Glacial varves can detail ice-margin positions and provide a proxy for meltwater discharge at resolutions comparable to those of the Greenland ice core archives, and thus they can be critical paleorecords for assessing the response of both ancient and modern ice sheets to climate change. Here we provide an ∼1500 yr varve chronology straddling the Younger Dryas (YD)–Holocene boundary (11.65 cal. kyr B.P.), the first such chronology in North America. The varves are from glacial Lake Agassiz (central North America). The chronology is pinned on accelerator mass spectrometry radiocarbon-dated terrestrial macrofossils at the base of a widespread red-clay bed deposited during flooding from the Lake Superior basin. We illustrate the utility of this record by examining ice-margin retreat and melting through the late Younger Dryas and across the Holocene boundary. The ice margin receded at a constant rate, not only during the late YD, but for at least 300 yr after the onset of the Holocene. In contrast, varve thicknesses increased at the boundary, and a moraine formed over a 50 yr period, perhaps in response to the warming climate. Our expectation is that this time series will continue to be developed, expanded, and refined because it promises to be the longest and most geographically extensive glacial varve data set in North America.
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ACKNOWLEDGMENTS: We are grateful to the many people who contributed to this project, including M. Gordon, R. Kane, P. Birschbach, C. Pearson, P. Douglas, and B. Bates. Field support was provided by M. Bursheim, W. Webb, and M. Pounder. At LacCore, K. Brady and A. Stone assisted with logging. Comments by A. Wickert, E. Leonard, J. Ridge, and an anonymous reviewer improved this manuscript. The U.S. National Science Foundation supported this research (EAR grants 1602788, 1602789, 1602791).
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