Benthic fluxes of iron, phosphate and silicate across the Peruvian Oxygen Minimum Zone

Oxygen Minimum Zones (OMZ) are key regions with respect to carbon and nutrient cycling as well as biological productivity of the global ocean. The Peruvian OMZ is one of the largest oxygen deficient water bodies worldwide. It impinges on the continental margin at water depths between ~ 50 and 750 m where oxygen levels of < 10 µM strongly influence biogeochemical processes and cycling of redox-sensitive elements. During Meteor cruise M77 in 2008 benthic exchange processes were studied by pore water analyses of surface sediments and benthic landers along a shelf slope transect at 11°S (water depth: 80-1000 m, O2: 1-40 µM). Benthic fluxes of iron, phosphate and silicate across the sediment-water interface were calculated using concentration gradients in pore waters and benthic chamber incubations. In most cases both approaches yielded similar results, thus diffusive fluxes from pore water data can be used as good approximations for total benthic fluxes, and hence are a useful alternative for quantification at sites where no lander deployments were conducted. Silicate fluxes decrease with water depth and distance from the coast which are important parameters controlling the export flux from primary production and input of terrigeneous material. Their magnitudes are among the highest reported so far (up to 3900 mmol m-2 yr-1 for in situ chamber fluxes) and reflect the high sedimentary input of biogenic opal in this productive upwelling zone. It is well known that iron and phosphorus are preferentially released from sediments under oxygen deficient conditions. Cross margin patterns of pore water profiles and exchange fluxes demonstrate this clear impact of oxygen availability on iron and phosphate mobilization. High subsurface peaks at the stations within the core of the OMZ (50-450 m) indicate a shift of the zone of dissimilatory iron reduction towards the sediment water interface. Iron and phosphate fluxes are highest at sites at the lower edge of the OMZ (O2 ~ 2 µM, ~ 400 m) and abruptly decrease at 700 m water depth where oxygen concentrations exceed 10 µM. However, considerable scatter in the data suggests that relationships controlling benthic exchange cannot be related to water depth and oxygen availability alone, but must be much more complex. Possible factors controlling variations of benthic fluxes on smaller scales are hydrodynamics and topographic features of the seafloor that may affect regional sediment accumulation rates