| Popis: |
In regions bordering productive eastern-boundary upwelling systems, extensive O2 minimum zones (OMZs) develop along the ocean shelf. Enhanced sedimentary release of Fe and PO4 under these low O2 conditions could drive enhanced N2-fixation and primary production in the ocean, thus ultimately increasing O2 consumption, and potentially creating a positive feedback to ocean deoxygenation. A similar feedback loop has been identified in shallow and enclosed coastal regions such as the Baltic where primary production is controlled primarily by N and P bioavailability, but it is unclear to what extent a similar mechanism operates at a larger scale in the open ocean where productivity is proximally constrained by Fe and/or N availability. This is largely because of uncertainties in the fate of the Fe released from shelf sediments.Here we present extensive Fe measurements from a series of 5 cruises and a mesocosm experiment on the Peruvian shelf which combined extensive measurements of Fe distribution with an inert tracer release experiment, to quantify off-shelf transport, and N2 fixation rates. As expected for a region with among the highest reported benthic Fe fluxes in the world, dissolved Fe concentrations were generally elevated along the inner-Peruvian shelf reaching >60 nM. Whilst concentrations rapidly declined across the shelf, reaching as low as 0.01 nM after the shelf break, ‘pockets’ of 1-2 nM elevated Fe concentrations were evident in some transects suggesting a significant role of eddies in off-shelf transport which was verified by the results of our CF3SF5 tracer release experiment. Never-the-less, benthic Fe release was rapidly attenuated close to the sediment interface, with the vast majority of Fe loss occurring on timescales of 2 fixation rates remained consistently low across the region (0-0.8 nmol N L-1 d-1). Furthermore, the maximum lateral transfer of Fe was de-coupled spatially from maximum benthic Fe release, limiting the potential for a P/Fe-fueled positive feedback loop to OMZ expansion. |
