Response of the Nitrogen Isotopic Composition of Planktic Foraminifer Tissue to Surface Ocean Nutrient Cycling: A Case Study in the Northern South China Sea

This study presents data on geographically small‐scale patterns of nitrogen (N) isotope signals (δ 15 N) within the northern South China Sea (SCS) imprinted on planktic foraminifers (PF). PF from net tows on the shelf, continental slope, and in pelagic waters from summer 2019 were analyzed for δ 15 N. PF tissue δ 15 N was observed to diverge from the subsurface nitrate supply. The divergences were shared by PF species at a given station and depth, and differed among regions, indicating an environmental (as opposed to physiological‐biochemical) driver. Moreover, the divergences were consistent with N cycle processes occurring in the respective oceanographic environments. In the oligotrophic open northern SCS, the PF tissue δ 15 N of shallow‐dwelling species was 1–2.4‰ lower than local subsurface nitrate δ 15 N. This likely reflects the documented tendency for upper ocean N pools to decrease in δ 15 N under intense euphotic zone N recycling. There may have also been low‐δ 15 N inputs from N 2 fixation and/or atmospheric N deposition. On the continental slope, in a mixing‐associated productivity plume, PF tissue δ 15 N was higher and varied around the subsurface nitrate supply, reflecting the ecosystem's consumption of nitrate entrained from the subsurface. Our data highlight the sensitivity of PF tissue δ 15 N to prevailing N cycling processes. Existing sediment data indicate that δ 15 N of PF microfossils dominantly reflects the δ 15 N of nitrate consumed in surface waters, but upper ocean studies such as ours show effects from seasonal and shorter‐duration processes, that may lead to second‐order controls on sedimentary PF δ 15 N.