STABLE ISOTOPIC EVIDENCE FOR MICROBIAL PRECIPITATION OF CALCITE IN CAVE POOL FINGERS

The caves of the Guadalupe Mountains of New Mexico, USA, contain numerous cave pools, mostly fossil, that were active during the wetter climate of the last glacial maximum. We have described over 250 pools from five different caves. Pools are relatively easy to identify by pool spar, isopachous calcite that coats pool sides and bottom and defines the former waterline like a bathtub ring. Approximately 60% of these pools also contain calcitic pool fingers, which are elongate, pendant speleothems that hang from submerged overhangs. Pool fingers are usually 1-2 cm in diameter and 5-20 cm long, but considerable variety has been observed. U-loops and webulite, draping calcite connecting adjacent fingers, are common. Since these features are all underwater, we hypothesize that the pendant nature of these features forms by following draping biofilm or microbial filaments.

In thin section, pool fingers range from clear calcite spar, often with fine filaments, to dense micrite to clotted micrite. The micritic fabrics suggest a microbial origin, which is supported by fossil microbes found by scanning electron microscope (SEM). However, the presence of microbes is not sufficient to show biogenicity of the calcite. To test for active participation, samples of pool fingers and pool spar (assumed abiologic) were micro-drilled and run for stable isotopes.

Oxygen stable isotopes of both pool spar and pool fingers reflect local meteoric waters (∂¹⁸O of -4‰ to -8‰) and are similar to reported values for other speleothems in the area. Pool spar has carbon isotopic values consistent with an origin from CO₂ degassing (∂¹³C = -1.9±1.7‰). The most recrystallized pool fingers have values overlapping pool spar (∂¹³C = -2.8±1.7‰), while the least recrystallized pool fingers are more depleted than pool spar (∂¹³C = -6.0±1.5‰). We interpret the 4‰ depletion as a microbial signal, suggesting the microbes are actively mediating the precipitation of the carbonate and not merely passive inhabitants of the pools. Thus, the isotopic results support the existing biosignature suite of field, petrographic, and SEM data and strongly support a biogenic origin for cave pool fingers.