Benthic iron and phosphorus fluxes across the Peruvian oxygen minimum zone

Abstract

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 .

Benthic nitrogen cycling traversing the Peruvian oxygen minimum zone

“…The fate of NH 4 + after being released from the Peruvian sediments is unknown, yet it may reach the euphotic zone and enhance primary production and the spread of the OMZ. This positive effect may be further exacerbated by the simultaneous release of excess phosphate and iron from the sediments that was also measured at the time of sampling (Noffke et al, 2010). At intermediate water depths (stations 3 -6), where NO 3 -and NO 2 -uptake was mainly attributed to denitrification (Fig.…”

Nitrogen cycling in shallow low-oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes

“…However, the assumption of Redfield stoichiometry may not be appropriate in this shallow environment due to preferential release of PO 3− 4 following iron and manganese oxyhydroxide dissolution in anoxic sediments (e.g., Noffke et al, 2012). An alternative method of calculating f makes use of our biogenic N 2 measurements to estimate expected N prior to N-loss (N expected -bio N 2 ) and f values based on it:…”

Effects of eustatic sea-level change, ocean dynamics, and nutrient utilization on atmospheric <i>p</i>CO<sub>2</sub> and seawater composition over the last 130 000 years: a model study

“…Hence, OMZ sediments are depleted in Fe / Mn-bound P, enriched in CFA and characterized by high POC / POP ratios exceeding the Redfield ratio by a factor of 2-8 (Schenau and De Lange, 2001;Lomnitz et al, 2015). Ratios between POC and reactive P (P reac : sum of POP, CFA, and Fe / Mn-bound P) amount to POC/P reac = 100-300 in OMZ sediments and 20-70 in continental margin sediments underlying oxygenated bottom waters (Schenau and De Lange, 2001;Noffke et al, 2012). Hence, the burial efficiency of reactive P and total P is reduced under low-oxygen conditions (Ingall and Jahnke, 1994;Schenau and De Lange, 2001) whereas POC is more efficiently buried in OMZ sediments covered by oxygendepleted bottom waters .…”