The Late Quaternary Paleolimnology of Lake Ontario

We examined the oxygen isotopic composition of biogenic carbonates, carbon and nitrogen abundances and isotopic compositions of bulk organic matter (OM), and the abundances and carbon isotopic compositions of individual n-alkanes (C17 to C35) for samples from three, 18 m long sediment cores from Lake Ontario in order to: (i) assess how changing environmental parameters affected the hydrologic history of Lake Ontario, and (ii) evaluate changes in organic productivity and sources since the last deglaciation. Knowledge of the hydrologic and ecological behaviour of the Lake Ontario basin during past climate change provides insight into its future sensitivity. During the glacial period, the average lakewater oxygen-isotope composition was –17.5 ‰ (VSMOW), which indicates a significant glacial meltwater contribution. Higher abundances of mid-chain n-alkanes (C23 and C25) with carbon-isotope compositions of –32.5 ‰ (VPDB) record allochthonous OM input, notwithstanding low bulk C/N ratios that normally indicate lacustrine productivity. These results suggest a degraded source, perhaps OM associated with clay minerals. Glacial retreat facilitated proliferation of terrestrial vegetation, as recorded in higher abundances of long-chain (terrestrial) n-alkanes (C27, C29, C31). Cessation of glacial meltwater supply is marked by an increase in lakewater oxygen-isotope composition to ~ –12 ‰ by 13,000 cal BP. This increase was interrupted by a final inflow of low-18O glacial meltwater that lasted ~500 years. Rerouting of the upper Great Lakes caused Lake Ontario to become hydrologically closed from 12,300 to 8,300 cal BP. The lakewater oxygen-isotope composition increased to –7 ‰ because of the end of glacial meltwater supply, and climate-related increases in evaporation and the oxygen-isotope composition of precipitation. A steady increase in terrestrial n-alkane abundances and their carbon-isotope compositions (–31 to –29 ‰) signified plant growth under water-stressed conditions until ~8,000 cal BP. Transition to a wetter climate (6,800 cal BP) and return of upper Great Lakes water supply (~5,300 cal BP) caused lake levels to rise. A decrease in carbon-isotope composition (~2 ‰) in all aquatic n-alkanes during this time signifies a change in the lacustrine carbon pool, whereas a decrease in the carbon-isotope composition of terrestrial n-alkanes signifies relief from more arid conditions