Study On The Biogeochemistry Of Sedimentary Silica In The Pearl River Estuary, China

Silicon, which represents 27% of the lithosphere, is an important component in the marine ecosystem. About 80% inputs of silicic acid to the world ocean come from rivers. Estuarine environments are potential locations for labile silica burial due to removal of dissolved silica during mixing processes of fresh and brackish water and deposition of biogenic opal, where commonly supporting abundant diatom blooms. However, small quantities of opaline silica detected by most operational analytical techniques exhibit that deltaic environments are relatively minor sites of biogenic silica burial, compared to abyssal sedimentary environments (e.g., the South Ocean, the northern Pacific Ocean). On the one hand, it lies in that high terrigenous debris flux dilutes biogenic silica content; on the other hand, it is due to reverse weathering of opaline silica, which is wildly present in tropical, subtropical deltaic environments. In high-turbidity shallow estuaries, the surfaces of biogenic silica particles are often quickly coated with aluminosilicate or Fe, Al-rich coatings, dramatically affecting solubilities and dissolution behavior of Si in both the oceanic water column and sedimentary deposits. These particles buried in reductive or suboxic delta deposits may be gradually altered into authigenic K-Fe-rich aluminosilicate minerals during the early diagenetic processes and be disseminated in the seasonally reworked sediment.In this thesis, the silicon release from the surficial sediments of the Pearl River estuary is quantitatively studied that are exposed in vitro under different environmental conditions of agitation time, pH value, and salinity. The consequent speciation distributions of silicon in the leached sediments are also analyzed using a sequential extraction method (Tessier et al., 1979. Analytical Chemistry 51(7), 844-851) through comparing with the silicon speciation in the initial (unleached) sediments. As to biogenic silica buried in the sediment cores, a wet-alkaline extraction method (Al correction) proposed by Ragueneau et al. (2005) (Continental Shelf Research 25(5-6), 697-710) is modified to remove metal-rich coatings on sedimentary particulates and then is performed; meanwhile, a mild acid-mild alkaline sequential extraction (Michalopoulos and Aller, 2004. Geochimica et Cosmochimica Acta 68(5), 1061-1085) is also adopted to analyze reactive silica. Results show that, silicon bound to Fe, Al oxides in the coatings of sedimentary particles, accounting for 30% of total reactive silica, derives from the removal of riverine dissolved silica. About 60% of biogenic silica is altered into authigenic aluminosilicates during the early diagenetic processes. The supply of biogenic silica and the presence of Fe, Al-oxide rich debris derived from the drainage basin are major controls on the amount of clay formed. Inclusion of authigenic alteration products of biogenic silica in estimates of reactive Si burial will markedly increase the deltaic storage of riverine Si. Deltaic environments are important sinks for silica. The lack of consideration for reverse weathering processes in deltas implies that the burial of biogenic silica and uptake of major elements therein may be substantially underestimated, and further suggests that global budgets for silica and associated elements are inadequate.