Early Chemical Diagenesis Of Iron In The Shelf Sediments Of The East China Sea

Marine sediments on continental shelf are active biogeochemical reactor. Iron is a redox-sensitive element with the highest abundance in the crust, and plays an important role in early diagenesis. The fractions and distribution of various speciations by sequential extraction can indicate their roles in early chemical diagenesis, which is of significance in revealing diagenetic cycle of iron, carbon, sulphur, phosphorus and heavy metals elements in offshore continental shelf sediments. One core from inner continental shelf mud, three cores from outer continental shelf mud and 21 samples of surface sediment were collected in the East China Sea. Surface and core samples were extracted using Poulton and Lord & Raiwell methods, respectively. The concentrations, fractions of various iron phases were investigated. Also potential factors influencing iron distributions were explorded. The main research results are as follows:1. The order of iron content is Fe_PRS > Fe_U > Fe_ox2 > Fe_mag > Fe_carb > Fe_ox1 > Fe_py in surface sediment of the East China Sea and the content of active iron is high, whose average content reaches 80.02%, while average content of active iron oxides is 28%. Content and distribution of total iron and total aluminum show obvious "grain size control". Fecarb, Feox1, Feox2 and FePRS have good correlations among each other, probably due to phase conversion, these four speciations have good correlation with the fractions of clay, silt and sand, especially silt.2. The proportion of Fe₃S₄ is the highest one (65.05%) in total active iron, Fe(III)-oxide is higher than FeS2 (2.36%) and FeS (1.55%) in inner shelf mud area of the East China Sea , while in outer shelf mud area of 0701, 0704, 0804 sites, the order of total active iron content is Fe(III)-oxide > Fe₃S₄ > FeS > FeS₂. Fe(III)-oxide and Fe₃S₄ are the main speciations of total active iron. As the transition between FeS and FeS₂, accumulation of Fe₃S₄ is determined by unique deposition conditions, such as lower active organic and higher activity iron and so on, high content of activity iron also inhibit pyrite formation. The distribution characteristics of different speciations of total active iron may be affected by the different sedimentary environments, redox-processes and so on.3. The sediments from the mud area of the outer continental shelf (C0508) have high amount and reactivity of organic matter, and high amounts of Fe₃S₄ and FeS₂, but low amounts of total iron and Fe(III)-oxide, with comparison with the sediments from mud areas of inner continental shelf (C0701, C0704, C0804). Under the condition of high amount and high reactivity of organic matter, the early chemical diagenetic reaction of Fe(III)-oxide in the inner continental shelf is much more rapid, with low production of metastable speciations such as FeS and high production of relatively stable speciations such as Fe₃S₄, FeS₂.