Award: OCE-2049416

The nitrogen and oxygen isotopic composition of nitrate (NO3-) in ocean can provide insights into modern ocean nutrient cycling and biological productivity, and these measurements also provide a basis for reconstructing past ocean changes using coral cores and microfossils buried in marine sediments. This project generated high-precision measurements of nitrate nitrogen and oxygen isotopes (δ¹⁵N and δ¹⁸O) along the U.S. GEOTRACES GP17-OCE Pacific transect (67°S–20°S) and integrated the data with existing measurements from the U.S. GEOTRACES GP15 transect to produce a continuous north-to-south view across the Pacific Ocean. The processes reflected included biological production in the surface ocean, the sinking of organic matter out of the surface ocean and interior, respiration and remineralization of organic matter by aerobic and anaerobic metabolisms in the interior, and ocean circulation and mixing at all levels.

Because the transect crosses multiple oceanic fronts and water-mass boundaries where nitrogen processes overlap, interpretation was aided by grouping samples using two complementary approaches. One approach classified samples based on geographic position and seawater density, while a second applied k-means clustering using multivariate biogeochemical properties alone. Despite relying on independent information, the two approaches yielded broadly consistent groupings, highlighting the close coupling between physical ocean structure and biogeochemical signals.

Overall, subgroup mean compositions lie close to a 1:1 δ¹⁵N–δ¹⁸O relationship, indicating that nitrate consumption provides the dominant isotopic imprint. while regional deviations record the overlapping influences of circulation-driven nitrate transport and in situ regeneration. Upper Circumpolar Deep Water (UCDW) is used as an isotopic reference point because it represents the dominant source of nitrate supplied to the Southern Ocean surface, from which much of the Pacific interior nitrate is ventilated through mode and intermediate water formation. Relative to UCDW, Subantarctic Mode Water and Antarctic Intermediate Water show strong nitrate drawdown and increases in both δ¹⁵N and δ¹⁸O near their formation regions due to nitrate assimilation by phytoplankton. Nitrate δ¹⁸O increases more than δ¹⁵N, signaling seasonal upper ocean cycling between partial nitrate assimilation and remineralization of the low-δ¹⁵N organic nitrogen back to nitrate. Remineralization of low-δ¹⁵N sinking organic N in the deep ocean lowers the nitrate δ¹⁵N of Antarctic Bottom Water, with a much weaker effect on δ¹⁸O. Farther north, waters show progressively higher δ¹⁵N and δ¹⁸O and increasing δ¹⁵N–δ¹⁸O difference, indicating nitrate supply to the lower latitude surface, biological uptake, sinking of resulting organic matter, and remineralization back to nitrate at depth (i.e., the conversion of preformed nitrate to to regenerated nitrate). Stepping back to a broader picture, almost all of the nitrate isotopic variation in GP17-OCE relates to the partial nitrate uptake in the Southern Ocean surface and the evolution of its signals as nitrate is transported into the lower latitude ocean. Water column denitrification, a critical mechanism of nitrate removal from the ocean, is only a minor driver of nitrate isotopic variation across the GP17-OCE section, which is a somewhat unexpected result.

Last Modified: 01/29/2026
Modified by: Daniel M Sigman