117-3 Pathways of carbon and water through proglacial lake sediments during the Last Termination

Abstract:

In the context of modern climate warming and deglaciation, proglacial wetland and lake systems are rapidly evolving, emergent environments. These dynamic systems are sensitive to cryosphere change and have recently received increased attention for their potential roles in mobilization/sequestration of organic carbon. Glacial Lake Hitchcock (GLH) was a transient proglacial lake that occupied the modern Connecticut River Valley from 18.2–12.5 ka as the Laurentide Ice Sheet retreated during the Last Termination. The varved sediments of GLH have been well-studied, establishing a robust geomorphic and depositional history of the proglacial lacustrine environment.

Post-depositional, authigenic carbonate concretions are distributed heterogeneously through GLH sediments and reflect the oxidation and mobilization of buried organic carbon within the proglacial sedimentary environment. We report stable and radiogenic isotopic geochemistry of concretions for two sedimentary sections (deposited 16–14.8 ka) to reconstruct pathways of water and carbon cycling through the GLH sedimentary environment. Carbonate concretion δ¹³C values  indicate buried organic matter as the source of dissolved inorganic carbon in the sedimentary environment. Carbonate concretion δ¹⁸O compositions correspond to water compositions ranging from -6 to -11 ‰ (VSMOW, assuming 5 ºC water temperature), several per mille heavier heavier than estimated LIS meltwater compositions (17–20 ‰). These compositions indicate influence(s) from meteoritic water and/or lake water evaporation. Both interpretations support formation in littoral settings (supported by sample sites) or a post-GLH environment.

Preliminary U-series data for concretions from ~14 ka GLH sediments are consistent with sedimentary carbon oxidation and concretion growth in the Late Pleistocene-Early Holocene. Ratios of ²³⁴U/²³⁸U near secular equilibrium reflect either short-lived water-sediment interaction or chemical weathering of fine-grained glacial sediment. In either case, the near-unity activity ratios contradict supply of groundwater with extended subsurface residence.

Collectively, these data indicate oxidation of buried organic carbon in proglacial GLH sediments shortly after sediment deposition or after lowering of the local water table. We will present refined U-series geochronology as well as radiogenic Sr isotopes to more fully reconstruct pathways of oxidizing waters into and through the GLH sedimentary system.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-16616

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