COSMOGENIC 10BE CHRONOLOGY FOR THINNING OF THE LAURENTIDE ICE SHEET IN NEW HAMPSHIRE DURING THE LAST DEGLACIATION

Well-constrained ages for the retreat of the Laurentide Ice Sheet (LIS) have been developed at several key sites throughout New England, providing a framework for the deglacial history of the region. Previous work has focused primarily on documenting retreat of the ice sheet margin, but only a few studies have attempted to reconstruct changes in ice sheet geometry and flow patterns during its recession. This study provides the first direct age control on the thinning of the LIS in central and southern New Hampshire during the last deglaciation.

In this study, we developed 13 new cosmogenic ¹⁰Be exposure ages from scoured bedrock (n = 12) and an erratic (n = 1) on four glaciated summits, in order to determine when each mountain top emerged from the ice as it thinned. Mean exposure ages for three of the dated peaks (Mounts Monadnock, Cardigan, Major) fall within ca. 15-14 ka, indicating the occurrence of rapid ice surface lowering across southern New Hampshire during this interval. On Mount Major, exposure ages from the sole erratic sampled in the study (14.8 ± 0.3 ka) and adjacent scoured bedrock (15.4 ± 0.5 ka) agree within uncertainty, suggesting negligible isotope inheritance. The northernmost dated peak (Mount Dickey) yields a mean exposure age of ca. 13 ka, which may indicate later emergence from thinning ice relative to the other more southerly peaks or alternatively could reflect potential issues with past surface cover on this peak.

Striation measurements collected both from this study and previously published literature were analyzed to observe how ice flow patterns evolved in this region. The striations at the lowest elevations display a wider azimuthal range than those found at the highest elevations, indicating the development of strong topographic control on ice flow as thinning occurred. Three-dimensional ice sheet surfaces were modeled to display changes in ice sheet geometry, and suggest an increased rate of thinning from 1 m/yr to 2 m/yr at ~15 ka coinciding with the start of the Bølling Allerød warm period. This study provides valuable new insight on LIS thinning and flow patterns in New Hampshire and suggests potential links between changes in ice sheet geometry and regional paleoclimate forcings.