Meltwater and seasonality influence on Subpolar Gyre circulation during the Holocene

Abstract

Dinoflagellate cyst assemblages in a marine sediment core from the Iceland Basin were used to carry out qualitative and quantitative assessments of upper ocean conditions in the northern branch of the Subpolar Gyre (SPG) during the Holocene. The data revealed that the early Holocene, i.e. prior to ~6.8 kyr B.P., experienced elevated summer sea surface temperatures (SST). In contrast, winter SST was lower compared to modern conditions. This stronger-than-present seasonality revealed by our data thus shows the influence of the early Holocene insolation forcing. Higher seasonality also dominated the reconstructed sea surface salinity (SSS) in the same period, with lower salinity during both summers and winters. The lower SSS suggests freshwater advection into the SPG, and the dinoflagellate cyst content from this interval indicates a dominant sourcing from the Canadian Labrador coast, and thus the eastern margin of the Laurentide Ice Sheet. Within this interval of low SSS, the onset of the Holocene Climate Optimum stands out by its distinct “marine” assemblage components and associated high summer SST and relative SSS increase between 9.3 and 7.8 kyr B.P. A prominent shift in the overall assemblage composition occurs at ~6.8 kyr B.P., with lower seasonality in sea surface conditions from that time onwards. Increased summer and winter SSS suggest that this change is linked to the halted influence of meltwater from the Laurentide Ice Sheet, a westward shift of the subpolar front and a contracted SPG allowing an easier northward movement of southern-sourced North Atlantic waters, contributing to and consequently sustained by the spin-up of the overturning circulation. The subpolar front appears to gradually have moved slightly eastward again between 2.8 and 1.9 kyr B.P. towards its modern-day position, with a subtle increase of elements characterized by a typical East Greenland Current signature in the Iceland Basin. Finally, spectral analyses indicate that a range of periodicities may be present in the new SSS and SST records, most notably a 2000-yr periodicity in SSS changes, although none of the periodicities are dominant in both records or throughout the time interval covered by the records.