Biogenic Silica Morphology And Composition In Different Environments And Its Dissolution Process

Silicon is one of the key element of matter cycle on earth surface and climate change due to its huge stock and interaction with carbon cycle. Biogenic silica(BSi) is the actual participants in the silica cycle and plays important role on terrestrial silicon transporting to the ocean, and thus its content and mechanisms of transport and transformation get increasing attention recently. In this paper, the content and composition of biogenic in four typical areas, i.e., the Ny-Alesund area in Arctic, the wetland in the Huanghe River estuary, the Changjiang River estuary and rivers in Malaysia, were studied, and also observed the process of BSi dissolution in different salinity. The main results were as follows:(1) The contents of BSi are various in different area. The average content in the topsoil of Ny-Alesund is 7.56 mg/g, while the topsoil in the Huanghe River estuary wetland has an average value of 9.91 mg/g. The BSi content in the water column of Changjiang River Estuary is 15.2 μmol/L, while its contents in core sediments are 2.73mg/g and 3.86 mg/g in B3 and C5 stations, respectively. The sediments in Malaysia rivers has an average BSi content of 8.55 mg/g. Being affected by vegeration, p H and particle size of soil, the BSi content has a seasonal and regional distribution in the soils; while,ocean and river have joint influence on BSi in wetland and estuary.(2) Reverse weathering of reactive silica could be obviously observed in the water column and sediments of Changjiang River Estuary, while the authigenic mineralization of BSi are negligible in water column, and ~28.0 % of BSi are involved in reverse weathering in sediments of Changjiang River estuary.(3) The biogenic silica particles are mainly comprised by phytolith, phytoplankton and sponge spicule, however, the contribution of them to BSi is different in various areas.The terrestrial BSi(i.e. phytolith) are the main form of BSi particles in most stations,while diatoms contribute more in estuary area. The size of phytoliths are in the range of5-150 μm, and are mainly in forms of elongate, elongate echinate, rondel, cuneiform andLanceolate.(4) Amount of erosion and dissolution signs of phytoliths and diatom segments could be obversed by the scanning electron microscope, which prove that part of BSi particles have dissolved during its transport. Expect for Si and O, elements of C、Al、K、Mg、Fe could also been detected in some phytoliths by X-Ray diffraction, which showed direct evidences of reverse weathering and carbon sequestration.(5) BSi particles showed the same dissolution trend in water with different salinity.BSi dissolved rapidly in the initial stage of the incubation experiments, then its specific dissolution rates decreased sharply and reached an equilibrium state in final. The accumulated leaching amount and specific dissolution rate of BSi were the highest in salinity of 5 system within all the incubations of this study.