The Carbon And Sulfur Characteristics Of Sediments In The Gas Hydrate Potential Area Of The Northern South China Sea And Their Implications For Methane Seepages

Natural gas hydrate is an ice-like solid composed of water and methane or other gas molecules,it is stable under low-temperature and high-pressure conditions.Due to the potential future clean energy and the huge resource reserves in marine sediments,natural gas hydrate has drawn great scientific and governmental attention since the 21 st century.The long geological processes,such as tectonic movement,sea-level changes,and other factors,would likely destroy the gas hydrate stability,resulting in the upward migration of methane gas along favorable pathways.In the course of methane migration,methane encounters the downward diffusing sulfate,resulting in the process of sulfatedriven anaerobic oxidation of methane(SD-AOM),which play s a role in mitigating the greenhouse effect of methane.Accordingly,this process leads to the precipitation of typical authigenic minerals(carbonates and pyrites)in marine sediments by affecting the geochemical characteristics of the local porewaters.Therefore,the study of carbonsulfur coupled biogeochemical processes in methane-seepage systems is quite vital for understanding the distribution of gas hydrate reservoirs,and for exploring the impact of methane release on environmental changes.In this study,the Shenhu area of the northern South China Sea,which has great potential for gas hydrate accumulations,was selected to characterize the features of carbon and sulfur interactions in sediments.This thesis relies on the National Natural Science Foundation of China project(Grant No.41773078 and 41276046).Our research group carried out the fieldwork,then several sediment column samples were collected in the Shenhu sea during the KK1804 voyage in 2018.Here,we focus on two gravity piston cores A27 and SH1.Firstly,the evolution of sedimentary environments was analyzed using sediment element contents,grain size,planktonic foraminifera AMS 14C dating and their δ18O tests.Next,the geochemistry of headspace gas and porewater was used to delineate the early diagenetic zones and to constrain the fluid processes that mainly occur in recent times.Then,the modification of sediment inorganic and organic carbon by methane seepage activities was analyzed by carbon and oxygen isotopes of bulk-sediment carbonates and foraminiferal shells,combined with the mass balance model.Finally,hand-picked pyrite aggregates from sediments were analyzed by pyrite texture,sulfur isotopic composition,framboid size distribution,and trace element content of pyrite through chemical extraction,to explore the formation mechanisms and the cycling of associated trace elements.The geological background and geochemical multi-parameters of the investigated sites are integrated to characterize the carbon-sulfur interaction in the sediments and to provide indications of anomalous methane seepage activities.The following main results and insights were obtained:1.The lithologies of sediment cores A27 and SH1 are relatively homogeneous,mainly composed of gray-black silt,with continuous sedimentary sequences and no discernible sedimentary interruptions or turbidites,reflecting stable deposition under normal marine conditions.The chronostratigraphic framework for the studied sites was established based on radiocarbon dating of planktonic foraminifera,the results showed that the bottom layer of core A27 corresponds to 17.2 kyr,while SH1 has a longer sedimentary burial history,the bottom sediment of this site is assumed to be older than 60 kyr.The evolution of sedimentary environments was reconstructed by analyzing the sediment grain size and elemental geochemical characteristics in conjunction with the chronostratigraphic framework and planktonic foraminiferal δ18O values.The results showed that the paleoenvironmental and paleoclimatic features such as material input,organic matter content,sedimentation rate,and calcareous biological productivity of the sedimentary records at the studied sites were mainly controlled by sea-level changes.During the period of lowstand with coastline retreat,stronger hydrodynamic systems brought abundant material input,significantly increasing sediment accumulation rates,and a relatively reducing seafloor environment was conducive to organic matter burial,the cold climate was usually accompanied by low calcareous biological productivity.In contrast,during the period of high sea level with transgression,weaker hydrodynamic conditions reduced the material input and the deposition rate,and gradually enhanced seawater vertical mixing conditions led to the oxygen-rich bottom water,which was not conducive to the preservation of organic matter,and the warm and humid climate at this time enhanced calcareous biological productivity.The stable depositional processes and cyclic patterns in the study area provide the basis for a deeper understanding of fluid dynamics and the characteristics of carbon and sulfur interactions in the early diagenetic stages.2.The porewater SO42-contents show nearly linear sulfate depletions below 400 cm and 450 cm at sites A27 and SH1,respectively,the patterns coincide well with the variations of increasing Ba2+concentration and decreasing dissolved Mo content,indicating that the occurrence of AOM promotes the sulfate reduction rate.Due to the large sulfate replenishment from the overlying seawater diffusion and OSR process,the upper columns of both cores exhibit nonlinear sulfate consumption.Based on a linear regression method,the depths of the sulfate-methane transition zone(SMTZ)for sites A27 and SH1 are estimated to be 9.5m and 9.8m,respectively,the results reflect that the studied sites are characterized by low methane fluxes compared to the seeping zone with SMTZ depths lower than 2-3 mbsf.According to Fick’s First Law,the calculated SO42-fluxes are 21.75-32.19 mmol m-2 yr-1 for A27 and 20.09-29.74 mmol m-2 yr-1 for SH1.Compared to the Shenhu sea,the relatively shallow SMTZ depths and high SO42fluxes from the studied sites suggest the relatively intense(micro)methane seepage activity in recent times.The studied sites are located in the bottom simulating reflectors(BSR)zone,and site SH1 is very close to the hydrate drilling area.Therefore,the methane seepage activity in this area is likely related to the underlying gas hydrate reservoirs.3.The carbon isotopes of bulk-sediment carbonate,benthic and planktonic foraminifera show no distinctly negative excursion,reflecting that these signals in the study area are invalid in identifying methane-carbon carbonates and seepages.In order to address the common problem that is prevalent in low methane flux environments,a mass-balance model was adopted to evaluate the δ13C value of authigenic carbonate(δ13CAC),aud paleo-methane seepage activities were effectively identified from the sedimentary record by the combination of sediment TS/TOC ratios and carbon isotopic compositions of organic matter(δ13CTOC).Within the paleo-SMTZs of sites A27 and SH1,significant negative excursions in the δ13CAC values are documented,as low as4.49‰ and-4.05‰,respectively.Scanning electron micrograph(SEM)studies show that foraminiferal tests have experienced post-depositional alteration,infilled with authigenic carbonate,which may be related to AOM.The investigated sites were characterized by relatively low-activity methane seepages since the primary authigenic carbonate minerals are high-Mg calcite based on the Ca,Mg,Sr,and REE geochemical characteristics of sediments.The dissociation of underlying gas hydrates might be responsible for the methane seepage owing to elevated δ18O values of carbonates at the paleo-SMTZs.In addition to the negative anomalies in authigenic carbonate δ13C value,the methane seepage may also influence sedimentary organic matter δ13C composition.Interestingly,the δ13CTOC values show positive excursions within the paleo-SMTZs,which can be explained herein by the diagenetic modification,rather than the organic sources.In the course of SD-AOM within the SMTZ,a high rate of(methylotrophic)methanogenesis preferentially consumes lighter carbon atoms in organic matter,with the remainder being gradually more positive.The δ13CTOC value from methane-seepage environments is a promising proxy,which is likely associated with the seepage intensity and the duration.4.Based on light microscopy and SEM observations,the authigenic pyrite in the studied core sediments occurs as tubes,organism-filling pyrite aggregates,and irregular massive shapes.Those aggregates are mainly composed of framboids,while individual framboids consist of uniform octahedral microcrystals.Small pyrite framboids and loosely aggregated microcrystals are commonly observed in shallow sediments.With increasing depth,pyrite framboids are gradually packed densely and become larger in diameter,and some of the pyrite framboids tend to exhibit radial overgrowths.At the same time,some of the tubes exhibit abnormally large forms with a length of 15 mm,which are usually preserved in the bottom of cores A27 and SHI.The alteration of diagenetic fluids during pyrite mineralization is the most important reason for the evolution of pyrite morphology,and the locally enhanced sulfidation of the bottom sediments promotes the development of pyrite microcrystals.Besides,the abundance of hand-picked pyrite aggregates and their δ34S values are tested.In shallow sediments,low pyrite abundances with negative δ34S are ascribed to the OSR formation,the low OSR reaction rate limits the pyrite content,resulting in the large δ34S fractionations.By contrast,high contents of 34S-enriched pyrite aggregates are documented at the lower part of both core sediments,with the low δ34S values of-29.5‰ for site A27 and-23.9‰for site SH1.The geochemistry of authigenic pyrite coincides with the morphologic anomalies,collectively indicating that the SD-AOM reaction significantly increases the sulfate reduction rate,causing the enrichment of 34S-enriched pyrite,and the favorable diagenetic microenvironment is beneficial for the development and growth of pyrite microcrystals.On the other hand,a sequential extraction procedure for iron and trace metal concentrations in sedimentary pyrite was performed.The results show that,with pyritization proceeding,Mo,Ni,Mn,and Cr are gradually incorporated into the pyrite phase,presenting a relatively high affinity of these elements for pyrite in the studied locations.However,the contents of trace elements in pyrite are positively correlated with its concentrations in the current ocean,from which we can get that the trace elements in diagenetic pyrite are basically affected by seawater chemistry,while diagenetic processes are insufficient to cause substantial changes in their variations.5.For the studied sites,the AOM-driven authigenic carbonates in the sedimentary layers are basically consistent with the pyrite-related morphological and geochemical anomalies,combined with the porewater geochemical evidence,the collective research indicates the occurrence of multi-stage methane seepage activities in the study area,our researches also deepens the systematic understanding of the carbon-sulfur coupling relationship.