Faunal change of benthic foraminifera in CAE-3 (middle Eocene) in the eastern Equatorial Pacific (IODP Exp 320)

Benthic foraminiferal studies have shown distinct step-wise assemblage changes during the late middle Eocene and the late Eocene. These changes might have responded primarily to the deep-water cooling in the oceans. For example, the abundance of common deep-sea species Nuttallides truempyi decreased in the bathyal depth at ~42 Ma and finally it became extinct at ~38 Ma (e.g., Tjalsma and Lohman, 1983). Recent high-resolution data about carbonate sedimentology and stable isotope geochemistry in ODP Leg 199 revealed that several carbonate accumulation events (CAEs) occurred during the middle to late Eocene (Lyle et al., 2005), suggesting that deepening of the carbonate compensation depth in CAE-3 (about 40.7 - 42.2 Ma) was consistent with the transient Antarctic glaciations through enhanced chemical weathering process (e.g., Tripati et al., 2005). Continuous and well-preserved middle Eocene to Neogene sediments were recovered at Site U1333, IODP Exp 320 in the eastern Equatorial Pacific. Samples at this site provide an opportunity to document paleoceanographic changes in CAE-3 entirely, compared to Site 1218, Leg 199. We investigate biotic response of benthic foraminifera to the paleoceanographic changes with respect to the Antarctic glaciation in the middle to late Eocene, particularly in CAE-3, based on faunal analysis of fossil benthic foraminifera and geochemical analysis of bulk sediments. Calcium carbonate (CaCO3) content at Site U1333 shows two intervals of high values in the middle to late Eocene, corresponding to CAE-3 and 4, respectively, which corroborates the results of Site 1218, Leg 199. Biogenic opal content basically shows the opposite trend to the variation of CaCO3 content. CaCO3 content is negatively correlated (p<0.01) to coarse particle fraction (>63μm). This suggests that CaCO3 content is basically balanced between large-size siliceous and small-size calcareous grains throughout the studied interval, presumably constituting radiolarian shells and calcareous nannofossils, respectively. As to benthic foraminifera, N. trumepyi, Oridorsalis umbonatus, Globocassidulina subglobosa and Cibicidoides grimsdalei are common species. Abundance of both benthic and planktonic foraminifera generally shows similar patterns to that of CaCO3 content in spite of more frequent fluctuations. However, these similarities changed at around 41.3 Ma. Fairly good correlation was observed between the abundance of benthic or planktonic foraminifera and CaCO3 content or coarse particle fraction prior to 41.3 Ma. However, there is no correlation after that time. This indicates that occurrences of benthic and planktonic foraminifera responded to the carbonate undersaturated deep-water in the early part of CAE-3, whereas other factors played important role in the latter period. The negative shift in stable carbon isotope ratio of benthic foraminifera (Tripati et al., 2005) and the specific diatom abundance (Lyle et al., 2005) were reported in CAE-3 at Site 1218. Changes in abundance of benthic foraminifera at around 41.3 Ma might be related to such paleoceanographic changes.