Marine diatoms take up silicic acid for the buildup oftheir opaline shells and discriminate against theheavier silicon isotope. For the frst time, the overalloceanic distribution of silicon isotopes has beenestimated by integration of the Hamburg Model ofthe Ocean Carbon Cycle, version 4 (HAMOCC4). Itis shown that the relationship between the silicicacid concentration and its silicon isotopecomposition is not a simple Rayleigh distillationcurve. Only the Southern Ocean and theEquatorial Pacifc show a clear functionaldependency similar to the Rayleigh distillationcurve. Model results can be used to predict opalsilicon isotope compositions in the sediment andconstrain the use of silicon isotopes as a proxyfor silicic acid utilization. Due to thestructure of the Pacific current system it might bevalid to apply a relationship between surfacesilicic acid concentrations and the silicon isotopesignal in the equatorial Pacifc sediments.