An oceanographic tracer based on variations of silicon stable isotope natural abundances(d30Si)has been developed. A procedure for the purification, recovery, and determination of isotopic abundances of silicon from dissolved and particulate matter was formulated. The subsequent observation of silicon isotope fractionation by marine diatoms during opal formation raised the possibility that d30Si may be used as a proxy for silicic acid utilization. As a test of this, d30Si variations were measured in sedimentary opal. d30Si indicates lower silicic acid utilization at the last glacial maximum than during the present interglacial in the Southern Ocean, south of the present day polar front. Purification of silicon occurs through the dissolution of particulate silica, quantitative precipitation of dissolved silicon as triethylamine silicomolybdate, and combustion of the collected precipitate to form high purity silica. Purified silica is fluorinated under laser-power to form silicon tetrafluoride for isotopic analysis. Yields for silicon recovery are 99.9% for precipitation and >95% for the overall procedure. Reproducibility of the measurement is plus or minus 0.1 permil. Silicon isotope fractionation was observed for three species of marine diatoms, Skeletonema costatum, Thalassiosira weissflogii, and Thalassiosira sp., grown in batch culture. The nearly identical fractionation factors, alpha, averaged 0.9989 plus or minus 0.0004 (s.d., n=13), which corresponds to the production of opal that is 1.1 permil more negative than its dissolved silicon source. Alpha did not vary with temperature and the consequent change in growth rate (ANOVA, p=0.61; tested at 12, 15, and 22C).