Climatic forcing of carbon-oxygen isotopic covariance in temperate-region marl lakes Available to Purchase

Carbon and oxygen stable isotopic compositions of lacustrine carbonate from a southeastern Michigan marl lake display linear covariance over a range of 4.0‰ Peedee belemnite (PDB) in oxygen and 3.9‰ (PDB) in carbon. Mechanisms of δ¹³C-δ¹⁸O coupling conventionally attributed to lake closure in arid-region 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-time temperate-region lake basins, summer meteoric precipitation is enriched in ¹⁸O relative to winter values, and summer organic productivity enriches epilimnic dissolved inorganic carbon in ¹³C. Thus, climate change toward longer summers and/or shorter winters could result in greater proportions of warm-month meteoric precipitation, longer durations of warm-month productivity, and net long-term enrichment in carbonate ¹⁸O and ¹³C. 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 winter season, or some combination of these three factors.