Date of Award

Summer 7-16-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

First Committee Advisor

Katherine A. Allen

Second Committee Member

Brenda Hall

Third Committee Member

Aaron Putnam

Additional Committee Members

Joellen Russell

Abstract

Abrupt, millennial-scale climate events characterize the most recent deglaciation. However, their nature remains poorly understood. Previous work has shown enhanced glacial melt signals from the European/Scandinavian Ice Sheet (Toucanne et al., 2015) and a rapid recession of the Laurentide Ice Sheet in Eastern Maine during Heinrich Stadial 1 (HS-1) (e.g., Hall et al., 2025). Globally, many proxies record a uniquely two-phased structure to HS-1, with a partition at ~16.1 cal ky BP (e.g. Broecker and Putnam, 2012). These observations challenge the idea that stadials were simply year-round cold excursions in the Northern Hemisphere. We aim to determine if these late HS-1 melt signals are replicated by Laurentide Ice Sheet meltwater in the Gulf of Maine marine sediment record, which was ice-proximal during much of the last deglaciation. Here we present geochemical records (δ18O, Mg/Ca, calculated δ18Oseawater) developed using benthic foraminifera Nonionellina labradorica and Islandiella helenae/norcrossi from a new Gulf of Maine sediment core, EN-669-GGC-40, spanning ~17.5- 11.5 cal ky BP. Because core EN-669-GGC-40 has a maximum age of 17.5 cal ky BP, we did not capture the earliest part of deglaciation nor the first part of HS-1, during which the LIS must have receded from much of the present-day GOM, including Franklin Basin (Schnitker et al., 2001; Todd and Shaw, 2012). However, the character of the documented portion of HS-1 in the GOM can be divided into two phases: the earlier (17.5 to ~16.0 cal ky BP) being characterized by extreme variability of δ18Osw, which we interpret as isotopically depleted meltwater runoff supply from the marine-terminating LIS occasionally interrupted by relatively enriched Atlantic water incursions. The later period (~16.0 to 14.7 cal ky BP) is characterized by a rapid isotopic depletion of δ18Osw from 15.7-15.3 cal ky BP which indicates a large-scale local melting event during late HS-1, consistent with a rapid collapse of the LIS in Eastern Maine. Our results lend support to the hypothesis that HS-1 was characterized by warming summers and cold, dry winters in the North Atlantic (Denton et al., 2005 and 2022), and provide new observations that may be used to reassess the cause of the phase shift midway through HS-1.

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