GEOCHEMICAL VARIATIONS IN CAVE WATERS AND SECONDARY MINERAL DEPOSITS AT LAVA BEDS NATIONAL MONUMENT, CALIFORNIA

We explored basaltic cave environments to determine the nature of interactions between subsurface water, rock, and microbes at Lava Beds National Monument (N. CA, USA). Diverse secondary mineral deposits (speleothems), containing varying amounts of microbial DNA, were collected from these caves and analyzed for their mineralogical and elemental compositions. The cave water from the puddles on the cave floor and drips from the cave walls and roof were also analyzed for inorganic and organic contents. The speleothems can be categorized as native basalt (plagioclase, clinopyroxene, olivine), opaline (cryptocrystalline silica), and carbonate dominated by calcite and/or aragonite. Opaline and carbonate speleothems contained >70 wt% SiO₂ and >14 wt% CaO respectively. Typically, opaline and carbonate speleothems exhibited polyp morphologies and microbialite-like textures in their interiors, indicative of a possible biogenic origin.

The cave waters were dominated by Si (21 ± 4 mg/L) followed by Na (8 ± 3 mg/L) and Ca (4 ± 2 mg/L); concentrations varied within and among the caves. Abundance of these elements is consistent with the dissolution of basaltic minerals such as plagioclase. Elevated levels of NO₃^- (9 ± 8 mg/L) in cave waters were detected which could reflect agriculturally influenced recharge (from lands adjacent to the caves) or in-situ microbial NH₄⁺ oxidation within caves. The cave waters also contained high concentrations of DOC (13 ± 6 mg/L), which could support microbial growth. Compositions of co-located speleothems and cave water correlated well: for example, higher Ca²⁺ (7 mg/L) was found in cave waters in the proximity where carbonate polyp speleothems containing (~20 wt% CaO) were found. With speleothem mineralogy and elemental composition, cave water chemistry, and metagenomic data, we expect to investigate the biological pathways involved in the formation of potentially biogenic speleothems. Geochemist’s Workbench is being utilized to calculate the dissolved precipitant constituent and speciations. The outcome of this work will contribute to the overarching goal of the NASA PSTAR BRAILLE (Biologic and Resource Analog Investigation in Low Light Environments) project (NNH16ZDA001N) to characterize potential mineral biomarkers capable of persisting over geologic time.