EVIDENCE OF MELTWATER MEGAFLOODS NEAR THE SOUTHERNMOST ICE MARGINS OF THE LAURENTIDE ICE SHEET BETWEEN 20.5 AND 18.5 KA

The last Northern Hemisphere ice sheet margins retreated in the late phase of the Last Glacial Maximum, peaked at 19 thousand calibrated years ago (ka) in response to large-scale changes in all components of the climate system. This recession is presumably to have weakened the Atlantic meridional overturning circulation via freshwater input triggering the Southern Hemisphere deglacial warming by a bipolar seesaw mechanism. The Southern Hemisphere warming was likely responsible for the increase of CO₂ in global atmosphere, resulting in the ice–CO₂ feedback and initiating the full process of the deglacial warming on the planet. Significant sea-level rise of 5-10 m at 19 ka provides a strong evidence for this hypothesis. However, the evidence of large-scale meltwater discharges from Northern Hemisphere ice sheets is rare. Here we provide AMS ¹⁴C and OSL chronologies to argue ice-margin retreat from the Lake Michigan Lobe of the Laurentide Ice Sheet and associated meltwater outwash deposits in the middle Illinois River valley between 20.5 and 18.5 ka and peaked at 19 ka. This event concurred with the immense ice-margin retreat from the Midwestern and Northeastern USA during Erie Interstadial warming period. Much of 900,000 km² area was free of ice during this interval. Our finding supports the Milankovitch hypothesis that the rising boreal summer insolation in mid-high latitudes triggered the last deglaciation.