Late Glacial and Holocene Flow Dynamics of the Denmark Strait Overflow Water

Abstract:

The overflow of dense water from the Nordic Seas to the North Atlantic across the Greenland-Scotland Ridge forms a major component of the deep branch of the Atlantic Meridional Overturning Circulation and influences the climate system in Northwest Europe. Research has focused on deep convection of the Iceland Scotland Overflow Water (ISOW) and its links to climate variability in the North Atlantic. Our understanding of the history of the Denmark Strait Overflow Water (DSOW) is significantly less constrained and yet it accounts for half of the total overflow production today. We focus on the Eirik Drift south of Greenland in the vicinity of the DSOW. Down-core ²³⁰Th_xs derived sediment focusing factors (Ψ) and measurements of the mean size of sortable silt reveal winnowed sediments during the Last Glacial Maximum and Heinrich 1 suggesting an influx of vigorous southern sourced waters and restricted DSOW production. Reduced overflow may be due to glacial isostatic processes which shoaled the Denmark Strait sill combined with a southward shift of deep convection sites in response to enhanced ice cover in the Nordic Seas. Intensification of the DSOW is evident between 9 and 13ka BP indicating initial deepening of the Denmark Strait sill and northward migration of the locus of deep water production. Ψ values for the Holocene suggest an active DSOW with a shift in the flow regime at 6.8 ka BP indicated by a reduction and subsequent stabilization of mean size sortable silt during the mid-late Holocene. This is corroborated by other studies showing a reorganization of the deep water after 7ka. An establishment of the Labrador Sea Water at intermediate depths altered the density structure of the deep western boundary current and weakened the ISOW. Changes in deep water circulation occur as North Atlantic climate entered Neoglacial cooling determined by Mg/Ca derived sea surface temperatures and abundances of the polar planktic foraminifera species N. pachyderma. They indicate a southward shift of the polar front in response to declining solar insolation during the late Holocene. Combined, our results suggest intimate links between the deep and surface ocean of the Labrador Sea.