Paleoproductivity and organic matter sources in Late Quaternary Lake Ontario
Abstract We have examined δ 13 C of ostracode valves and clam shells, carbon and nitrogen mass accumulation rates (C- and N-MAR), atomic C/N and δ 13 C OM from Lake Ontario since ~ 16,000 cal BP to evaluate the impacts of the Laurentide Ice Sheet (LIS) and regional paleohydrology on primary lacustrine productivity and sources of OM. Samples were obtained from piston cores from each of the Niagara, Mississauga and Rochester basins, which geographically span Lake Ontario from west to east. During the glacial period, variations in inputs directly from the LIS versus up-stream glacial Lake Algonquin and (or) ancient Lake Erie controlled the DIC pool in ancestral Lake Ontario. Cold and dry conditions limited primary productivity in the Ontario basin (low C/N ~ 7.5). Increased contributions of glacial meltwater from glacial Lake Algonquin during the latter stages of deglaciation provided a uniform supply of DIC to ancestral Lake Ontario and other lakes then occupying the Huron and Erie basins, stabilizing the δ 13 C DIC pool at ~− 4‰ (VPDB). Upon hydrologic closure of early Lake Ontario at the onset of post-glacial conditions (12,300 cal BP), a growing divergence between the δ 13 C of ostracode valves (decreases to − 6‰) and clam shells (increases to + 1‰) indicates differences in summer versus spring DIC pools. The end of glacial meltwater input to early Lake Ontario also made it possible to detect changes in primary lacustrine productivity using the isotopic and C/N proxies. Increasing δ 13 C OM (from ~− 29 to − 27.5‰) and low C/N (