Early Eocene palaeoceanography and palaeoclimatology of the eastern North Atlantic: stable isotope results for DSDP Hole 550

Abstract High-resolution oxygen and carbon isotope records for benthonic and planktonic foraminifera have been established through the early Eocene at DSDP Site 550, Goban Spur in the North Atlantic. Benthonic stable isotopic records from Site 550 (estimated palaeodepthc.4000 m) represent the first documentation of actual deep water properties during the early Eocene. At Site 550 in the earliest Eocene (end of NP10 Zone), a 0.5‰ vertical δ18O (i.e. thermal) gradient between subsurface and bottom water was eliminated, probably reflecting upward displacement of subsurface water by intermediate and deep water. Thereafter vertical isothermal conditions prevailed for the rest of the early Eocene. This is consistent with earlier findings suggesting unusually low vertical and latitudinal temperature gradients in the early Eocene ocean. A comparison of the isotopic records from Site 550 and nearby DSDP Site 401 (palaeodepth 1800 m), reflecting intermediate water properties, confirms the isothermal nature of the water column in the eastern North Atlantic. This is in contrast to earlier findings for the Antarctic region (ODP Sites 689 and 690) in the early Eocene, where supposedly warm bottom waters originating in low latitudes were overlain by cooler intermediate waters originating in high latitudes. Both the oxygen and the carbon isotopic records for Sites 550 and 401 indicate that the eastern North Atlantic region was influenced by a single source of intermediate-to-deep water through most of the early Eocene. The benthonic δ18O and δ13C values for Site 550 are generally low compared with coeval sections worldwide, indicating that during most of the early Eocene the North Atlantic was dominated by warm and nutrient-enriched deep water formed in a low latitude region. The earliest stages of the long-term cooling trend, which characterizes the second part of the Paleogene, can be traced by a 2°C decrease in bottom water temperatures across the NP13–NP14 transition at both Sites 550 and 401.