A 3450-YEAR VARVE CHRONOLOGY FROM MINNESOTA (USA) AND ONTARIO (CANADA) THAT SPANS THE BØLLING-ALLERØD TO EARLY HOLOCENE

Pro-glacial varves from large lakes rimming the Laurentide Ice Sheet (LIS) have the ability to not only resolve LIS sensitivity to late-glacial climate fluctuations, but also reconstruct lake levels and meltwater paleohydrology. Here we provide a varve chronology from glacial lakes Norwood and Agassiz, which fronted the Rainy Lobe of the LIS. The chronology is compiled from sediment cored from 19 modern lakes, which preserve underlying glacial lake sediment, plus correlations to regional varve thickness records in Ontario by Rittenhouse (1933) and Antevs (1951). Radiocarbon and optically-stimulated luminescence results provide direct dates, and an independent check on the varve counts. In total, the varve chronology spans from 14,300 to 10,850 cal. yr BP. We illustrate the utility of the record by: 1) reconstructing century-scale, ice-margin isochrons, 2) comparing ice-margin behavior to Greenland ice-core climate archives, and 3) connecting changes in varve sedimentology to a major drop in Lake Agassiz level known as the Moorhead lowstand. Results indicate that the Moorhead lowstand did not occur until long after the onset of the Younger Dryas, therefore models that link this major Lake Agassiz drawdown to meltwater drainage impacting ocean circulation and global climate should be abandoned.