| In this study, porewaters were sampled from shallow sediments of three sites (XS-2012, DS-2012-A and DS-2012-B) in the Xisha and Dongsha areas in the northern South China Sea. This drill cores were collected during the 2012 gas hydrate exploration expedition carried out by the Guangzhou Marine Geological Survey. We have conducted a series of geochemical analyses for the sediment porewaters in these three sites, including concentrations of trace elements, concentrations of anions and catiosn, carbon isotopic compositions of dissolved inorganic carbon (DIC) and sulfur isotopic compositions of the dissolved sulfate in porewaters. These data have been used to trace the biogeochemical processes, such as the anaerobic oxidation of methane (AOM) and the microbial reduction of sulfate with organic matter (SOM), and to indicate the microbial activity and fluid transportation in the areas, which will help to guide the exploration and development of natural gas hydrate in the South China Sea.Concentration of salinity elements including Cl", Na+ and K+ of sediment porewater at the site XS-2012 remains basically unchanged with the depth, no obvious abnormality is found, and the value is basically the same as that of seawater, which indicates that salinity characteristics of surface sediment porewater at the site are mainly inherited from the seawater instead of other foreign fluids (such as fluids reformed during formation of biological methane gas or natural gas hydrate). Value of carbon isotope δ13C for DIC in porewater is around -20‰ on the whole while only at 0-1.5m show a small fluctuation within the range of-15 to-20‰ possibly due to perturbation caused by seawater diffusion. These characteristics indicate that the source of DIC at the site was mainly from decomposition of organic matters via SOM process and the biogechemiacl process in this site was mainly controlled by SOM. Concentration of Ca2+, Mg2+ and Sr2+ decreases with increase of the sediment depth, especially the latter two elements, which is likely related to formation of authigenic carbonate and indicates existence of decomposition of organic matters to provide carbon sources (HCO3-or CO32-) at the site. Calculated value of iodine flux at the site is 13.09×10-3 μmol·cm-2·a-1, which is obviously higher than that of site 997 in the Blake Ridge, which indicates existence of intense decomposition of organic matters at the site and release of biological iodine. The calculated SMI depth at the site XS-2012 is 14.5m using the sulfate concentration curve. This site is free of AOM, and oxidation of SOM was the main factor leading to consumption of sulfate radical. However, the calculated high reduction rate (2.53×10-11mol m-3 s-1) of sulfate for porewater of the site seems to indicate existence of overwhelmingly intensive microbial activities at the site, which makes SOM consume sulfate radical greatly. In addition, sulfur isotope enrichment factor (ε=21.3‰) and sulfur isotope fractionation coefficient (α=1.021) at the site are both low, compared with sulfur isotope data at Xisha area obtained by previous researchers, which further confirms the existence of high organic matters concentrations, intensive microbial activities and obvious SOM characteristics at the site.No obvious abnormality is found for salinity elements including Cl-, Na+ and K+ of sediment porewater at the site of DS-2012-A and DS-2012-B in the Dongsha area with depth, which also indicates that porewater at these two sites mainly inherits salinity characteristics of seawater instead of other foreign fluids. When comparison of characteristics of Ca2+, Mg2+ and Sr2+at the two sites, negative anomaly of gradient is found more intensive at the site DS-2012-B that at the site DS-2012-A. Iodine flux at the site DS-2012-A is 4.60×10-3 μmol·cm-2-a-1, and that at the site DS-2012-B is 11.50×10-3μmol·cm-2·a-1, the former is obviously lower than the iodine flux of Blake Ridge, and the latter is obviously higher than that of Blake Ridge. Value of carbon isotope δ13C for DIC of porewater at the site DS-2012-A decreases with increase of burial depth (-5 to-25‰), and value of δI3C in the bottom is slightly lower than that of sedimentary organic matters (-21‰), which indicates that the source of DIC at the site is mainly from the convective seawater with small proportions of SOM release, as well as accompanied unobvious AOM. There is intensive negative anomaly of gradient for value of carbon isotope δ13C at the site DS-2012-B (+0 to-42%o), and value of 813C in the bottom approximates to carbon isotope value of biological carbon gas (<-50%o), which indicates that the DIC source of the site was mainly anaerobic oxidation of methane (AOM). SMI depth of the two sites obtained through linear fitting calculation is 18.6m and 8.4m, respectively. Since the two sites are within a short distance and there is certain degree of SOM and unobvious SOM process at the site DS-2012-A, we may use this site as the reference site available to represent initial sedimentary environment of the whole region. Through comparison of sulfate radical reduction rates between the two sites, it is possible to qualitatively and semi-quantitatively estimate the effect of AOM at the site DS-2012-B up to 91.3% of total sulfate radical consumption, which indicates existence of anaerobic oxidation of methane at the site DS-2012-B. Since the same sedimentary environment shows entirely different sulfur isotope fractionation characteristics, microbial activity was likely the main factor making the two sites subject to different degrees of sulfur isotope fractionation regardless of temperature-pressure and pH conditions, and to be specific, it is mainly caused by AOM process. In addition, sulfur isotope enrichment factors of sites DS-2012-A and DS-2012-B are respectively 27.2%o and 16.7‰ (corresponding sulfur isotope fractionation coefficients are 1.027 and 1.107), which indicates the latter has higher sulfate reduction rate; it also agrees well with the calculated sulfate reduction rates of the two sites (respectively 0.66×10-11 mol m-3 s-1 and 7.63×10-11mol m-3 s-1), which further indicates the site DS-2012-B has geochemical conditions for accumulation of natural gas hydrate. |
PDF Full Text Request