GEOCHEMISTRY AND PALEOHYDROLOGY OF SEARLES LAKE (CALIFORNIA) TUFA

Pleistocene Searles Lake was alkaline such that fresh, calcium-rich inflow rapidly precipitated CaCO₃ in the form of shoreline tufa found along the western margin of the basin and from the high-stand elevation of a 200 m-deep lake down to ~40 m above the modern salt pan. Closed-basin lake levels are sensitive to changes in precipitation/evaporation, and oxygen isotopes from carbonate tufa are important tools for reconstructing lake level and hydrologic variability. However, an important prerequisite for reconstructing lake level variability from isotopic records of tufa is establishing that the tufa formed directly from lake water. Here, carbonate mineralogy, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr were obtained from shoreline tufa (n = 110) and deep lacustrine sediments (n = 20) of Searles Lake. Searles Lake tufa occur as both calcite and aragonite. Calcite tufa is less enriched in ¹⁸O (δ¹⁸O = -12.0 – 0.3 ‰) compared to aragonite tufa (δ¹⁸O = -0.1 – 3.5 ‰). Aragonite tufa ⁸⁷Sr/⁸⁶Sr (0.70857 – 0.70906) agree with lacustrine values (0.70851 – 0.70921) and indicate that aragonite tufa precipitated directly from lake waters. In contrast, calcite tufa ⁸⁷Sr/⁸⁶Sr (0.70758 – 0.70980) extend beyond the lower and upper values measured for lacustrine sediments and indicate additional source waters (likely springs) influenced tufa growth. Calcite tufa display diverse depositional textures including subaerial precipitation (pendant cements and straws), lacustrine precipitation (coated algal filaments, cemented gas bubbles), and subsurface precipitation (cemented sands/gravel and bedrock fracture fill). ⁸⁷Sr/⁸⁶Sr values for lacustrine tufa mostly fall below the Searles Lake values, indicating that calcite tufa, which precipitated within the water column, were strongly influenced by non-lacustrine water sources. Various types of tufa occupy the same range of elevations, and yet exhibit different δ¹⁸O values that reflect different mixing ratios of spring and lake water. Thus, without knowing the δ¹⁸O of the ancient spring water and the relative mixtures of spring to lake water, isotopic records of tufa are not representative of lake level. This study demonstrates that multiple geochemical proxies can be used to clarify the paleohydrology of tufa formation and determine the reliability of tufa as records of lake water δ¹⁸O variability.