A Test of the Diatom-Bound Paleoproxy: Tracing the Isotopic Composition of Nutrient-Nitrogen Into Southern Ocean Particles and Sediments

Sedimentary nitrogen isotope (as delta N-15) records from the Southern Ocean provide critical constraints on surface nutrient consumption in the past and the role of Southern Ocean biophysical changes in setting atmospheric pCO(2). We present a field assessment of how surface nitrate consumption is reflected in delta N-15 values of total nitrogen and diatom-bound nitrogen pools of particles and sediments across the Southern Ocean along 170 degrees W during late austral summer. Mixed layer nitrate delta N-15 values increase northwards associated with greater nitrate drawdown. Particles and sediments are expected to follow this trend. Contrary to expectations, surface ocean particle total nitrogen and diatom-bound delta N-15 values decreased northward during the late summer, likely due to recycling of nitrogen and the assimilation of regenerated ammonium, as well as nitrate. The relationship between delta N-15 values of the total nitrogen and diatom-bound pools remains relatively constant across this Southern Ocean transect, suggesting that the isotopic composition of these two surface ocean nitrogen pools are largely set by the delta N-15 value(s) of the assimilated nutrient(s). Surface sediment delta N-15 values do increase away from the region of maximum biogenic silica deposition, suggesting that the recycled nitrogen isotopic signal observed in late summer particles may not significantly impact the sedimentary record. However, the enrichment in delta N-15 values of the diatom-bound pool is greater than what is expected from progressive utilization of the surface nitrate alone and not yet explained.