| The behavior and fate of material elements in the in the transitional area-estuarine wetland from ocean to land which dominated by the large river, play a key role in studying the sea land matter cycle. The world’s rivers transport tens of millions of particulate matter to the estuary area and larges of terrigenous materials burial in the estuarine wetland. Based on the land-ocean mass transport path " river-estuary-inshore-ocean", it can be proved that the estuarine wetland is the only way when the material elements transport to the ocean from the river.The Chongming East Tidal Flat wetland located in the Yangtze River mouth is the typical semi closed and semi opened estuarine wetland with large inputs of particulate matters via Yangtze River. Due to the large river inputs, the relative semi closed sedimentary environment and relatively complicated hydrodynamics forced the sink of material elements in this area. Therefore, Chongming East Tidal Flat Wetland, is one of the best place to study the burial characteristic of material elements.Taking Chongming East Tidal Flat Wetland as the typical study area of the Yangtze Estuary. The dissertation focused on investigating and analysising on the contents level and distribution characteristics of organic carbon and trace (Cd,Cr,Cu,Pb,Zn,Ni, Zr,Sr,Rb) in the sediments. By using the pipe bury method, the contribution of Scirpus mariqueter to the organic carbon storage of Chongming east tidal flat wetland sediment was calculated. Recent (≤1 year) wetland sedimentation rate was measured with feldspar marker horizons. From this, the short-term burial fluxes of organic carbon and heavy metals in the sediments of Yangtze Estuarine were investigated. The main conclusions can be summed up as follows:(1) In the surface sediments of Chongming East Tidal Flat wetland, organic carbon contents had some obvious seasonal variations. The variation of organic carbon turns a "U" shape curve. In the spring (from April to May), the organic carbon content was increased slightly because of the sediment transportation from the upper of Yangtze River. In the summer (from July to August), organic carbon contents declined obviously, due to the mineralization rate of sediment organic carbon accelerated under the high temperature. When it comes to Autumn (from September to November), the organic carbon mineralization rate declined after July, the defoliated and lodged Scirpus mariqueter began decomposing, so the peak value of organic carbon occurred in the fall.(2) Vertical distribution characteristic of organic carbon in sediment was jagged multimodal distribution, and fluctuated greatly. The contents of organic carbon in sediment had a descending trend along the profile and changed from 4.98 mg/g to 8.08 mg/g. From the sediment surface to the depth of 20 cm, the tidal flat accepts the fine particular deposition, the decomposing of defoliated and dead Scirpus mariqueter was the main factor determing the highest organic carbon content in the surface sediment profiles during the dry period. The contents of organic carbon were high at the interval 20-90cm, due to the absorption immobilization and bury of Scirpus mariqueter, much organic carbon stored in the middle layer. Below the 90 cm, the tidal flat accepts the coarse particles deposition, The reduction of organic carbon input to the deep sediment and the mineralization losing process, further to a substantial decrease of deep sediment organic carbon storage.(3) The organic carbon content in the buried pipe sediment profiles showed significant variation due to the decomposing of dead and buried Scirpus mariqueter and the microbe respiration in sediment. The internal distance between the organic carbon accumulation layer is about 5-12cm, which was corresponding to the short-term (≤1 year) marsh accumulation rate (11.6cm/yr, from August 28th,2012 to September 2th,2013) measured by feldspar marker horizons. Ignoring the carbon fractionation during the process of organic carbon degradation, contribution of Scirpus mariqueter input to the Chongming East Tidal Flat sediments organic carbon pool was estimated by using the reference on a simple two component mixing equation. Net input of organic carbon from Scirpus mariqueter into the sediments was about 0.65mg/g in flood period and 2.06mg/g in the dry season, account for 7.35% and 31.20% to the sediments carbon pool respectively.(4) The characteristics of seasonal and vertical section change in heavy metal content of Cd,Cr, Cu, Pb, Zn, Ni were distinctive and complicated. The temperature increasing was favorable to the desorption of Cu, Pb, Zn from the sediments to the pore water. The major pollutant such as Cr, Ni from runoff erosion mainly enriched in the wetland sediments in the periods of flood season. Since the Yangtze Estuarine wetland sediments showed weak alkaline, the release rate of Cd was very low. In the case of vertical profiles change of heavy metals in sediment, the content of Cd,Cr, Cu,Pb,Zn,Ni showed multi peak patterns. The contents of heavy metals in sediment had a decreasing trends from the bottom to the top.The trends of heavy metal contents reflected a great impact of artificial pollution and early diagenesis. Grain size, the redox process and the nonlinear sedimentation of East Tidal Wetland are important factors in vertical distribution. The average concentration of trace elements of Zr、Sr、Rb are 278.44、205.74、151.79 μg/g, the mineral elements of K、 Ca、Na are Yangtze wetland sediment was characterized by high Ca content.(5) During the short time scale(from August 28th,2012 to September 2th,2013), the average accumulation rate of organic carbon in Chongming East Tidal Flat Wetland was calculated as approximately 760.19 tC/(km2·yr). Combined with the study area, there was 0.34 Mt/yr organic carbon depositions in East Tidal Flat Wetland, account for 15%～20% of the organic carbon transport fluxes from the Yangtze mainstream to the East China Sea in flood and dry season. High accumulation rate of organic carbon is the key factor which determines carbon sink function of Yangtze Estuary. The average accumulation rates of Cd, Cr, Cu, Pb, Zn, Ni in Chongming East Tidal Flat Wetland were calculated as approximately 40.54, 4457.20,1889.38,1033.66,4548.51,2092.92 mg/(m2·yr). Combined with the study area, the burial fluxes of Cd, Cr, Cu, Pb, Zn, Ni were 1.84,201.87,85.57,46.81, 206.00,94.79 t/yr respectively in East Tidal Flat Wetland, account for 5.3% of the heavy metals transport fluxes from the Yangtze mainstream to the East China Sea.The innovation point of the research is about:(1) Ignoring the influence of exogenous sediment input, by using the feldspar pipe bury method, The contribution of Scirpus mariqueter to the organic carbon storage of Chongming east tidal flat wetland sediment was calculated by using the reference on a simple two component mixing equation.(2) The short-term (≤1 year) marsh accumulation rate of Chongming East Tidal Wetland was measured by feldspar marker horizons. With the third largest sediment-load river in the world-Yangtze, the burial rate and burial flux of organic carbon and trace elements were calculated. |
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