A Study On The Dissolution Behavior Of Biogenic Silica In Coastal Sea

Biogenic silica (BSi) is produced in the surface layer of the world's ocean mainly by diatoms, which are the predominant algae in the ocean, espicially in the coastal seas. Diatoms play an important role in the carbon export not only from euphotic layer to deep sea, but also to the higher trophic levels. Due to long-term decreases in riverine Si:N and Si:P nutrient ratios, changes in phytoplankton dynamics and food webs have been observed in many coastal ecosystems. Therefore it's necessary to study the dissolution behavior of BSi in the offshore sediment.We established a new technology, the continuous flow-through experiment, to study on the dissolution behavior of BSi in offshore sediment in our lab on the base of alkaline dissolution experiment and batch experiment.Alkaline dissolution experiment is used to determined the content of BSi in offshore sediment and also offer the specific dissolution rate constant in the condition of 85℃2% Na2CO3. We found the BSi in offshore sediment have different phases, one of them avoided from dissolution in natural seawater and there was change of struture in the surface of BSi as for the aging effect.The batch experiment is also show us the way to unstanded the BSi dissolution behavior in offshore sediment. The BSi in the sediment cannot reach its solubility in seawater. The offshore sediment also has effect on the fresh diatom's preservation in sediment.Our study shows that the dissolution behavior of BSi is affected by sediment type, the highest specific dissolution rate constant of BSi is observed in sand sediment (6.88 yr-1 in Hongshiya tidal flat), which is close to the suspended BSi's in the surface seawater. As for its high specific dissolution rate, most reactive BSi has dissolved, therefore the dissolution rate of BSi is also low. Maybe due to the effect of clay, the specific dissolution rate constant of BSi is lower in silt-clay sediment and it's helpful for BSi preservation in sediment.when fresh diatom deposit to seabed sediment, the apparent dissolution rate of BSi in sediment can be increased. As a result, concentration of DSi in pore water is high, but the surface reactive site on fresh diatom can be decreased due to the formation of authigenic clay in high concentration of DSi. Therefore, fresh diatom can be preserved in sediment. The solubility of BSi in sediment decreases from the water column to deeper sediment.