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

Rafter, PA, Sigman DM, Charles CD, Kaiser J, Haug GH.  2012.  Subsurface tropical Pacific nitrogen isotopic composition of nitrate: Biogeochemical signals and their transport. Global Biogeochemical Cycles. 26

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13-degrees-c water, deficient waters, denitrifier method, eastern Pacific, inventories, marine-sediments, north pacific, nutrient, south-pacific, subtropical cell, western equatorial pacific


We report measurements of the nitrogen isotopic composition of nitrate (the delta(15)N of NO(3)(-)) across the equatorial Pacific, for zonal transects from 165 degrees E to 95 degrees W and meridional transects across 95 degrees and 110 degrees W. The delta(15)N of NO(3)(-) is similar in the equatorial thermocline (approximate to 100 m) and intermediate depth waters (approximate to 150 to 600 m), averaging (7.1 +/- 0.3)parts per thousand and (7.1 +/- 0.1)parts per thousand, respectively. These values are more than 2 parts per thousand higher than subthermocline waters of the Southern and Atlantic Oceans and are approximate to 1 parts per thousand higher than putative source waters in the high latitude South Pacific (Subantarctic Mode Water, SAMW). The combined constraints of nitrate concentration and delta(15)N 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 delta(15)N of NO(3)(-) indicates rapid transport within the equatorial zone, which works to homogenize the delta(15)N of NO(3)(-) across the Pacific basin. Against this backdrop of high delta(15)N of NO(3)(-) in the tropical Pacific, we find a discrete off-equatorial core of lower delta(15)N of NO(3)(-) (5.5 +/- 0.3)parts per thousand concentrated at 5 degrees S and 150 to 200 m along the 110 degrees and 95 degrees W 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 delta(15)N of NO(3)(-) in these waters.






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