Deep flow variability under apparently stable North Atlantic Deep Water production during the last interglacial of the subtropical NW Atlantic

The environmental history of the last interglacial period is equivocal, since its climate has been reported to be anything from unstable, with at least one significant cold event, to generally stable and comparable to the Holocene. Here the activity of the southerly Deep Western Boundary Current (DWBC) in the similar to 130-110 ka interval is studied at two sites (3481 and 4760 m) in the subtropical NW Atlantic presently bathed by Lower North Atlantic Deep Water (LNADW). Proxies for both bottom water chemistry (delta C-13) and near-bottom flow vigor (SS, the mean size of the "sortable silt" fraction) are employed. The data indicate the interval of peak interglaciation when LNADW bathed both sites studied is followed by the development of a vertical water mass gradient between them, with nutrient-rich waters of southern origin flooding the deeper parts of the area. After similar to 113 ka this gradient disappears as the deeper, low delta C-13 waters homogeneously invade the whole of the depth range investigated. The SS data imply these events are likely to be associated with the vertical migration of the flow axis of the DWBC. Furthermore, they also reveal unexpected fluctuations in near-bottom flow activity during the period of minimum ice volume which are tentatively ascribed to small changes in the convective activity of the Nordic Seas. Alternatively, DWBC flow fluctuations may be controlled by processes occurring farther downstream from the areas of deep water formation, therefore not requiring changes in the production rates of LNADW. In either case, this series of small fluctuations during the peak of the last interglaciation may ultimately be linked to the insolation-related evolution of latitudinal sea surface temperature and salinity gradients in the North Atlantic.