EXPERIMENTAL DETERMINATION OF THE BIOGENICITY OF MOONMILK, AND THE CHARACTERIZATION OF MOONMILK AND ITS DEPOSITIONAL ENVIRONMENT IN SPIDER CAVE, CARLSBAD CAVERNS NATIONAL PARK, NEW MEXICO

Microorganisms affect many of earth’s processes, including cave formation. Within caves, microbe/rock interactions are thought to play a role in the formation of speleothems, such as moonmilk (a two-phase system of calcite and water). Moonmilk is interpreted as forming by either primary deposition (subaqueous, subaerial or both) or secondary deposition (degradation product of cave popcorn). Moonmilk from Spider Cave of Carlsbad Caverns National Park, NM (known as “Crisco”) was analyzed to determine the geochemistry, fabric, depositional setting and extent of biogenicity. Crisco moonmilk is composed of filamentous, calcitic carbonate that is associated with microbes and has a greasy texture. Crisco is no longer growing and has a thick, detrital (?) silt surface coating, which renders the present day cave environment inadequate for determining the depositional environment of the moonmilk. Microbes derived from Crisco were cultured using four types of media to study their precipitates. Solid and liquid cultures were made with different nutrients and incubated for 2-7 months at 18ºC (ambient temperature of Spider Cave) and at 24ºC for faster growth. Moonmilk samples were analyzed using SEM and thin sections stained with Alizarin Red S. All media types show bacterial and fungal growth, but none have produced visible precipitates. SEM examination at low magnification shows a smooth, curd-like, biofilm-like texture and at high magnification shows both an organic filamentous fabric with calcite coatings and calcite rhombohedrons. Thin sections show two types of crystalline fabrics: 1) continuous, crenulated laminations that vary in thickness and degree of stain absorption; 2) irregular and mottled, with a much lighter but regular stain. Within the cave, moonmilk is associated with cave rafts (thin layers of carbonate that form at the air/water interface) and a silt coating. Silt coats the rafts and moonmilk and has been radiocarbon dated at ~ 4,000 ybp. The relative timing of moonmilk versus cave raft deposition is currently uncertain but indicates subaqueous deposition if concurrent. Preliminary results suggest there is a biotic component to moonmilk formation, and the deposition was accretionary and primary.