The Study On BSi In The Water Column And Sediment Of Tonkin Gulf

Biogenic Silica (BSi ) is a sort of amorphous silica from diatom, radiolarians, sponges and other siliceous organisms. BSi may reflect the spatial and temporal distribution of primary productivity and the environmental changes of aquatic systems. Presently, BSi has been widely used as a proxy of paleoceanography events and paleoclimate. Furthermore, the cycle of BSi is the important section of Si cycle in the sea and BSi has a close coupling in the global carbon cycle.In this study, in order to improve the BSi determination method suited for Tonkin Gulf sediment samples, we screened out the BSi extraction conditions through experiments, and found that continuous shaking in BSi-extracting process and using round bottom centrifuge tube as reaction vessel could improve BSi extractive efficiency. In the ameliorated method, 0.5 mol/L Na₂CO₃ is used as extracting solution and 50 mL round bottom centrifuge tube as reaction vessel, the solid/liquid ratio is 1 g/L, and sediment samples are digestd for 5 h at 90°C. The method was validated by measuring the interlaboratory comparison sediment samples with it, and the results were in the range of standard content. In the study, a technique was also established to distinguish the diatom sourced BSi from total BSi in the sediment by different extracting time, thus to coupling BSi in sediment with the primary productivity in water column more directly.In 2007, two cruises for BSi investigation at Tonkin Gulf had been carried out in spring and autumn, and the results are as followes:1. In the study area of Tonkin Gulf, the concentration of PBSi in the water column was higher in the northern gulf and in the nearshore waters along Hainan Island than that in the southern gulf and in the offshore deep waters. Due to comparatively sufficient vertical mixing in the water column in shallow waters, PBSi showed no obvious difference in the surface and bottom of the euphotic zone in the northern gulf and the nearshore waters along Hainan Island. The seasonal fluctuation revealed that the PBSi concentration was higher in autumn than in spring, with positive correlation to nutrients, chlorophyll a concentration and primary productivity in the water column.2. The BSi content of surface layer sediments in Tonkin Gulf was 0.58 %～1.68 %, 1.1 % in average, which is close to value in the southern continental shelf of the South China Sea. The horizontal distribution of diatom-sourced BSi (DBSi) was similar to that of total BSi (TBSi), showing higher value in the surface sendiemnts of the northern gulf and the nearshore waters along Hainan Island than that in the central gulf and southern gulf deep waters.3. According to the vertical profile of BSi value in the sediment samples and the proportion of DBSi to TBSi, the sedimentary rate of Tonkin Gulf was estimated to be 2.0～3.0 cm/yr in avarage, with higher rates in the shallow nearshore waters than that in the deep offshore waters. The BSi recovery rate, which reflects the release of BSi from sendiment via dissolving, in sediment of Tonkin Gulf was calculated to be 0.49 %～1.52 %. Higher recovery rate of BSi was found in the sediments of the northern gulf and the nearshore waters along Hainan Island. Sediments in the central gulf deep waters had lower BSi recovery rate. However, it seemed that sediment in the southern gulf deep waters also had high BSi recovery rate, maybe due to the higher salinity in this region.4. In most stations, the content of TBSi and DBSi was found to increased gradually from sendiment surface to more than 14 cm in depth, indicating that, in most parts of Tonkin Gulf, the amount of sedimentary diatom in sediment was decreased in recent years. This phenomenon implies that there maybe a descending trend for the biomass of diatom and other siliceous organisms in the plankton community of Tonkin Gulf.