Evidence For A Series Of Large Freshwater Discharge Events To The North Atlantic Immediately Preceding The Younger Dryas

Sedimentary evidence including a thick sand unit between two rhythmically laminated glacial lacustrine clay units in the northern Champlain Valley of New York State indicates that a flood was directed through the valley during the last deglaciation. This flood occurred when ice-marginal retreat north of the Adirondack Uplands allowed the confluence of Glacial Lakes Iroquois (Ontario Basin) and Vermont (Champlain Valley). A rough discharge estimate of 0.16 Sv has been calculated for the flood. There is also evidence that this event was preceded by a smaller flood with an estimated discharge of 0.11 Sv. Both floods would have been directed southward to the North Atlantic through the Hudson Valley. Varve counts from the northern Champlain basin indicate that these floods occurred 150 - 300 years before the Champlain Sea marine incursion. Radiocarbon ages from terrestrial macrofossils date these floods to about 13,000 calibrated years B.P. A third flood event (estimated at about 0.10 Sv) was caused by ice retreat north of the St. Lawrence Lowland, and was immediately followed by the incursion of the Champlain Sea. This flood would have been directed through the Gulf of St. Lawrence sometime between 12,850 - 12,700 calibrated years B.P., about the time of initiation of the Younger Dryas. This was also the beginning of the re-routing of all Great Lakes discharge (estimated at around 0.05 Sv) from the Hudson Valley to the Gulf of St. Lawrence. A brief re-occurrence of freshwater ostrocodes in early Champlain Sea sediments may indicate that another freshwater flood, perhaps originating from the Great Lakes, discharged into the North Atlantic through the Gulf of St. Lawrence.