The marine Silicon cycle
There is a close coupling of silicon and carbon in global biogeochemical cycling. About three quarters of the primary production in coastal and nutrient replete areas of the world oceans is carried out by diatoms, a phytoplankton group that essentially needs silicon (Si) for the build up of their opaline shells. In low nutrient areas diatoms still contribute to about one third of the marine primary production. Silicic acid, Si(OH)4 and its ions, is the biologically available form of Si in the marine environment and its surface water concentration can severely limit diatom biomass build up. Hence, efforts to understand the marine carbon cycle should also take into account the silicon nutrient cycle. Three stable isotopes of Si, 28Si, 29Si, and 30Si, exist with 28Si being the most abundant (92.2 % to 4.7 % to 3.1 %). Diatoms taking up silicic acid prefer the lighter isotope 28Si and, thus, progressively enrich the surface ocean silic acid in 29Si and 30Si relativ to 28Si.