IN THE LAND OF BLACK AND WHITE, MICROBIAL DEPOSITION OF FERROMANGANESE ON THE WALLS OF SNOWY RIVER, FORT STANTON CAVE, NEW MEXICO

The Snowy River section of Fort Stanton Cave, NM contains an unusual passage with walls and ceilings coated with a layer of jet black manganese oxide and a white calcite "river" on the floor of the borehole-like passageway. This single passage has currently been surveyed to over 18 km in length, with no end in sight. Black Mn-oxides on the walls of the passage coat thin clay and silt layers that were deposited on the limestone bedrock prior to the deposition of the white Snowy River calcite. The Mn layer is in places separated by thin clay partings, and a thin Mn layer is often observed deeper in the substrate; the layers on the wall and in floor sediments suggest episodic deposition during an earlier era when the passage was apparently open to the surface. In cross-section, the Mn-oxide coating consists of discontinuous dendritic clusters in a layer 0.2-0.5 mm thick. The surface of the coating, imaged by scanning electron microscopy, exhibits grape-like clusters of Mn-oxide nodules or spherules a few micrometers in diameter. Carbon-free polished sections were prepared for EPMA analysis by embedding the coated clay substrate with sodium silicate instead of epoxy and then evaporated with copper to avoid adding carbon. Manganese oxide makes up the bulk of the nodules at around 48 wt%, and SiO₂, Al₂O₃ and Fe₂O₃ comprise the remaining major components. Carbon was measured up to 1.5 wt% in the nodules but less than 0.5 wt% in the clay substrate. Culturing experiments produced rapid growth on reduced-Mn culture media and deposition of dark Mn-oxides in the media. 16S rRNA analysis reveals a microbiome dominated by bacteria (over 80%), of which Proteobacteria are most common, and archaea (mainly Euryarchaeota) compress less than 20%. The data provide evidence that this unusual Mn-oxide deposit is a product of a chemolithotrophic microbial community surviving in the subsurface.