| Excessive phosphorus in seawater will lead to eutrophication,hypoxia and other environmental problems,while the upward release of internal phosphorus in coastal sediments is an important source of phosphorus in seawater.Dissimilar iron reduction and sulfate reduction under anoxic condition are not only important paths for early diagenesis of organic matters in coastal sediments,but also main ways for remobilization of solid active phosphorus.However,the presence of iron oxides effectively controls the adsorption and passivation of phosphorus under oxic condition.The redox cycle of iron and sulfur are key factors driving the phosphorus cycle and exchange between the sediment-water interface.However,under the influence of rapid global changes and human activities,the sedimentary environment in sensitive coastal zones has become more dynamic and complex.The coupling relationship between phosphorus,iron and sulfur in sediments as well as the biogeochemical cycle mode may have undergone great spatio-temporal changes,and the material exchange with the overlying water may also be more active and complex.Nevertheless,there are still some problems in understanding how various environmental factors affect the biogeochemistry of phosphorus,iron and sulfur and how they interact with the overlying water materials under different spatial and temporal scales.With the support of the National Natural Science Foundation of China:"The geochemical processes and substance transportation of submarine groundwater discharge(SGD)and associated contribution and impacts on the coastal ocean" and"Study on submarine groundwater discharge and related water cycle processes in the Qianhu Bay,Fujian",some muddy coast of Xiamen Bay were chosen,and the in situ high-resolution HR-Peeper and DGT technology were applied in this thesis to,describe concentration-depth profiles of sensitive components for phosphorus,iron and sulfur in surface sediments and porewater in coastal zones,analyze the variation rules and influencing factors under different spatial and temporal scales via high frequency sampling.On this basis,the iron-sulfur redox cycle and accompanied phosphorus cycle model under different spatial and temporal scales were preliminarily summarized.Finally,the molecular diffusion flux of phosphorus and hydrological exchange flux of phosphorus related to biological disturbance were evaluated and compared,respectively.The main findings are as follows:(1):The phosphorus concentration in the overlying water of each study area exceeded the standard to some extent,indicating that the phosphorus pollution in Xiamen Bay area is serious.Under anoxic condition,the availability of ASC-Fe and TOC is the main limiting factor of iron reduction and sulfate reduction,respectively,which controlling the spatial distribution of dissolved Fe and DGT-Labile S,respectively.Sulfate reduction showed more temperature-dependence and obvious competition with iron reduction,that is,sulfate reduction rate was higher in spring and summer and lower in autumn and winter,while iron reduction rate was higher in autumn and lower in winter and moderate in spring and summer.(2):Preliminary studies on phosphorus and sulfur at 5 sites in Xiamen Bay showed that DGT-Labile P and DGT-Labile S presented a significant trend of synchronous change,which was attributed to sulfate reduction-induced mineralization of active organic phosphate and reduction dissolution of Fe-P,leading to synchronous activation of phosphorus and sulfide.In addition,the reoxidation of sulfide induced the adsorption and passivation of phosphorus by sediments under the oxic condition,leading to the synchronous removal of phosphorus and sulfide.(3):According to results in the Jiulong River Estuary,dissolved Fe,DGT-Labile S and SRP showed a relatively consistent and synchronous change trend among each other,indicating that the overall strength of iron reduction and sulfate reduction was not much different,both of which occurred together in local micro-environment and participated in the reactivation of phosphorus.Changes in the redox condition caused by variation of tidal intensity and biological disturbance in intertidal zone are important factors affecting iron reduction and sulfate reduction.Adsorption and passivation of phosphorus in the surface oxic zone limited the release of phosphorus to the overlying water,which accumulated continuously in the anoxic zone and was finally released to the overlying water during the river flood season.(4):According to results in the Eastern coast of Xiangan,SRP and dissolved Fe showed a very consistent and synchronous change trend,indicating that the iron redox cycle dominates the adsorption/passivation and desorption/reactivation of phosphorus.The competition and zonation between iron reduction and sulfate reduction were obvious in the vertical direction.Sulfate reduction dominated the surface layer,while iron reduction dominated the subsurface layer and below.The diffusion flux of SRP were negative in most months,indicating that SRP were mainly resupplied from overlying water to sediments.The partial anoxic in surface layer was not conducive to phosphorus accumulation,which lead to the low concentration of SRP and DGT-Labile P,as well as the lack of regularity for monthly distribution.(5):According to results in the Jiuxi River Estuary,the change trend of SRP and dissolved Fe was inconsistent,while DGT-Labile P and DGT-Labile S showed an extremely significant consistent change trend almost all year round,indicating that the strong sulfate reduction and sulfide accumulation dominated the reactivation of phosphorus in sediments.However,as the reduction rate of sulfate decreased in autumn,the iron reduction increased rapidly and co-precipitated with the high concentration of SRP.In winter,as the reduction rate of iron decreased,the precipitated vivianite in turn supplied SRP and DGT-Labile P through precipitation dissolution.(6):Using the DIFS model to simulate the phosphorus adsorption-desorption dynamics between surface sediment and porewater,the results showed that in July,from the Jiulong River Estuary to the Eastern coast of Xiangan and then to the Jiuxi River Estuary,the SRP resupply capacity from sediments to porewater increased successively.The redox conditions,iron reduction and sulfate reduction,iron oxide content and capacity on phosphorus adsorption and other factors cumulated influenced the adsorption and desorption kinetics of phosphorus.(7):DGT-Labile S and DGT-Labile P in intertidal surface sediments were enriched during FT and removed during ET,while the cave sediments basically showed opposite changing characteristics.It not only confirmed and revealed the important influence of tidal movement on the geochemical behavior of phosphorus and sulfur,but also showed that the superposition of biological disturbance and tidal movement affect the geochemical behavior of phosphorus,iron and sulfur,as well as greatly enhance the phosphorus exchange flux between sediments and overlying water through hydrological exchange.In addition,the results of tidal simulation experiments further confirmed and discovered the important influence of tidal movement on the geochemical behavior of phosphorus,iron and sulfur.On this basis,it is concluded that the early diagenetic model dominated by oxidation and reduction of iron in sediments is conducive to control the reactivation and release of internal phosphorus.To sum up,the biogeochemistry of phosphorus,iron and sulfur in the coastal sediments and the material exchange with the overlying water in Xiamen Bay are affected by many environmental factors and show a certain cycle with the changes of these factors in different spatial and temporal scales.In this thesis,we summarized several kinds of iron-sulfur redox cycle and accompanied phosphorus cycle model under different sedimentary environments,as well as the cycle model under the short time scale of local microenvironment,which provides scientific basis for further understanding the early diagenesis of marine sediments and the release of internal phosphorus. |
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