A quantitative assessment of methane cycling in Hikurangi Margin sediments (New Zealand) using geophysical imaging and biogeochemical modeling
Luo, M.; Dale, A. W.; Haffert, L.; Haeckel, M.; Koch, S.; Crutchley, G.; De Stigter , H.; Chen, D.; Greinert, J. (2016). A quantitative assessment of methane cycling in Hikurangi Margin sediments (New Zealand) using geophysical imaging and biogeochemical modeling. Geochem. Geophys. Geosyst. 17: 4817–4835. dx.doi.org/10.1002/2016GC006643
Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by awell-defined subsurface seismic chimney structure 80,500 m2in area. Subseafloor geophysical data basedon acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimneystructure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrainmethane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methanedynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas(up to 3761 mmol m22yr21) dwarfed the rate of methanogenesis within the simulated sediment column(2.6 mmol m22yr21). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) atthe base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximumrates in the central part of the chimney (946 and 2420 mmol m22yr21, respectively). A seep-wide methanebudget was constrained by combining the biogeochemical model results with geophysical data and led toestimates of AOM rates, gas hydrate formation, and benthic dissolved methane fluxes of 3.68 3 104molyr21, 73.85 3 104mol yr21, and 1.19 3 104mol yr21, respectively. A much larger flux of methane probablyescapes in gaseous form through focused bubble vents. The approach of linking geochemical model resultswith spatial geophysical data put forward here can be applied elsewhere to improve benthic methaneturnover rates from limited single spot measurements to larger spatial scales.
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