USING 10BE SURFACE EXPOSURE DATING TO RECONSTRUCT THE LATE PLEISTOCENE HISTORY OF ICE SHEET RETREAT AND RELATIVE SEA-LEVEL CHANGES IN THE SEACOAST REGION OF NEW HAMPSHIRE

During the Last Glacial Maximum (LGM), erosional and depositional processes beneath the Laurentide Ice Sheet (LIS) dramatically modified the landscapes of northern North America, including New Hampshire. The resulting glacial landforms and deposits can be studied to reconstruct regional glacial history. However, age control for the timing of deglaciation in New Hampshire following the LGM has been developed at only a few sites. Specifically, the nature and timing of ice retreat in the Seacoast region of New Hampshire remains poorly documented due to the scarcity of organic material available for radiocarbon dating and complexities related to glacio-isostatic effects and relative sea-level changes following ice recession. Here we reconstruct the style and timing of deglaciation and the rate of associated marine regression in coastal New Hampshire following the LGM. Our approach involves the application of ¹⁰Be surface exposure dating to glacial features in the Seacoast region that are found both above and below the marine limit (approximately 70 meters above present-day) and hence are expected to have experienced a variable and complex exposure history. We collected 27 samples from glacial erratics distributed throughout the Seacoast along an elevation transect from present-day sea level to approximately 100 meters in altitude. ¹⁰Be ages from samples below the marine limit are anticipated to indicate the timing of their emergence from inundation, while samples from above the marine limit will yield exposure ages that reflect when ice retreated from the landscape. These ¹⁰Be ages should reveal different marine limit levels caused by varying magnitudes and rates of localized subsidence and post-glacial rebound throughout the Seacoast. We hypothesize that the northern Seacoast of New Hampshire experienced a rapid rate of postglacial marine regression comparable to coastal Maine, but that locations farther south will deviate from this trend and exhibit a spatially complex pattern of marine regression. Our ¹⁰Be ages will also help to determine when the retreating LIS margin transitioned from marine- to land-terminating. Understanding the transitional stage is valuable for monitoring modern glaciers that are currently undergoing a similar transition and for evaluating future sea-level changes.