The Onset of Planktic Foraminifera in the Mid-Cretaceous of the Boreal Realm
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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 tec- tonic 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 circula- tion permitted the occupation and partition of new habitats. Diversity and abundance of ma- rine 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 ba- sins 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 sig- nificant 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 evolu- tion 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 signifi- cant 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 cosmo- politan 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