Marine diatoms take up silicic acid for the buildup of theiropaline shells and discriminate against the heavier siliconisotopes. Rayleigh distillation in the upper mixed layer duringthe growing season increases the mean isotopic compositionof diatom shells. In principle it should be possible toreconstruct the relative utilization of silicic acid from theisotopic composition of diatom shells archived in sediments.In the real ocean, however, several other factors perturb thissimple picture. The impact of mixing, advection andremineralization on the isotopic composition of silicic acidand diatom shells have been investigated by integration andanalysis of a biogeochemical general circulation model aswell as a conceptual model. We will discuss the applicabilityof silicon isotopes in the Southern Ocean.