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.