GEOMICROBIOLOGY OF CAVE PEARLS: NEW EVIDENCE FROM CARLSBAD CAVERN

Cave pearls are coated grains best known for forming in shallow pearl nests where water drips onto nearly flat cave floors. Cave pearls can also form in rimstone dam pools, where several workers have proposed a significant role for microbes. We report on the petrography and SEM fabrics of three locations in Carlsbad Cavern, including the Rookery, a classic area of cave pearl nests, and two new localities where pearls occur in shallow pools, which are neither nests nor rimstone dams.

The Rookery pearls were broken out from cemented-in pearls next to recently active nests. These pearls are 1-2 cm spherical pearls, with continuous laminae and no detrital core. Individual laminae are mainly bladed to equant low-Mg calcite (LMC) to high-Mg calcite (HMC). In the center third, 5-20 micron equant microspar (LMC to HMC) laminae alternate with 1-5 micron laminae of aragonite needles, suggesting control by varying Ca:Mg ratio in pool waters. Evidence of microbes is very rare.

In Lower Cave (LC) and Secondary Stream Passage (SSP), pearls formed in shallow 1-3 cm deep pools in areas of nearly flat flowstone (now dry) without evidence of dripping water. Pearls are typically elongate (<1 to 2 cm), lumpy in appearance, and fit together like a jigsaw puzzle (indicating no rotation). LC pearls have cores of speleothem fragments mixed with clay, quartz silt and biofilm. SSP pearls have cores of porous micrite. LC pearls are thicker on top while SSP pearls are not. The cortex of both is mainly laminated bladed to equant microspar (LMC). LC pearls also include detrital layers (to 1 mm thick) of clay/silt with biofilm, often overlain by aragonite needle layers. SSP pearls have no aragonite, but include discontinuous patches of clay-biofilm with reticulated microbial filaments. The association of clays with biofilm and reticulated filaments suggests a burst of microbial growth following influx of clastic material. Microspar laminae rarely contain any evidence of microbes.

Investigations of cave pearls from Carlsbad Cavern therefore support a largely abiologic origin driven by changes in water chemistry with concentric layers forming without rotating. Influx of clays in flowstone areas produced short-term microbial growth that led to more asymmetric pearls. Later calcite layers follow this asymmetry, leading to lumpy irregular pearls.