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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

View larger version (32K): [in this window] [in a new window]   Figure 1. A: Seismic profile of Aarhus Bay sediment (56°05'55''N, 10°24'43''E) showing acoustic blanking due to the presence of free gas in Holocene mud. Top of gas horizon is termed the free gas depth (FGD). B: If CH4(aq) is efficiently consumed by anaerobic oxidation of methane in sulfate-methane transition zone (SMTZ), flux of dissolved methane from FGD [F(aq)] is proportional to ratio of methane solubility (CH4 SOL) to the distance between the FGD and the depth of SMTZ. C: Example of measured CH4(aq) and SO4 2– concentration data at a sampling site in Aarhus Bay (56°05'55''N, 10°27'42''E). As shown in inset, to limit degassing artifacts only CH4(aq) lower than the solubility at atmospheric pressure were used to calculate F(aq) by Fick's first law.

View larger version (23K): [in this window] [in a new window]   Figure 2. Map of Aarhus Bay showing sampling locations (crosses) and approximate free gas depth (FGD) in sediments (gray shaded) detected by seismic survey (after Laier and Jensen, 2007).

View larger version (33K): [in this window] [in a new window]   Figure 3. A: Prognostic nomogram of F(aq) (nmol cm–2 d–1) applicable for given ranges of free gas depth (FGD) (100–600 cm) and CH4SOL(SOL—in situ solubility) (4–10 mM). White isopleths represent lines of equal F(aq) calculated from model simulations. Fluxes are indicated on contours. Plotted symbols correspond to 19 stations in Aarhus Bay where FGD was determined by seismic profiling and CH4 SOL was calculated from in situ salinity, temperature, and pressure. B: Dependence of F(aq) on FGD for three values of CH4 SOL (5, 7, and 9 mM) indicated in A by vertical lines. Curves are mathematically described by power law functions (r 2 > 0.99) where a and b are the coefficients.

View larger version (16K): [in this window] [in a new window]   Figure 4. A: Methane flux (nmol cm–2 d–1) to the sulfate-methane transition zone (SMTZ), calculated from CH4(aq) (see Fig. 1C), versus flux predicted by nomogram for 19 stations plotted in Figure 3A. Regression curve (solid black line; y = 0.88 x + 2.3, r 2 = 0.77) was calculated using reduced major axis linear regression (Model II of Sokal and Rohlf, 2001) and dashed black lines are 95% confidence intervals. B: Regression curves showing effect of doubling (i) fluxes of particulate organic carbon POCfast and POCslow to sediment-water interface; (ii) rate of POCfast and POCslow mineralization; (iii) intensity of bioturbation; and (iv) intensity of bio-irrigation. See the GSA Data Repository (see footnote 1) for further details.