Centennial Honors College Western Illinois University Undergraduate Research Day 2014 Poster Presentation Stable Isotope Analysis of Pool Finger Fabrics from Cottonwood Cave and Hidden Cave, Lincoln National Forest, New Mexico Brian Knecht Faculty Mentor: Leslie Melim Geology Pool fingers are interesting hanging encrustations that form underwater within cave pools. Pool fingers are interpreted to be the result of microbial activity in cave pools because of their pendant shape and the presence of fossil filaments and biofilms. However, conclusively attributing a biologic origin for these rocks is difficult because the pools have been dry for a long time. Internally pool fingers have distinct alternations between a brown crust composed of dense micritic calcite (considered microbial) and a light crust composed of dogtooth spar (abiologic). If microbes are playing a part in the deposition of pool fingers they would be expected to alter the ratio between the stable isotopes of carbon, the 13C/12C value, which should then be preserved in the rock. A relatively small negative shift in the micritic fabrics of fingers when compared to spar layers can be interpreted as evidence of fractionation driven by microbial activity. Thirty-six samples from six different pool fingers were collected from micrite and spar layers using a micro-drill. The powdered samples were analyzed by the stable isotope lab at Southern Methodist University in Texas. The resulting data was then plotted on high resolution images of the pool fingers for analysis. Carbon isotope values were compared to values obtained not only from adjacent layers within a single sample but reveal large changes between contrasting fabrics supporting microbial involvement, while the values from the Cottonwood Cave samples are less conclusive.