SEA-SURFACE CONDITIONS AND FRESHWATER EXPORT TROUGH THE ST. LAWRENCE LOWLAND AND GULF DURING THE DEGLACIAL CHAMPLAIN SEA EPISODE

The opening of the Champlain Sea, which has been re-assessed to have occurred 13 ka ago (Occhietti and Richard, Geogr. Phys, Quat., 2003), corresponds with a time of major climate changes during the late Alleröd -Younger Dryas transition as recorded in Greenland Ice (Rasmunssen et al., JGR, 2006). The reconstruction of sea-surface conditions based on organic walled dinoflagellate cysts (or dinocysts) indicates that the Champlain Sea was characterized by very cold conditions (<5°C in summer), low salinities (30-32) and extensive seasonal sea-ice cover (~ 10 months/yr), with modern analogues found in the Western Arctic Ocean (Paiement, MSc thesis, UQAM 2007). Offshore, in the Gulf of St. Lawrence and the adjacent Atlantic margins, the use of updated database to reconstruct sea-surface condition from dinocyst assemblages in sediment cores indicate that a major cooling in surface waters, from 15 to 7°C in summer, with the development of harsh freezing conditions in winter occurred at about 13 ka BP. Small salinity variations are also recorded, but they seem insignificant given the accuracy of the approach. Hence, despite difficulty with absolute dating of marine cores, the data suggest that whereas the opening of the Champlain Sea might have occurred at the very end of the Alleröd, it was rapidly accompanied by an offshore spreading of sea-ice with significant cooling, during the onset of the Younger Dryas. Although we failed to identify a significant low salinity anomaly that would result from the drainage of Lake Agassiz when the route of the St. Lawrence axis opened, one may hypothesize that the offshore spreading of sea-ice cover could have been triggered by the opening of the Champlain Sea. Alternatively, a slowdown of the Atlantic Meridional Overturning during the Younger Dryas might have been as efficient in this respect. Due to the impossibility to set a precise 14C-chronology during the onset of the Younger Dryas, causal mechanisms, might remain open to discussion. Nonetheless, the spreading of sea-ice off the St. Lawrence system has played a role on sea-surface condition in the western North Atlantic during the interval and certainly has added to feedback processes in the initial cooling of the Younger Dryas.