Active Nordic Seas deep-water formation during the last glacial maximum

Abstract

The Nordic Seas are the primary location where the warm waters of the North Atlantic Current densify to form North Atlantic Deep Water, which plays a key part in the modern Atlantic Meridional Overturning Circulation. The formation of dense water in the Nordic Seas and Arctic Ocean and resulting ocean circulation changes were likely driven by and contributed to the regional and global climate of the last glacial maximum (LGM). Here, we map the source and degree of mixing of deep-water in the Nordic Seas, and through the Arctic Gateway (Yermak Plateau) over the last 35 thousand years using neodymium isotopes (εNd) measured on authigenic phases in deep-sea sediments with a high spatial and temporal resolution. We find that a large-scale reorganisation of deep-water formation in the Nordic Seas took place between the LGM (23-18 thousand years ago) and the rapid climate shift that accompanied the subsequent deglaciation (18-10 thousand years ago). We show that homogeneous εNd signatures across a wide range of sites support LGM deepwater formation in the Nordic Seas. In contrast, during the deglaciation disparate and spatially variable εNd values are observed leading to the conclusion that deep-water formation may have been reduced during this time. Deep-water formation processes in the Nordic Seas regulate the global climate via the redistribution of heat by the surface ocean and the capacity of the deep ocean to store carbon 1 . At present the Atlantic Meridional Overturning Circulation (AMOC) links polar and sub-polar climate with the formation of North Atlantic Deep Water (NADW), a major component of the global oceanic thermohaline circulation. The densest northern-sourced waters in the modern AMOC are formed in the Nordic Seas, primarily by deep convection and gradual transformation of North Atlantic surface waters 2 .These dense waters formed in the modern Nordic Seas overflow the Greenland-Scotland Ridge (GSR), eventually contributing to NADW accumulating carbon and nutrients as it flows throughout the deep ocean 2 (Fig. 1). The extent, mechanism, and importance of deep-water formation in the Nordic Seas during glacial periods and periods of ice rafting during meltwater events (Heinrich Events/Heinrich Stadials) are still not adequately understood. The canonical view is that the glacial AMOC was displaced from the Nordic Seas to south of Iceland in the form of a fast and shallow overturning cell forming Glacial North Atlantic Intermediate Water and that there was Southern-sourced water in the deep (> 2.5 km) Atlantic e.g,3 . Contrary to this several studies e.g.4,5 argue for the presence of glacial NADW and speculate that this dense water may have been sourced from the Nordic Seas. Keigwin and Swift 6 similarly suggest that a Northern-sourced water mass may have been present in the deep (~ 5000 m) Atlantic, which could plausibly have been sourced from the Nordic Seas 7 . However, proposed scenarios of LGM deep-water formation in the Nordic Seas range from near-cessation to vigorous present-day-like deep-water f...