Temporal Variation in the Stable Isotopic Composition of Water and Sediment in Seneca Lake, NY (USA): Implications for Paleoclimate Reconstructions
Wednesday, 17 December 2014
Paleoclimatic studies often use stratigraphic changes in the oxygen isotope composition (δ18O) of lacustrine carbonate to infer changes in water temperature and/or the δ18O of lake water. To better understand the environmental factors that influence the isotopic composition of lacustrine carbonates and the assumptions that need to be made to infer past changes in climate, we undertook a four-year study of the water chemistry and sediment trap material in Seneca Lake (NY, USA), a large, monomictic, glacial, hardwater lake. Sediment trap material collected weekly between May 2009 and 2013 together with isotopic monitoring of surface and bottom water allow for analysis of the controls on calcite precipitation in the lake. We show that calcite mainly accumulates in traps July through September when the lake water column is stratified and epilimnetic water temperature exceeds 20°C. Up to ~70% of the sediment is comprised of calcite during summer and only ~10% of the sediment is calcite during autumn, winter, and spring. The δ18O of Seneca Lake epilimnetic water varies by only ~0.6 ‰ throughout the year whereas the δ18O of bulk carbonate varies by as much as 2.4‰. As calcite precipitates in the eplimnion, the δ18O declines. Likely due to the large volume and residence time of water, the δ18O of Seneca Lake water appears to track changes in temperature. Our temperature reconstruction using the δ18O of calcite and epilimnetic lake water reveals that calcite was a reasonable proxy for lake surface temperature from July through September. During the remainder of the year, reconstructed temperatures exceed actual temperatures by as much as 18°C. Sediment resuspension (including calcite) during isothermal conditions may explain why calcite is a poor predictor of lake surface temperature in late autumn though spring. The δ18O of calcite in this lake records summer temperatures rather than year-round conditions.