Oceanographic regimes in the northwest Labrador Sea since Marine Isotope Stage 3 based on dinocyst and stable isotope proxy records

摘要

Sea surface temperature (SST), salinity and density gradients in the upper water column of the northwest Labrador Sea have been reconstructed based on high resolution analysis of a core (HU2008-029-004PC) spanning the last ~36 ka, raised off Hudson Strait. The modern analogue technique was applied to dinocyst assemblages and combined with stable isotope data from Neogloboquadrina pachyderma leftcoiled (Npl) for this purpose. Three oceanographic regimes were identified, broadly corresponding to the "glacial", "deglacial" and "postglacial" intervals. The site remained under the direct influence of the Laurentide Ice Sheet (LIS) margin until the postglacial and did not record the B?lling-Aller?d warming and weakly recorded the Younger Dryas event. The "glacial" regime lasted until ~12.2 cal ka BP. It was characterized by generally low concentrations of dinocysts within an assemblage indicative of quasiperennial sea ice. The "deglacial" regime (ca 12.2-8.3 cal ka BP) was marked by increased biogenic fluxes and more diversified dinocyst assemblages and possibly an enhanced subsurface inflow of North East Atlantic Deep Water. Warm summer (~11 °C) but low winter (~0 °C) sea surface temperatures, sea ice cover during about 3 months per year, and low summer salinity (~28) suggest strong stratification in the upper water layer in relation to meltwater supply from the LIS. Following the final drainage of glacial Lake Agassiz through Hudson Strait, which is dated here at ~8.3 cal ka BP, and the subsequent LIS collapse, increased summer salinity (up to ~35) was accompanied by a reduced seasonal gradient of sea surface temperature from winter (~3.8 °C) to summer (~8.6 °C) suggesting enhanced penetration of North Atlantic Water. Weakened stratification of the surface water layer then allowed for winter convection and Labrador Sea Water formation, which is consistent with increased Npl-δ~(13)C values in response to higher ventilation of the subsurface water layer.