New Mexico Geological Society Annual Spring Meeting — Abstracts

Many secondarily formed deposits within caves (known as speleothems) are the result of primarily physiochemical processes. Moonmilk is a unique speleothem whose origin does not appear to be explainable via the more usual abiotic mechanisms employed to explain traditional speleothems (i.e. stalagmites, stalactites). Moonmilk is exceptional due to its high biomass and water content, highly variable mineralogy, and unusual texture. Moonmilk is currently loosely defined as a microcrystalline aggregate cave deposit composed of one of a variety of possible mineralogies and with a distinguishable texture that is soft, plastic, and pasty when wet, and crumbly and powdery when dry. Visible micropits in bedrock are often associated with microbial bodies, filaments, and hold-fasts. These suggest a microbial role in dissolution of parent material, apparently caused by organism attachment and associated carbonate dissolution via organic acids. Evidence of mineral precipitation by organisms can be seen in encrustation around microbial filaments, and significant overall biofilm content of the material. We hypothesize that such moonmilk is the product of a passive, microbially-mediated disaggregation of host rock and reprecipitation of carbonate from bedrock in a groundwater seepage-driven evaporative process.

We are investigating calcite and monohydrocalcite moonmilk within four different cave environments in order to help determine the relative importance of biotic versus abiotic mechanisms. Each cave system provides different environmental parameters,(e.g. temperature and lithology). We will discuss results to date including 1) organisms isolated from the moonmilk, 2) SEM images and EDS of organisms and their associated minerals 3) stable isotope analyses of S and C in organisms and resulting mineral precipitation, 4) electron microprobe elemental mapping, and 5) petrography.