Biogeochemical Processes Of Nutrients And Trace Metals In The Hypoxia Zone In The Adjacent Area Of Yangtze Estuary

The hypoxia zone in the bottom water in the adjacent area of Yangtze estuary is one of the largest in the world. As a natural laboratory, studies of the Yangtze Estuary maybe provide insight into for examining biogeochemical processes of redox sensitive elements in hypoxia environments. In order to understand the biogeochemical processes in the hypoxia zone in Yangtze Estuary, we investigated the distributions of nutrients and trace metals (Fe, Mn, U, Mo, Cr, Ni, Cu, Zn) in water column, sediments and interstitial water. Results showed that the distribution of nutrients in the water column is mainly controlled by salinity. Nutrients release in the high turbidity zone were observed. In the bottom water, two different biogeochemical processes can be identified. When the salinity is less than 33 PSU, the negative correlation between nutrients and apparent oxygen utilization (AOU) is observed. When the salinity is more than 33 PSU, the positive correlation between nutrients and AOU can be observed. The causes can be ascribed to current and the mineralization of particulate organic matter, respectively. Trace metals can be grouped into conservative-type (Mo, U, Ni) and mixing-type (Fe, Mn, Cu, Pb, Zn).The distribution of dissolved trace metals are controlled by salinity, dissolved oxygen and plankton. There exists significant positive correlation between turbidity and distribution coefficient when the turbidity values more than 20 NTU are eliminated. Interstitial Fe and Mn profiles indicated that there exist strong iron and manganese reduction phenomenon in Yangtze estuary. Two-layer diagenetic model are used to fit the interstitial iron and manganese profiles. The fitting results coincides with the observed profiles for Mn, but mediocre for iron. Interstitial uranium profiles suggested that iron is the main factor that controlled the distribution of uranium. And what's more, other factor such as organic matter content also can affect the distribution of the uranium and molybdenum. Interstitial nutrients profiles suggested that there exist two different biogeochemical processes in Yangtze estuary. In the offshore area, the interstitial nitrogen profiles is controlled by organic matter diagenetic processes, while in the nearshore area, other processes, such as adsorption and ammnox, influence the interstitial nitrogen profile. Diagenetic fitting curves for NH4+ matched well with the observed interstitial NH4+ profiles. Moreover, the fitting curves indicate that there exist bioturbation phenomena in the surface sediments. The calculated nutrients fluxes indicated that the fluxes of silicate, nitrogen and phosphorus for plankton need daily only account for 15%, 10% and 0.1%, respectively. Pyritization of trace metals in the Yangtze estuary is much lower, which suggests that pyrite is not the sink for trace metals in Yangtze estuary. The reasons that cause the lower degree of pyritization can be ascribed to the high sedimentation rate, the poor organic matter and sulfur concentrations in Yangtze Estuary sediments.