Award: OCE-1736652

Nitrate (NO3-) is the dominant form of "fixed" (biologically available) nitrogen in the ocean. The ratio of heavy to light stable isotopes of nitrogen (N) and oxygen (O) in nitrate reflects the processes that have produced, consumed, and modified thte nitrate. Our core goal in this project was to generate nitrate 15N/14N ratio (δ15N) and 18O/16O ratio (δ18O) measurements along the US GEOTRACES Pacific Meridional GP15 transect (Figure 1). The nitrate isotope measurements were merged with the other data generated from the GP15 expedition to investigate modern ocean processes and to provide ground-truthing of tools used for reconstruction of past ocean conditions and changes. Nitrate δ15N records a range of physical, biological, and chemical processes in the ocean that combine to form "feedbacks" (interactions among processes) that ultimately control the productivity of the ocean and the ocean/atmosphere partitioning of carbon dioxide. While nitrate consuming processes have a similar impact on the δ15N and δ18O of nitrate, nitrate consuming processes can affect nitrate isotopes differently. Thus, nitrate δ18O measurements are complementary to δ15N measurements, helping to separate and quantify overlapping imprints of multiple N fluxes. This is particularly important in the Pacific, which hosts all major N fluxes, including the processes of the so-called fixed N budget (N2 fixation and denitrification, the source and the sink of the global ocean?s r fixed N) and the processes of the ocean's internal N cycle (the cycling among different forms of fixed N; for instance, nitrate assimilation in which phytoplankton build their tissues using dissolved nitrate). The distribution of the nitrate isotopes along US GEOTRACES GP15 (extending from 60°N to 20°S of the Pacific Ocean at 152°W) reveals a strong North-to-South asymmetry, with the North and South Pacific being dominated by different N fluxes. In the southern equatorial-to-tropical Pacific, the impact of denitrification (signaled by a deficit in nitrate relative to phosphate that is also characterized by high nitrate δ15N and δ18O) is restricted to ~300 m depth (Figure 2). This implies that denitrification in the East Pacific alters the δ15N and δ18O of the background nitrate in the interior, but it does not lead to strong isotopic gradients in the nitrate that is supplied to the surface for phytoplankton consumption. Instead, large isotopic gradients in the shallow thermocline develop through the coupling between upwelling and phytoplankton production, leading to what we call a "fractionating fountain" that lowers the δ15N of the thermocline nitrate (to ~6?) close to the equator and raises it (to ~10?) at 15°S (Figure 2). North of the equator, the isotopic impact of the equatorial upwelling is almost negligible (Figure 2). Instead, the nitrate isotopes of the equatorial to subtropical North Pacific are dominated by the processes of the whole ocean?s fixed N input-output budget. At ~11°N, we identify a secondary upwelling location in which high nitrate δ15N from denitrification reaches the surface, exerting a major influence on the δ15N of the nitrate supply to phytoplankton. North of 11°N, the existence of a shallow subsurface region of low nitrate δ15N centered at 20°N signals the occurrence of N2 fixation in the North Pacific. The latitude of greatest N2 fixation appears to coincide with the northern margin of upwelling of the denitrified waters. These results are consistent with a previously proposed feedback in which denitrification encourages N2 fixation at a comparable rate, the net result of which is stabilization of the global ocean's fixed N reservoir. The project supported the researchers' involvement in workshops for middle and high school teachers and the development of web portal for displaying the published nitrate isotope data available for the global ocean. Last Modified: 04/26/2023 Submitted by: Daniel M Sigman