Abstract

Stratigraphic variations in the delta 18O values of sedimentary carbonate in Lake Mendota (Wisconsin, USA) correlate with changes in atmospheric circulation patterns over the past century. Carbonate delta 18O values trend toward 18O depletion in progressively younger sediments. Archives of historical water-column data and climate data from the surrounding drainage basin indicate that this trend cannot reflect changes in water temperature, timing of carbonate precipitation, disequilibrium isotope fractionation, air temperature, or seasonal precipitation patterns. However, significant correlations with the frequency and direction of cyclones indicate that the isotopic record has been significantly influenced by subtle changes in atmospheric circulation over the past 100 yr. Two atmospheric circulation models can account for this relationship: (1) greater contributions of 18O-depleted precipitation associated with air masses from the Pacific Ocean or (2) a progressive increase in the altitude of condensation leading to 18O depletion of precipitation originating from the Gulf of Mexico. Because the Pacific Ocean generally contributes little moisture to the Midcontinent, the first model is an unlikely mechanism to generate the isotopic trends observed in the sediments. Intrusions of dry, dense air from the west, however, could force moisture originating from the Gulf of Mexico to higher altitudes of condensation and thus account for the trends in the carbonate isotopic record. This study demonstrates that historic data used in conjunction with lacustrine delta 18O stratigraphy provide a method to reconstruct past atmospheric circulation regimes in continental environments. Comparisons of sedimentary isotopic records from lakes in North America and Europe indicate that these reconstructions of regional atmospheric circulation patterns can be integrated to assess circulation changes on the global scale.

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