THE N-ALKANE ABUNDANCES AND ORIGIN OF ORGANIC MATTER IN LAKE ONTARIO SINCE 14,000 CAL YR BP

Here we present a 14,000 cal yr BP record of terrestrial and aquatic organic matter (OM) contributions to Lake Ontario as inferred from n-alkane abundances. Glacial sediments in Lake Ontario contain ~0.2-0.3 % OM, and have bulk OM carbon-isotope compositions of ~ –27 per mil. The carbon/nitrogen (C/N) ratios (<10) of the glacial samples suggest a lacustrine (algal) origin rather than a terrestrial (land plant) (C/N >20) OM source. However, some allochthonous organic matter that entered Lake Ontario could also have had low C/N ratios. For example, OM fixed to soil clays typically has C/N ratios <20 and in some cases <10, and clay minerals are more abundant in glacial than non-glacial sediments in Lake Ontario. We have measured n-alkane abundances to discriminate among these possible OM sources. Typically, long-chain n-alkanes (C27 to C35) dominate terrestrial (land plant) OM, short-chain n-alkanes (C17 and C19) characterize lacustrine algal OM, and mid-chain n-alkanes (C23 to C25) indicate submergent macrophytes. Mid-chain n-alkanes are the most abundant in the Lake Ontario glacial sediments; the origin of this OM is remains enigmatic (soil clays, residual OM in carbonate detritus?). Upon hydraulic closure of Lake Ontario at 12,300 cal yr BP, bulk OM carbon-isotope compositions decrease to ~ –29 per mil and the abundances of short-chain n-alkanes (C17 and C19) increase. These results are interpreted to indicate an OM source dominated by in-lake algal productivity. At 8,800 cal yr BP a shift occurred from postglacial, cold/dry to early hypsithermal, warm/dry conditions in the Ontario Basin. By then, the OM contents of the lake sediments had increased to ~1 %, with this change accompanied by a slight increase in bulk OM carbon-isotope composition (–29 per mil to –28 per mil) and increased abundances of long-chain n-alkanes. These data are interpreted to indicate greater terrestrial OM contribution to the lake. By the onset of the main hypsithermal period (7,000 cal yr BP), OM content had further increased to ~2 %, and there was a three-fold increase in C27 n-alkane abundance. These results are interpreted to indicate a ‘flush’ of terrestrial OM into Lake Ontario, associated with increased precipitation and runoff during this warm and moist period of time.