Subsurface tropical Pacific nitrogen isotopic composition of nitrate: Biogeochemical signals and their transport

We report measurements of the nitrogen isotopic composition of nitrate (the δ ¹⁵N of NO ₃ ^-) across the equatorial Pacific, for zonal transects from 165E to 95W and meridional transects across 95 and 110W. The δ ¹⁵N of NO ₃ ^- is similar in the equatorial thermocline (100 m) and intermediate depth waters (150 to 600 m), averaging (7.1 0.3)‰ and (7.1 0.1)‰, respectively. These values are more than 2‰ higher than subthermocline waters of the Southern and Atlantic Oceans and are 1‰ higher than putative source waters in the high latitude South Pacific (Subantarctic Mode Water, SAMW). The combined constraints of nitrate concentration and δ ¹⁵N of NO ₃ ^- in the equatorial Pacific require (1) lateral exchange between the high-latitude source waters and the zones of denitrification in the eastern tropical Pacific and (2) the accumulation of remineralized nutrients at depth. The zonal uniformity of the subsurface equatorial Pacific δ ¹⁵N of NO ₃ ^- indicates rapid transport within the equatorial zone, which works to homogenize the δ ¹⁵N of NO ₃ ^- across the Pacific basin. Against this backdrop of high δ ¹⁵N of NO ₃ ^- in the tropical Pacific, we find a discrete off-equatorial core of lower δ ¹⁵N of NO ₃ ^- (5.5 0.3)‰ concentrated at 5S and 150 to 200 m along the 110 and 95W transects and in apparent association with the Southern Subsurface Counter Current (SSCC). We propose that the remineralized products of nitrogen fixation, at the source of the SSCC in the western south Pacific, are the origin of the low δ ¹⁵N of NO ₃ ^- in these waters.