In the biogeochemical dynamics of marine ecosystems, silicon is a major element whose role has, for a long time, been underestimated. It is however indispensable to the activity of several biomineralizing marine organisms, some of which play an essential role in the biological pump of oceanic carbon. This book presents notions indispensable to the knowledge on the silicon biogeochemical cycle in ocean systems, first of all describing the main quantitative analysis techniques and examination of the major organisms involved in the cycle. The author then moves on to study the most up-to-date processes to control the use of silicon and its regeneration in natural conditions, before mentioning the central role played by this original element in the control of all the biogeochemical cycles in the global ocean. The available information finally enables the global biogeochemical budget of silicon in the marine environment to be quantified.

Silicon is among the most abundant elements on earth. It plays a key but largely unappreciated role in many biogeochemical processes, including those that regulate climate and undergird marine food webs. The Silicon Cycle is the first book in more than 20 years to present a comprehensive overview of the silicon cycle and issues associated with it. The book summarizes the major outcomes of the project Land-Ocean Interactions: Silica Cycle, initiated by the Scientific Community on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (ICSU). It tracks the pathway of silicon from land to sea and discusses its biotic and abiotic modifications in transit as well as its cycling in the coastal seas. Natural geological processes in combination with atmospheric and hydrological processes are discussed, as well as human perturbations of the natural controls of the silicon cycle.

In the biogeochemical dynamics of marine ecosystems, silicon is a major element whose role has, for a long time, been underestimated. It is however indispensable to the activity of several biomineralizing marine organisms, some of which play an essential role in the biological pump of oceanic carbon. This book presents notions indispensable to the knowledge on the silicon biogeochemical cycle in ocean systems, first of all describing the main quantitative analysis techniques and examination of the major organisms involved in the cycle. The author then moves on to study the most up-to-date processes to control the use of silicon and its regeneration in natural conditions, before mentioning the central role played by this original element in the control of all the biogeochemical cycles in the global ocean. The available information finally enables the global biogeochemical budget of silicon in the marine environment to be quantified.

Silicon is the second most abundant element in Earth's crust and one of the key nutrient in aquatic ecosystems. There are strong interactions of Si with carbon cycle and biogeochemical processes. The present thesis focused on variability of silicon (amorphous-ASi, lithogenic-LSi and dissolved-DSi) and Si isotopes along the land to ocean continuum. We investigated the seasonal and spatial variability of ASi, LSi & DSi and Si isotopes in ~20 Indian estuaries. We categorize the estuaries using statistical analysis (PCA and cluster analysis). Diatom uptake seems to be the main process controlling ASi during dry period, especially in the South. Weathering and erosion control the variability of LSi in the remaining estuaries. Similarly lithogenic supply controls Si during wet period in all estuaries and no impact of diatoms was seen because of high suspended load. Si isotopic compositions trace the Si sources and biogeochemical pathways. The isotopic results exhibit clear seasonal difference with high impact of type of weathering during both seasons. They show that southwest watersheds are very special in terms of weathering regime compared to the other watersheds because of topography and climate. The impact of agriculture and forest cover on Si cycle is also clearly evidenced in all the basins during wet period. We show that groundwater Si isotopic variability results from a combination of dissolution and production of minerals. Overall, this study shows the preponderant influence of weathering and type of secondary clays on Si isotopes irrespective to the seasons, rather than the biological uptake or mixing as reported elsewhere.

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