Geochemistry of Mineral-Water-Microbe Interactions in Lava Caves: Exploring Terrestrial Correlations to inform Future Planetary Life Detection Missions

Abstract:

Volcanic caves at Lava Beds National Monument (N. CA, USA) provide a valuable terrestrial analog for lava tubes on Mars and the Moon. The caves house secondary mineral deposits (speleothems), formed in the presence of liquid water, of diverse morphology and chemistry and preserve a record of microbial life. In order to elucidate the processes leading to their formation, we studied cave water and speleothem biogeochemistry in seven lava caves of variable age, temperature, moisture content, light intensity, and frequency of human visitation. Cave water was characterized by major and trace elements, cations, anions, dissolved organic matter, and stable isotope (O, H) composition. Mineralogy and chemical compositions of speleothem samples were determined using X-ray diffraction, fluorescence spectroscopy, and electron microprobe analysis. Geochemical modeling (PHREEQC) was used to test possible pathways of speleothem precipitation. Results support our interpretation that moderately evaporated meteoric water entering the caves becomes primarily Na-K-Mg-HCO₃-flavored and enriched with Si (~20 mg/L) as a result of silicate weathering. High dissolved organic carbon (15 mg/L) in the waters suggests a predominance of autotrophy. The speleothem chemistry was dominated by opaline silica (47–96 wt% SiO₂), calcite (1–25 wt% CaO), and high Mg-calcite (0.25–18 wt% MgO), along with clay minerals and biomarker elements (Fe, Mn, Cu, S and V). Microprobe analyses and transect imaging of speleothems revealed microstromatolite-like structures with alternating bands of SiO₂ and CaO. Geochemical modeling supports our hypothesis that evaporation within the caves plays a critical role in speleothem formation. This work is part of a multi-disciplinary project (NASA BRAILLE) funded by the PSTAR Program (NNH16ZDA001N), one aspect of which is to characterize biomarkers preserved in lava caves to inform future life detection missions to planetary caves.