Seawater-mediated interactions between diatomaceous silica and terrigenous sediments: Results from long-term incubation experiments

Reactors containing frustules of the cultured diatom Thalassiosira punctigera suspended in seawater were incubated with or without added sediment from the Mississippi River Delta or the Congo River Fan. The diatom frustules were separated from the terrigenous sediments by a dialysis membrane, thereby only allowing the exchange of dissolved species. One series of incubations was carried out in the laboratory, at room temperature (21 °C) and for a period of 10 months. Another series of reactors was deployed along a mooring in the Mozambique Channel at three water depths (500, 1250, and 2000 m), for a period of 22 months. Chemical analyses after total destruction of frustules collected at the end of the incubations showed elemental transfer from seawater (Mg and K) and the sediments (Al, Fe, Mn, P and Ca) to the frustules. In the presence of the terrigenous sediments, the dissolved silicate concentrations at the end of the incubations were systematically lower that those measured in the incubations without the sediments. In addition, electron microscopy revealed the formation of new mineral precipitates. These included amorphous deposits on the frustules containing Si, Fe, Al, Mg, K and P, as well as euhedral clay crystallites. Differences were observed between the incubations performed in the laboratory and those deployed at sea, likely as a result of differences in redox conditions, temperature and reaction time. Overall, the interactions between biogenic silica, seawater and lithogenic minerals reduce the regeneration of nutrient silicon fixed by siliceous organisms. These interactions take place on relatively short time scales (months to years), and affect not only the marine cycle of silicon, but also those of other major and minor elements, such as Al, Fe, Mn, K, and Mg.