DEVELOPING A PROCESS-BASED UNDERSTANDING OF LACUSTRINE CARBONATE CLUMPED ISOTOPE THERMOMETRY IN FAYETTEVILLE GREEN LAKES, NY, USA

Clumped isotope temperatures (TΔ₄₇) of modern and ancient lacustrine carbonates have been interpreted to reflect equilibrium carbonate precipitation in near-surface waters during the warm season, but uncertainties remain regarding the processes and seasonal biases of lacustrine carbonate formation. We address this issue using modern to preindustrial carbonate from Fayetteville Green Lakes (FGL), a well studied lake in NY, USA, that experiences seasonal surface water temperature swings (0-25°C) and abundant carbonate deposition. Carbonate was filtered from the water column (0-15 m depth, June-September 2019) and sampled from the top 12 cm of a sediment core (~94-445 years old) for ẟ¹³C, ẟ¹⁸O, Δ₄₇, Δ₄₈, and XRD analysis to test the hypothesis that calcite crystals precipitate 1) in chemical and isotopic equilibrium, 2) primarily during spring “whiting” events and throughout the warm season, and 3) within the upper 8 m of the water column. Preliminary XRD analyses indicating that the core-top sediments are >90% calcite suggest that lake-precipitated calcite is most likely preserved in lake sediments. Core-top TΔ₄₇ values average to 33±4°C (2σ), which exceeds the highest observed water temperatures of 25°C. Cooler pre-industrial climate cannot explain this offset. Instead, the offset suggests disequilibrium, possibly due to microbially-mediated calcite precipitation, which has previously been proposed in FGL. To test the role of microbial activity on disequilibrium, core-top carbonate Δ₄₈, ẟ¹⁸O, and ẟ¹³C compositions will be evaluated next. For example, if ẟ18O is higher relative to expected, this would support kinetic fractionation associated with CO2 degassing, whereas if ẟ18O is lower relative to expected, this would support pH-driven effects. Filtered carbonate Δ₄₇, Δ_48, ẟ¹⁸O, and ẟ¹³C can also test this hypothesis and refine predicted core-top values based on the volumetric contributions from water column carbonates. Our preliminary results suggest disequilibrium processes may alter lake carbonate Δ₄₇ independent of temperature alone. Fortunately, additional isotopic tools may help identify and correct for these disequilibrium effects to accurately reconstruct temperature.