The isotopic composition of diatom‐bound nitrogen in Southern Ocean sediments

Treatment of diatom microfossils from Southern Ocean sediments with hot perchloric acid leaves a “diatom‐bound” N fraction which is 0–4‰ lower in δ 15 N 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 15 N 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 15 N of diatom‐bound N, suggesting that diatom‐bound δ 15 N, like bulk sedimentary δ 15 N, varies with nitrate utilization in the overlying surface waters. The δ 15 N 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 15 N, 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 CO 2 .