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  1. Abstract

    The southern Benguela upwelling system (SBUS) supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fueled not only by upwelled nutrients but also by nutrients regenerated on the broad and shallow continental shelf. We measured nutrient concentrations and the nitrogen (N) and oxygen (O) isotope ratios (δ15N and δ18O) of nitrate along four zonal lines in the SBUS in late summer and early winter to evaluate the extent to which regenerated nutrients augment the upwelled nutrient reservoir originating offshore. During summer upwelling, a decrease in on‐shelf nitrate δ18O revealed that 0–48% of the subsurface nutrients derived from in situ remineralization. The nitrate regenerated on‐shelf in the more quiescent winter (0–63% of total nitrate) extended further offshore along the mid‐shelf. A shoreward increase in subsurface nitrate δ15N and a greater N deficit in on‐shelf bottom waters further indicated N loss to benthic (and at times, watercolumn) denitrification coincident with the on‐shelf remineralization. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer through early winter, enhancing the nutrient pool that can be upwelled to support surface production. We hypothesize that this process is aided by a number of equatorward‐flowing hydrographic fronts that impede the lateral exchange of surface waters. The extent to which nutrients remain trapped on the shelf has implications for the occurrence of hypoxic events in the SBUS.

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  2. Abstract

    In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom‐bound15N/14N (δ15Ndb) and reduced export production during the ice ages indicates more complete nitrate (NO3) consumption. This evidence points to an ice age decline in gross NO3supply from the deep ocean to the surface wind‐mixed layer, which may help to explain the reduced CO2levels of the ice age atmosphere. We use a seasonally resolved, two‐layer model of the N isotopes in the Antarctic Zone upper ocean to quantify the ice age decline in gross NO3supply implied by the data. When model parameters are varied to reflect reduced gross NO3supply, the concentration of wintertime upper ocean NO3is lowered, but with a much weaker increase in NO3δ15N than predicted by analytical models such as the Rayleigh and steady state models. Physical mixing is the dominant cause, with a modest contribution from foodweb dynamics. As a result, the observed δ15Ndbrise of ~3‰–4‰ must be explained mostly by a greater summertime increase in NO3δ15N during the ice ages. The high degree of NO3consumption required to generate this summertime δ15N rise indicates a >80% reduction in gross NO3supply. Half or more of the modern gross NO3supply is from wind‐forced Antarctic upwelling that drives the upper cell of Southern Ocean overturning. Thus, the decrease in NO3supply cannot be achieved solely by a decline in vertical mixing or wintertime convection; rather, it requires an ice age weakening of the upper cell.

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