The DIC (dissolved inorganic carbon) of interstitial waters from sediments of a small carbonate-precipitating lake in southern Ontario has highly enriched δ13C values of up to +13‰ relative to PDB (Peedee belemnite). This post-depositional isotopic enrichment is attributed to the metabolism of methanogenic bacteria. The δ13C values of marl in the sediment profile are also enriched by an average of +3 to +4‰ relative to the associated molluscs.Photosynthetically induced carbonate precipitation and post-depositional recrystallization are among the processes that are considered to contribute to the isotopically enriched marls. Evidence for the latter process is drawn from geochemical analyses on interstitial waters, which show them to be at or very close to saturation with respect to calcite, indicating that net calcite dissolution has occurred, probably under the influence of high pCO2's generated by methanogenic bacteria. Such freshwater carbonate sediments with interstitial waters enriched in 13C provide a unique environment for study of isotope relations during carbonate diagenesis under accessible conditions and with a time scale of geologic relevance.