Unsteady Diagenesis Of Sulfur And Iron In The East China Sea Inner Shelf Muds

Sulfur(S) and iron(Fe) are the most important elements in the early diagenetic processes, which govern coupled biogeochemical reactions such as remineralization and preservation of organic matter, nutrient regeneration and authigenic mineral formation. Sedimentary S and Fe cycling are usually conceptualized and interpreted within the context of steadily accreting transport-reaction regimes. However, in the inner shelf of East China Sea (ECS), the surface sediments are commonly subject to episodic or periodic reworking by waves, tides and bioturbation. The reworked surface deposits act as unsteady batch reactors, unconformably overlying more consolidated relic sediments, and can result in dramatically different S and Fe reaction balances and diagenetic products with steady conditions.In this study, detailed S and Fe speciation in the two cores of ECS were characterized by traditional chemical extraction techniques. And sediment incubation experiments were carried out to investigate the interaction of C, S, Fe during the early diagenesis and the primary limiting factor for the diagenetic processes. The geochemical features of S and Fe in the ECS and tropical mobile muds were comparative analyzed. The main results and conclusions are as follows:1.The low TOC contents (～0.6%) with no clear depth variability in core DH5-1 and DH7-1 suggest that degradation of organic matter was not large enough to reveal the gradient of TOC loss due to low availability of labile organic matter. The low degradability of organic matter in the early diagenesis can be ascribed to the extensive mobility of sediments, where organic matter is decomposed efficiently during frequent reworking and leave the refractory components finally buried.2. In both sediment cores of ECS, the pyrite sulfur (Spy) was the predominant speciation of solid phase S pools, accounting for～85% of the total reduced S, while acid volatile sulfide (AVS) and elemental sulfur (S0) accounting only 15%. The rapid increase downcore of pyrite in the upper 40 cm sediments indicate that sulfate reduction occured mainly whthin the 10-40 cm intervals, below 40 cm the sulfate reduction weakened but not disappeared. The total reduced S content was low (8-40 μmol/g) because of weak sulfate reduction, that was the results of low availability of labile organic matter and competitive inhibition of microbial iron reduction. The comparatively higher C/S ratio (-7.5) in ECS than the general marine average (-2.8), on the other hand, demonstrated that the organic carbon was the primary limiting factor for the sulfate reduction and pyrite fomation.3. The total highly reactive Fe (ΣFeHR) was in the range of 143-247 μmol/g, and approximately 50-80% of the ΣFeHR was diagenetically reduced to Fe(II). The low DOP (0.8-6.1%) and Py-Fe(II) (0.07-0.26) index indicates that only a small proportion of the diagenetically reduced Fe(II)was present as Fe-sulfides in the ECS. And values of Sid-Fe(II) index suggests that～40% of diagenetically reduced Fe(II) was present as siderite or Fe-rich carbonates. A primary conclusion derived from both Py-Fe(II) and Sid-Fe(II) index is that about 35-50% of the reduced Fe(II) was present in a form other than sulfides or carbonate minerals in the sediments of ECS (e.g. authigenic Fe-silicate and vivianite).4. The occurence of Fecarb (43.2-53.4 μmol/g) in the ECS sediments reflects Fe-rich and nonsulfidic suboxic to anoxic environment, where microbial Fe reduction was strongly coupled to organic matter mineralization. Although the contents of Feoxl and Feox2 were almost constant downcore due to regeneration during the reworking, the reductive reactivity of Fe oxides quickly decreased in the upper 10 cm. It’s reasonable to conclude that the microbial iron reduction mainly occur in the upper 10 cm sediments. According to the algorithm raised by Jensen et al., the contribution of iron reduction to anaerobic carbon oxidation in the upper mobile leyers of ECS was approximately 87%. Below this depth, the decreased reactivity of Fe oxides and the increasing sulfate reduction (though still weak) suppressed the iron reduction to some extent.5. Both the ECS sediments and the tropical mobile muds were characterized by low concentration of total solid phase S(<100 μmol/g) and high C/S ratio (-7), which reflect nonsulfidic environment in the unsteady diagenesis. The differences of S speciation and δ34Spy between the ECS and tropical sediments were related to the thickness of the mobile layer. In the ECS, where the mobile layer was thin (～10 cm), AVS and S0 did exist and isotopic fractionation of 834Spy (-21- -34‰)act as in open system. While in the tropical sediments, where the mobile layer was more than 50 cm, the AVS and S0 did not exist and the 34Spy was unusually heavy (0-4‰) due to different diffusion-diagenesis fractionation of 32SO42- and 34SC42-.6. The contents of FeT, Fecarb and ΣFe(II) in the ECS sediments were comparative to that of tropical mobile muds. The deposits in the unsteady environments were all characterized by low DOP (2-8%) and Py-Fe(II) (<0.3). ΣFeHR/FeT ratio in the ECS sediments was～0.2, roughly half of the tropical sediments (-0.44), that can be ascribed to the more intensive weathering in tropical areas than in temperate latitudes. Moreover, the Sid-Fe(II) index in the ECS sediments was higher than that of tropical mobile sediments, though the reason still remain unknown.