Post-Glacial Development of Western North Atlantic - Labrador Sea Oceanographic Circulation

The subpolar North Atlantic Ocean – Labrador Sea region is an important site for both oceanographic and atmospheric circulation. The convergence of ocean currents causes downwelling of cold, saline water in the subpolar gyre, helping to drive the world-wide thermohaline circulation system. The main surface currents involved in the gyre are the south-flowing, cold and relatively fresh Labrador Current and the north-flowing, warm and relatively saline Gulf Stream. The oceanic front between these two major currents moves north and south, dependent on the relative strengths of the currents, impacting the climate around the North Atlantic region. Holocene variability of the western subpolar gyre has been elucidated based on marine sediment cores from Placentia Bay and Trinity Bay in southeastern and northeastern Newfoundland, respectively. Changes in the northeastern part of the subpolar gyre, and its impact on the Greenland margins after the end of the last glacial period and through the Holocene, are examined based on analyses of sediment cores from the Uummannaq Trough, West Greenland. Marine sediment cores were taken from the Uummannaq Trough on the continental shelf of central West Greenland. The core sites were chosen in an effort to track the retreat of the edge of the ice sheet after the Last Glacial Maximum. Similar to the other core sites, the cores were analysed using benthic foraminiferal assemblages in addition to bathymetry, quantitative x-ray diffraction analyses, and lithological analyses, all constrained by radiocarbon dates. At the height of the Last Glacial Maximum, there was a large, confluent ice stream draining the Uummannaq fjords on central west Greenland which extended to the shelf edge. By 15,000 years BP, the ice had begun to retreat from the outer shelf, aided by the presence of the warm West Greenland Current. The ice stream subsequently stabilised on the mid-shelf during the cold Younger Dryas event, as evidenced by the bathymetry. After the Younger Dryas, the ice stream rapidly retreated back toward the fjords, leaving the shelf free of grounded ice. A combined record is based on three cores taken from Placentia Bay that span the time since the Younger Dryas, approximately the last 13,000 years, while the core from Trinity Bay represents the last 7200 years. The cores were dated using radiocarbon measurements obtained from benthic foraminifera. The proxies used to examine these cores include foraminiferal analyses used to determine changes in botom-water conditions, x-ray fluorescence scanning to determine relative levels of elements and therefore sediment provenance, quantitative x-ray diffraction analysis to determine mineralogical content, and laser diffraction grain size analysis to determine relative bottom current speed. For selected cores dinoflagellate cyst (Trinity Bay) or diatom (Placentia Bay) analyses were also available. After the end of the Younger Dryas, the Labrador Current weakened, allowing a stronger Gulf Stream influence to impact the western North Atlantic. A brief stratification event was recorded in Placentia Bay, likely tied to the drainage of glacial Lake Agassiz, after which the Labrador Current strengthened. The Labrador Current remained the major influence around Newfoundland and the western North Atlantic. During the late Holocene, the subpolar gyre weakened, which carried less Gulf Stream-derived water to the western North Atlantic Ocean via the West Greenland Current and the Slopewater Current, south of Newfoundland. Changes in the subpolar gyre circulation had developed to be analogous to the modern climate by approximately 2 cal kyr BP.

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