Quantification of Fossil Microbe Distribution In Pool Fingers from Cottonwood Cave, NM, USA

The objective of this study was to determine the distribution of fossil microbes in pool fingers from Cottonwood Cave, NM. These pool fingers are composed of three layers: laminated micrite, spar, and a dense, clotted micrite and developed on the submerged ends of previously formed stalactites. In the upper to middle portions, the crust is smooth to knobby and consists of a mixture of alternating layers of micrite and spar, accompanied by a dense, clotted micrite that fills the bulbous knobs. At the ends of the stalactites, where the pool fingers are located, the dark, dense clotted micrite is the dominant petrographic fabric.

Thin sections of each fabric were scanned for fossil microbes in the Scanning Electron Microscope (SEM) at the UNM, at a uniform magnification of 4000X to allow quantification of microbes per square micron for specific petrographic fabrics.

In smooth crust, the micrite layers have 0.5 x 10-4 filaments/micron2, the spar layers have 2.0 x10-4 filaments/micron2, and the outer-most micrite layers have 4.0 x 10-4 filaments/micron2. In knobby crust, the spar layer has 3.8 x 10-4 filaments/micron2, the laminated micrite area has 3.3 x 10-4 filaments/micron2, and the clotted micrite has 6.0 x 10-4 filaments/micron2. In the pool finger itself, a clotted micrite area contains 19.4 x 10-4 filaments/micron2, while an outer-most micrite area has 4.0 x 10-4 filaments/micron2, and a second dense clotted micrite area has 5.3 x 10-4 filaments/micron2.

These data indicate that the dense, clotted micrite is more microbial than the layered micrite and/or layered spar. In particular, the ends of pool fingers, which are commonly connected by U-loops, contain the most fossil evidence. This is significant, because it confirms that the combination of clotted micritic fabric with u-loops is a biosignature.