UNIQUE EVAPORITES IN CAVES: COMPARISON TO SURFACE-DERIVED DEPOSITS

Evaporite deposits are usually associated with precipitation of minerals in surface water bodies. In caves we also find numerous examples of apparent evaporite minerals deposited in water. Additionally, we find deposits formed subaerially at saturated relative humidity. The latter formation mechanism can produce evaporite minerals that have no counterparts in surface systems. Caves also protect these materials. For example, delicate black manganese "fluff" forms on the surface of calcite speleothems, lace-like gypsum crusts form on microbial mats, and stringy Mn/Fe minerals dangle from passage ceilings. Microbial influences contribute significantly to many examples we have studied. However, other apparently similar deposits and speleothems bear no evidence of biological influence. For example, "Crisco" moonmilk seems to form around microbial filaments, but similar moonmilk in alpine caves appears to be abiogenic.

How similar are cave and surface evaporites and their formation mechanisms? Microorganisms, microbial mats, and biofilms are known to play important roles in modern surface evaporite-depositing systems. These often occur in high irradiance environments where photosynthetically-based trophic webs provide plenty of biological energy to contribute to mineralization processes. In typically oligotrophic cave systems, the geochemistry and geomicrobiology are considerably different, however we still find non-photosynthetic microbial mats and biofilms and associated mineral precipitation. In a few unusual caves, influx of reduced materials augments the local food chain with chemoautotrophically-derived energy and dominates the geochemistry and geomicrobiology.

We will detail the major microbial interactions with carbonate, manganese, iron, and sulfur cave evaporites that we have catalogued to date. Our attempts to distinguish between active microbial participation, passive microbial participation, and microbial irrelevancy to different mineral suites and specific speleothems will be discussed. Discrimination between biogenic and abiogenic evaporite processes can add to our knowledge of biosignatures; a subject of vital interest to the study of environments on Earth, Mars, and other planetary bodies.