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

Abstract

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.