A NEW UNIFIED MODEL FOR CAVE PEARLS: INSIGHTS FROM CAVE PEARLS IN CARLSBAD CAVERN, NEW MEXICO, USA

摘要

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.