Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical South Pacific

Abstract

AbstractNitrogen isotopic ratios of nitrate (δ15N– ) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ15N– and N* values were as high as 17‰ and as low as −18 μmol N L−1, respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L−1, respectively, in subsurface waters west of 170°W. The relationships among concentrations, N* values, δ15N– values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite‐accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high‐δ15N– and negative‐N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ15N– and N* values, as well as Trichodesmium abundances and size‐specific nitrogen fixation rates at the surface, suggest that the low‐δ15N– and positive‐N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ15N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific.