Geochemistry And Isotope Systematics Of Pyritic And Organic Sulfur In Mud Sediments Of The East China Sea Shelf

Pyritic (S_py) and organic sulfur (S_org) are two the most important reduced sulfurforms in marine sediments. The study on contents and isotope compositions of pyriticand organic sulfur is a useful tool for understanding limiting factors for pyriteformation, sources and pathways of S_orgformation, the relationship between pyriteand diagenetic organosulfur formations. The researches of pyretic and organic sulfurin sediments of East China Sea which is one of the world’s largest marginal sea havean important significance on the biogeochemistry cycling of carbon, sulfur and iron inglobal. Simultaneously the analysis of isotope compositions of pyritic and organicsulfur in sediments of East China Sea provides an important basis about sources andpathways of S_orgformation, the relationship between pyrite and diageneticorganosulfur formations which raised recently. Three cores from inner continentalshelf mud were collected in the East China Sea. S_pywas extracted from the sedimentsusing Cr（II） reduced method and was determined using the methylene blue method.S_orgwas quantified following Eschka’s procedure. Both S_pyand S_orgisopotecompositions were also determined. Other basic parameters of the sediments, forexample, TOC, highly reactive iron (Fe_HR), total reactive iron （Fe_R）, were alsodetermined. One of the three cores was also determined humic acid sulfur and fulvicacid bound-sulfur. Several issues were discussed in the study, for instance,concentration and distribution of S_pyand S_org, limiting factors for pyrite formation,sources and pathways of S_orgformation, fraction of biosynthetic and diageneticorganosulfur, the relationship between S_pyand diagenetic organosulfur. The mainconclusions are as follows:1. Core0702,0802and0803, which were from mud sediments of the ECS, arerich in reactive iron oxides （80¹20μmol/g） and sulfate remains undepleted to thedepth of⁶0cm. Therefore, it is expected that the content and reactivity of organicmatter are the limiting factors for pyrite formation in the three cores. 2. The differences of δ³⁴S between sulfate of modern seawater and pyritc sulfur(³⁴Ssulfate-pyrite) in core0802and0803（C0802:49.0⁵3.6‰; C0803:50.6⁵3.7‰）and in the bottom layer of Core0702（47.3⁵1.6‰） exceed46‰, which could be theresults of either additional isotope fractionation during disproportionation ofintermediate sulfur or single-step microbial sulfate reduction at low sulfate reductionrate, or both. Although Δ³⁴Ssulfate-pyritefor core0702was less than46‰in the upperlayer （41.0⁴5.8‰）, contribution of disproportionation to isotope fractionation cannotbe excluded. In the ECS sediments, both disproportionation and low sulfate reductionrate may have contributed to large sulfur isotope fractionation.3. Low degree of pyritization （DOP） for the three cores （DOPHCl:0.24³.45%;DOPox:0.61¹6.8%） indicates that reactive iron has not been limiting pyriteformation.4. The sedimentary rate and organic sulfur concent in Core0702are higher thanin Core0802and0803, implying that higher sedimentary rate can conducive to thepreservation of organic sulfur.5. The sulfur isopote compositions of S_org(³⁴S_org:5.39⁷.88‰) for the threecores are between those of sulfate of modern seawater (³⁴Ssulfate) and S_py(³⁴S_py),implying that there are two sources of S_orgin the sediments, that is, biosyntheticorganosulfur and diagenetic sulfide organosulfur.6. Based on isotope mass balance model, relative contributions of diageneticorganosulfur has a small fraction （26³1%）, biosynthetic organosulfur is the maincomposition of S_orgin the sediments of three stations.7. Both S_pyand Sdiagformed in the whole depth with Core0702,0802and0803which are rich in reactive iron, implying that pyrite and S_orgcan form simultaneouslyin sediments rich in reactive iron.