HR: 0830h
AN: PP41B-0844    [PDF]
TI: Calcification Changes of Mesozoic Calcareous Nannofossils
AU: * Bornemann, A
EM: andre.bornemann@web.de
AF: Institut fuer Geologie, Universitaet Bochum, Universitaetsstr. 150, Bochum, 44780 Germany
AU: Mutterlose, J
AF: Institut fuer Geologie, Universitaet Bochum, Universitaetsstr. 150, Bochum, 44780 Germany
AB: Studies on plankton samples and cultures revealed a variety factors which presumably control calcification and the size of coccoliths. Among others temperature, nutrients and seawater pH are thought to influence nannoplankton calcification. Whereas these studies only provide information of very short time intervals from hours to years, global climatic and oceanographic changes occur, however, on geological timescales. Thus their impact on nannofossil calcification and carbonate production can only be studied from the fossil record. We investigated DSDP sites from the western Atlantic of late Jurassic to early Cretaceous age in order to better understand long-term variations of the size of common nannofossil taxa and the resulting carbonate accumulation. The studied interval is characterized by two events in the pelagic carbonate record: (1) the onset of pelagic carbonate accumulation in the Tithonian, and (2) the Valanginian 'nannoconid crisis'. The Tithonian event went along with high abundances of strongly calcified nannofossils which presumably have an affinity to more oligotrophic surface water conditions. The mid Valanginian is marked by a positive carbon isotope excursion (CIE). This coincides with a sea level rise, volcanic activity and elevated atmospheric pCO2 levels. Greenhouse climate and an accelerated hydrological cycle presumably intensified weathering processes causing enhanced nutrient transfer from the continents into the oceans. Increasing surface water fertility is indicated by high abundances of nannofossils which possibly indicate more eutrophic conditions. In the western Tethys the CIE is predated by a sharp decrease in the abundance of rockforming nannoconids. This event is less pronounced in the western Atlantic due to a general scarcity of nannoconids. Low nannofossil carbonate accumulation rates and a dominance of less calcified taxa were observed and may reflect a general marine biocalcification crisis. Possible factors, which may have caused changes in calcification and carbonate accumulation are (1) the trophic system and (2) seawater pH. Low nutrient conditions coincide with high abundances of strongly calcified taxa in the Tithonian, whilst high nutrient levels are presumed for the Valanginian. In addition, low pH levels induced by high atmospheric pCO2 is believed to reduce the calcification rates of modern coccolithophores.
DE: 3030 Micropaleontology
DE: 4267 Paleoceanography
DE: 9609 Mesozoic
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting