Climate and oceanography of the Tasmanian Gateway during the Middle Eocene Climatic Optimum (MECO)

Abstract

Based on improved biostratigraphic control of Cores 9R to 38R from ODP Hole 1170D, the material is dated between ~43 and ~35 Ma. According to TEX86 based SST reconstruction, sea surface temperatures generally decreased from ~25°C to ~21°C during this period. Superimposed on this temperature decrease, a temperature peak from ~21.5°C to ~25.5°C was observed. Biostratigraphy confirms that this peak corresponds with the Middle Eocene Climatic Optimum (MECO). Endemism in the Eocene Tasmanian Gateway seems to have been influenced by paleoceanography rather than by sea surface temperatures. Biogeography of dinocyst taxa suggests that ODP Site 1170 was located under direct influence of the Tasman Current until the MECO, whereafter it was located out of direct reach of any current. It is unclear if water mass exchange occurred through the Tasmanian Gateway during the MECO. The shift of the Tasman Current away from the Site could be somehow related to the sea level rise during the MECO, which was observed from dinocyst assemblages as well as smear slide analysis. Before and after the MECO, either increased terrestrial runoff from the nearby continents or the melting of seasonal sea ice could have caused stratification in the Southern Ocean. During the MECO, climate was possibly relatively dry. It is not possible to draw conclusions on paleoproductivity during the MECO based on P cyst abundance or absolute abundances, as these variables show ambiguous signals and could both be influenced by more factors than nutrient levels only. It remains unclear why general absolute dinocyst abundances in ODP Hole 1170D are relatively high compared to other Sites in the Southern Ocean, and why in particular Enneadocysta dictyostila thrived during the MECO.