Benthic fluxes of dissolved ferrous iron (Fe 2+ ) and phosphate (TPO 4 ) were quantified by in situ benthic chamber incubations and pore-water profiles along a depth transect (11uS, 80-1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom-water O 2 levels were , 2 mmol L 21 down to 500-m water depth, and increased to , 40 mmol L 21 at 1000 m. Fe 2+ fluxes were highest on the shallow shelf (maximum 316 mmol m 22 yr 21 ), moderate (15.4 mmol m 22 yr 21 ) 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. TPO 4 fluxes were high (maximum 292 mmol m 22 yr 21 ) throughout the shelf and the OMZ core in association with high organic carbon degradation rates. Ratios between organic carbon degradation and TPO 4 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 Fe 2+ and TPO 4 .

ABSTRACT

[1] Empirical transfer functions are derived for predicting the total benthic nitrate loss (L NO3 ) and the net loss of dissolved inorganic nitrogen (L DIN ) in marine sediments, equivalent to sedimentary denitrification. The functions are dynamic vertically integrated sediment models which require the rain rate of particulate organic carbon to the seafloor (RRPOC) and a proposed new variable (O 2 -NO 3 ) bw (bottom water O 2 concentration minus NO 3 À concentration) as the only input parameters. Applied globally to maps of RRPOC and (O 2 -NO 3 ) bw on a 1 Â 1 spatial resolution, the models predict a NO 3 À drawdown of 196 Tg yr À1 (L NO3 ) of which 153 -155 Tg yr À1 is denitrified to N 2 (L DIN ). This is in good agreement with previous estimates using very different methods. Our approach implicitly accounts for fixed N loss via anammox, such that our findings do not support the idea that the relatively recent discovery of anammox in marine sediments might require current estimates of the global benthic marine N budget to be revised

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