Early and Middle Wisconsin glaciolacustrine sediments exposed at Scarborough Bluffs, Toronto, Canada, contain a low-diversity benthic ostracode fauna dominated by
Candona subtringulataand C. caudata. We have studied the oxygen and carbon isotopic compositions of the two ostracode species from the base of the Scarborough section to its top immediately below a Late Wisconsin till. We find an upward decrease in δ18O in both species that is more marked in C. caudata. The shift in δ18O of ostracode calcite from interglacial values of −6 to −8‰, to values of −17‰ immediately below the Late Wisconsin till is the result of (1) lowering of δ18O of the lake as a result of addition of isotopically light meltwater from an advancing and thickening Laurentide Ice Sheet combined with (2) lowering of δ18O of local precipitation entering the lake, due to falling temperatures. In the last stages of the lake, δ18O of the lake water was between −17 and −21‰, corresponding to a lake body composed of between 35 and 55% glacial meltwater. The lake appears to have been isotopically and thermally stratified for part of its history. Differences in δ13C values between C. caudataand C. subtriangulataincrease upward in the stratigraphic section. This may record enhanced partitioning of carbon isotopes between surface and bottom waters as a result of increased water depths, photosynthesis by algae, and changes in the input of dissolved organic carbon from the oxidation of organic matter. If the age assessments of these sediments are correct, then these data provide valuable information regarding the isotopic composition of ancestral Lake Ontario and also confirm that the basin was only fully glaciated during the Late Wisconsin.