Variable Marine Reservoir Effect in Bivalves From Champlain Sea Sediments in the Lake Champlain Valley, USA

The Champlain Sea was an inland sea that existed in the St. Lawrence lowlands and Lake Champlain Valley of eastern North America during the last deglaciation. At the beginning of its formation the Champlain Sea was in contact with the retreating Laurentide Ice Sheet, although the ice margin eventually retreated beyond the sea's extent. Its only connection to the open ocean was through the relatively narrow Gulf of St. Lawrence. The Champlain Sea must have had significant salinity stratification because it received freshwater runoff from the ice margin, Adirondack Mountains of New York State, and the Green Mountains of Vermont, as well as the entire discharge from the Great Lakes in the west. Age estimates for the Champlain Sea have traditionally been based on an abundance of bivalve and other marine fossils collected throughout the region; however, more recent studies based on terrestrial organic radiocarbon ages indicate that the Champlain Sea may be at least 500 - 1000 years younger than suggested by bivalve ages. We will present paired radiocarbon ages on terrestrial organic material and bivalve shells showing that a simple reservoir correction cannot be applied for all Champlain Sea bivalve ages. Hiatella arctica shells deposited in water depths of less than 100 m appear to be 500 - 800 14C years too old. Portlandia arctica shells deposited in water depths of greater than 200 m appear to be 1000 - 1500 14C years too old. The difference may be related to salinity stratification. Macoma balthica shells appear to be 1500 - 2000 14C years too old, possibly because they are filter feeders and were taking up old carbon from carbonate rich sediment.