Mixed-layer dynamics in the Nordic Seas during MIS 11: Implications for the AMOC

Abstract

Deep-water formation in the high latitudes partially controls the strength of the Atlantic Meridional Overturning Circulation (AMOC). While recent research has illustrated a reduction in ocean convection likely attributable to enhanced freshwater fluxes in this region, elevated surface freshening of the Nordic Seas occurred in tangent with a strong AMOC during the marine isotope stage (MIS) 11 interglacial, 424,000 – 374,000 ka. It was previously hypothesized that the prolonged introduction of freshwater resulted in a thicker mixed layer relative to other quaternary interglacials, which is supported by bulk nitrogen reconstructions of nutrient utilization. Here, we further investigate the phasing of nutrient utilization during MIS 11 using foraminifera-bound stable nitrogen isotope ratios, which are better protected from diagenetic alteration and thus allow for higher-confidence interpretation. Our record faithfully tracks the global AMOC trend as reconstructed from benthic carbon data, where lower levels of nutrient utilization occur in phase with stronger AMOC conditions. While this may reflect the enhanced advection of nutrient-rich North Atlantic waters into the Nordic Seas, reconstructions of sea surface temperature and salinity suggest that the influence of southern-sourced waters was delayed relative to the observed AMOC and nutrient trends. We therefore hypothesize that the coupling of nutrient utilization and AMOC strength reflects the physical conditions of a deeper mixed layer, allowing for an increased mixing of nutrients from below.