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. Therefore, the silicon isotope distribution in the surface ocean contents information on the Si utilisation by diatoms. Since the diatom shells, as they are buried in the ocean sediments, reflect this distribution, the analysis of these shells can give information on the silicon utilisation at the time of shell formation, i.e. also the geological past. Up to now there has not yet been a lot of Si isotope measurements. One way to gain more information is with the help of biogeochemical ocean circulation models. One of these models, the Hamburg Model of the Ocean Carbon Cycle (HAMOCC) by Ernst Maier-Reimer, is also run in our project group. Through the implementation of isotope fractionation during opal formation the model calculates a possible silicon isotope distribution during both, modern times and at the last glacial maximum (20000 years ago). The combination of model results with the available water column and sediment measurements will help to constrain the use of silicon isotopes as a paleoproductivity tracer.