Nitrogen isotopic composition of planktonic foraminifera from the modern ocean and recent sediments

We investigated the controls on the δ 15 N values of shell‐bound organic matter of planktonic foraminifera (foraminifera‐bound δ 15 N, or FB‐ δ 15 N). The bulk biomass δ 15 N of live foraminifera collected from plankton tows at Sta. S in the Sargasso Sea is within ∼1‰ of the FB‐ δ 15 N of the same species picked from surface sediments from the low‐latitude North Atlantic. The FB‐ δ 15 N value in the surface sediments is strongly correlated with the δ 15 N of thermocline nitrate, the dominant source of new N to the euphotic zone. The three euphotic‐zone‐dwelling, symbiotic, spinose species, Globigerinoides ruber , Globigerinoides sacculifer , and Orbulina universa , have a FB‐ δ 15 N similar to or slightly higher than that of the nitrate supply to the euphotic zone, whereas the deeper‐dwelling, non‐spinose, and/or asymbiotic forms have higher δ 15 N. In the Cariaco Basin sediment trap samples, the FB‐ δ 15 N of O. universa varies substantially (1.2‰ between the lowest and highest value), in some cases in step with δ 15 N changes in the bulk sinking N, while the subeuphotic‐zone‐dwelling, asymbiotic, and/or non‐spinose species are generally higher in FB‐ δ 15 N and less variable through the time series. The higher and less temporally variable FB‐ δ 15 N values of the deeper dwellers are consistent with their partial reliance on subsurface suspended particulate nitrogen, the δ 15 N of which is elevated and relatively stable over time. As an alternative, possibly additional explanation for the lower FB‐ δ 15 N of the euphotic‐zone dwellers (despite their likely tendency to feed on high‐ δ 15 N zooplankton), the dinoflagellate symbionts may reduce the δ 15 N elevation of a foraminifera's biomass relative to its diet by reducing the efflux of low‐ δ 15 N ammonium.