THE N-ALKANE AND CARBON-ISOTOPE SIGNATURES OF ORGANIC CARBON IN LAKE ONTARIO SINCE 14,000 CAL YR BP

The n-alkane abundances and carbon-isotope compositions of organic matter (OM) from Lake Ontario sediments reveal a complex history of variation in both source and lacustrine productivity over the last 14,000 cal yr BP. Glacial sediments containing ~0.2-0.3 % organic carbon (OC) are dominated by C23 through C29 n-alkanes. These compositions most likely represent allochtonous contributions of peat, higher terrestrial plant matter and clay-associated OM delivered by glacial meltwaters from the periglacial environment. Subordinate amounts of C17-C19 n-alkanes may indicate limited primary lacustrine productivity at this time. Transition from glacial to post-glacial conditions in the Lake Ontario basin was marked by a lowering of water levels, rising OC contents, and increased abundances of C17-19 n-alkanes. Little systematic carbon isotopic variation was found for individual n-alkanes in the glacial and transitional sediments: C17-C19, –30 per mil; C21, –33 to –30 per mil; and C23+, –33 to –32 per mil, which likely reflects well-mixed, multiple OM sources. Hydraulic closure of Lake Ontario beginning at about 12,300 cal yr BP produced its lowest recorded levels. Approximately equal abundances of C17-C19, C23-C25, and C27+ n-alkanes at this time are tentatively interpreted to indicate lacustrine, (submergent) macrophyte and terrestrial OM contributions. C17-C19 n-alkanes showed little carbon isotopic variation from the older sediments. However, a 2 per mil decrease in carbon-13 for C23 and enrichments of ≤8 per mil for C25+ n-alkanes (C25> C27> C29) occurred by the end of hydraulic closure. Increased littoral zone productivity may have been favoured by low lake levels. Warming beginning at 8,300 cal yr BP triggered a transition from cold/dry to warm/wet conditions, during which water levels gradually rose. OC in associated lake sediments reaches 2 % and contains the highest fraction of C27+ n-alkanes, which have carbon isotopic compositions of –33 to –30 per mil. These data suggest that Lake Ontario received significant terrestrial OM from its catchment at this time. The progressive 5 per mil depletion of carbon-13 in C17 and C25 n-alkanes upwards through this interval remains to be explained.