STRUCTURE OF THE LAST GLACIAL MAXIMUM AT THE SOUTHERN MARGIN OF THE LAURENTIDE ICE SHEET

North American ice sheets were a primary driver of sea-level changes during the last glacial period. Given this, changes in ice sheet extent are less well understood than desired. Here we report on the timing and structure for the maximum southernmost extent of the Laurentide Ice Sheet (LIS), recognizing that extent and volume are not related in a simple way. This ice is sourced from the Labrador dome on the eastern side of the ice sheet.

We focus her on the Connersville Interstadial, originally identified in western Ohio ands eastern Indiana. This inter-till organic bed suggests that the maximum extent of the ice sheet was achieved more than once. However, despite extensive organic remains, the radiocarbon chronology was unable to resolve the timing of the interstadial and the maximum extent of the ice. Here we present recent efforts that have increased the number and quality of radiocarbon ages allowing the following sequence of ice-marginal fluctuations to be put forth.

The L IA margin was south of the present-day Great Lakes by 28.8 ka reaching a stable (?) position by 25.8 ka with subsequent expansion to a maximum by 23.7-23.1 ka depending on location. We take 23.4 ka as an estimate for the maximum ice-margin extent. The subsequent Connersville Interstadial lasted between 0.7 and 2.0 ka and terminated by a readvance dated to 21.5 ka. A slow retreat from that readvance in the Erie Lobe began by 21.4 ka that was interrupted by readvances or standstills between 19.7 and 16.8 ka. Subsequently recession rates increased.

The details of this chronology can not yet be replicated west of influence of the Labrador dome where ice was sourced from the Keewatin Dome. Thus, the extent of the ice-marginal fluctuations , the the forcing of them remain unknown. Nevertheless, it appears that the LIS remained nears its southern most position from at least 23.7 to 19.7 ka ~ 4 ka. The relatively minor changes in ice-sheet extent may not show up in sea-level curves, but this activity implies the ice-sheet margin was not static during the Last Glacial Maximum.