Carbon-sulfur Cycle In Shallow Sediments Of Cold Seep And Gas Hydrate Bearing Area And Its Influence In The Northeastern Of The Northern Slope Of The South China Sea

The shallow sediments of cold seep and gas hydrate bearing areas are active province of carbon and sulfur cycle.The upward migration of methane from the sediment columns reacting with downwards diffusive sulfate from seawater is the trigger of series biogeochemical reactions occurring in shallow sediments,which is an important part of global carbon cycle.Although a previous study estimated diffusive fluxes of methane and sulfate in marine sediments based on sedimentation rates rather than measured porewater sulfate profiles,the diffusive fluxes of methane and sulfate are highly spatial variable in cold seep and gas hydrate bearing areas with nearly unchanged sedimentation rates.Therefore,considering the widespread of submarine cold seep and gas hydrate bearing areas,it is necessary to use more measured pore-water sulfate profiles to constrain the regional and even the global carbon and sulfur fluxes near the seafloor.We compiled pore water data at 238 sites from the areas of Shenhu and Dongsha-Taixinan in the northern slope of South China Sea and used the linear fitting and Fick’s fist law to obtain the depth of sulfate-methane transition（SMT）,diffusive flux of sulfate,flux of authigenic carbonate precipitation,and efflux of dissolved inorganic carbon（DIC）.Combined with the Inverse Distance Weighting plotting,we get the budget of sulfate consumption,authigenic carbonate precipitation,and DIC efflux in the area of Shenhu and Dongsha-Taixinan.The calculation from pore water data at 94 sites in the Shenhu region with an area of 5.2×10⁴km² reveals that the SMT depth is about 7-89 meters below seafloor（mbsf）with the most in a range of 7-42 mbsf.The southern area has a deeper depth of the SMT than in the north,the sulfate fluxes are thus lower than those in the north.There is a good double-logarithmic linear relationship between the SMT depth and the sulfate flux（R²=0.95）.The sulfate fluxes range from 3 to 21 mmol m^-2 a^-1,and consumed 5.39×10¹¹mmol of sulfate annually.The flux of authigenic carbonate precipitation is 2 to 26 mmol m^-2a^-1 and precipitated 3.21×10¹¹ mmol of carbon annually.From the SMT zone the sediments discharged the DIC with rates from 2 to 44 mmol m^-2 a^-1 with the maximum rates corresponding to the highest fluxes of sulfate and authigenic carbonate precipitation.The total efflux of DIC from sediments to seawater reached 5.38×10¹¹mmol a^-1.The estimated results from pore water data compiled at a total number of 123 sites in the Dongsha-Taixinan region with an area of 3.2×10⁴km² reveal that the SMT depths range from 1 to 43 mbsf,showing three high valued centers.The sulfate fluxes are3-171 mmol m^-2 a^-1.SMT depths and sulfate fluxes have a weak double logarithmic linear correspondence（R²=0.45）.Regional sulfate consumption flux in Dongsha-Taixinan area reaches 1.14×10¹² mmol a^-1.Authigenic carbonate precipitation rates vary from 3 to 106 mmol m^-2 a^-1,whose peaks occur in the northwest of the area.The regional authigenic carbonate precipitation rate is 4.53×10¹¹ mmol a^-1.The DIC emission fluxes were in the range of 3-596 mmol m^-2 a^-1 with the peaks appearing in the northwest-southwest of the area.The regional emission rate of DIC from the sediments to seawater is 1.31×10¹²mmol a^-1.It is obvious that significant changes of DIC efflux occur in a limited area of 1 km² in the“Jiulong methane reef”and the“HY4sedimentation body”,which indicate the influence from the uneven distribution of cold seeps and gas hydrates in space.Because of this feature,it is probably imprecise to constrain the regional carbon-sulfur fluxes through the method of sedimentation rates.In general,Dongsha-Taixinan area has higher regional fluxes of sulfate consumption,authigenic carbonate precipitation and the DIC emission compared with the Shenhu area.After an area average,the regional fluxes of sulfate consumption,authigenic carbonate precipitation,and DIC emission in the Dongsha-Taixinan area are 2-4 times higher than those in the Shenhu area,suggesting that more active carbon-sulfur turnover rates occur in the Dongsha-Taixinan area.Meanwhile,we preliminary analyzed pore water data from published literatures in Xisha-Qiongdongnan area,combining with the results from the Shenhu and Dongsha-Taixinan areas,the budgets of carbon-sulfur cycle（i.e.,the fluxes of sulfate consumption,authigenic carbonate precipitation and DIC emission）are likely larger.Meanwhile,Xisha-Qiongdongnan area has a higher organic carbon content in sediments,much wider BSRs signal,well channel systems for gas’migration,make the area with a much active carbon-sulfur cycle flux,which may exert greater influences on the carbon-sulfur cycling and its driven ecosystem near the seafloor.