Nitrogen isotopic evidence for micronutrient control of fractional NO3- utilization in the equatorial Pacific

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During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio (delta (15)N) and their control by relative NO(3)(-) drawdown were examined in detail. Near-surface delta (15)NO(3)(-) data clearly conform to Rayleigh isotopic fractionation during phytoplankton utilization of NO(3)(-), with a fractionation factor of about 5 parts per thousand. This isotopic signal both propagates into particulate nitrogen pools and is a persistent, large-scale characteristic of the system. Decreasing near-surface [NO(3)(-)] with distance from the equator strongly correlates with increasing delta (15)N in euphotic zone particulate organic matter (POM) as well as sinking particles during both Surveys 1 (El Nino) and 2 (non-El Nino) cruises. Despite a doubling of [NO(3)(-)] accompanying relaxation of El Nino conditions, delta (15)N values in near-surface POM and deeply sinking particles as a function of latitude were similar for the two periods. Since delta (15)N varies with relative nutrient drawdown and not its concentration, this parameter appears to not have varied significantly. In both cases, about 50% of upwelled NO(3)(-) was consumed in the immediate vicinity of the equator, as estimated from the delta (15)N data. The moored sediment trap time series provided a more highly resolved temporal view of the (15)N dynamics of this system. At the equator and to the south, there is little temporal variation in delta (15)N and, hence, in relative NO(3)(-) utilization over the 1-yr duration of the program. At 2 degrees and 5 degreesN, there is a large 5 parts per thousand decrease in delta (15)N during the transition to non-El Nino conditions in response to the northward movement of NO(3)(-)-rich waters, which probably reflects the intensification of the north equatorial current. Overall, % NO(3)(-) utilization at the equator remained between 40 and 60% over the observation period, despite large hydrographic and dynamical changes. This observation implies a tight control of relative NO(3)(-) utilization, consistent with iron (Fe) limitation. Since Fe is supplied in this region by upwelling, relative NO(3)(-) utilization is likely determined by the product of the ratio of Fe to NO(3)(-) in upwelled waters and the NO(3)(-):Fe utilization ratio. Given no change in the chemistry of source waters, increases in upwelling, as observed during the transition away from El Nino, will affect surface [NO(3)(-)] new and export production, but it will not affect relative NO(3)(-) utilization. In this light, downcore delta (15)N records in the equatorial Pacific should reflect past changes in the limitation of NO(3)(-) drawdown by Fe, perhaps through changes in equatorial undercurrent chemistry.

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  • Mark a. Altabet

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