic surface-water conditions. Due to the poor preservation in the Aptian/Albian boundary interval, our palaeoceanographic and palaeoecologic interpretations are not con- clusive. We assume that the original fauna was more diverse and yielded more specimens.

4.6.2. Ecology and palaeoceanography of benthic foraminifera The Barremian-Albian benthic foraminiferal assemblages of BGS Borehole 81/40 are clearly dominated by calcareous specimens. Epifaunal calcareous morphotypes (CM1, CM3) are the prevailing component within the Barremian and earliest Aptian, the infaunal species (CM4, CM5, CM6) reach 10-20% in this interval (Fig. 4.7). In the Upper Aptian and Albian, epifaunal morphotypes (CM1, CM2, CM3) are the most common representatives within this group. At the transition from the lithological units I and II a significant peak of the infaunal morphogroup CM6 was observed. The arenaceous assemblage of the studied interval is dominated by epifaunal and phytal

60 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ...

mesotroph to eutroph to mesotroph aerob to dysaerob to aerob Nutrients

BOTTOM WATER BOTTOM Oxygenation, eo odseo aerob dysaerob to aerob

Oxygenation SURFACE WATER SURFACE

rich surface water surface rich stratified surface water surface stratified

cool, nutrient cool, warmer, presumably warmer, 100 P4 80 60 P2 40 barren of Planktic 20 P3 planktic foraminifera P1 0 morphotypes (%) C6 100 80 C4 C5 60 C3 40 20 Calcareous C2 C1 0 morphotypes (%) 100 80 A4 60 A3 40 A2 poor preservation FORAMINIFERA 20 Arenaceous A1 0 morphotypes (%) I 100 80 B 60 40 P/ B - P ratio (%) 20 0 100 80 A 60 40 C / A - C ratio (%) 20 0 RFCA Repagulum parvidentatum VR spp. RFCA Crucibiscutum VR CALCAREOUS NANNOFOSSILS spp. RFCA Nannoconus VR

65 70 75 80 85 90 95 100 105 110 Depth [m] Depth UNIT I UNIT II UNIT V

UNIT III UNIT IV

Lithology

Upper L. Upper

Aptian Albian Barremian Stage

Fig. 4.7: Palaeoceanographic synthesis based on distribution patterns of selected calcareous nannofossils and planktic and benthic foraminiferal morphogroup analyses. Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 61 taxa (AM2), although in the Barremian infaunal specimens of morphogroup AM4 reach high ratios. The arenaceous ratio increases towards the Aptian-Albian boundary interval where rounded planispiral, shallow infaunal specimens of morphogroup AM3 occur and reach up to 40%. Koutsoukos and Hart (1990) suggested that aerobic, mesotrophic to oligotrophic, pelagic conditions are characterised by a dominance of calcareous epifaunal deposit feeders (CM1, CM2, CM3). According to Tyska (1994) high ratios of infaunal arenaceous species (AM3, AM4) suggest a high organic carbon flux, whereas high values of epifaunal morphotypes (AM1, AM2) are indicative for a low organic carbon flux. Under normal marine conditions Nagy (1992) observed an increased occurrence of the morphotype AM2 within shallower water depositions. On the other hand he interpreted increasing numbers of the morphogroups AM1, AM3 and AM4 as an index for slightly deeper waters. A dominance of one morphogroup is therefore indicative for a restricted environment. Following the characteristics of the morphogroups tabulated in Table 1 the bottom- water conditions of the Barremian and earliest Aptian were aerobic to dysaerobic. Since both “aerobic” and “dysaerobic” types occur together in all samples investigated in varying numbers, the oxygen conditions fluctuated, but may be best described as “suboxic”. Nutrients varied from mesotrophic to oligotrophic with short periods of increased burial of organic matter. According to our findings the water depth of the Central North Sea Basin did not exceed 200 m. The Late Aptian is characterised by oceanographic changes displayed by a significant short-termed decline of calcareous foraminifera. This excursion is accompanied by a marked increase of the biconvex morphotype CM6. The arenaceous specimens of this interval are unfortunately too poorly preserved to allow an identification. The oxygenation of the bottom water is still fluctuating in the Late Aptian. An increasing abundance of arenaceous specimen as well as a rising ratio of infaunal species of the morphogroup AM3 within the Aptian/Albian boundary interval may indicate a deepening of the North Sea Basin.

4.6.3 Palaeoceanographic implications of calcareous nannofossils Based on the study of calcareous nannofossils from BGS Borehole 81/40 two phases characterised by specific palaeoenvironmental conditions have been distinguished. The first phase is indicated by high abundances of the nannolith taxon Nannoconus. Nannoconids have been interpreted by Busson & Noël (1991) as calcareous dinoflagellate cysts, which flourished in shallow-water environments under oligotrophic conditions with low terrigenous supply. Other authors (e.g., Erba, 1987; Coccioni et al., 1992; Mutterlose, 1996) proposed that the group preferred warm, oligotrophic surface waters. Erba (1994) suggested that Nannoconus spp. inhabited the lower photic zone similar to the recent Florisphaera profunda (Molfino and McIntyre, 1990; Ahagon et al., 1993). In nowadays oceans abundance variations of F. p ro- funda are linked to changes in nutricline depth and stability (Molfino and McIntyre, 1990; 62 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ...

Beaufort et al., 1997). A shallow nutricline causes a greater nutrient transfer into the upper photic zone causing blooms of coccoliths and low percentages of F. profunda. During periods of a deep nutricline mesotrophic conditions in the lower photic zone prevailed and led to high percentages of F. profunda. Evidence for such ecological preferences have been provided most recently by Herrle (2003) and Bornemann et al. (in press). Nannoconids are an important nannofloral component of the mid-Barremian, which may point to enhanced stratified and/or warm, oligotrophic surface-water conditions (Fig. 4.7). Low sea level and a relatively restricted environment have been described for epicontinental basins for this period (Below and Kirsch, 1997; Mutterlose and Böckel, 1998). A restricted environment is supported by the high abundances of endemic taxa like N. abundans and N. borealis (Mutterlose, 1992b; Mutterlose and Böckel, 1998) as well as by other organisms, e.g., belemnites (e.g., Mutterlose, 1998). The second phase covers most of the uppermost Aptian and lower Albian. In this interval the cool-water taxa Crucibiscutum spp. and Repagulum parvidentatum (e.g., Mutterlose and Kessels, 2000; Street and Bown, 2000; Herrle et al., 2003) dominate the calcareous nannofossil assemblage. Based on this observation the particular interval seems to indicate a cooling trend, which is supported by other faunal and floral changes as well as by sedimentological evidence as the occurrence of glendonites and ice drafted sediments (e.g., Kemper, 1987; Mutterlose and Bornemann, subm.).

4.7. Conclusions The foraminiferal and calcareous nannofossil composition of the mid-Cretaceous succession of BGS Borehole 81/40 suggests a distinctive palaeoceanographic shift from a restricted sea in the Late Barremian-earliest Aptian to an open-oceanic environment in the Late Aptian and Early Albian. Our observations of sporadic occurrences of planktic foraminifera in the Barremian and earliest Aptian indicate a restricted marine setting with temporary open marine conditions during this interval. The composition of the benthic foraminiferal assemblage indicates aerobic, sometimes dysaerobic bottom-water conditions, which were caused by the oceanographically restricted setting. The calcareous nannofossil assemblage, dominated by nannoconids, hints toward warm, oligotrophic conditions and/or enhanced stratified surface water for the Barremian. The microfaunal and nannofloral composition of the Late Aptian and Early Albian displays a shift towards hemipelagic conditions. Presumably cool, eutrophic and aerobic conditions prevailed in the surface water, punctuated by several short-termed intervals of either dysaerobic or mesotrophic surface water. The abundance and diversity of planktic foraminifera increased rapidly pointing to a more open-oceanic setting of the Cretaceous North Sea having a connection to the Tethys in the south. Sporadic findings of planispiral, clavate and trochospiral-flattened planktic foraminifera are an evidence for a sea-way connecting the Boreal Realm with the Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... 63

Tethys. The palaeoceanographic change did not affect the bottom water, the composition of benthic foraminifera indicates aerobic to dysaerobic bottom-water conditions for the Late Aptian and Early Albian. High abundances of arenaceous benthic foraminifera suggest a deepening of the North Sea Basin towards the Aptian/Albian boundary interval, causing the poor oxygenation of the basin. Increasing abundances of Crucibiscutum spp. and Repagulum parvidentatum indicate a cooling across the Aptian/Albian interval.

Acknowledgements We acknowledge financial support by the Deutsche Forschungsgemeinschaft (Mu 667/18-1, 18-2). We are grateful to R. Knox (British Geological Society, Keyworth) and G. J. Tulloch (British Geological Society, Edinburgh) who enabled the sampling of BGS Borehole 81/40. J. Eggenstein, J. Onneken and D. Riechelmann are thanked for sample preparation. Thoughtful reviews by xx and yy are gratefully acknowledged. 64 Chapter 4: Palaeoecological and palaeoceanographic implications of planktic foraminifera ... Conclusions 65

5. Conclusions

Planktic and benthic foraminiferal analysis, supported by calcareous nannofossil data and stable carbon isotope records, have been used to establish a Boreal planktic foraminiferal zonation. Further on the palaeoenvironmental conditions of the Central North Sea Basin and the adjacent Lower Saxony Basin (NW Germany) were reconstructed for the late Early Creta- ceous (Barremian-Albian). In the following, the main results of these studies are summarised:

• Findings of distinctive, short-ranged planktic forminiferal taxa like Globigerinelloides ferreolensis or Leupoldina cabri in the Boreal Realm are suitable for stratigraphical purposes. A new Boreal planktic foraminiferal zonation scheme was developed. We differentiate six Boreal Planktic Foraminiferal zones (BPF 1-6) encompassing the Barremian- Albian interval, wich can now be compared with the established Tethyan zonation schemes. Nevertheless a correlation of Boreal and Tethyan sections is still difficult because of the absence of marker species or their different ranges in the Boreal.

• A refined stratigraphical framework of Barremian to Albian sediments of the North Sea borehole BGS 81/40 has been established. The applied stratigraphical tools,calcareous nannofossils, planktic foraminifera, stable carbon isotopes, indicate a hiatus of Early Aptian age. A combination of the three methods resulted in a useful multi-stratigraphical scheme, refining the duration of the hiatus.

• Findings of Tethyan derived planktic foraminifera (Clavihedbergella, Globigerinelloides, Guembelitria, Leupoldina, Ticinella) throughout the Barremian-Albian interval suggest several short-termed influxes of warm Tethyan water masses into the Boreal Realm.

• The palaeoceanographic setting of NW Europe changed significantly in the Early Aptian. The Barremian and earliest Aptian is characterised by marine, but restricted surface-water conditions. The Early Aptian transgression gave rise to the development of a hemipelagic environment with well-oxygenated surface waters. These oceanographic situation prevailed until the Early Albian.

• For the Lower Saxony Basin a palaeoceanographic change from a restricted marginal sea with a stratified water column in earliest Aptian times to a hemipelagic, well-oxygenated setting in the late Early Aptian is suggested. The analysis of the foraminiferal composition of the Rethmar section shows a distinctive shift within the planktic assemblage in the Early Aptian. These changes do not occur at the lithological transition of the OAE 1a black shales and the overlying Hedbergella marl, as expected, but almost immediately above the base of the marls. Hence the changes of sedimentological patterns predate this shift in 66 Conclusions

the composition of the planktic foraminiferal assemblage. By contrast, the composition of the benthic assemblage changed rapidly in response to the enviromnental changes.

• A marine, temporary restricted setting of the Central North Sea Basin is suggested for the Barremian and earliest Aptian. The composition of the calcareous nannofossil assemblage indicates enhanced water stratification and/or warm, oligotrophic surface-water conditions for this interval. The Late Aptian is characterised by open-oceanic conditions with cooler and more eutrophic surface waters. In contrast, no significant changes within the bottom water were observed. These conditions were still aerobic to partly dysaerobic throughout the Barremian-Albian interval, but a deepening of the Basin in the Albian is suggested by an increase in abundance of arenaceous foraminifera.

The reconstruction of the palaeogeographical and palaeoceanographic setting of NW Europe is hitherto mainly based on calcareous nannofossils, benthic foraminifera, ammonites and belemnites. The results of the planktic foraminiferal analyses presented in this study support the idea of major palaeoenvironmental changes in the Early Aptian. Moreover, it has been demonstrated that the use of planktic foraminifera for stratigraphical purpose is practicable in high latitudinal sections. Due to the fragmentary occurrence of planktic foraminifera in Early Cretaceous sediments, an integrated bio- and chemostratigraphical zonation scheme is suggested in order to get more specific age assignments of Boreal marine sequences. Further studies of mid-Cretaceous planktic foraminifera from the northern latitudes should be carried out in order to obtain a better understanding of their migration patterns and biogeo- graphical distribution. The early evolution of this group in the Boreal is still under discussion. The often incomplete mid-Cretaceous successions in NW Europe may, however, hamper planktic foraminiferal studies. Thus an exhaustive inquiry of adequate sections has to be done first in the future. Overall References 67

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Curriculum Vitae

Personal data

Name: Sylvia Rückheim Day and place of birth: 01.06.1973; Essen Marital status single Nationality: German

Education

10/1992 - 05/2000 Study of geology, Ruhr-University Bochum degree: Diplom-Geologist Diploma Thesis: “Die Entwicklung der planktonischen Foraminiferen im Apt NW-Deutschlands“ Mapping Thesis:“Geologische Kartierung des Ostteils von Poros (Griechenland): Südlicher Bereich“ 1989 - 1992 Bischöfliches Gymnasium Am Stoppenberg, Essen University entrance diploma (Abitur) 1983 - 1989 Bischöfliche Realschule Am Stoppenberg, Essen 1979 - 1983 Zollverein Grundschule, Essen

Career since 06/2002 Freelance at the Ruhrlandmuseum in Essen 01/2001 - 06/2004 Research assistant at the department of Geology, Mineralogy and Geophysics, Ruhr-University Bochum 08/2000 - 06/2001 Teacher of Palaeontology at the Training College for Preparation Techniques in Bochum