Use Of Pore Water Sulfate Sulfur And Oxygen Isotopes Tracing Methane Flux At Submarine Cold Seeps

The anaerobic oxidation of methane coupled with microbial sulfate reduction（AOM-SR）is a key biogeochemical process that plays a significant role in removing of most of the methane released from the deep towards the seafloor,thereby controlling its emission to the atmosphere.It contributes to marine sulfur burial second only to the more widespread organoclastic sulfate reduction（OSR）.This process is thought to occur extensively in shallower layers of marine sediments thought geological history,where their influence on the amount of sulfur burial and regulation of greenhouse gas methane emissions is more significant.Therefore,identifying the AOM-SR process in both current and geological records is essential for evaluating ecological environment effect related seafloor methane seepage activities.Sulfur and oxygen isotope composition of porewater sulfate(δ³⁴S_SO4andδ¹⁸O_SO4)provide deep insight into the MSR progress and sulfur cycle driven by MSR in marine sediment.In recent years,δ¹⁸O_SO4vs.δ³⁴S_SO4 features have been widely used to distinguish sulfate reduction driven by OSR or AOM.The feature of low slope driven by AOM-SR can be preserved in reservoirs such as authigenic carbonate rocks,serving as a good indicator for identifying methane seepage activities in the geological history.However,sulfur-oxygen isotopic signatures of pole water sulfate in sediments were affected by OSR and sulfide oxidation to varying degrees due to the strength and dynamic changes of methane seepage.In the sense of methene-diffusing environment with few methane fluxes,the structure ofδ³⁴S_SO4andδ¹⁸O_SO4 is mainly controlled by OSR;and the shallow layers in active cold seeps represented by high-methane fluxes have more active sulfide oxidation processes.Uncertainties in the sulfur-biogeochemical signature in the context of dynamic methane fluxes can significantly interfere with the environmental indicative significance of the lowδ¹⁸O_SO4/δ³⁴S_SO4 slope signature.Therefore,it is necessary to carry out an analysis of the characteristics of sulfur and oxygen isotope composition of porewater sulfate and its controlling factors under different methane flux,in order to clarify its feasibility in tracing methane fluxes during geological history periods.We compiled porewater data at 4 gravity piston cores from the Shenhu gas hydrate-drilling regions and Haima cold seep in the northern slope of South China Sea and used the linear fitting coupled Fick’s first law to obtain diffusive flux of sulfate and methane.Combined with the consumption-depth profile of geochemical parameters,we discussed difference in sulfur and oxygen isotope fractionation in varying methane fluxes.Focusing on two specific questions following:（a）the signature ofδ¹⁸O_SO4 vs.δ³⁴S_SO4 plot and its indication of AOM-SR in methane-diffusing environment;（b）varyingδ¹⁸O_SO4/δ³⁴S_SO4 slope values of porewater sulfate and their indicative significance for methane flux.Porewater geochemical results from sites SH-B01 and SH-W19 in Shenhu area showed that sulfate reduction is driven by different biogeochemical processes during early diagenetic processes in methane-diffusing environment.Based on the simulation results,the diffusion fluxes at sites SH-B01 and SH-W19 of downward total sulfate are12.7 mmol m^-2a^-1and 25.4 mmol m^-2a^-1.And concentrations of Mg²⁺,Ca²⁺,DIC andδ¹³C_DICvalues with no obvious change in the sulfate consumption-depth profile at site SH-B01 indicated the main biogeochemical processes was OSR.Additionally,porewater(ΔDIC+ΔCa²⁺+ΔMg²⁺)vs.ΔSO₄^2-ratio suggested that sulfate was consumed by OSR and AOM at SH-W19,influenced by diffuse methane.Compared to the background,the input of diffuse methane led to faster growth and larger equilibrium value ofδ¹⁸O_SO4at site SH-W19.Furthermore,a higher initialδ¹⁸O_SO4/δ³⁴S_SO4 slope found in core SH-W19 reflected the potential influence of hydrogen sulfide oxidation flowing in the pore water.Respectively,A series of porewater data show that the depths of SMTZ before the seafloor at sites QDN-1 and QDN-2 of Haima cold seep are about 1.6 m and 5.4 m.The extremely negative anomalies ofδ¹³C_DIC suggested the presence of AOM in QDN-1and QDN-2.And diffusion fluxes of downward total sulfate from the simulation results at sites QDN-1 and QDN-2 are 125.3 mmol m^-2a^-1and 83.5 mmol m^-2a^-1,correspond to the upward methane diffusion fluxes.On theδ¹⁸O_SO4 vs.δ³⁴S_SO4 plot,sulfur and oxygen isotopic compositions grow rapidly and develop a typical smallδ¹⁸O_SO4/δ³⁴S_SO4slope value（0.37）in core QDN-1;at site QDN-2 with reduced methane flux,theδ¹⁸O_SO4 continues to increase in the sediments below 4 m of seafloor while theδ³⁴S_SO4remains unchanged,indicating a transition from a high slope in the shallow to a low slope signature（0.35）in the deep part.The feature of highδ¹⁸O_SO4/δ³⁴S_SO4 slope in the methane-in-excess sediment reflects the influence of active reoxidation-disproportionation of flowing sulfide.The comparison of porewater geochemical characteristics and simulation results revealed that dominated sulfate reduction in methane-diffusing environments.The structure ofδ³⁴S_SO4 andδ¹⁸O_SO4 were modified during early diagenetic periods.Therefore,attention should be paid to the following issues when using theδ¹⁸O_SO4/δ³⁴S_SO4 slope signature.:（1）the rapid equilibrium ofδ¹⁸O_SO4 in the OSR process makes it difficult to trace AOM-SR process in methane-diffusing environment（2）In methane-in-excess sediments,changes in seepage activities were leading to an increase or decrease in theδ¹⁸O_SO4related active cycle of porewater sulfate,which affects the fitting of sulfate oxygen-sulfur isotope compositions in geological records.Therefore,theδ¹⁸O_SO4/δ³⁴S_SO4 slope proxy of pore water sulfate,combined with pore water geochemical parameters,can be used to trace methane flux in marine sediments,but the impact of strong in-situ biogeochemical processes on the use of this proxy needs to be fully considered.In the future,the combined use of sulfateδ¹⁸O_SO4/δ³⁴S_SO4proxy and multi-sulfur isotope methods can be used to explore the impact of dynamic methane flux on marine carbon-sulfur cycling.