New Mexico Geological Society Annual Spring Meeting — Abstracts

Lechuguilla and Spider caves provide the setting for a multidisciplinary study of geochemical and microbial interactions in an extreme environment (dark, nutrient-limited, carbon-poor). Corrosion residue (CR) is a multi-colored secondary mineral deposit associated with a diverse microbial community that includes iron and manganese oxidizers. CR occurs as coatings (up to 2 cm thick) overlying a layer (2 to approximately 10 cm) of altered carbonate host rock (termed 'punk rock' or PR) on walls of the caves. This study describes the geochemistry of CR, PR, and the underlying, unaltered host wall rock (WR). The host carbonate (permian Capitan reef complex; dominantly dolomite and calcite) contains up to 2.3% insoluble residue. The insoluble residue consists of quartz and illite. The PR is dominantly carbonate with virtually no insoluble residue. The chemical composition of CR is quite different. Carbonates and quartz are either absent or limited, and high concentrations of manganese and iron are present. There is a good correlation between color and chemical composition: colors grade from white through pink, red-brown, to black. Iron is associated with red, red-brown, brown and black residues (40, 50, 80, and 25 wt% oxide, respectively). Manganese is associated with the black residues (up to 20 wt% oxide). Utilizing manganese concentrations in host rock and corrosion residues, mass balance calculations indicate concentration factors of 50 times by weight; but the low density of CR translates to a thickness of 2.2 cm of host rock required for 1 cm of residue. The CR is too chemically distinct from the WR insoluble residue to have formed simply through dissolution of host carbonate during cave formation or later chemically-driven processes. We propose that the CR has built up over long time periods through microbially-assisted dissolution and leaching of underlying host carbonate, and microbial oxidation of reduced iron and manganese.