8th International Nannoplankton Association Conference


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Silke Kruse, Jörg Mutterlose:
Coccoliths under stress? - Lower Cretaceous calcareous nannofossils assemblages of nearshore and pelagic settings (NW Africa)

Calcareous nannofossils are widely used as biostratigraphic markers. In recent years, however, interest has more and more focused on the usage of this group as an ecological proxy, helping to reveal palaeoclimatic and palaeoceanographic problems. The degree to which various ecological factors are controlling the composition of calcareous nannofossil assemblages is poorly known. In order to better understand the role of these ecological parameters, calcareous nannofossils were investigated for the Berriasian/Valanginian interval, a period characterised by a long-term sea-level lowstand.

Onshore sections from Morocco, reflecting a shallow-water setting, were compared to DSDP Site 416A, representing pelagic conditions. Stratigraphically, the sections cover the interval of Nannofossil Biozones CC2 to CC4. The preservation of calcareous nannofossils from the Moroccan sections is moderate to good, and that of the offshore site good. Simple smear-slides and 'random settling' slides were counted. The latter allow a quantification of the calcareous nannofossils. Further, morphometric data for the genus Watznaueria were obtained in order to check a possible link between ecology and size variations of nannofossils.

The onshore sections (Morocco) are dominated by Watznaueria spp. (upto 70%) with higher percentages of Nannoconus spp. Rhagodiscus spp. and Biscutum spp. are very rare. The most abundant species in the offshore section (DSDP Site 416A) is Watznaueria spp. but Rhagodiscus spp., Zeugrhabdotus spp. and Biscutum constans are more common than in the equivalent onshore samples. Zeugrhabdotus spp. and Biscutum spp. indicate cool, nutrient-rich surface-waters, whereas Rhagodiscus spp. is known as an indicator of oligotrophic conditions and warm surface-waters. Nannoconus spp. and Micrantholithus spp. are proxies for warm water but are also seen as nearshore species. High abundances of Nannoconus spp. may correlate with eutrophic conditions.

The onshore assemblages hint towards warm waters. The offshore sections yield proxies indicative for cool, nutrient-rich waters. An upwelling area was described for the Cretaceous along the NW margin of Africa. The high percentages of Zeugrhabdotus spp. and Biscutum spp. support this hypothesis.


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

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