EARLY DEGLACIAL THINNING OF THE LAURENTIDE ICE SHEET FOLLOWED BY RAPID REGIONAL DEGLACIATION

Future changes in sea level will largely be dictated by changes in the world’s ice sheets. Yet, uncertainty still exists over the magnitude and rate to which these ice sheets will respond to climate change. To better understand ice-sheet sensitivity, accurate dating of past changes in ice sheet configurations is critical. Knowledge of past changes in the Laurentide Ice Sheet (LIS) are particularly valuable, as this ice mass held the equivalent of 80 m of global sea level at its maximum extent. Numerous studies have documented the timing and pattern of LIS margin retreat since the Last Glacial Maximum (LGM) (e.g. Dyke, 2004), yet few studies have provided vertical constraints that detail thinning histories necessary for accurate sea-level contribution estimates. Here we present 21 new ³⁶Cl ages from boulder and bedrock samples along vertical transects spanning >1000 m from multiple peaks in the Adirondack Mountains of northeastern New York, USA. Ages range from 24.1±0.5 ka to 13.4±0.4 ka excluding two inherited outliers (76.3±1.3 ka and 43.1±1.5 ka). Two distributions from the highest elevation sites (~1500 m) return average ages of 22.9±0.8 ka and 21.7±1.3 ka, and range between 15.7±0.6 ka to 13.6±0.8 ka from sites <1000 m. Comparison of sample age against elevation indicates an initial gradual thinning of the southeastern LIS during the LGM (~23-21 ka) followed by rapid surface lowering around 16-14 ka. Reconstructed ice-surface profiles of the LIS based on glaciological constraints and independently dated margin histories (Dyke, 2004) are consistent with this pattern of gradual early thinning in the Adirondacks, and rapid mid-deglacial thinning from lateral retreat of the steeper ice-sheet flank across the study region. Our new work demonstrates that the LIS responded to initial deglacial climate change at 21 ka with gradual thinning (~300 m over 6 kyr) accompanied by slow ice margin retreat, and subsequently followed by rapid thinning (~800 m over 2 kyr) and large scale lateral retreat.