A Geomicrobiological and Geochemical Approach to the Biogenicity of Moonmilk Formation: Thursday Morning Cave, Colorado; Spider Cave and Pahoehoe Cave, New Mexico

Megan Curry

New Mexico Institute of Mining & Technology,

Department of Earth & Environmental Sciences,

Socorro, New Mexico

[email protected]

Many secondarily formed deposits within caves are the result of primarily physiochemical processes. Moonmilk is a unique speleothem (i.e. secondary cave decoration) that does not appear to be explainable via same abiotic mechanisms as traditional speleothems (i.e. stalagmites, stalactites). Moonmilk is exceptional due to its biological content, highly variable mineralogy, and texture. Visible pits in bedrock suggest a microbial role, apparently caused by organism attachment, mineral encrustation around microbial filaments, and significant biofilm content. We hypothesize that 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. The purpose of this project investigates the formation of calcite moonmilk within three different megaporosity environments in order to help determine the relative importance of biotic from abiotic mechanisms. Our primary goal is to distinguish biotic from non-biological mechanisms of production of cave moonmilk, one of the numerous carbonate break-down products found in many limestone caves.

Microbiological techniques include scanning electron microscopy for evaluating physical biosignatures, microprobe coupled with energy dispersive spectroscopy for compositional chemical analyses, and attempts to culture live microorganisms found within moonmilk. Trace element analysis evaluates possible microbial disaggreagation of host rock and residual trace elements. Stable isotopes of the solid component analyze if biological processes are working over carbon deposited as calcite coatings on microbial filaments or contributing to other moonmilk minerals. Petrographic analysis investigates the morphological differences, and possible diagenetic alteration.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid