Structure of the upper water column in the northwest North Atlantic: Modern versus Last Glacial Maximum conditions

During the Last Glacial Maximum, the northwestern North Atlantic constituted a major conduit for Labrador and Greenland ice sheet meltwaters. Vertical density gradients in its upper water masses have been reconstructed by combining information from transfer functions based on dinocysts and from oxygen isotope measurements (δ 18 O) in planktonic foraminifera. Transfer functions yield temperature and salinity and thus potential density (σ θ ) for the warmest (August) and coldest (February) months in the photic zone. The δ 18 O values in different size fractions of epipelagic ( Globigerina bulloides ) and mesopelagic ( Neogloboquadrina pachyderma left‐coiled (Npl)) foraminifera allow us to assess σ θ gradients through the pycnocline between surface and intermediate waters, based on the calibration of a σ θ versus δ 18 O relationship from transfer function reconstructions. The size and density of Npl shells provide further constraints on these σ θ gradients. The results show the development of a very strong pycnocline during the LGM with a difference of about 3 (summer) to 1.5 (winter) σ θ units between surface and underlying waters. They indicate conditions unfavorable for vertical convection and support the hypothesis of the spreading of a shallow, low‐salinity buoyant layer over the northern North Atlantic. This layer depicted a strong E‐W gradient, with maximum seasonal contrast and minimum absolute σ θ values westward.