TRACKING THE CLIMATE SIGNAL FROM THE LAKE TO THE MUD: USING CARBONATE TO ASSESS THE FIDELITY OF THE SEDIMENT RECORD IN SENECA LAKE, NY

Previous study of piston cores collected from Seneca Lake, NY indicates that calcite comprises an important component of post-glacial rhythmites preserved in the deep lake basin. By combining concurrent meteorological and limnological monitoring with sediment-trap samples and freeze cores, we investigated how the climate signal is recorded by modern profundal sediment in the lake to validate the use of authigenic carbonate to reconstruct past climate and environmental conditions. At present, Seneca Lake is a borderline mesotrophic-oligotropic, warm monomictic lake that undergoes seasonal shifts driven by changes in incoming solar radiation. Loss-on-ignition and X-ray diffraction analysis of sediment-trap samples were used to identify the seasonal sequence of sedimentation. Two seasons are preserved that reflects the mixis conditions of the lake. When the lake is thermally stratified during summertime, most of the sediment that accumulates is comprised of calcite. During the mixing period, the sediment is comprised of organic-rich (~10-15%), terrigenous (quartz, mica, feldspar) material. Minor calcite also accumulated during winter when the lake was isothermal. The most sediment accumulates during late autumn and winter when lake bottom sediment is likely re-worked when the lake is isothermal. Freeze cores preserve evidence for this seasonal variation. Alternating layers of calcite-rich and organic-rich silty clay are preserved in detail. These cores demonstrate how mixing at the sediment-water interface due to storms and bioturbation, even at or below 60-m water depth, impacts how the climate signal is recorded by the sediment laminae.

Concurrent sediment-trap and water-column sampling allowed for analysis of the controls on calcite precipitation in the lake during summertime. Calcite begins to precipitate in the epilimnion when temperatures exceed 20°C, the pH is greater than 9, and chlorophyll-a concentrations are at least 3.1 mg/L. Calcite precipitates in isotopic equilibrium with the δ¹⁸O of lake surface waters. Our temperature reconstruction using the δ¹⁸O of calcite and epilimnetic lake water reveals that calcite is a reasonable proxy for temperature from July through September. Despite sediment reworking, calcite records and preserves only summertime surface water temperatures in Seneca Lake.