Novel Applications for Biogeophysics:
Prospects for Detecting Key Subseafloor Geomicrobiological Processes or Habitats

Authors: Rick Colwell, Oregon State University, and Dimitris Ntarlagiannis, Rutgers University.

Venue: American Geophysical Union’s 2007 Joint Assembly, Acapulco Mexico, May 21-25, 2007 (http://www.agu.org/ [external site]).

Abstract: The new subdiscipline of biogeophysics has focused mostly on the geophysical signatures of microbial processes in contaminated subsurface environments usually undergoing remediation. However, the use of biogeophysics to examine the biogeochemistry of marine sediments has not yet been well integrated into conceptual models that describe subseafloor processes. Current examples of geophysical measurements that have been used to detect geomicrobiological processes or infer their location in the seafloor include sound surveillance system (SOSUS)-derived data that detect seafloor eruptive events, deep and shallow cross-sectional seismic surveys that determine the presence of hydraulically conductive zones or gas-bearing sediments (e.g., bottom-simulating reflectors or bubble-rich strata), and thermal profiles. One possible area for innovative biogeophysical characterization of the seafloor involves determining the depth of the sulfate-methane interface (SMI) in locations where sulfate diffuses from the seawater and methane emanates from subsurface strata. The SMI demarcates a stratum where microbially driven anaerobic methane oxidation (AMO) is dependent upon methane as an electron donor and sulfate as an electron acceptor. AMO is carried out by a recently defined, unique consortium of microbes that metabolically temper the flux of methane into the overlying seawater. The depth of the SMI is, respectively, shallow or deep according to whether a high or low rate of methane flux occurs from the deep sediments. Presently, the SMI can only be determined by direct measurements of methane and sulfate concentrations in the interstitial waters or by molecular biological techniques that target the microbes responsible for creating the SMI. Both methods require collection and considerable analysis of sediment samples. Therefore, detection of the SMI by non-destructive methods would be advantageous. As a key biogeochemical threshold in marine sediments, the depth of the SMI defines methane charge in marine sediments, whether it is from dissolved methane or from methane hydrates. As such, a biogeophysical strategy for determining SMI depth would represent an important contribution to assessing methane charge with respect to climate change, sediment stability, or potential energy resources.

Related NETL Project: The goal of the related NETL project entitled “Methanogenesis in Hydrate-Bearing Sediments: Integration of Experimental and Theoretical Approaches” (FLU5A425/FWP100400) is to improve the understanding of processes that control the distribution, occurrence, and behavior of gas hydrate systems over time, especially with respect to the role played by these systems in global climate change.

NETL Project Contacts:
NETL – Robert Vagnetti (robert.vagnetti@netl.doe.gov or 304-285-1334)
OSU – Rick Colwell (rcolwell@coas.oregonstate.edu)