New insights into the relationship between mid latitude North Atlantic hydrography and productivity during the intensification of northern hemisphere glaciation

Abstract:

In the North Atlantic Ocean today, the North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with key feedbacks on global climate. The mean latitudinal position of the NAC dictates whether the mid-latitude North Atlantic is dominated by warm, nutrient-poor subtropical surface waters (as it is during Late Pleistocene interglacials) or cold, nutrient-rich Arctic surface waters (as it is during Late Pleistocene glacials). The origin of this glacial-interglacial pattern in North Atlantic productivity is hypothesised to be linked to the onset of southward deflections of the NAC into the mid latitude North Atlantic after 2.7 Ma, during the late Pliocene intensification of northern hemisphere glaciation (iNHG). Yet, changes in wind-driven Ekman divergence and the upwelling of nutrient-rich thermocline waters are also hypothesised to influence surface water productivity in this region from this time. To shed new light on the origin of these large-amplitude productivity cycles, we present new suborbital-resolution records of surface and thermocline water properties from IODP Site U1313 (41°N), a benchmark mid-latitude drillsite at which this transition in productivity is recorded, during iNHG (MIS G11 to 95, ~2.82-2.41 Ma). We reconstruct surface and thermocline temperatures using Mg/Ca ratios of the tests of two species of planktic Foraminifera with distinct depth habitats, Globigerinoides ruber (surface mixed layer) and Globorotalia crassaformis (500-800 m water depth), and upper water column stratification using co-registered thermocline-surface water δ¹⁸O and δ¹³C gradients. With these new data and other published proxy records from Site U1313, we compare competing hypotheses to explain drivers of the large export productivity changes previously documented at this site during iNHG.