Role of marine silica and carbon in biogeochemical cycles
The MUSICC (Multi-proxy approach of marine silicon and carbon biogeochemical cycles) project was based on two distinct research axes. The first axis involved Si and C cycles in the Southern Ocean and was developed through two international projects, KEOPS-2 and SAZ. KEOPS-2 investigated a naturally iron-fertilised phytoplankton bloom in the Southern Ocean (Kerguelen Plateau), which is dominated by diatoms, a group of algae that requires Si to build their glass frustule. Results highlighted the importance of summer Si supply, which enabled scientists to propose a seasonal budget of the Si cycle above the Kerguelen Plateau, one of MUSICC's specific objectives. This budget includes winter and summer Si nutrient supply from the lower water column and bulk and net silica production as well silica export. The SAZ project used Si isotope measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, polar frontal and subantarctic) of the open Southern Ocean. Variations in the exported flux of biogenic silica revealed the sinking rate of diatoms decreases to very low values in winter, enabling particles to remain in the water column until the following spring. This is relevant to the carbon biological pump, which transfers carbon from the surface to the deep ocean, where carbon is stored. The second research axis concerned the Si cycle at the interface between the continents and the ocean. Previous studies have shown that dissolved Si isotope (d30Si) is controlled by land use at the sub-basin level, and therefore it can be used as a proxy sensitive to anthropogenic pressures on the Si cycle. MUSICC collaborated with Indian scientists to sample 18 estuaries in both the dry and monsoon season, in order to study the Si cycle. Results indicate that the Si source to the estuary is insensitive to the season and is little changed during the monsoon. Therefore, Si does not appear to be the limiting nutrient in Indian estuaries, and Si isotopes of rivers upstream of estuaries are more related to weathering and land use than seasonal discharge. The MUSICC project showed that natural isotopic composition of Si allows identification of Si source and quantitative estimates of Si fluxes and mixing on an annual scale. In addition, Si isotopic dilution is the most precise and sensitive technique for determining short-term uptake-dissolution fluxes.
Keywords
Carbon, biogeochemical cycles, silicon, MUSICC, Southern Ocean
