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 (δ¹⁵N) 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 δ¹⁵N. PF tissue δ¹⁵N 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 δ¹⁵N of shallow-dwelling species was 1–2.4‰ lower than local subsurface nitrate δ¹⁵N. This likely reflects the documented tendency for upper ocean N pools to decrease in δ¹⁵N under intense euphotic zone N recycling. There may have also been low-δ¹⁵N inputs from N₂ fixation and/or atmospheric N deposition. On the continental slope, in a mixing-associated productivity plume, PF tissue δ¹⁵N 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 δ¹⁵N to prevailing N cycling processes. Existing sediment data indicate that δ¹⁵N of PF microfossils dominantly reflects the δ¹⁵N 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 δ¹⁵N.