Magnetic Susceptibility as a biosignature: Green River Formation (Eocene) Stromatolites as a test case

PETRYSHYN, VICTORIA A.; CORSETTI, FRANK A.; SHAPIRO, RUSSELL S.; MILLS, DANIEL B.; COX, CAITLIN; ACHBERGER, AMANDA; BOIDI, FLAVIA; DURAN MEJIA, VERONICA

Charlotte, North Carolina

Exposición; 2012 GSA Annual Meeting & Exposition; 2012

The Geological Society of America

Stromatolites are commonly considered evidence for ancient life. However, abiotic processes are known to create structures indistinguishable from microbial stromatolites at some scales. Micro-scopic investigation can in some cases reveal biogenicity, but many, if not most, putative micro-bialites in the rock record have experienced post-depositional alteration that obscures the original microfabric, rendering the biogenicity of most ancient stromatolites ultimately ambiguous. Given that silt or smaller sized magnetic grains are present in nearly all depositional envi-ronments in some fraction, we hypothesized that magnetic grains in an abiogenic structure should be swept off areas beyond the agle of repose and those adhered to biofilms (e.g., biogenic) will appear in positions inconsistent with simple physical sorting, such as along steeply dipping lamina. Experimental results from both biogenic and abiogenic systems are consistent with the hypothesis, but remain to be tested rigourously in natural samples. Stromatolites from the La Clede Beds of the Eocene Green River Formation contain pe-torgrphic evidence for biogenicity in the form of rare grains trapped at high angles, and draping lamina filling underlying topography and enveloping grains. Thus, they constitute an excellent natural test for magnetic susceptibility as a biosignature. Stromatolite samples were cut, polished, and microdrilled along single laminae from 0 degrees to 90 degrees (and in some cases, overhang-ing beyond 90 degrees). The resulting powder was measured for  and normalized to the weight of the sample. Results reveal that  is broadly consistent along lamina at all angle, even well beyond the angle of repose. Thus, the magnetic susceptibility corroborates the petrographic evidence of bi-genicity in the La Clede Bed stromatolites, and shows promise as a means to evaluate biogenicity in other putative microbialite structures.