Volcanic (lava tube) caves at Lava Beds National Monument (N. CA, USA) provide a valuable terrestrial analog for volcanic caves on Mars and the Moon. Terrestrial volcanic caves host a diverse microbial life, liquid water, and a variety of secondary mineral deposits (speleothems) with diverse morphologies and chemical compositions. Speleothems may preserve records of past and present microbial life and signatures of paleoenvironmental changes in terrestrial volcanic caves. Distinguishing between speleothems via chemical processes and microbially-mediated processes in terrestrial volcanic caves will provide valuable insights for future exploration of martian volcanic caves. To elucidate the formation of speleothems, we studied the chemical makeup (inorganic and organic) of cave waters in seven volcanic caves of variable ages, temperature, moisture content, light intensity, and frequency of human visitation. Cave water was characterized by stable isotopic composition (δ¹⁸O and δ²H), concentrations of major and trace elements, cations, anions, and characteristics of dissolved organic matter (DOM). A forward reaction model (PHREEQC) was used to test possible pathways for secondary mineral precipitation that formed these speleothems. The source of cave water was primarily regional meteoric precipitation that entered the caves through cave openings or through the cave overburden and fractured basalt walls as indicated by cave floor puddle water line δ²H = 8.32*δ¹⁸O + 9.55 parallel to the global meteoric water line (GMWL, δ²H = 8.3*δ¹⁸O + 10). A line formed by cave ceiling drip water δ²H = 3.39*δ¹⁸O – 44.77 intersecting the GMWL indicated that the water may be undergoing evaporation within the caves. Silicate weathering was found to be a primary process resulting in cave water enriched in Si (22 ± 7 mg/L), and contained trace levels of Al, Fe, Zn, Li, Sr, Cu, B, V, Ba, Cr and Mn. Geochemical calculations indicated that cave waters were undersaturated with respect to both amorphous silica (SiO_2am) and calcite (CaCO₃) which were the major components of speleothems observed within the caves. Results of a forward reaction model showed that evaporation of cave waters could lower the solubility of SiO_2am and CaCO₃ by increasing their saturation and ultimately precipitate these two secondary minerals forming the speleothems. The cave water DOM was characterized by high concentrations of dissolved organic carbon (DOC, 12 ± 8 mg/L) with a molar C/N ratio ranging from 2 to 22. The DOM was found to be aromatic (SUVA₂₅₄, 1.2–2.9 L/mg.m), terrestrially derived and humic-like (humification index, 7–26) and contained molecules of 100 Da and 5000 Da approximate molecular weight (AMU). Our results indicated that the terrestrially derived carbonaceous organic matter transported into the caves was not utilized for heterotrophic microbial metabolisms as DOC was accumulated over dissolved inorganic carbon (DIC). Both findings suggest that with minimal heterotrophy, chemo-litho-autotrophy may be important pathways that cycle the elements within these volcanic caves with low light conditions. Together, this study proposes a potential pathway of speleothem precipitation through the interaction of water, dissolved mineral constituents, and microbial life where dissolved ions are concentrated in cave drip water through cyclic condensation-vaporization processes. This work is part of a multi-disciplinary project Biologic Resource Analog in Low Light Environments (BRAILLE) funded by the NASA PSTAR Program (NNH16ZDA001N), which focuses on studying volcanic caves as terrestrial analogs for the Moon and Mars.

• Source of water in volcanic (lava tube) caves is regional meteoric precipitation.
• Water-rock interactions controlled cave water chemistry.
• Evaporation of cave water played an important role in precipitation SiO_2(am.).
• Cave waters were enriched in plant-derived organic matter.
• Enrichment of organic carbon in cave waters indicate net lack of heterotrophic metabolism.