T36 Oceans and Climates throughout Earth’s History: From Proxy Reconstructions to Model Assessments (Posters)

Sunday, 22 September 2019
9:00 AM - 5:30 PM
Hall AB, North Building

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

Paper number: T36-27
Program: Technical Sessions
Time: 9:00 AM – 5:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-339226

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