Reduced meltwater outflow from the laurentide ice margin during the Younger Dryas

The cause of the Younger Dryas cold event, which interrupted the last deglaciation, is still a matter of debate(1). A prevalent hypothesis, proposed by Broecker et al.(2) is that the abrupt climate change was driven by a decrease in the rate of North Atlantic Deep Water production, triggered by a sudden dilution of North Atlantic surface water in response to the diversion of Laurentide ice-sheet melt water from the Mississippi drainage system to that of the St Lawrence river. Here we investigate the feasibility of this triggering mechanism by reconstructing sea-surface temperature, salinity and sea-ice cover records for the outlet of the Gulf of St Lawrence into the North Atlantic Ocean. These reconstructions-based on dinoflagellate-cyst assemblages(3,4) in sediment cores from the region-show reduced meltwater runoff, low temperatures and extension sea-ice cover during the Younger Dryas, dated here between 10,800 and 10,300 BP. Meltwater pulses did occur before and after the Younger Dryas event: as early as 11,700 BP, during the development of the Champlain Sea in the St Lawrence Lowland, and afterwards, until similar to 10,100 BP. At the resolution of our salinity proxy (0.7 parts per thousand), the meltwater pulses preceding the Younger Dryas did not affect sea-surface salinity off the shelf break. These constraint on the meltwater outlflow through the St Lawrence drainage system do not support the triggering mechanism of the Broecker et al. hypothesis(2), unless the North Atlantic thermohaline circulation is more sensitive to small salinity changes than most models suggest (23).