New Mexico Geological Society Annual Spring Meeting — Abstracts

Microbe/rock interactions within caves may 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 or subaerial) or secondary deposition (degradation product). Moonmilk from Spider Cave of Carlsbad Caverns National Park, NM (known as “Crisco”) was analyzed to determine the fabric, depositional setting and extent of biogenicity. Crisco moonmilk is composed of filamentous, calcite CaCO₃ associated microbes. Crisco is no longer growing and has a thick, detrital silt surface coating, rendering the present day cave environment inadequate for determining the depositional environment. Microbes derived from Crisco were cultured using four types of media to study their precipitates. 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 organic filamentous fabric with calcite coatings and calcite rhombohedrons. Thin sections show three types of crystalline fabrics: 1) laminations varying in thickness and degree of stain absorption; 2) irregular and mottled laminations 3) recrystallized fabric. Within the cave, moonmilk is associated with cave rafts (thin layers of CaCO₃ that form at the air/water interface). 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 deposition was accretionary and primary.