Profiles of δ18O in two marl cores from southern Ontario, Canada, are compared in detail with the records of meteoric water isotopic composition and photosynthetic humidity obtained from previous isotopic studies using wood cellulose at Brampton, Ontario. By accounting for the inferred changes in meteoric water composition and temperature, the changes in the oxygen-isotope content of the marl arising from fluctuations in the evaporative enrichment of the lakewater can be estimated. The resulting synthetic profiles of relative evaporative enrichment from both marl cores exhibit strong correspondence to the photosynthetic humidity profile generated from the studies at Brampton, which substantiates the importance of evaporation in the water budgets of these two lakes. A striking analogy exists between the isotopic evaporative enrichment response of such lakewaters and that of the waters in terrestrial plants; both of the marl lakes approach close to isotopic steady state during the summer season, when carbonate precipitation occurs.The results of these studies strengthen the basis for paleoclimatic interpretations suggested in previous reports. Most notably, the marl data yield further evidence for relatively dry conditions in southern Ontario during the early part of the Hypsithermal interval, at a time that coincides with maximum eastward extension of prairie in the midwest United States.