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Ute Aschwer, Jörg Mutterlose:
Paleobiogeography of early Cretaceous calcareous nannofossils: sea-level versus paleoclimate
(Talk)

The distribution of calcareous phytoplankton in modern oceans shows clearly defined floral zones, following latitudinal patterns. Cosmopolitan species, not bound to particular latitudes, show, however, latitudinally-controlled morphological differences (e.g. Gephyrocapsa spp.). Comparable marine floral belts exist for the Early Cretaceous. The objective of this study is to compare nannofossil data (diversity, abundance, morphological parameters) from the Tithonian-Berriasian interval. Samples from three DSDP/ODP sites of different latitudes have been analysed from the central Atlantic, in order to better understand the palaeoceanography and palaeoclimatology of the Jurassic/Cretaceous boundary interval. Site 367 (palaeolatitude 6°N) and Sites 105 (17°N) and 901 (19°N) have been studied. The upper part of the Tithonian of the equatorial Site 367 (Capverde Basin) is characterised by the cosmopolitan species, Watznaueria barnesiae (15%), Cyclagelosphaera margerelii (15%), Watznaueria manivitiae (30%), and the tethyan species, Conusphaera mexicana (30%). This gives way to an equatorial W. manivitiae-C. mexicana assemblage. Samples of Site 105 (Hatteras Abyssal Plain) yield the tethyan species, Polycostella beckmannii (45%), Conusphaera mexicana (35%), Watznaueria spp. (20%), resulting in a P. beckmannii-C. mexicana assemblage. This assemblage characterises a setting situated further north (17°N).

A short-term sea-level rise in the Berriasian causes a shift in the composition of the assemblages, with increasing diversities from the south to the north. Biometric measurements of Watznaueria spp., an euryoce, Mesozoic-cosmopolitan genus, show a slight increase of size for the higher latitudes in Tithonian times. The data show distinctive diversity patterns for different latitudes possibly controlled by temperature. Differences in the size of Watznaueria spp. may be controlled by other factors such as, for example, evolution or sea-level changes.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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