Although long thought to be largely the result of physicochemical processes, speleothems are now recognized as potentially including a microbial component. Cave pearls, a type of speleothem, are coated grains that form in caves and mines. Three diverse cave pearl sites in Carlsbad Cavern, New Mexico, U.S.A., provide insight into physicochemical and microbial controls on cave pearl formation and, by analogy, other speleothems. Pearl growth beneath active drips in the classic pearl nests of The Rookery was largely abiologic with autochthonous nuclei coated by laminae of columnar low-Mg calcite (LMC) alternating with laminae of LMC, high-Mg calcite (HMC), and aragonite. Changing mineralogy likely reflects a changing Mg/Ca ratio during evolution of the pool water during mineral precipitation, degassing, and evaporation. Pearls in Lower Cave, in contrast, grew on detrital nuclei in shallow pools from water that slowly evolved while flowing across a large, relatively flat cave floor. The pearl layers are variable amounts of LMC, authigenic clay mixed with biofilm (EPS), and aragonite with episodes of erosion and truncation. Here, abiologic precipitation dominated most precipitation but episodes of biofilm development aided precipitation of authigenic Mg clay, likely sepiolite. The third location, Secondary Stream Passage, had faster throughput of water, such that only LMC precipitated. Periodic influx of detrital Mg-Al clay, likely a trioctahedral smectite, supported a microbial community, including abundant reticulated microbial filaments.
Combining these results with previous work on cave pearls allows development of a unified model for cave-pearl formation. Required elements include 1) saturated water entering a cave (present in most caves); 2) an available detrital or autochthonous crystal to act as a nucleus (again, readily available); 3) the water must enter into a relatively flat floor in a cave or mine, either as drips from the ceiling or as water slowly flowing across the floor; 4) a shallow pool must form, generally < 3 cm but up to 6 cm deep to allow agitation of the pearls; 5) conditions in the pool must change so as to create layers; and 6) additional chemicals or particles may enter with the groundwater to feed a microbial community and/or add detrital or authigenic clays to the cave pearls. It appears that the most restrictive control is the slope of the cave floor: pearls do not occur without at least a small area of nearly horizontal floor. In addition, the documentation of microbial and authigenic clay laminae in cave pearls suggest any dark and/or organic-rich layer in a speleothem should be thoroughly evaluated for possible authigenic and/or microbial origins and not be assumed to represent detrital, washed-in material.