Treatment of diatom microfossils from Southern Ocean sediments with hot perchloric acid leaves a diatombound N fraction which is 0--4? lower in Δ 15N than the bulk sediment, typically 3? lower in recent Antarctic diatom ooze. Results from Southern Ocean surface sediments indicate that early diagenetic changes in bulk sediment N content and Δ 15N are not reflected in diatom-bound N, suggesting that diatom-bound N is physically protected from early diagenesis by the microfossil matrix. A meridional transect of multicores from the Indian sector of the Southern Ocean shows a northward increase in the Δ 15N of diatom-bound N, suggesting that diatom-bound Δ 15N, like bulk sedimentary Δ 15N, varies with nitrate utilization in the overlying surface waters. The Δ 15N of diatom-bound N is 3--4? higher in glacial age Antarctic sediments than in Holocene sediments, supporting the hypothesis, previously based on bulk sediment Δ 15N, that nitrate utilization in the surface Antarctic was higher during the last ice age. While there are important uncertainties, the inferred range of utilization changes could potentially explain the entire ~80 ppm amplitude of observed glacial/interglacial variations in atmospheric CO2. ¿ 1999 American Geophysical Union