3-6 A Hydrologically and Biologically Complex Proglacial Lake System at the Margins of the Collapsing Laurentide Ice Sheet

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources

Presenting Author:

Authors:

Abstract:

In the context of modern climate warming and widespread deglaciation, proglacial lacustrine environments are (and will be) emergent environmental systems that both reflect and impact the state of retreating glaciers and ice sheets. To explore how these glacial-proglacial systems interact, we examine the paleoclimate and environmental history of Glacial Lake Hitchcock (GLH), a transient proglacial lake that occupied the modern Connecticut River Valley from 18.2–12.5 ka as the Laurentide Ice Sheet (LIS) retreated during the Last Termination. GLH has been well-studied with respect to its geomorphic and depositional history, with a continuous annual chronology constrained by the North American Varve Chronology. Yet, the highly clastic nature of the lake sediments have, to-date, limited geochemical interpretations of GLH environmental history and paleoclimate.

Here, we present novel data that establishes authigenic carbonate concretions, which formed within GLH sediments, as powerful paleoenvironmental proxies. Preliminary comparison between GLH concretions and adjacent ostracod valves reflect similar δ¹⁸O compositions, indicating shared formation from lake waters and concretion formation shortly following deposition of lake sediments. Concretion (and ostracod) δ¹⁸O compositions correspond to water compositions ranging from -6 to -14 ‰ (VSMOW, assuming 5ºC water temperature), suggesting that lake water compositions are predominantly meteoric, as LIS meltwater compositions would be more depleted at ≤-20 ‰. In contrast, adjacent clay mineral δ¹⁸O values correspond to approximately -20 ‰ water compositions, indicating a more dominant meltwater component and authigenic clay mineralization in subglacial or ice-proximal (rather than lacustrine) environments.

Carbonate δ¹³C values are consistent with fractionation from a range of organic carbon sources. A group of concretions with abundant fossil leaf casts on their basal surfaces reflect a narrow isotopic composition (-12.5 to -11 ‰, VPDB), indicating organic littoral debris as the direct source of dissolved inorganic carbon in the sedimentary environment. Concretions that formed in deeper water sediments exhibit a broader range of C isotopes, suggesting a broader variety of lacustrine carbon sources.

Our preliminary data indicate a GLH system supported by abundant meteoric water inputs and hosting abundant organic carbon sources from both littoral and deeper water settings. We will report on forthcoming U-series ages of concretions to more rigorously evaluate the temporal relationship between concretions and lake sediment deposition as well as potential groundwater contributions to the proglacial lacustrine system.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8850

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