A GEOMICROBIOLOGICAL AND GEOCHEMICAL ANALYSIS OF THE BIOGENICITY OF MOONMILK CAVE FORMATIONS: POTENTIAL AS BIOSIGNATURES

Moonmilk is a pasty carbonate deposit that occurs within various subterranean systems. Many secondarily formed deposits within caves (speleothems) are thought to be the result of primarily physiochemical processes. However, moonmilk does not appear to be explainable via the same abiotic mechanisms, nor have the same morphologies and textures as traditional speleothems such as stalagmites and stalactites. Moonmilk is currently loosely defined as a microcrystalline aggregate cave deposit composed of various mineralogies with a distinguishable texture. Moonmilk differs from other speleothems due to its biological content, highly variable mineralogy, and texture. A microbial role is suggested by the presence of visible pits in bedrock apparently caused by organism attachment, mineral encrustation around microbial filaments, and significant biofilm content.

To explain the observed physical, chemical, and biological properties of moonmilk, we hypothesize that it is the product of microbially mediated disaggregation of host rock and reprecipitation of carbonate from bedrock in a groundwater seepage-driven evaporative process. In order to test the degree to which moonmilk is the product of biological activity, we have employed a combination of molecular phylogenetic methods, culturing and isolation, and geochemical and stable isotopic analyses. It is clear that moonmilk can be produced at both near freezing and warm tropical cave temperatures. Not all moonmilk is biologically active when collected. In situ diagenesis may be occurring. There is overlap in the suite of microbial strains present but also numerous differences.

This research can aid the understanding of microbial roles in biomineralization, particularly of carbonates. Microbial carbonate precipitation plays a significant role in many processes, including microbial formation of kidney stones and dental plaque; corrosion of buildings and pipes; corrosion of statues and cave paintings; development of banded iron formations, stromatolites, and concretions. Further, such material can serve as a biosignature representative of a significant subsurface set of processes here on Earth of potential application to future life detection missions in extraterrestrial subsurface sites.