Estimating δ15N of remineralized NO3- in water masses of the eastern tropical south Pacific using NO3- isotope measurements from the GEOTRACES Eastern Pacific Zonal Transect

Nitrogen and oxygen isotope measurements of nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3, respectively) are useful tools in interpreting biogeochemical modification of water masses. In particular, changes in δ¹⁵N_NO3 and δ¹⁸O_NO3 can signify the occurrence of processes such nitrate consumption and remineralization, as well as mixing between water masses. We explored distributions in the parameter Δ(15-18), or δ¹⁵N_N03  − δ¹⁸O_N03 , along the GEOTRACES Eastern Pacific Zonal Transect. Δ(15-18) values in the deep ocean (3‰), are altered by the addition of NO₃^- derived from remineralization of organic matter, which has different effects upon the δ¹⁵N_NO3 and d¹⁸O_NO3. Elevated Δ(15-18) values (4 to 5‰) were found at ~1000 m across the entire transect. We combined NO₃^- isotope measurements and a water mass analysis to estimate the δ¹⁵N of remineralized NO₃^- in the upper 1000 m across the zonal transect. The highest estimates of remineralized δ¹⁵N_NO3 values (~8‰) were associated with Antarctic Intermediate Water (AAIW) in the waters underlying the subtropical gyre where surface NO₃^- consumption is nearly complete. Closer to the to coast, where NO₃^- is not entirely consumed at the surface, remineralized δ¹⁵N_N03 estimates were lower (~4‰) in subsurface waters. Thus, our preliminary results suggest that the elevated Δ(15-18) in AAIW could be driven by elevated δ¹⁵N values of remineralized NO₃^-, rather than decreased δ¹⁸O_NO3. Elevated Δ(15-18) values (~5 to 7‰) were also observed at ~200 m in the central Pacific, just below the nutrient-poor surface waters. As shown in previous studies, these increased Δ(15-18) values appear to be driven by a decrease in δ¹⁸O_NO3 relative to δ¹⁵N_NO3, most likely from complete consumption of NO₃^- in surface waters and remineralization of organic matter with the same δ¹⁵N value as that in subsurface waters and a lower δ¹⁸O value close to that of seawater (~0‰).