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

Abstract

During the U.S. JGOFS EqPac program, variations in nitrogen isotopic ratio (d15N) and their control by relative NO3− drawdown were examined in detail. Near‐surface d15N data clearly conform to Rayleigh isotopic fractionation during phytoplankton utilization of NO , with a fractionation factor of about 5‰. This isotopic signal both propagates into particulate nitrogen pools and is a persistent, large‐scale characteristic of the system. Decreasing near‐surface [NO3−] with distance from the equator strongly correlates with increasing d15N in euphotic zone particulate organic matter (POM) as well as sinking particles during both Surveys 1 (El Niño) and 2 (non‐El Niño) cruises. Despite a doubling of [NO3−] accompanying relaxation of El Niño conditions, d15N values in near‐surface POM and deeply sinking particles as a function of latitude were similar for the two periods. Since d15N varies with relative nutrient drawdown and not its concentration, this parameter appears to not have varied significantly. In both cases, about 50% of upwelled NO3− was consumed in the immediate vicinity of the equator, as estimated from the d15N data. The moored sediment trap time series provided a more highly resolved temporal view of the 15N dynamics of this system. At the equator and to the south, there is little temporal variation in d15N and, hence, in relative NO3− utilization over the 1‐yr duration of the program. At 28 and 58°N, there is a large 5‰ decrease in d15N during the transition to non‐El Niño conditions in response to the northward movement of NO3− ‐rich waters, which probably reflects the intensification of the north equatorial current. Overall, % NO3− 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 NO3− utilization, consistent with iron (Fe) limitation. Since Fe is supplied in this region by upwelling, relative NO3− utilization is likely determined by the product of the ratio of Fe to NO3− in upwelled waters and the NO3− : Fe utilization ratio. Given no change in the chemistry of source waters, increases in upwelling, as observed during the transition away from El Niño, will affect surface [NO3−] new and export production, but it will not affect relative NO3− utilization. In this light, downcore d15N records in the equatorial Pacific should reflect past changes in the limitation of NO3− drawdown by Fe, perhaps through changes in equatorial undercurrent chemistry.