New Mexico Geological Society Annual Spring Meeting — Abstracts

Accumulations of low density, soil-like material line the walls, floors and ceilings of some caves. In Lechuguilla and Spider Caves in Carlsbad Caverns National Park, NM, this material is particularly well developed, but similar material has recently been found in Jewel Cave, SD. The deposits are diverse in composition with variable amounts of clay and AI-oxide minerals and all are rich in Mn- and/or Fe-oxides. The mineralogy and microbiology of these deposits bear striking resemblance to terrestrial soils in which iron, aluminum and manganese oxides have accumulated.

Lithiophorite [(AI,Li)Mn⁴⁺O(OH)₂], nordstrandite and gibbsite [AI(OH)₃], goethite, kaolinite, and illite have been identified by XRD, and abundant nanocrystalline Fe-oxides were found by TEM examination, similar to minerals found in laterite soils. Likewise, todorokite [(Mn²⁺,Ca, Na, Mg,K)Mn₃⁴⁺O₇•H₂O] and birnessite [(Ca,Na)_0.5(Mn⁴⁺,Mn³⁺)₂O₄•1.5H₂O], often observed in soil nodules, are also present in these cave deposits. A diverse microbial community has been identified in these ferromanganese deposits by 16S rDNA sequence analysis, and includes microorganisms whose closest relatives are manganese-and iron-oxidizing bacteria and nitrogen-fixing bacteria.

The similarities in mineral and microbial species composition to that in soils suggests that these cave deposits undergo a process similar in some ways to terrestrial soil development. Both chemical and microbial processes may influence the formation of these cave deposits. Chemical weathering from the condensation of weak carbonic acid in the cave atmosphere may contribute to the dissolution of the bedrock carbonate, but microbial breakdown of bedrock probably plays a more significant role. The oxidation of reduced iron and manganese releases H⁺ ions, and the microorganisms themselves may release organic acids, both of which contribute to dissolution of carbonate. The residual weathered products, rich in secondary minerals and organic matter, are essentially subterranean soils.