Subsurface waters from both hemispheres converge in the Western Equatorial Pacific (WEP), some of which form the Equatorial Undercurrent (EUC) that influences equatorial Pacific productivity across the basin. Measurements of nitrogen (N) and oxygen (O) isotope ratios in nitrate (d15N-NO3 and d18O-NO3), the isotope ratios of dissolved inorganic carbon (d13C-DIC), and complementary biogeochemical tracers reveal that northern and southern WEP waters have distinct biogeochemical histories. Organic matter remineralization plays an important role in setting the nutrient characteristics on both sides of the WEP. However, remineralization in the northern WEP contributes a larger concentration of the nutrients, consistent with the older "age" of northern thermocline- and intermediate-depth waters. Remineralization introduces a relatively low d15N-NO3 to northern waters, suggesting the production of sinking organic matter by N2 fixation at the surface - consistent with the notion that N2 fixation is quantitatively important in the North Pacific. In contrast, remineralization contributes elevated d15N-NO3 to the southern WEP thermocline, which we hypothesize to derive from the vertical flux of high-d15N material at the southern edge of the equatorial upwelling. This signal potentially masks any imprint of N2 fixation from South Pacific waters. The observations further suggest that the intrusion of high d15N-NO3 and d18O-NO3 waters from the eastern margins is more prominent in the northern than southern WEP. Together, these north-south differences enable the examination of the hemispheric inputs to the EUC, which appear to derive predominantly from southern hemisphere waters.

D(15-18) refers to the difference in d15N and d18O of nitrate. Si* represents the difference between silicic acid and nitrate concentrations.

Supplement to: Lehmann, Nadine; Granger, Julie; Kienast, Markus; Brown, Kevin S; Rafter, Patrick A; Martínez Méndez, Gema; Mohtadi, Mahyar (2018): Isotopic evidence for the evolution of subsurface nitrate in the Western Equatorial Pacific. Journal of Geophysical Research: Oceans, 123(3), 1684-1707