Abstract
In the northern South China Sea (SCS) we explored methane dynamics in the water column during SONNE-cruise SO266 in October/November 2018. Two depth zones contained elevated methane concentrations: the upper 400 m ( 10°C and > 20°C, respectively. Both 16S rRNA gene and pmoA amplicon analyses revealed distinct microbial and methanotrophic communities in water with temperature of 27°C, ~10°C, and 3°C. Second, we found elevated methane concentrations in 200-400 m in the FWCR-region whereas increased methane concentrations occurred in the uppermost 100 m above SSFR. The deeper plume above FWCR might be due to an intrusion of the Kuroshio water mass into SCS keeping the methane from being aerobically oxidized in the warm surface water and vented to the atmosphere. Finally, all peak methane concentrations occurred in water depth, with rather low backscatter, i.e. in water depth with less suspended matter. At the seafloor, ocean currents and long-term seepage appeared to control methane dynamics. We derived methane fluxes of 0.08-0.12 mmol m-2 d-1 from a 4.5 km2 area at FWCR and of 3.0-79.9 mmol m-2 d-1 from a 0.01 km2 area at SSFR. Repetitive sampling of the area at SSFR indicated that changing directions of ocean currents possibly affected methane concentrations and thus flux. In contrast to these seepage sites with distinct methane plumes, retrieval of drilling equipment produced no methane plume. Even gas emission triggered by seafloor drilling did not supply measureable methane concentrations after 3 hours, but caused an increase in methanotrophic activity as determined by rate measurements and molecular-biological analyses. Apparently, only long-term seepage can generate methane anomalies in the ocean.
Highlights
Methane is after water vapor and CO2 the most abundant greenhouse gas on Earth
Offshore southwest Taiwan, elevated methane concentrations occurred in the uppermost 400 m of the water column and at seepage sites at Four Way Closure Ridge and Southern Summit Formosa Ridge, which were partly associated with authigenic carbonates
Peak methane concentrations occur in different depth, which seem linked to water mass interactions in the ocean
Summary
Methane is after water vapor and CO2 the most abundant greenhouse gas on Earth. Currently the ocean contributes 2–10% to the atmospheric methane content despite the many seepage sites in the ocean (Skarke et al, 2014; Mau et al, 2017; Riedel et al, 2018) and the generation of methane in the oxic surface and subsurface water (Reeburgh, 2007; Conrad, 2009).Methane in the ocean originates either from discharge of methane from sediments or from methane formation in the water column itself. Serpentinization and Fischer–Tropsch reaction generate abiotic methane (Johnson et al, 2015; McCollom, 2016) In addition to these sedimentary sources, conspicuous methane concentration maxima in oxic water layers provided indications for methane production under oxic conditions. As microbial methanogenesis relies on reduced, anoxic conditions, this phenomenon was called the “ocean methane paradox” (Reeburgh, 2007). Studies of this phenomena identified fecal pellets (Karl and Tilbrook, 1994; Karl et al, 2008) and the guts of zooplankton (de Angelis and Lee, 1994; Tang et al, 2011; Stawiarski et al, 2019) as anaerobic micro-niches allowing anaerobic growth and methane production. Methylated compounds like methylphosphonates (MPn) (Karl et al, 2008; Repeta et al, 2016) or dimethylsulfoniopropionate (DMSP) (Damm et al, 2010) correlate with methane concentration and were postulated as source of methane in the water column
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