THE LATE PALEOGENE EVOLUTION OF SOUTHERN OCEAN DEEP-WATER FORMATION – THE ONSET OF GLOBAL THERMOHALINE CIRCULATION

The Meridional Overturning Circulation (MOC) during the Early Paleogene was characterized by separate overturning in the Atlantic and Pacific. Convection occurred in the North Pacific, the Ross Sea region (Southern Pacific Deep Water, SPDW) as well as the Atlantic sector of the Southern Ocean (Southern Component Water, SCW). Deep waters that formed in the Southern Ocean regions had distinct Nd isotope signatures –SPDW had an ε_Nd(t) value of ~-5 from at least ~70 to 30 Ma, while the composition of SCW was lower and increased from ~-10 to -8 over the ~70 to 30 Ma interval. The overall increase in SCW deep-water values likely reflects an increasing proportion of relatively radiogenic Pacific surface waters circulating through the Drake Passage as it opened at shallow water depths. This radiogenic surface water mixed with regional dissolved weathering inputs prior to convection in the Atlantic sector. However, the fact that SPDW remained distinct from SCW as late as ~30Ma suggests that the MOC and was not influenced by the Eocene-Oligocene transition – separate overturnings persisted in each basin and the “global conveyor” had not yet formed.

When was the “global conveyor” established, and more importantly, how did this transition impact meridional heat transport? The major step in the transition to a global conveyor involved the Southern Ocean and the convergence of SPDW and SCW compositions. Here we present new data from South Pacific DSDP and IODP Sites 323, 596 and 1370 to investigate the timing of the Southern Ocean convergence. The three sites record an isotopic decrease from ~-5.5 to -6.3 epsilon units from 50 to 32 Ma, and by ~24 Ma ε_Nd(t) values decreased further to -7.1. This value is ~1.5 epilon units higher than Late Oligocene – Early Miocene values recorded at ODP Site 689 in the Weddell Sea, however some of this difference may be due to the water depths of the sites – the South Pacific sites were all situated at >3000m water depth while Site 689 was <2000m. The timing of the apparent convergence between SPDW and SCW coincides with the onset of large-volume Antarctic Circumpolar Current formation indicated by a significant hiatus recorded in the South Pacific at ~23-25Ma, suggesting that establishment of the ACC and global thermohaline circulation were closely related.