BIOFILM AND MOONMILK IN COMPLEX CAVE PEARLS, ARTHUR SINGLETON CAVE, ROCKCASTLE COUNTY, KENTUCKY

Cave pearls are layered speleothems the form with a range of microbial input. The largest, and most biologic, cave pearls (>10 cm) form in rimstone-dam pools, generally without associated smaller pearls. In contrast, cave pearls in Arthur Singleton Cave, Rockcastle County, Kentucky, display a continuous range of pearl sizes from <3 mm to >20 cm in rimstone-dam pools and adjacent shallower pools and ephemeral streams. Deeper pools have scattered large (10-20 cm) pearls resting on detrital mud. The shallowest pools, in contrast, are covered in a thick layer of reverse graded pearls, ranging from 3-5 cm down to <3 mm.

All pearls, regardless of size, are cauliform and primarily composed of micritic textures. Small grains (<1 mm) are just micrite to clotted micrite. Small pearls (2-5 mm), and the nuclei of most larger pearls, are composite grains with smaller micritic grains fused by thick laminated to clotted micrite layers. Pearl layering is micrite/clotted alternating with microspar, with larger pearls having more microspar; the largest pearls add thick layers of radiating bladed spar. Thicker layers are continuous, but individual laminae are often not. In the SEM, micritic and mud-rich intervals contain abundant 1-3 µm-thick biofilm to clay/biofilm mats, rare reticulated filaments and collapsed cells. EDS shows biofilm is C-O, trace N; clay/biofilm is Si-O-Al-C, trace Mg. Porous areas in larger pearls contain abundant moonmilk (needle-fiber calcite) growing in biofilm.

During heavy rain, infiltrating water fills pools and brings in detrital clays and surface organic matter to feed the microbial ecosystem. The shallow pools rapidly drain (hours to days), but the deeper pools retain water that slowly evaporates over longer periods (months to years). Micritic layers form within biofilms as each pool evaporates to calcite saturation, and likely continues until each pool dries completely. Moonmilk apparently requires longer submersion, as it is restricted to larger pearls from deeper pools. Bladed spar only forms in deeper pools that remain saturated for longer periods between floods. The large (10-20 cm) pearls from rimstone-dam pools are part of a spectrum of pearls with greater size related to longer exposure to saturated waters. The large ratio of microbial fabrics is attributed to frequent input of nutrients during flood events.