The Distribution And Environmental Significance Of Redox Sensitive Elements In The Hypoxia Zone Of The Changjiang Estuary And Its Contiguous Area

The hypoxia zone near the Changjiang Estuary have been found, its influence on environment attract the attention of scientists. Bottom water dissolved oxygen depletion zone locate around 122° 45' -123° 00' E and 30° 50' N. The area of hypoxia zone where dissolved oxygen ≤ 2mg/l is about 13 700 km2. The mud off Changjiang Estuary area are the modern accumulative centers on the East China Sea shelf since Holocene epoch, their formative process and storage of pollutant materials are essential for understanding of martial flux,it is a important question of Land-Ocean Interactions in the Coastal Zone(LOICZ) in the East China Sea.Based on the concentrations and enrichment characteristic of redox sensitive elements, such as Cu, Pb, Ni, U, Mo, V and Cd of the surface sediments and sediments cores, combined with field survey data of water dissolved oxygen in June 2003 and the measured concentrations of redox sensitive elements of the suspension, the paper discusses the bottom water hypoxia's influence on the distribution and enrichment of the redox sensitive elements in sediments. This paper has studied the distribution, environment significance of redox sensitive elements and environmental changing trend of the Changjiang Estuary oxygen depletion zone.This is a first time researches on the relation of hypoxia zone and the distribution of redox sensitive elements in sediments and discusses the questions as follows: the reductive extent of the hypoxia zone, the changing trend of the hypoxia zone.The paper has analyzed the stability and uniformity of sediments, and believes that the components of the sediments are uniform and stable. So we can get rid of the influence of provenience and sediments pattern. There are no significant differences in grain size and clay minerals concentrations between the hypoxia zone and non-hypoxia zone off the Changjiang Estuary mud area. "Grain size effects" andadsorption by clay minerals have no effects on the distribution and enrichment of ihu redox sensitive elements in sediments.The paper believes that hypoxia zone off the Changjiang Estuary has important influence on the distribution of V, Cd, Mo in sediments. The distribution of redox sensitive element of V, Cd, Mo is not uniformity in the research area. They are significantly enriched in sediments of the hypoxia zone, and depleted in water dissolved oxygen normal zone off the Changjiang Estuary mud area. The results indicate that the distribution pattern and enrichment characteristic of V, Cd and Mo can be used as redox environment indicators of the bottom water hypoxia status, and can be used to study the redox status and changing trend of the environment.Redox sensitive elements, such as Pb, Cu, Ni, and U are also sensitive to redox conditions of bottom sediments theoretically. Many factors, such as the influence of adsorption by organic carbon^ scavenging by suspension and terrigenous component, can affect the distribution and enrichment of these redox sensitive elements, in that having no enrichment in hypoxia zone off the Changjiang Estuary, So the redox sensitive elements Pb, Cu, Ni, and U have no environment significance in the hypoxia zone of the research area.Based on the rules of the distribution and enrichment characteristic of redox sensitive elements of the surface and core sediments, compared with concentrations of V, Mo and Cd in the sediments from the different sea area in the world, the paper believes that Cd with low potential energy is enriched in hypoxia, V and Mo with high potential energy are also enriched in hypoxia zone. So the reductive extent of the Changjiang Estuary hypoxia zone is in moderate. It is weaker than that of Arabia Sea, but stronger than that of the Africa continental margin, the Northwest of American continental margin and the deposit environment of the shale.Environment of oxygen normal zone of the Changjiang Estuary approach to that of the Africa continental margin, and its reductive extent is weaker than that of Northwest of American continental margin. Compared with the deposit environment of shale, the environment of oxygen normal zone off the Changjiang Estuary is in oxidation state.The paper found the redox sensitive elements V\ Cd> Mo in core 17 begin to enrich at the depth of 28cm, and it is slightly enhanced in shallower depth. The paper concludes that the oxygen depletion zone off the Changjiang Estuary came into being since 1964, and its extent is enhanced these years.