CLIMATIC FORCING OF CARBON-OXYGEN ISOTOPIC COVARIANCE IN TEMPERATE-REGION MARL LAKES

Carbon and oxygen stable isotopic compositions of lacustrine carbonate from a southeastern Michigan marl lake display linear covariance over a range of 4.0 parts per thousand Peedee belemnite (PDB) in oxygen an d 3.9 parts per thousand (PDB) in carbon. Mechanisms of delta(13)C-del ta(18)O coupling conventionally attributed to lake closure in arid-reg ion basins are inapplicable to hydrologically open lake systems. Thus, an alternative explanation of isotopic covariance in temperate region dimictic marl lakes is required. We propose that isotopic covariance is a direct record of change in regional climate. In short-residence-t ime temperate-region lake basins, summer meteoric precipitation is enr iched in O-18 relative to winter values, and summer organic productivi ty enriches epilimnic dissolved inorganic carbon in C-13. Thus, climat e change toward longer summers and/or shorter winters could result in greater proportions of warm-month meteoric precipitation, longer durat ions of warm-month productivity, and net long-term enrichment in carbo nate O-18 and C-13. Isotopic covariance observed in the Michigan marl lake cores is interpreted to reflect postglacial warming from 10 to 3 ka followed by cooler mean annual temperature, a shift toward greater proportions of seasonal summer precipitation, a shortening of the wint er season, or some combination of these three factors.