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 parts per thousand lower in delta(15)N than the bulk sediment, typically 3 parts per thousand lower in recent Antarctic diatom ooze. Results from Southern Ocean surface sediments indicate that early diagenetic changes in bulk sediment N content and delta(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 delta(15)N Of diatom-bound N, suggesting that diatom-bound delta(15)N, like bulk sedimentary delta(15)N, varies with nitrate utilization in the overlying surface waters. The delta(15)N of diatom-bound N is 3-4 parts per thousand higher in glacial age Antarctic sediments than in Holocene sediments, supporting the hypothesis, previously based on bulk sediment delta(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 similar to 80 ppm amplitude of observed glacial/interglacial variations in atmospheric CO2.