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Remote quantification of methane fluxes in gassy marine sediments through seismic survey

¹Department of Earth Sciences–Geochemistry, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, Netherlands
²School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0340, USA
³National Environmental Research Institute, Department of Marine Ecology, Vejlsøvej 25, 8600 Silkeborg, Denmark
⁴Denmark Greenland Geological Survey, Thoravej 8, 2400 Copenhagen, Denmark
⁵Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany
⁶Center for Geomicrobiology, University of Aarhus, Ny Munkegade, Building 1540, DK-8000 Århus C, Denmark

A novel methodology for predicting upward diffusive fluxes of dissolved methane in gassy marine sediments is presented. The predicted fluxes are derived from a set of theoretical simulation data gener ated using a diagenetic reaction-transport model. The model calculates the upward methane flux for a given free gas depth (FGD) below the seafloor and a given in situ gas solubility, which together define the methane concentration gradient. Fluxes can thus be extracted from a nomogram of FGD and solubility parameter space. Because, in general, microorganisms anaerobically oxidize all dissolved methane before it can escape the sediment, the estimated fluxes are equivalent to the amount of methane trapped by this subsurface microbial barrier. A test of the approach using measured methane fluxes from Aarhus Bay, Denmark, reveals a statistically significant correlation between the observed and predicted fluxes. The predicted fluxes further show a low sensitivity toward enhanced sediment mixing by faunal activity, as well as the deposition flux and reactivity of organic matter. Therefore, only a limited amount of data at strategic coring sites is required to constrain the major physical and geochemical forcings for a particular study area in order to extrapolate fluxes at a regional scale. Because the FGD can be mapped over large areas of the seafloor from shipboard seismic survey, the new approach represents a means to estimate regional methane flux budgets for gassy sediments in a cost-efficient manner.