Variations in the silicon stable isotopic composition (δ30Si) of sedimentary biogenic silica (BSi) are used to reconstruct the utilization level of dissolved silicic acid (DSi) by diatoms in the geological past and to explore the influence of diatoms on past oceanic biogeochemistry and climate. A Last Glacial Maximum (LGM) climate simulation has been performed with a coupled ocean-sediment model that includes a prognostic formulation of BSi production with concurrent silicon isotopic fractionation. The model results show reduced DSi utilization by diatoms in high latitudes during the LGM, likely due to the extended ice cover in the model. There is a decrease in BSi export production in the Southern Ocean during the LGM compared to a present-day climate experiment. This leads to an increased equator-ward transport of DSi in Subantarctic Mode Water and Antarctic Intermediate Water, and a shift in the distribution of DSi from the deep Pacific into the deep Atlantic. The mean δ30Si value of DSi in the upper ocean shows a 0.14 per mil decrease in the LGM experiment, while there is an increase in the low-latitude Pacific compared with the present-day experiment. In the Pacific and Indian Ocean the slopes of the surface Si(OH)4~