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. Copyright 2012 by the American Geophysical Union.
Global Biogeochemical Cycles