THE POSSIBLE FORMATION OF MAGNETIC ANOMALIES VIA METHANE-FUELED SULFATE REDUCTION AT A THERMOGENIC METHANE SEEP

Acid volatile sulfide (AVS) and sulfate measurements in soils overlying a thermogenic coal-bed methane seep near Durango, Colorado indicate the occurrence of sulfate reduction.  Both AVS and sulfate concentrations in the soils (0.2 mmoles /g dry soil and ~3 mmoles / g dry soil, respectively) near the water table (~1.5 mbsf) are elevated compared to the shallower, overlying soils.  Increased AVS (~10x) was measured after anaerobic incubation of these soils with added sulfate, indicating that sulfate-reducing bacteria (SRB) are actively producing sulfide near the water table.  Magnetic anomalies that correlate with high soil gas methane measurements (up to 90% CH₄ at 1 mbsf) have been identified along a transect of the methane seep.  We hypothesize that these anomalies arise from the reduction of iron oxide minerals by microbially produced sulfide.  The formation of iron sulfide minerals may result in changes in magnetic properties and thus create the observed magnetic anomalies.

We further hypothesize that sulfate reduction at this site is driven by microbially mediated anaerobic methane oxidation (AMO), whereby CH₄ serves as the electron source and sulfate as the ultimate electron acceptor.  Several studies in marine settings have identified a bacterial consortium of SRB and methanogen-like archaebacteria believed responsible for AMO.  Previous evidence for this consortium in marine settings includes the detection of archaebacteria biomarker lipids with d¹³C values between -100 and -130‰ and iso- and anteiso-C15:0 phospholipid-derived fatty acids (PLFAs) indicative of SRB with d¹³C values between -63 and -75‰.  Iso- and anteiso-C15:0 PLFAs have been detected in the soils overlying the methane seep with d¹³C values depleted by as much as -60‰.  These results are consistent with SRB utilizing the thermogenic methane (-47‰) as a carbon and energy source.  We are currently investigating whether a consortium similar to that found in marine settings is responsible for the sulfate reduction observed above the thermogenic methane seep by analyzing soils for ¹³C-depleted archaebacteria lipids.