104-4 Stable Isotope Survey of the Lakes and Freshwater Carbonates of the Grand River, Michigan Watershed

Session: A Showcase of Undergraduate Research in Hydrogeology (Posters)

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Abstract:

Stable isotopes of water (δ2H and δ18O) are well-established tracers of environmental and climate signals, including moisture source, humidity, and temperature. These data in lakes are also influenced by the relative contributions of precipitation and groundwater, as well as evaporation. Global-scale water isotope systematics are relatively well understood, but lake isotopic variability across regional scales has not been widely studied, and never investigated in detail in Michigan. And there are very few correlative studies with freshwater mollusc carbonate δ13C and δ18O data across the same spatial region. Such data can help evaluate lake-specific differences in inflow and evaporation, and also establish a more authentic modern baseline for paleo-applications of mollusc isotopes.

To do this we sampled water and carbonates across 37 freshwater lakes within the Grand River watershed, as well as rivers (including tributaries), groundwater, and precipitation. The current data set largely spans Spring and Summer 2025. Lake water δ2H and δ18O vary considerably across the dataset (-57.7 to -26.2 ‰, and -8.5 to -2.0 ‰, respectively). Calculated d-excess values (often reflective of evaporation when negative) are below zero in 24 samples across 16 lakes. Fennessey, Bostwick, and Big Crooked Lakes show the most negative d-excess values (-6 to -11 ‰) and are likely the most evaporative. Carbonate δ18O data follows this water δ18O variability closely as expected, ranging from -9 to -2 ‰ VPDB). Though of course well correlated with measured water values, this large amount of carbonate isotopic variability is remarkable in a paleoclimate context. Calculated temperatures are normally distributed around 19°C, implying a summertime bias in mollusc shell growth. Mollusc genera do seem to have meaningful differences in their recorded δ13C and δ18O in some cases, implying real micro-environmental differences as well as possible vital effects.

Overall, these data reveal that surface water isotopic variability is underestimated in many cases, and will support ongoing efforts to evaluate overall watershed hydrology. They also can support future studies of climate and hydrological change and reveal the magnitude of smaller-scale spatial isotopic variability in a humid temperate watershed. 

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

doi: 10.1130/abs/2025AM-9294

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