High resolution Mg/Ca and calcareous nannofossil records from middle Eocene of Blake Nose, ODP Site 1052

Significant short-term, cyclic variations have been identified in the δ18O record of middle Eocene planktonic foraminifera from Blake Nose, subtropical western North Atlantic. The causes of the variations (e.g., the nature of the environmental change) are not well understood. To constrain the nature of the environmental change as well as the response of phytoplankton communities to these changes, we generated high resolution planktonic foraminifera Mg/Ca and calcareous nannofossil absolute abundance data. Planktonic foraminiferal Mg/Ca values increased from ~5.0 mmol/mol at 37.85 Ma to ~5.7 mmol/mol at 37.78 Ma, followed by a gradual decrease to ~4.0 mmol/mol at 37.50 Ma. This translates to a decrease in temperature from ~ 34°C to 28°C, a cooling of ~6°C over 280 ky, and is consistent with the gradual global cooling during the middle Eocene. However, this result is inconsistent with the previously published δ18O data that showed an overall decrease through this interval. This suggests that salinity, not temperature, is the major factor influencing the oxygen isotope record. The nannofossil assemblage at Blake Nose is diverse with 55 taxa identified, and is composed mainly of small Reticulofenestra (0-6μm), Pemma spp., Coccolithus pelagicus, Reticulofenestra bisecta, Reticulofenestra reticulata, Discoaster barbadiensis, Zygrhablithus bijugatus and Discoaster saipanensis. Other less common taxa include Braarudosphaera spp., Calcidiscus protoannulus, Sphenolithus moriformis, Discoaster tanii, and Coccolithus formosus. We compared the nannofossil absolute abundance data with the Mg/Ca paleotemperatures. No single taxa showed a direct similarity with the Mg/Ca paleotemperature trend suggesting that other factors such as salinity and nutrient level could be affecting their abundance.