Record of postglacial organic matter delivery and burial in sediments of Lake Ontario

A 12 m piston core obtained from Lake Ontario has allowed us to study the accumulation of organic matter in sediments deposited since the retreat of the Laurentian ice sheet about 12,500 years ago. Discrepancies between radiocarbon dates of disseminated organic matter and ostracod shells emphasize the magnitude of the recycling and retention of organic carbon in the Great Lakes. Concentrations of CaCO3 and organic carbon decrease down core due to the presence of fine grained glaciolacustrine clays at the base of the sedimentary sequence. Increases in sediment grain size indicate periods with enhanced fluxes of terrigenous material. C/N ratios, %organic carbon and %CaCO3 vary proportionally with fluctuations in terrigenous input. C/N ratios indicate that lacustrine algae have been the main source of organic matter to Lake Ontario sediments. Organic δ13C values become heavier at the bottom of the core, suggesting a shift in carbon sources for the bulk organic matter. Anthropogenic effects are well documented in total hydrocarbon and total fatty acid profiles of modern sediments, whereas postglacial trends representing natural changes of organic matter are relatively undetectable. Sedimentary profiles of terrigenous/aquatic ratios of n-alkane and n-alkanoic acids indicate that early variations in postglacial sedimentation rates may have impacted the preservation of aquatic organic matter. Changes in watershed vegetation and organic matter delivery to Lake Ontario altered these ratios as deglaciation progressed. Organic geochemical properties of modern sediments have recorded how anthropogenic activity has augmented algal productivity in Lake Ontario by increasing nutrient input.