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