Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone

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 (11uS, 80–1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom-water O2 levels were , 2 mmol L21 down to 500-m water depth, and increased to , 40 mmol L21 at 1000 m. Fe2+ fluxes were highest on the shallow shelf (maximum 316 mmol m22 yr21), moderate (15.4 mmol m22 yr21) 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 m22 yr21) 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. Oxygen minimum zones (OMZ), water layers with oxygen concentrations , 20 mmol L21, are persistent hydrographic features in large parts of the ocean, in particular the eastern Pacific, the northern Indian Ocean, and the eastern Atlantic off southwest Africa (Helly and Levin 2004). One of the most extended and intense OMZs (dropping to oxygen concentrations close to anoxia in core regions; Stramma et al. 2008) is located in the eastern South Pacific, underneath the productive coastal waters of the Humboldt Current System. This OMZ stretches from 37uS, Chile, to the equatorial belt (0–5uS) and reaches its greatest extension off Peru between 5 and 13uS, with . 600-m thickness to about 1000 km offshore (Fuenzalida et al. 2009). The complex maintenance mechanisms and dynamics of OMZs still have not been sufficiently resolved. However, the principal factors leading to their formation are intense oxygen consumption in response to high surface productivity, sustained by high amounts of upwelled nutrients, and sluggish ventilation due to the hydrographic regime (Wyrtki 1962). Climate models predict an overall decline of dissolved oxygen in the ocean interior to emerge from global warming (Matear and Hirst 2003). For the tropical OMZs this decline was recently confirmed by 50-yr time series analyses of O2 data (collected since 1960; Stramma et al. 2008). In response to the expansion of the hypoxic water masses, major changes in nutrient cycling could occur and affect the marine carbon, nitrogen, phosphorus, and iron cycles via various feedback mechanisms. Thus, improving current knowledge on the key biogeochemical and physical processes governing today’s OMZs by quantitative approaches remains critical to estimating the ocean’s responses to global warming and becomes a future research challenge. Oxygen depletion substantially affects the biogeochemical reactions of redox-sensitive elements. This in particular applies to iron (Fe) and phosphorus (P), whose individual cycles are strongly linked in the marine environment. A number of previous studies have resulted in the observation that under oxygen-deficient bottom-water conditions dissolved ferrous iron (Fe2+) and phosphate (TPO4) are preferentially released into the pore fluids and overlying bottom water (Sundby et al. 1986; Ingall and Jahnke 1997; McManus et al. 1997). Particulate ferric iron oxides and hydroxides (hereafter referred to as iron oxyhydroxides) scavenge phosphate, and oxygen deficiency promotes their reduction to soluble states by microbial induced dissolution and the concomitant liberation of metal-oxide-bound phosphate (Sundby et al. 1992). Phosphate release from iron oxyhydroxides may further be enhanced through reductive dissolution by hydrogen sulfide (Jensen et al. 1995). Furthermore, in addition to these metal oxide interactions, growing evidence has been presented that phosphate is preferentially regenerated from P-bearing organic matter (as compared to C) under hypoxic and anoxic bottom-water conditions (Ingall et al. 1993). Despite the obvious biogeochemical significance of organic rich sediments underlying productive upwelling systems for global element cycles and related feedbacks on ocean–climate interactions, relatively few systematic in situ studies on benthic nutrient turnover have been conducted to date. Phosphate fluxes derived from benthic chamber incubations are available for the continental margins off Washington State (Devol and Christensen 1993; Hartnett and Devol 2003), central California (Ingall and Jahnke * Corresponding author: [email protected] Limnol. Oceanogr., 57(3), 2012, 851–867 E 2012, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2012.57.3.0851