Late Glacial and Holocene Paleolimnology of two temperate lakes inferred from sediment organic delta C-13 chronology

The stable carbon isotope (delta(13)C) and elemental C/N ratios in Total Or ganic Carbon (TOC) extracted from radiometrically dated cores from two Midw estern USA lakes were determined to investigate the factors that control th ese values in temperate lakes. The range of delta(13)C values (-26 to -32 p arts per thousand) and C/N ratios (mean value similar to 10.8) are typical of values reported for other temperate lake organic matter in this region. In the core from Lake Winnebago, Wisconsin, a negative correlation was seen between the TOC and delta(13)C, which can be interpreted in terms of a re- mixing and consumption of sedimented organic carbon along with rapid equili bration throughout the water column. No correlation was seen between the TO C and delta(13)C in the record from Ladd Lake, Ohio, implying that in this latter lake productivity alone was not a singular process controlling the i sotope ratio. Here, it is suggested that equilibrium conditions are maintai ned such that the DIC of the water is never depleted of aqueous CO2 during high organic production and the resulting delta(13)C Of the organic carbon lacks correlation with the TOC. Further, in this lake a fine resolution ana lysis was carried out which indicated a possible anthropogenic influence on the isotope ratio around times when human settlement (similar to 300 yrs a go) and enhanced agricultural practices (similar to 80 yrs ago) were signif icant. The study shows that carbon isotope studies are useful in paleolimno logic investigations.