Geochemical interactions among water, minerals, microbes, and organic matter in formation of speleothems in volcanic (lava tube) caves

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 (SiO2am) 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 SiO2am 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