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

Oxygen-isotope Variations in Post-glacial Lake Ontario

The role of glacial meltwater input to the Atlantic Ocean in triggering the Younger Dryas (YD) cooling event has been the subject of controversy in recent literature. Lake Ontario is ideally situated to test for possible meltwater passage from upstream glacial lakes and the Laurentide Ice Sheet (LIS) to the Atlantic Ocean via the lower Great Lakes. Here, we use the oxygen-isotope compositions of ostracode valves and clam shells from three Lake Ontario sediment cores to identify glacial meltwater contributions to ancient Lake Ontario since the retreat of the LIS (~16,500 cal [13,300 14C] BP). Differences in mineralogy and sediment grain size are also used to identify changes in the hydrologic regime. The average lakewater δ18O of –17.5 ‰ (determined from ostracode compositions) indicates a significant contribution from glacial meltwater. Upon LIS retreat from the St. Lawrence lowlands, ancient Lake Ontario (glacial Lake Iroquois) lakewater δ18O increased to –12 ‰ largely because of the loss of low-18O glacial meltwater input. A subsequent decrease in lakewater δ18O (from –12 to –14 ‰), accompanied by a median sediment grain size increase to 9 μm, indicates that ancient Lake Ontario received a final pulse of meltwater (~13,000-12,500 cal [11,100-10,500 14C] BP) before the onset of hydrologic closure. This meltwater pulse, which is also recorded in a previously reported brief freshening of the neighbouring Champlain Valley (Cronin et al., 2012), may have contributed to a weakening of thermohaline circulation in the Atlantic Ocean. After 12,900 cal [11,020 14C] BP, the meltwater presence in the Lake Ontario basin continued to inhibit entry of Champlain seawater into early Lake Ontario. Opening of the North Bay outlet diverted upper Great Lakes water from the lower Great Lakes causing a period (12,300-8,300 cal [10,400-7,500 14C] BP) of hydrologic closure in Lake Ontario (Anderson and Lewis, 2012). This change is demarcated by a shift to higher δ18Olakewater (~ –7 ‰), driven in part by strong evaporative conditions in the Ontario basin and in part by increasing δ18Oprecipitation at this time. The δ18Olakewater then fluctuated only slightly upon the eventual return of the upper Great Lakes water during the Nipissing phase at 5,800 cal [5,090 14C] cal BP (Anderson and Lewis, 2012), after which shelly fauna are no longer preserved in the sediment record