Speciation And Early Diagenesis Study Of Sulfur And Iron In Sediments Of The East China Sea Continental Shelf

Continental shelf sediments are important locations not only for burial andmineralization of organic matter, but also for energy conversion and nutrient cyclingduring the early diagenesis. Early diagenesis of sulfur and iron in marine sedimentsexerts a strong influence on global cycle and budgets of carbon, phosphorus, andinteracted trace elements, and, thereby, has important geochemical implications. Inthis study, several selective extraction techniques were applied to determine depthdistributions of acid volatile sulfide （AVS）, elemental sulfur （S⁰） and pyrite (S_py)in3sediment cores collected in the East China Sea （ECS） continental shelf.30surfacesediments were also collected in the ECS for speciation of solid-phase iron using acombined method of sequential extractions and the Lord method. The abovemeasurements together with total organic carbon （TOC） contents, grain-sizedistribution, and other geochemical parameters were used to investigate spatialpatterns distributions of main solid-phase iron species in surface sediments of the ECS,diagenetic processes of sulfur and iron, as well as the relationships among C-S-Fe.The main results and conclusions are as follows:1. In core0702,0802, and0803, the contents of AVS, S⁰and S_pyincreasedowncore, with rapid increase occurring in the subsurface depth. In the deepersediments, the decreases in AVS and S⁰correspond well with the increase in S_py,which indicates that pyritization of iron is largely controlled by AVS and S⁰. Thegenerally low content of AVS, S⁰and S_pyin the ECS sediment can be ascribed to thecombined results of low availability of labile organic matter, strong bioturbation andphysical mixing, and the inhibition of from microbial sulfate reduction bydissimilatroy iron reduction to some extent.2. Low degrees of pyritization （DOP）（0.62¹7.7%） and high contents of reactiveiron （65.3¹50μmol/g）, and non-depletion of sulfate to a great depth in the core0702,0802, and0803suggest that the availability of labile organic matter, but not reactiveiron, is the limiting factor for pyrite formation.3. Reactive iron in core0702,0802, and0803has high buffering capacity towards free sulfides with βLranging from75.3mol/g to165mol/g, and β_R:ranging from91.0mol/g to272mol/g. The results imply that free sulfide producedby microbial SO₄^2-reduction can be rapidly buffered by reactive iron oxides, and,thereby, cannot accumulate in the pore water and diffuse up to overlying water to poseadverse impacts on benthic ecosystems.4. The good spatial couplings among TOC, Fe_Tand clay contents in the surfacesediments in the ECS indicate that the spatial distributions of TOC and Fe_Tarecontrolled largely by the spatial pattern of the clay fraction, terrestrial fine-grainedminerals and iron oxide coatings on the clay particles are the main sources of Fe_T.5. The order of iron pools in ECS surface sediments is Fe_PR> Fe_U> Fe_O2> Fe_mag> Fe_carb+AVS> Fe_O1> Fe_py. The spatial pattern of Fe_O1and Fe_O2is mainly controlled bythat of the clay fraction. Oxic and dynamic environments of the surface sedimentsconfine the generation and accumulation of Fe_carb+AVSand Fe_py, leading to lowcontents. The absence of spatial coupling of Fe_carb+AVS, Fe_pyand Fe_magto the clayfraction can be attributed to the facts that the low contents of the three Fe pools andcomplex size effect of Fe_pyand Fe_magin the sediments render insensitivity of the threepools to the spatial variations of clay fraction.6. The average FeHRin the ECS surface sediments is apparently lower than thatin the Yangtze River due to FeHRsequestration within the Yangtze Estuary filter, butcomparable to that in the global marine sediments. The average Fe_PRin the ECSsurface sediments is substantially higher than that in the global continental marginsediments, but comparable to that in the global riverine particulates, and only slightlyless than in the Yangtze River particulates, indicating that Fe_PRin ECS surfacesediments is controlled by Yangtze River particulates. The average Fe_Uin the ECSsurface sediments is much less than that in the global marine sediments, which can beattributed to the characteristically low Fe_Uin its source, the Yangtze Riverparticulates.7. Simple and reliable predication of Fe（III）HR, FeHR, and Fe_PRcan be achievedby using routinely and readily measured items, such as clay faction, TOC, Fe_Tand Al,in geochemical/environmental survey of marine sediment as master variables inmultiple linear regression. The statistical predications are helpful to avoid muchtroublesome chemical analysis in determination of the three iron pools.