Enhanced Intermediate‐Depth Nutrient Import to the Late Last Interglacial Atlantic

Abstract The delivery of nutrients from intermediate waters that form in the Southern Ocean is thought to be a key control on tropical ocean surface productivity. In this paper, we present geochemical evidence that an increase in low‐latitude productivity during the Last Interglacial (LIG) was driven by an increase in the preformed nutrient content of Subantarctic Mode Water (SAMW). We generated records of benthic foraminiferal δ 13 C, δ 18 O, Cd/Ca and Mg/Li which are used to reconstruct seawater cadmium, dissolved oxygen, and temperature from a core site in the Florida Straits. The Florida Straits is a location of mixing between SAMW and Northern Component Water, the ratio of which is dependent on the strength of the Atlantic Meridional Overturning Circulation. We find that Late LIG seawater cadmium—which in today's ocean is correlated to phosphate—was substantially higher than the Late Holocene (LH) average at this location, while apparent oxygen utilization was similar during these two periods. Thus, we invoke higher preformed phosphate in the Florida Straits during the Late LIG relative to the LH. Increased SAMW preformed phosphate could be the result of reduced Antarctic Zone winter mixed layer residence time and greater Southern Ocean surface nutrient supply during the Late LIG compared to the LH, as supported by published reconstructions of Southern Ocean biogeochemistry and dynamics. We therefore hypothesize that higher SAMW preformed phosphate would cause an increase in the transport of nutrients into the low latitudes, thereby increasing productivity there.