Evidence for Increased Thermal Differentiation in the Deep Atlantic in the Middle-Late Eocene (40-33 Ma)

New high-resolution benthic foraminiferal δ18O data from ODP Site 1053 (North Atlantic) reveal paleoceanographic changes in the middle to late Eocene. The comparison of the δ18O data from this site with those published for ODP Site 689 (Southern Ocean) shows that in the late middle Eocene (~40 Ma), the Southern Ocean began to cool relative to the western North Atlantic. At the start of the late Eocene (~37-36 Ma), Site 1053 δ18O values started to decrease, indicating a warming. We conclude from the divergence in δ18O values between Sites 1053 and 689 that the thermal differentiation between Northern and Southern deepwater source regions that is characteristic of the Neogene began in the middle Eocene. We hypothesize that this early reorganization in deep-sea circulation was driven by the opening of the Drake Passage (~40 Ma), allowing the development of a proto-Antarctic Circumpolar Current (ACC). This proto-ACC, in its shallow, early stages of development, initiated the thermal isolation of the Antarctic continent from the warm subtropical South Atlantic surface gyre; cooler surface water south of the ACC resulted in cooling of deep Southern Component Water (SCW) at Site 689. Our data are also consistent with published modeling experiments showing that the ACC results in a strong cross-equatorial heat transport, contributing to warmer deep Northern Component Water (NCW). We speculate that the δ18O decrease of ~0.6%, registered in the benthic foraminiferal δ18O values from ODP Site 1053 at 37-36 Ma, is evidence for cross-equatorial heat transport warming the northern hemisphere.