CLIMATE CHANGE AND THE DEGLACIATION OF WISCONSIN

The state of Wisconsin contains one of the best-preserved glacial landform and sedimentary records for the last deglaciation though its chronology is less well constrained, particularly during the earlier deglacial period. The poor chronology is likely due to the presence of permafrost in the region during and following the Last Glacial Maximum (LGM), confounding the application of traditional radiocarbon dating techniques. Here we review the current deglacial chronology for the Green Bay Lobe (GBL) in Wisconsin and compare it against the much better dated chronology of the adjacent Lake Michigan Lobe (LML) as well as runoff to the Gulf of Mexico. The onset of GBL retreat is constrained by 4 bedrock cosmogenic dates from ~60 km within the LGM moraine. The mean of the 4 dates indicates retreat was underway prior to ~18 ka. If retreat commenced shortly before ~18 ka, then there is a 2-4 kyr lag between the onset of GBL retreat relative to the LML, though this has yet to be actually determined. Nevertheless, these few dates confirm that ice in Wisconsin was retreating prior to initial increases in carbon dioxide and tropical temperature implying that retreat was triggered by gradually warming summers in response to rising boreal summer insolation. During the Oldest Dryas cold period of the North Atlantic, both the GBL and LML retreated rapidly 17-16 ka, which is recorded as a freshwater pulse into the Gulf of Mexico. Subsequent readvances occurred after ~15 ka and ~13.4 ka during the Bølling and Allerød warm periods of the North Atlantic with retreat during the intervening Older Dryas cold period. If these oscillations were in response to North Atlantic climate, then the GBL and LML may have responded to fluctuations in precipitation. Alternatively or in concert, the oscillations may reflect the rise in carbon dioxide and tropical temperature ~17 ka, and their cessation during the Bølling/Allerød. Based on the above chronology, we identify three persistent questions in regards to the deglacial history in Wisconsin. 1) When did the GBL retreat from its LGM extent? 2) Was it synchronous with the LML and the other ice lobes in Wisconsin, which have even less chronologic constraint? 3) When did ice finally retreat from Wisconsin?