Carbonate AssociatedSulfate as a Proxy for Lake Volume Fluctuations, Walker Lake, Nevada
Closed-basin lakes can undergo significant volume changes in response to climate change, and thus constitute excellent systems to investigate past climate change. Various approaches are used to estimate paleo-lake level and volume (d18O, “shoreline” tufa dating, biotic proxies, etc.), and all carry certain caveats that limit their usefulness. Ultimately, the relationship between the chemistry of the lake, the volume of the lake, and the response of the proxy will determine how well a proxy serves a paleolimnologic purpose. Here, we propose the use of carbonate associated sulfate (CAS), the sulfate contained within the lattice of carbonate minerals that precipitate in lake water, as a proxy for lake water chemistry and by extension, lake volume.
Walker Lake, an alkaline closed-basin lake in western Nevada, has experienced a well-documented lake level decline since 1880 and provides a test case for CAS as a lake level proxy. Sulfate acts conservatively with changing lake volume. By extracting the CAS from sedimentary carbonate and tufas that have been age dated, we can relate CAS values to lake sulfate content based on historical or other proxy data. We confirm that CAS tracks lake sulfate. Our study of sedimentary carbonates demonstrates that CAS is a linear function of lake sulfate through a range of 10-25 mM, which corresponds to a change in lake level of 30 m.
As confirmation of the CAS technique, we analyzed a stromatolitic tufa dated via AMS 14C. The CAS trend in the stromatolite suggested that it grew during a lake level decline, a result consistent with other proxy data. In addition, we applied the CAS technique to stromatolites from the Green River Formation, Wyoming, a well-known Eocene lake system. If applicable, our results suggest the Green River stromatolites grew in lake water with ~1.5 mM sulfate, significantly lower than Walker Lake.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009