A quantitative assessment of methane cycling in Hikurangi Margin sediments (New Zealand) using geophysical imaging and biogeochemical modeling. (2nd December 2016)

• Record Type: Journal Article
• Authors: Luo, Min; Dale, Andrew W.; Haffert, Laura; Haeckel, Matthias; Koch, Stephanie; Crutchley, Gareth; De Stigter, Henko; Chen, Duofu; Greinert, Jens
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 17:Number 12(2016:Dec.)
• Page Start: 4817
• Page End: 4835
• Publication Date: 2016-12-02
• Subjects: cold seep -- methane cycling -- reaction‐transport modeling -- geophysics -- Hikurangi Margin; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2016GC006643
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well‐defined subsurface seismic chimney structure ∼80, 500 m 2 in area. Subseafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction‐transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m −2 yr −1 ) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m −2 yr −1 ). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m −2 yr −1, respectively). A seep‐wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation, and benthic dissolved methane fluxes of 3.68 × 10 4 mol yr −1, 73.85 × 10 4 mol yr −1, and 1.19 × 10 4 mol yr −1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales. Key Points: Porewater geochemistry at the Takahe seep was linked to upward methane bubble transport and dissolution Modeling results showed that AOM and gas hydrate precipitation were the dominant processes in the methane cycle Model data were combined with geophysical data to constrain a seep‐wide methane budget