Process And Behavior Of Terrestrial Reactive Silica In The Changjiang And Huanghe River And The Impact Of Silica Cycle In The Estuary

Silicon is a vital biogenic element which plays important roles in aquatic ecosystems.With the enhanced intensity of human disturbance on the major river system in recent decades,such as dam construction,increased release of nitrogen and phosphorus to the river and climate change,the transport process of silica carried with river water has deeply changed,which results in the variation of coastal ecosystem.However,the silica cycling in the river and coastal systems has poorly documented.It is therefore important to corporate the study of silica transport and transformation in the major river basin and its estuary,and to explore the response of ecological environment to the variation of silicon and carbon cycle.The corresponding study of silicon in the large dammed river has a great significance to reveal the impact of human activity on the coastal ecosystem and improve our knowledge of the biogeochemical process of silicon deeply.This study focused on the Changjiang and Huanghe River and the Changjiang Estuary.Based on the determinations of reactive silica,particle organic carbon and stable carbon isotope,the sources and distribution of reactive silica and the seasonal variation of reactive silica riverine flux to the sea in the Changjiang and Huanghe rivers have been discussed.Combined with the historical and regarding data,the major controlling factors of reactive silica flux to the sea and the tracer function of particle organic carbon and stable carbon isotope have been determined.Refer to the buried content and distribution of reactive silica in sediment corer off the Changjiang Estuary,the source and transformation of reactive silica and their controlling factors have been discussed combined with the observation of ²¹⁰Pb radioactivity and X-ray core scanning.Our result would provide the basis on the study of silica biogeochemical cycling in the basin and estuary and explore the evolution of coastal ecosystem.Major results in the thesis are summarized as follows:?1?The composition of reactive silica is quite different between Changjiang and Huanghe River;and concentration of dissolved silicate?DSi?in the low reach of Huanghe River is significantly lower than that in the Changjiang River,while the concentration of biogenic silica?BSi?in the low reach of the Huanghe River is three times higher than that in the Changjiang River.The atomic ratios of BSi/RSi were0.22 and 0.49 in the Changjiang River and Yellow River,respectively.The shares of RSi among the wet season,normal season and dry season were 5.3:3.1:1.6 and3.8:3.4:2.8 in the Changjiang River and the Yellow River,respectively.More than half of RSi discharge occurred during the wet season in the Changjiang River.The RSi export in the Yellow River mainly occurred during the water and sediment regulation.The flux of DSi in Changjiang and Huanghe River has changed dramatically in recent decades.Dam construction and climate change is the key point caused the decreasing of DSi concentration.Increased temperature promotes the ratio of silicate minerals weathering,which improve the export of DSi from the river basin.While damming effect can lead to the trapping of DSi and limit the contribution of weathering to the DSi flux to some extent.The combined effects of runoff and DSi concentrations resulted in the decreasing DSi flux in the Yellow River.?2?According to the spatial distribution,DSi concentration in upper reach was higher than that in middle and low reaches in the Changjiang River due to the retention effect of Three Gorges Dam?TGD?.The content of BSi is reversely related to the concentration of suspended particle matter?SPM?in the Changjiang River.The spatial distribution of DSi and BSi concentration are similar to the concentration of SPM in the Changjiang River,which are mainly controlled by weathering and erosion processes in the river basin.The major source of POC in the main stream of the Changjiang River was originated from terrestrial C3 plant,and the spatial distribution of POC was closely related to the concentration of SPM.Terrestrial POC input enduced by basion erosion was lower in tributaries compared to that of the main stream in the Changjiang River.In comparison with the Changjiang River,the relatively higher value of ¹³C_bulk in the Huanghe River indicated that terrestrial C4 plant input was an important source of POC in the Huanghe River.Generally,the share of phytolith from C4 plant to the BSi in the Yellow River is higher than Changjiang River.The spatial distribution of POC in Huanghe was controlled by the combined effect of plant species and soil erosion.Terrestrial phytolith could be an important source of BSi in the Changjiang River,and thus the contributions of phytolith in middle and low reach were higher than those in the upper reach and its regarding tributaries.Shares of POC_BSi fluxes to the POC fluxes in the Changjiang and Huanghe River are 0.09%and 0.02%,respectively.?3?The mineralization of BSi and RSi were 0.52 in the past 60 years.POC_BSi/POC_bulk in sediments in the Changjiang estuary was close to that in SPM in the Changjiang River.The shares of terrestrial input to the POC and BSi in sediment in the Changjiang estuary were 44-73%and 16-83%,respectively.Since 1980,burial rate of BSi decreased compared to that of C as a result of the changes in primary production in the Changjiang Estuary.The retention of DSi caused by the Three Gorge Dam accelerated the loss of burial efficiency of BSi.The fluxes of DSi released from pore water to bottom water at station A5-4 and A6-6 were 316?mol/?m²·d?and 159?mol/?m²·d?,respectively.According to the burial budget of RSi,BSi and amorphous were the major form in burial processes.About 20%RSi was transformed to Auth-Si during the burial process in the Changjiang River estuary.This study suggests that the human disturbance like as the riverine damming has important influence on the estuarine silica cycling.The alteration of river discharge would decline the burial of silica in the coastal area due to the change in primary production,which should be paid more attention in the future study.