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 δ¹⁵N 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 δ¹⁵N 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 δ¹⁵N of diatom-bound N, suggesting that diatom-bound δ¹⁵N, like bulk sedimentary δ¹⁵N, varies with nitrate utilization in the overlying surface waters. The δ¹⁵N 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 δ¹⁵N, 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.