Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a re-evaluation

Overflow from glacial Lake Agassiz has been implicated in affecting late-glacial ocean circulation and climate. The timing of the diversion of Lake Agassiz overflow away from its southern routing (to the Gulf of Mexico) correlates closely with the Younger Dryas ca 11-10 14C ka. New models of paleotopography in the eastern outlet region of Lake Agassiz, adjusted for differential isostatic rebound, show that overflow to the Superior basin would have occurred when the lake formed its Herman, Norcross, and Tintah beaches, just after ∼10.9 14C ka, if the Laurentide Ice Sheet (LIS) had retreated from the Thunder Bay, Ontario, region by that time. However, the absence of coarse flood deposits normally associated with large overflows, plus the morphology of possible overflow channels in the Thunder Bay region, suggest that Agassiz overflow may not have been routed into the Great Lakes during the Younger Dryas, as previously believed. In addition, non-carbonate radiocarbon dates in the Thunder Bay area are all younger than 10 14C ka, suggesting that this area may not have been deglaciated until after the Younger Dryas, thus preventing Lake Agassiz overflow through this area. An alternative route of overflow from Lake Agassiz during the Younger Dryas could have been through the northwestern (Clearwater) outlet to the Arctic Ocean, although all research to date shows that this region was covered by ice until ∼10 14C ka. On the other hand, it is possible that older and coarser overflow sediment may now lie buried beneath finer deposits associated with the 10 14C ka Marquette readvance of the LIS, which ponded waters in valleys between Lake Agassiz and Lake Superior, or that eastward overflow in this area was not catastrophic.