SEARLES LAKE EVAPORITE FACIES: A NEW PROXY FOR PALEO-PCO2

Searles Lake, California, was a Pliocene-Pleistocene saline-alkaline lake which deposited a suite of >25 different evaporite mineral phases under a range of climate-related conditions, such as temperature and pCO₂. The behavior of alkaline lakes with respect to atmospheric CO₂ (i.e., equilibrium vs. sinks vs. emitters) is debated but some suggest that alkaline lakes may have pCO₂ concentrations >6X atmospheric. The sensitivity of certain evaporite minerals to pCO2 makes the Searles Lake deposit an ideal candidate for discovering new mineral proxies for pCO2 conditions of ancient alkaline lakes. The computer program EQL/EVP was used to identify pCO₂-dependent mineral facies by modeling the evaporation of Searles Lake source water with various concentrations of dissolved CO₂. High-CO₂ facies (≥1,000 ppm pCO₂) is defined by the minerals Nahcolite [NaHCO₃] and Thenardite [Na₂SO₄]. The elevated-CO₂ facies (400-1,000 ppm pCO₂) is defined by Hanksite [Na₂₂K(SO₄)₉(CO₃)₂Cl]. The low-CO₂ facies (200-400 ppm pCO₂) is not defined by specific minerals, but model runs for this pCO₂ range account for all other minerals in the Searles Lake deposit.

This new pCO₂ proxy was applied to drill core SLAPP-SRLS17, collected from Searles Lake in January 2017, which extends to 76 mbs and records continuous lake deposition through the past ~150 kyr. The Bottom Mud (~37-150 kyr) is largely comprised of the pCO₂-independent mineral Gaylussite [Na₂Ca(CO₃)₂ · 5H₂O]. However, sparse thin-bedded Nahcolite within the Bottom Mud indicate intervals of high (>1000 ppm) pCO₂. The Lower Salt (~23-37 kyr) is dominated by the low-CO₂ facies and represents a time when the lake was likely in equilibrium with atmospheric CO₂. Hanksite occurs near the top of the Lower Salt, indicative of an increase in CO₂ concentration to at least 400 ppm at ~23 kyr. The Upper Salt (~6-13 kyr) contains Hanksite throughout, as well as ~5m of Nahcolite and Thenardite, indicating an extended period of high-CO₂ lake conditions during the Holocene. Elevated pCO₂ conditions in Searles Lake, in disequilibrium with atmospheric concentrations, is likely due to permanent stratification of Searles Lake (meromixis) which allowed dissolved CO₂ to accumulate in the hypolimion, either from biological processes or from volcanic CO₂ outgassed along faults into the lake bottom-waters.