Silicon isotopes provide a useful means of tracing the silica cycle both in the modern day and for times in the past. Silicon isotopes are fractionated during the formation of biogenic opal and possibly also during weathering and clay formation. Fractionation factors vary from about -1 permil to -4 permil, generally averaging about -1 permil. This fractionation of isotopes during the movement fo silicon from different reservoirs results in the variationd in d30Si of about 7 permil observed among marine and terrestrial samples. Rayleigh distillation of the isotopes during nutrient utilization by phytoplankton means that silicon isotopes can be used to track changes in the draw down of nutrients in the ocean. d30Si of diatoms in the Southern Ocean indicate that percent silicic acid utilization was at its lowest point over at least the last 80 000 years at the Last Glacial Maximum, 20 000 years ago. The analysis of sponge spicules from ODP site 689B (Maud Rise, Antarctica) suggests both that the reconstruction of deep water d30Si is possible and that the use of considerably older samples (e.g. from about 34 Ma) is plausible. From d30Si of sponges, which live in the relatively stable deep sea and are present several hundred million years earlier than diatoms in the fossil record, it may be possible to identify global changes in the silica cycle associated with climate change or the appearance and radiation of the diatoms.