MAGNETIC SUSCEPTIBILITY AS A BIOSIGNATURE: GREEN RIVER FORMATION (EOCENE) STROMATOLITES AS A TEST CASE
Given that silt or smaller sized magnetic grains are present in nearly all depositional environments 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 LaClede Bed of the Eocene Green River Formation contain petrographic 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 (Χ) as a biosignature. Stromatolite samples were cut, polished, and microdrilled along single laminae from 0 degrees to 90 degrees (and in some cases, overhanging beyond 90 degrees). The resulting powder was measured for Χ and normalized to the weight of the sample.
Results reveal that Χ is broadly consistent along lamina at all angle, even well beyond the angle of repose. Thus, the magnetic susceptibility corroborates the petrographic evidence of bigenicity in the LaClede Bed stromatolites, and shows promise as a means to evaluate biogenicity in other putative microbialite structures. The work presented here was conducted in association with the 2012 International Geobiology Summer Course.