A BIOMARKER SUITE FROM CAVE POOL PRECIPITATES, COTTONWOOD CAVE, NEW MEXICO

Calcite cave pool precipitates often display a variety of potential biomarkers from the macroscopic to the submicroscopic. Although individually suspect, a suite of biomarkers provides strong evidence that these speleothems are biogenic.

A particularly interesting fossil pool in Cottonwood Cave, New Mexico is several hundred feet below the surface and well into the dark zone. Older stalactites and stalagmites are completely coated in brown, laminated calcitic crust that defines a horizontal waterline for a pool that reached about 4 m in depth. For 5 cm below the water line, the micritic crust is smooth; from 5-15 cm small knobs protrude; below that, masses of thin pool fingers (pendant, finger-like speleothems), 2-4 mm in diameter, extend as much as 50 cm from all overhangs (i.e. cave walls, stalactites) and most are connected by u-loops, draping filamentous calcite long suspected of being fossil slime. On the floor beneath the pool finger masses, delicate irregular spires are composed of the same brown calcite. Since pool fingers hang down but form underwater, we hypothesize they are biogenic with hanging microbial filaments or biofilm acting as nuclei for calcite precipitation.

In thin section, the uppermost crust is composed of laminated micrite and spar. Moving down, the small knobs are cored by clotted micrite and then also coated in micrite/spar laminations. The pool fingers, u-loops, and floor spires are mainly micrite to clotted micrite with minimal spar layers. Micrite, particularly clotted micrite is usually interpreted by carbonate workers as microbial.

SEM examination of etched pool fingers, u-loops and the upper crust reveals abundant calcified microbial filaments and biofilm. EDX analysis shows these features have excess carbon, above that found in pure calcite. These same samples contain up to 2% organic carbon (determined using a Carlo Erbo elemental analyzer after dissolving the carbonate).

Although each of these lines of evidence could be interpreted in other ways, their combined weight strongly suggests the cave pool precipitates in Cottonwood Cave are biogenic. Because of the abundance of micrite and fossil filaments, we hypothesize that these pendant features formed during a period of plentiful nutrients and active hydrological activity when the pool was literally dripping with microbial slime.