CARBONATE ASSOCIATED SULFATE: A NOVEL PALEOLIMNOLOGIC AND GEOBIOLOGICAL TOOL APPLIED TO WALKER LAKE, NEVADA

The ability to recognize past lake levels is fundamental to understand the evolution of a lake and how it responded to a changing environment/climate. Lake chemistry can be a diagnostic measure of lake level, especially for elements and compounds whose concentration is closely related to lake volume. A variety of lake level proxies have been applied in the past, with variable success. We propose a new lake level proxy based on Carbonate Associated Sulfate (CAS=sulfate incorporated into the carbonate lattice during mineral precipitation) that will allow us to investigate changing lake biogeochemistry. Gravity cores were collected from Walker Lake for CAS analysis and an age model was developed using 137Cs and 210Pb. CAS was measured down-core in order to compare its value to the known historical sulfate concentration in lake water (past 100 years). CAS abundance tracks lake water sulfate concentration excellently, becoming more abundant as the lake level fell. As a second test, we analyzed the 14C record and CAS content of a laminated tufa (a microdigitate stromatolite). Radiocarbon data indicate that this form ‘accreted' during lake level fall (as determined by others), so we would hypothesize that CAS should have increased as the stromatolite grew. Our measurements indicate that CAS did increase as the stromatolite grew, as predicted. Although our results appear promising, little is known about CAS incorporation, especially under alkaline conditions. Therefore, we have started to conduct laboratory experiments to investigate CAS systematics. Carbonate precipitated in lake water had similar values to natural Walker Lake samples, but they were much lower than CAS values from carbonates precipitated in artificial waters. The effect of different ionic strengths and temperatures was not nearly as great in Walker Lake water as with the artificial waters, suggesting an important avenue for investigation both for CAS and S isotopes.