Rapid Retreat of a Marine-Terminating Margin of the Laurentide Ice Sheet in the Seacoast Region of New Hampshire, USA

• Main Author: Brazo, Julia E
• Format: Text
• Language: unknown
• Published: University of New Hampshire Scholars' Repository 2023
• Subjects: deglacial chronology; geospatial analysis; Laurentide Ice Sheet; marine limit; marine-terminating; surface exposure dating; Ice Sheet
• Online Access: https://scholars.unh.edu/thesis/1749; https://scholars.unh.edu/context/thesis/article/2789/viewcontent/Brazo_unh_0141N_11626.pdf

A better understanding of the transition of marine-terminating margins of the Laurentide Ice Sheet (LIS) to land-based termini is instructive for monitoring modern glaciers that are currently undergoing a similar transition and influencing sea-level. Expanded knowledge of regional ice retreat rates and the timing of deglaciation of the LIS also provides insight on mechanisms of ice sheet retreat and collapse. Here we reconstruct the style and timing of ice recession and attendant marine regression in coastal New Hampshire during the last deglaciation. Our approach involves geospatial analysis of glaciomarine deposits to assess regional marine limit variability combined with 10Be surface exposure dating of glacial features to evaluate the potential for contrasting exposure histories above and below the marine limit during deglaciation. Geospatial analyses reveal a variable marine limit from north to south in New England with maximum elevations of glaciomarine deposits ranging from 20 meters near Boston, Massachusetts, 57 to 103 meters within the Seacoast of New Hampshire, and up to 152 meters in central Maine. We report 13 new 10Be exposure ages from erratics and bedrock distributed throughout the Seacoast region along an elevation transect that extends above and below the marine limit. Our results suggest rapid retreat of the LIS margin through the Seacoast of New Hampshire and indicate that the glacier front transitioned from marine- to land-terminating at ~17 ka. Exposure ages of erratics below the marine limit suggest rapid postglacial marine regression from ~ 17–16 ka. Our reconstructions of rapid rates of deglaciation and marine regression are consistent with previously published regional ice recession rates and sea-level histories. However, the timing of deglaciation is ~1-2 ka earlier than suggested by recently compiled isochron maps of former ice margin positions in New England during the last deglaciation.