BASALTIC CAVES AND POTENTIAL FOR BIOSIGNATURE DEVELOPMENT AND PRESERVATION

Putative lava tube sky lights and cave-like features have been identified on Mars based on orbital imagery. On Earth, lava tube caves are ubiquitous in terrestrial basaltic flows, offer constant, temperate environments, and provide sources of shelter and water. In caves, both secondary mineral precipitation and microbial growth are protected by stable physico-chemical conditions, and traces of microbial life (biosignatures) may be better preserved in minerals. We are beginning a study of the microbial diversity and its relation to secondary mineralogy at the Lava Beds National Monument (LABE) in N California, situated on the NE flank of the Medicine Lake Volcano. Notable features observed in this study include (1) a gold-colored, hydrophobic biofilm that exhibits a metallic-like appearance produced by countless water droplets beaded upon its surface, and (2) abundant fractures in the cave walls outlined by secondary mineral phases and/or microbialites. LABE caves typically show features of undersaturated and saturated areas, reflecting changes in depth beneath the vadose zone; we are using stable major and trace element data and O and H isotopes to constrain cave hydrology. Preliminary water signatures indicate an evolved rock-water interaction, and cation values have a pronounced basaltic fingerprint. Surface waters have substantially lower total dissolved solids (TDS) than ground (well) waters and water sampled inside caves, consistent with the hypothesis that meteoric water reacts with the surrounding mineral phases as it passes through the vadose zone. Preliminary water chemistry analyses from two caves, sampled during an initial visit, record variations that may mirror different water sources, dilution, flow path length, and temperature variations. The various secondary minerals postulated to precipitate are sulfates and carbonates such as hydromagnesite and epsomite. Nitrate concentrations were high in some of the waters. Different major and trace element concentrations in the waters sampled are consistent with the variations in path length from the surface to the cave interiors as well as the different ages of the host lava flows. Older flows would have thicker, more mature soil horizons, resulting in water with higher concentrations of dissolved solids.