Paper No. 3-4
Presentation Time: 9:00 AM
THE POTENTIAL OF GLACIAL LAKE AGASSIZ VARVES TO RECONSTRUCT LAURENTIDE ICE SHEET RESPONSE TO CLIMATE CHANGE: PRELIMINARY RESULTS FROM LAKE KABETOGAMA (MN) SEDIMENT CORES
Pro-glacial varves from large lakes have the ability to resolve LIS sensitivity to deglacial warming with annual precision, but most varve records are under-developed. Varves from glacial Lake Agassiz found within lakes from northern Minnesota (MN) and northwestern Ontario (ON) should archive a 3000+ year record of ice sheet response from the mid-continent of the LIS, spanning from the Bølling-Allerød through the Younger Dryas (YD). Here we present Lake Agassiz stratigraphic and varve thickness data from Lake Kabetogama (MN) sediment cores. Lake Kabetogama is the oldest of three varve series from the region, which includes Steep Rock Lake (ON) and multiple sites from Lake Wabigoon (ON). Sediments from all three can be correlated with a series of ~24 anomalously thick red clay varves, previously interpreted to have resulted from the western flow of red Lake Superior sediments into the Lake Agassiz basin due to ice re-advance to the Marks and Dog Lake moraines, presumably near the end of the YD. Macrofossils within the Lake Kabetogama sediments should provide the first radiocarbon dates of this event. The oldest glaciolacustrine sediments in Kabetogama consist of 523 varves that record ice retreat from the core site. These varves pre-date the oldest varves at Steep Rock and Wabigoon. The stratigraphy above the 523 basal varves is complex, and includes units of laminated sand and silt, superseded by periods characterized by the abrupt onset of anomalously thick varves that rapidly thin up section. Based on comparisons with other glacial varve sediments, the sand and silt units most likely resulted from lake level lowering. The thick varves could be explained by rapid lake level rise due to ice margin fluctuations closing an eastern Lake Agassiz outlet into the Lake Superior basin (during or before the YD.) Alternatively, the thick varves may reflect a sudden and dramatic increase in sediment flux from the LIS, presumably due to summer melting. Better dating precision would place this varve record, which clearly documents abrupt and short-lived events, in context with high resolution climate records, such as those from the Greenland Ice Sheet.