Relative lake level reconstructions for glacial Lake Agassiz spanning the Herman to Campbell levels

Abstract

Lake Agassiz was the largest North American pro-glacial lake during the last deglacial period, and the routing of the overflow is hypothesized to have affected thermohaline circulation and in turn, global climate. Here an existing glacial isostatic adjustment model is applied to Campbell beach strandline data to synthesize and correlate published radiocarbon- and OSL-dated lake level indicators in an effort to produce relative lake-level curves and a drainage history. The curves highlight where the water level history of Lake Agassiz is well understood, and where additional work is necessary. Many OSL-dated strandlines between 14.5 and 13.2 ka record meltwater routing through a southern outlet to the Gulf of Mexico during this time, and that lake levels fell due to sill incision and glacial isostatic adjustment (GIA). Oxygen isotope records from marine sediments tracking continental runoff, plus deglacial isochrones, suggest eastern outlets to the Atlantic Ocean opened at around 13.2 ka, near the onset of the Younger Dryas (YD) stadial, but the evidence within the basin is scant. At some point during the YD, Lake Agassiz overflow shifted to a northwestern outlet into the Arctic Ocean, and the reconstructed lake levels track lake-level rise driven by uplift of the northwestern outlet, which eventually transferred water to the southern outlet to create the Campbell level of Lake Agassiz. The apparent shallow water depth (∼4 m) of overflow at the southern outlet relative to earlier phases suggests multiple active outlets at this time (northwestern, southern, and perhaps eastern). Oxygen isotope records from the Gulf of Mexico indicate overflow via the southern outlet occurred in two brief phases, likely explained by an oscillating ice margin that regulated overflow. Considerable work remains to assess the timing of deglaciation to understand when Lake Agassiz first drained east and north. Lacking are details of eastern drainage and any dated stratigraphic record of likely lake level fluctuations during the Younger Dryas.