Controls Of Local Sedimentary Environment On Pyrite Sulfur Isotopes In Surface Marine Sediments On The Northern Slope Of The South China Sea

The sulfur isotopic composition of syndepositional pyrite(δ³⁴S_py)in the marine sediment is the most commonly used method for reconstructing the marine sulfur cycle in geological history.However,the range ofδ³⁴S_py values in sedimentary layers in different environments during the same period generally exceeded 40‰,which is obviously not related to the long-term changes of marine environment such as seawater sulfate reservoir and seawater chemical stratification,but is affected by the different sedimentary environments during the early diagenesis.Therefore,to clarify the influencing factors and controlling mechanism of early diagenesis on pyrite sulfur isotopic composition is a prerequisite for reconstructing the ocean sulfur cycle and marine environment changes through pyrite.Recent studies have found:（1）The phenomenon that theδ³⁴S_py value decreases with depth in the surface sediments of the continental margin generally occurs,which is completely different from the previous traditional research understanding that"the phenomenon of theδ³⁴S_py value in the surface sediments increases with depth"different,however the mechanism that leads to this phenomenon is still unknown;（2）sulfate reduction coupled with the anaerobic oxidation of methane（AOM-SR）contributes to the burial flux of pyrite,which is second only to Organic sulfate reduction（OSR）,and theδ³⁴S_py value of pyrite derived from AOM-SR has a variation range of 150‰,but the relationship betweenδ³⁴S_py and methane flux is still unknown.Therefore,this study carried out the analysis of the pyrite sulfur isotopic composition characteristics of two piston gravity cores from the northern continental slope of the South China Sea and an intubation core from the cold seep area.The ¹⁴C dating analysis of microscopic morphology,grain size of sediment terrigenous components and planktonic foraminifera explored the following two key scientific problems:（1）The mechanism of the decrease ofδ³⁴S_py value with increasing depth in marine surface sediments and its Influencing factors;（2）The key controlling factors ofδ³⁴S_py in the seabed methane seepage environment and their significance to methane flux.In the surface sediments of the northern South China Sea,the decrease ofδ³⁴S_pyvalues with increasing depth occurs widely.Taking the gravity piston columnar sample DS-08（length 7.0 m,water depth 889 m）in the Dongsha sea area as the research object,the morphology,Content andδ³⁴S value,pore water sulfate sulfur isotope composition(δ³⁴S_SO4)and concentration,and total carbon（TC）,total organic carbon（TOC）,total nitrogen（TN）,total sulfur（TS）content and particle size parameters of sediments,and performed AMS¹⁴C dating of planktonic foraminifera.The results show that:（1）Different pyrite crystal forms（framboidal,octahedral）and framboidal pyrite in the core DS-08 have a large average grain size variation range（6.3μm-16.6μm）,indicating that pyrite is formed by precipitation in sediment pore water,which can be used to explore the sulfur isotopic composition of pyrite during early diagenesis;（2）with the increase of depth,the value ofδ34Spy decreases from-16.8‰to-42.8‰,indicating that The degree of sulfur isotope fractionation produced by the OSR process in the most surface sediments is<40‰,and it increases rapidly with depth;（3）Theδ³⁴S_py value in the sediments is more negative under the characteristics of higher deposition rate and high organic matter supply,and this characteristic It is completely different from the previous understanding that high deposition rate/organic matter supply leads to positiveδ³⁴S_py values.In summary,this study reveals the mechanism of the decrease ofδ³⁴S_py value in the shallow surface layer of sediments with increasing depth,highlights the response characteristics of different depositional environments forδ³⁴S_py values in sediments,and provides new information for usingδ³⁴S_py to reveal the early diagenetic environment and marine sulfur cycle.Methane flux is the key factor affectingδ³⁴S_py in sediments.The gravity piston columnar sample in the seepage area of the Dongsha area on the continental slope of the South China Sea（low methane flux environment,DS17-15,length 7.0 m,water depth2023 m）and the interpolation of the seepage area of the Haima cold seep in the Qiongdongnan Basin were studied.The pipe column sample（high methane flux environment,ROV05-CS01,length 0.6 m,water depth 1353 m）was used as the research object,and the pyrite content andδ³⁴S_py,pore water sulfate concentration andδ³⁴S_SO4,hydrogen sulfide sulfur isotope composition(δ³⁴S_H2S),and sediment TC were analyzed,TOC,TN,TS content.The results show that:（1）The linear fitting results of the pore water sulfate concentration show that the methane sulfate transition interface（SMI）depth of the columnar sample ROV05-CS01 is lower than that of the columnar sample DS17-15,which are 32 cmbsf and 659 cmbsf respectively.The methane flux calculated by the first law shows that the columnar sample ROV05-CS01（1.26 mol/m²/yr）has a higher methane flux than the columnar sample DS17-15（0.08 mol/m²/yr）;（2）the columnar sample Theδ³⁴S_py value in ROV05-CS01 ranges from-3.2‰to 11.2‰,which is lower than the seawater sulfateδ³⁴S value（21.2‰）.This is due to the rapid consumption of sulfate radicals diffused to the shallow surface of the sediment under high methane flux,and the product sulfides The hydrogen concentration increases rapidly with depth and theδ³⁴S values are close to the seawater value(theδ³⁴S_H2S values ranges from-0.9‰to 19.6‰from the surface to the deep).（3）The range ofδ³⁴S_py in the columnar sample DS17-15 is-45.1‰to 41.2‰,Among them,it can reach 41.2‰near the methane sulfate transition zone（SMTZ）,which exceeds the seawater sulfateδ³⁴S value,which is due to the more enriched ³⁴S sulfate diffused from the overlying pore water and the higher sulfate reduction rate at the SMTZ jointly decided.This study clarified the composition characteristics and influencing factors of pyrite sulfur isotope values under different methane fluxes,and highlighted the key control effect of sulfate and hydrogen sulfide diffusion in pore water on the sulfur isotope composition of pyrite during the early diagenetic process.This provides new insights into tracing past methane fluxes.In summary,this study,through the analysis of the sulfur isotope composition characteristics of pyrite in the surface sediments of the northern continental slope of the South China Sea,improved the control mechanism of pyrite sulfur isotope in the early diagenetic process,and established the key control effect of methane flux on pyrite sulfur isotope composition.The results show that the sulfur isotope composition of whole-rock pyrite is the product of continuous accumulation of pyrite in the early diagenetic stage,with a huge range of variation and different environmental response characteristics.The sulfur isotopic composition of pyrite micro-area can trace the sulfur isotopic composition of pyrite formed in different periods.Therefore,further research on the sulfur isotopic composition characteristics of pyrite micro-regions in modern marine sediments combined with pyrite morphology will provide new enlightenment for the application of pyrite geological carriers in exploring the evolution of the earth’s surface environment.