Compound-specific isotopic analyses track changes in seasonal precipitation regimes in the Northeastern United States at ca 8200 cal yr BP

Abstract

We analyzed the hydrogen isotopic composition of sedimentary leaf wax (C 28 n-acid) and palmitic acid (C 16 n-acid) in cores from Berry and Crooked Ponds in Massachusetts. The results show a dramatic positive shift in leaf wax δD values around 8200 cal yr BP, when regional lake levels rose and plant communities shifted in favor of plants intolerant of dry conditions. To aid our interpretation, we studied modern lake sediment samples and found that the δD values of leaf wax and palmitic acid record the hydrogen isotopic composition of water available during the synthesis of these compounds. Palmitic acid δD values follow lake water values often derived from winter precipitation, and leaf wax δD values track the water available to terrestrial plants during the growing season. Based on these results, we interpret the downcore isotopic shift as an increase in the fraction of precipitation falling during the growing season. The increase coincides with evidence from pollen and palmitic acid δD values for a multi-century period of cold conditions at ca 8200 cal yr BP. The close timing of the brief cool period and a step-change in summer precipitation levels in the northeastern United States is consistent with the hypothesis that the collapse of the Laurentide ice sheet occurred in association with cold North Atlantic conditions from 8400 to 8000 cal yr BP. The location of the northeastern US downwind of the ice sheet and close to the North Atlantic makes it sensitive to both factors. Our results also provide insight into later changes in the northeastern US because they indicate a progressive decrease in summer precipitation levels after ca 8000 cal yr BP.