ALBERT

Description: Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slope-parallel elongated depressions that link to headwalls of sediment slides on upper slope. The depressions of about 250 m in width and several kilometers in length are areas of focused gas discharge indicated by bubble-release into the water column and methane enriched pore waters. Sparker seismic profiles running perpendicular and parallel to the coast, show gas migration pathways and trapped gas underneath these depressions with bright spots and seismic blanking. The data indicate that upward gas migration is the initial reason for fracturing sedimentary layers. In the top sediment where two young stages of landslides can be detected, the slope-parallel sediment weakening lengthens and deepens the surficial fractures, creating the elongated depressions in the seafloor supported by sediment erosion due to slope-parallel water currents.

Description: Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH4+, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13CDIC ranges from −7.2 to −26.3‰ VPDB).

Description: Highlights • Three distinct types of carbonates exist in cold seeps in the Okinawa Trough. • Carbonate forms between lower part of SMTZ and upper part of methanogenic zone. • Hydrothermal has the potential to foster Fe-AOM in cold seep sediment. • Metal-AOM might have an influence on early Earth's methane flux and metal cycle. Abstract Anaerobic oxidation of methane (AOM) is an important process that regulates methane budget in the global carbon cycle. Sulfate is traditionally regarded as the most important electron acceptor for AOM. However, recent studies reveal that reactive metal reduction-driven AOM (metal-AOM) may also present on natural environments. Partitioning the methane-C sink from anaerobic oxidation between sulfate reduction-driven AOM (sulfate-AOM) and metal-AOM is thus becoming a scientific challenge to be addressed. This study aimed to show that metal-AOM has a potential to complement sulfate-AOM, especially when methane seeps met sediments enriched in iron and manganese oxides supplied by hydrothermal plumes. Samples of authigenic carbonate were collected from the cold seeps on the western slope of the Okinawa Trough (OT), a semi-enclosed back-arc basin with cold seeps and hydrothermal activities coexist spatially. Petrologic and mineralogical observations provided faithful evidence of a close genetic relationship between goethite and authigenic carbonates, likely indicating the presence of metal-AOM although some contributions of organoclastic Fe reduction cannot be ruled out. The most possible place of metal-AOM was between the lower part of the sulfate–methane transition zone (SMTZ) and the immediate upper part of the methanogenic zone. The characteristics of δ13Ccarb [−53.7‰ to −3.3‰ Vienna-PeeDee Belemnite (VPDB)] also point to our carbonate samples as the products of AOM. These authigenic carbonates was further identified as three possible types of carbonate of different sources: sulfate-AOM, metal-AOM, and hydrothermal carbonates, depending on the correlations among concentrations of carbonate-associated sulfate (CAS), δ13Ccarb, and δ18Ocarb. Based on these findings, we tentatively suggested that the reduction of reactive metals carried by hydrothermal plumes might drive the methane oxidation in the adjacent cold seep areas, resulting in the precipitation of unique carbonates and enhancement the efficiency of “the benthic methane filter”. This study reported the coupling or interaction between the two extreme submarine environments by metal-AOM for the first time, which might help in understanding and improving the global carbon and metal cycles now and in the past. Previous article in issue