Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone

A. Noffke, C. Hensen, S. Sommer, F. Scholz, L. Bohlen, T. Mosch, M. Graco and K. Wallmann

Limnol. Oceanogr., 57(3), 2012, 851-867 | DOI: 10.4319/lo.2012.57.3.0851

ABSTRACT: Benthic fluxes of dissolved ferrous iron (Fe2+) and phosphate (TPO4) were quantified by in situ benthic chamber incubations and pore-water profiles along a depth transect (11°S, 80-1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom-water O2 levels were < 2 µmol L-1 down to 500-m water depth, and increased to ∼ 40 µmol L-1 at 1000 m. Fe2+ fluxes were highest on the shallow shelf (maximum 316 mmol m-2 yr-1), moderate (15.4 mmol m-2 yr-1) between 250 m and 600 m, and negligible at deeper stations. In the persistent OMZ core, continuous reduction of Fe oxyhydroxides results in depletion of sedimentary Fe : Al ratios. TPO4 fluxes were high (maximum 292 mmol m-2 yr-1) throughout the shelf and the OMZ core in association with high organic carbon degradation rates. Ratios between organic carbon degradation and TPO4 flux indicate excess release of P over C when compared to Redfield stoichiometry. Most likely, this is caused by preferential P release from organic matter, dissolution of fish debris, and/or P release from microbial mat communities, while Fe oxyhydroxides can only be inferred as a major P source on the shallow shelf. The benthic fluxes presented here are among the highest reported from similar, oxygen-depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean. The shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom-water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of Fe2+ and TPO4.

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