BIOTIC RESPONSE TO SHORT TERM CLIMATIC CHANGES RECOGNIZED IN THE MIDDLE EOCENE OF BLAKE NOSE, WESTERN NORTH ATLANTIC

Significant short-term, cyclic variations have been identified in the δ¹⁸O record of Middle Eocene planktonic foraminifera from Blake Nose, Western North Atlantic (Wade and Kroon 2002). The causes of the variations are not well understood and the response of marine microorganisms to these changes is also not clear. We have investigated the response of marine phytoplankton to the inferred environmental changes through a new random settling technique (RST) combined with a spiking method (SM) to obtain absolute abundance data for calcareous nannofossil. We then compare the nannofossil assemblage data to the published δ¹⁸O record. The combined RST and SM produces two independent estimates of absolute abundances, which can be compared with each other to corroborate results. The slides produced from this technique are more randomly and evenly distributed than those prepared by the smear slide technique because the nannofossils and spiking beads are allowed to settle down from a suspension onto the cover-slip. Preliminary analyses of samples from ODP Site 1052 show that nannoplankton respond to short-term environmental changes. Discoaster barbadiensis, for example, is considered a warm-water taxon and is expected to have an inverse relationship with the oxygen isotope data. Results show a significant inverse relationship between D.barbadiensis and the δ¹⁸O, albeit with some inconsistencies that may be due to the influence of other environmental parameters. At 37.59 Ma there was a major shift (~1.5‰) in the δ¹⁸O, which corresponds to significant changes in the nannoplankton composition. Coccolithus pelagicus, Reticulofenestra bisecta, and Pemma spp. dominated the samples prior to 37.59 Ma and Cyclicargolithus floridanus was among the least abundant species. At 37.59 Ma the abundances switched and C. floridanus became one of the most abundant species in the samples while the rest decreased significantly. Following the switch at 37.59 Ma, preservation of the nannofossils improved and the total number of specimens increased. Further investigation of other paleoenvironmental proxies will give more information on the causes of these changes.