MELTWATER ROUTING FROM GLACIAL LAKE AGASSIZ: VALIDATING THE MOORHEAD PHASE BUT NOT ITS SYNCHRONICITY WITH THE YOUNGER DRYAS EVENT

During the last glacial hemicycle the intricate interplay of ice margin geometry, isostatic rebound, and topography dictated the locations of meltwater storage and routing. For over 15 years drainage events from glacial Lake Agassiz have been implicated as triggers for abrupt climate change. However, the chronology of Lake Agassiz events is poorly known with only a few moraines, beaches, and outlets accurately dated. Lake phases corresponding to high and low water are stratigraphically well understood in the main basin, but poorly constrained in time. New radiocarbon dates from the Grand Forks Air Force Base on the Ojata Beach suggest that, contrary to previous interpretations, the Ojata Beach is not a record of lake level drop at the beginning of the Younger Dryas event, but instead it is a record of lake level rise at the end of the Younger Dryas. In situ radiocarbon ages from a stump, roots, and macrofossils are overlain by sand of the Ojata Beach. The drop in lake level to the low-water Moorhead Phase, thought to have triggered the Younger Dryas, is now bracketed between 10,675±60–10,470±75 14C BP well after the beginning of the Younger Dryas. The youngest wood at the base of the Ojata beach deposit is 10,000±70 14C BP (11,470 cal. yrs; 1 sigma mean) consistent with 11,000±300 cal. yrs, the average of two OSL ages from the overlying beach sand. We interpret the Ojata Beach as recording the transgression of Lake Agassiz at the study site, which eventually overflowed the southern outlet at about 9400 14C BP. While this data does not resolve the mechanism by which Lake Agassiz dropped to some elevation below the Ojata Beach at the beginning of the Moorhead Phase, it does force us to look elsewhere to what may have triggered the Younger Dryas cold event.