The fractionation of silicon isotopes by diatoms during silicification (i.e., opaline cell wall formation) provides a new tool for paleoceanographic studies of the silicon cycle. Here we examine the natural variations of the 30Si:28Si ratio in silicic acid (Si(OH)4) and biogenic silica (bSiO2) in surface waters of the Antarctic Circumpolar Current (ACC) along 170¿W. The results provide direct evidence of biologically mediated fractionation of silicon isotopes, with an enrichment factor ($varepsilon$) of between -1.1 and -1.9?, depending on the model employed. Comparison of the mass flux of bSiO2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles. We calculated Si(OH)4:NO3- utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4:1 in this region. This work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative Si(OH)4 utilization in surface waters.