Description: Oxygen minimum zones (OMZs) that impinge on continental margins favor the release of phosphorus (P) from the sediments to the water column, enhancing primary productivity and the maintenance or expansion of low-oxygen waters. A comprehensive field program in the Peruvian OMZ was undertaken to identify the sources of benthic P at six stations, including the analysis of particles from the water column, surface sediments, and pore fluids, as well as in situ benthic flux measurements. A major fraction of solid-phase P was bound as particulate inorganic P (PIP) both in the water column and in sediments. Sedimentary PIP increased with depth in the sediment at the expense of particulate organic P (POP). The ratio of particulate organic carbon (POC) to POP exceeded the Redfield ratio both in the water column (202 ± 29) and in surface sediments (303 ± 77). However, the POC to total particulate P (TPP = POP + PIP) ratio was close to Redfield in the water column (103 ± 9) and in sediment samples (102 ± 15). This suggests that the relative burial efficiencies of POC and TPP are similar under low-oxygen conditions and that the sediments underlying the anoxic waters on the Peru margin are not depleted in P compared to Redfield. Benthic fluxes of dissolved P were extremely high (up to 1.04 ± 0.31 mmol m−2 d−1), however, showing that a lack of oxygen promotes the intensified release of dissolved P from sediments, whilst preserving the POC / TPP burial ratio. Benthic dissolved P fluxes were always higher than the TPP rain rate to the seabed, which is proposed to be caused by transient P release by bacterial mats that had stored P during previous periods when bottom waters were less reducing. At one station located at the lower rim of the OMZ, dissolved P was taken up by the sediments, indicating ongoing phosphorite formation. This is further supported by decreasing porewater phosphate concentrations with sediment depth, whereas solid-phase P concentrations were comparatively high.

Description: The isotope composition of reactive iron (Fe) in marine sediments and sedimentary rocks is a promising tool for identifying Fe sources and sinks across ocean basins. In addition to cross-basinal Fe redistribution, which can modify Fe isotope signatures, Fe minerals also undergo diagenetic redistribution during burial. The isotope fractionation associated with this redistribution does not affect the bulk isotope composition, but complicates the identification of mineral-specific isotope signatures. Here, we present new Fe isotope data for Peru margin sediments and revisit previously published data for sediments from the California margin to unravel the impact of early diagenesis on Fe isotope compositions of individual Fe pools. Sediments from oxic California margin sites are dominated by terrigenous Fe supply with Fe release from sediments having a negligible influence on the solid phase Fe isotope composition. The highly reactive Fe pool (sum of Fe bound to (oxyhydr)oxide, carbonate, monosulfide and pyrite) of these sediments has a light isotope composition relative to the bulk crust, which is consistent with earlier studies showing that continental weathering shifts the isotope composition of Fe (oxyhydr)oxides to lighter values. Ferruginous sedimentswithin the Peruvian oxygen minimumzone are depleted in Fe relative to the lithogenic background, which we attribute to extensive Fe release to the water column. The remaining highly reactive Fe pool has a heavier isotope composition compared to California margin sediments. This observation is in agreement with the general notion of an isotopically light benthic Fe efflux. Most of the reactive Fe delivered and retained in the sediment is transferred into authigenic mineral phases within the topmost 10 to 20 cm of the sediments. We observe a first-order relationship between the extent of pyritization of Fe monosulfide and the isotope composition of authigenic pyrite. With increasing pyritization, the isotope composition of authigenic pyrite approaches the isotope composition of the highly reactive Fe pool. We argue that the isotope composition of authigenic pyrite or other Fe minerals that may undergo pyritization may only be used to trace water column sources or sinks if the extent of pyritization is separately evaluated and either close to 100% or 0%. Alternatively, one may calculate the isotope composition of the highly reactive Fe pool, thereby avoiding isotope effects due to internal diagenetic redistribution. In depositional settings with high Fe but lowsulfide concentrations, source and sink signatures in the isotope composition of the highly reactive Fe pool may be compromised by sequestration of Fe within authigenic silicate minerals. Authigenic silicate minerals appear to be an important burial phase for reactive Fe below the Peruvian oxygen minimum zone.

Description: We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO3−, NO2−, NH4+) and oxygen (O2) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O2 concentrations at the shallowest station were hypoxic (42 μM) and increased to 125 μM at the deepest site (1113 m). Total oxygen uptake rates were highest on the shelf (−10.3 mmol O2 m−2 d−1) and decreased quasi-exponentially with water depth to −3.2 mmol O2 m−2 d−1. Average denitrification rates estimated from a flux balance decreased with water depth from 2.2 to 0.2 mmol N m−2 d−1. Overall, the sediments acted as net sink for DIN. Observed increases in δ15NNO3 and δ18ONO3 in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0‰ (15εapp) and 14.1‰ (18εapp). Measurements of δ15NNO2 further demonstrated that the sediments acted as a source of 15N depleted NO2−. These observations were analyzed using an isotope box model that considered denitrification and nitrification of NH4+ and NO2−. The principal findings were that (i) net benthic 14N/15N fractionation (εDEN) was 12.9 ± 1.7‰, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO2− (−22 ± 1.9‰), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported εDEN for fine-grained sediments are much lower (4–8‰). We speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater–seawater exchange in permeable sediments and the hypoxic, high productivity environment. Although not without uncertainties, the results presented could have important implications for understanding the current state of the marine N cycle.

Description: The upwelling area in the eastern equatorial Pacific off Peru is one of the most pronounced oxygen minimum zones (OMZs) of the modern ocean. Modeling scenarios predict an expansion of the OMZs in the course of global change in the coming decades. As a consequence, the Peruvian continental margin represents a key locality for studies on biogeochemical dynamics in the future ocean. We present pore water and sediment data for redox-sensitive metals (Fe, Mn, V, Mo, and U) that have been collected along a transect across the Peruvian margin at 11°S. The results are used to evaluate the behavior of trace metals in a wide range of biogeochemical and hydrodynamic settings. In the core of the OMZ, where permanently anoxic conditions prevail, redox sensitive metals exhibit diagenetic behaviors largely consistent with previous studies. Vanadium and Mo are released from Fe oxihydroxides and subsequently recycled through diffusion across the benthic boundary or trapped through formation of authigenic V phases and sequestration of Mo by authigenic pyrite. Some U is delivered through diffusion across the benthic boundary, reduction and precipitation of UO2 and incorporation into phosphorites. The utmost part of the buried U, however, is delivered in particulate form, most likely as bioauthigenic U which cannot be recycled in the suboxic waters overlying the anoxic sediments. In contrast to sediments in the core of the OMZ, sediments on the shelf experience frequent oxygenation episodes related to the passage of internal waves and the regular recurrence of El Niño events. These oxygenation episodes lead to the re-oxidation and remobilization of authigenic U and V. In contrast to that, the authigenic accumulation of Mo is favored by the occasional occurrence of slightly oxidizing conditions. This is most likely due to enhanced formation of sulfur intermediates necessary for pyrite formation and the increased stability of pyrite, the major Mo sink, under oxidizing conditions, compared to authigenic V and U phases. Redox oscillations in the Peruvian OMZ thus lead to a discrimination of U against Mo, a mechanism that should be considered in the interpretation of U/Mo systematics in paleo redox studies. Overall our results provide valuable constraints on how trace metal inventories of marginal sediments may respond to expanding shelf anoxia and to short term perturbations of sediment redox conditions.

Description: Worldwide oxygen minimum zones (OMZs) as well as coastal oxygen-deficient regions have been shown to be expanding during recent decades. When such oxygen minima impinge on the sea floor, the retention capacity of sediments for phosphate (TPO4), ferrous iron (Fe2+), as well as ammonium (NH4+) is strongly reduced, resulting in high sea-bed release rates of these key nutrients into the bottom water. Despite the significance of the benthos exerting a major positive feedback on surface-water primary productivity and in turn maintenance of oxygen (O2) deficiency, the nutrient release in OMZ and coastal O2-deficient regions has hardly been quantified. The aim of this study was to investigate the benthic nutrient turnover in two different highly O2-deficient systems: i. the intense OMZ off Peru and ii. the landlocked Gotland Basin, Baltic Sea, which suffers from anthropogenically induced eutrophication. The focus was on the phosphorus (P) cycle but associated cycles of iron (Fe) and nitrogen (N) were also included. Off the coast of Peru, benthic fluxes of TPO4 and Fe2+ were quantified in situ using benthic landers and were calculated from pore-water profiles across a latitudinal depth transect at 11°S. This transect extended from 80 m to 1000 m water depth and covered anoxic to oxic bottom-water conditions. The working area was divided into three different zones: the shelf that is subjected to periodically fluctuating bottom-water O2 conditions, the core of the OMZ where anoxia can be assumed to be permanent, and the depth range below 500 m where O2 levels increased again. TPO4 fluxes were high (maximum 292 mmol m-2 yr-1) throughout the shelf and in the core of the OMZ. In contrast, Fe2+ fluxes were high on the shallow shelf (maximum 316 mmol m-2 yr-1) but moderately low (15.4 mmol m-2 yr-1) in water depths between 250 m and 600 m due to the continuous reduction of Fe oxides and Fe hydroxides (henceforth referred to as Fe oxyhydroxides). Below 600 m, where O2 concentrations increased, Fe2+ fluxes became negligible due to the precipitation of Fe2+ in the oxic sediment surface. Ratios between organic carbon degradation and TPO4 flux indicated an excess release of P over carbon (C) when compared to Redfield stoichiometry. This was most likely caused by preferential P release during organic matter degradation, dissolution of fish debris, and/or P release from sulfide-oxidizing microbial mat communities. Fe oxyhydroxides were relevant as a P source only on the shallow shelf. The benthic fluxes are among the highest reported from similar O2-deficient continental margin systems, and highlight the efficiency of OMZ sediments returning TPO4 and Fe2+ to the bottom water. The shelf region is particularly important in this regard since O2 fluctuations likely trigger a complex biogeochemical reaction network of P, Fe and sulfur turnover resulting in transient, high TPO4 and Fe2+ release under anoxia. Sources for P release were further constrained by combining P speciation data, based on sequential extraction of sediment samples, with a mass balance and benthic modeling. P speciation revealed that authigenic calcium phosphate (Ca-P; including carbonate fluorapatite, biogenic apatite from fish remains, and calcium carbonate-bound P), was the major fraction along the transect. It accounted for 35 to 47% of the depth-averaged total extracted P on the shelf and upper slope, but for 〉 70% below 300 m water depth. Further extraction of fish-P showed that below 259 m water depth this fraction dominated the authigenic Ca-P pool by 60 to 69%. Organic P was present in considerable amounts (18 to 37%) only at the shelf and the upper slope, whereas detrital P and P bound to Fe oxyhydroxides was generally of minor importance at all sites. Organic matter in surface sediments was highly depleted in P relative to Redfield stoichiometry with C:P ratios of up to 516. The benthic model found preferential P mineralization in the water column or, alternatively, preferential P release during organic matter degradation in the sediment surface as possible pathways explaining such high C:P ratios. Nevertheless, both model and mass balance calculations revealed that irrespective of which pathway prevails, organic P was only of minor importance for the benthic P budget of Peruvian OMZ sediments. According to the solid phase speciation, authigenic Ca-P, with a high contribution of fish debris, is a likely candidate for the missing source of P required to close the P budget. These sediments were identified as weak sinks for P, as more than 80% of the imported P was recycled back into the water column. In the Gotland Basin, TPO4 and DIN fluxes were quantified in situ across an oxic to anoxic depth-transect using benthic landers. A CTD-water sampling rosette was deployed to record the nutrient and O2 distribution in the water column and thereby investigate the benthic-pelagic coupling because of its significance for the euthrophication state of the Baltic Proper. The study area was divided into three different zones: the oxic zone at 60 m to 〈 80 m water depth, the hypoxic transition zone between 〉 80 m and 120 m, and the deep anoxic and sulfidic basin at 〉 120 m. The hypoxic transition zone was characterized by fluctuating O2 levels as well as the occurrence of extended mats of sulfur bacteria. Beside the deep anoxic basin, the hypoxic transition zone was revealed as a major release site for TPO4 and NH4+ with rates of up to 0.2 mmol m-2 d-1 and 1 mmol m-2 d-1, respectively. There are clear indications that the bacterial mats converted NO3-/NO2- into NH4+ during dissimilatory nitrate reduction to ammonium (DNRA), thereby retaining reactive N in the ecosystem. The transient release and uptake of TPO4 during oscillating anoxic and oxic conditions by these bacteria, however, can only be speculated as the entire TPO4 release from the sediment could be potentially covered by preferential P release during organic matter degradation. Extrapolation of benthic fluxes to the Baltic Proper resulted in internal TPO4 and DIN loads of 109 kt yr-1 and 295 kt yr-1, respectively, which is significantly higher than external P and DIN loads. This up-scaling of fluxes revealed the importance of the hypoxic transition zone for the internal nutrient loading, which only covered 51% of the total considered area, but released as much as 70% of the total TPO4 load. Likewise, 75% of the internal NH4+ load (200 kt yr-1) was released from this particular environment; however, this NH4+ did not reach the surface mixed layer. This resulted in the supply of water with a low N:P ratio to the euphotic zone. In summertime, such low N:P ratios favor the development of N2-fixing cyanobacterial blooms which, by different feedback processes, counteract the recovery of the Baltic Proper from eutrophication.

Description: Iron (Fe) is a key element in the global ocean’s biogeochemical framework because of its essential role in numerous biological processes. A poorly studied link in the oceanic Fe cycle is the reductive release of Fe from sediments in oxygen depleted ocean regions - the oxygen minimum zones (OMZs). Changing rates of Fe release from OMZ sediments may have the potential to modulate ocean fertility which has far-reaching implications considering the high amplitude oxygen fluctuations throughout earth history as well as the ongoing ocean deoxygenation projected for the near future. In order to explore spatial and temporal trends of Fe cycling in OMZs, we present here Fe isotope and speciation data for surface sediments from a transect across the Peruvian upwelling area, one of the most pronounced OMZs of the modern ocean. Because of continuous dissimilatory Fe reduction and diffusive loss across the benthic boundary, sediments within the OMZ are strongly depleted in reactive Fe components, and the little reactive Fe left behind has a heavy isotope composition. In contrast, surface sediments below the OMZ are enriched in reactive Fe, with the majority being present as Fe oxides with comparably light isotope composition. This lateral pattern of Fe depletion and enrichment indicates that Fe released from sediments within the OMZ is reoxidized and precipitated at the oxycline. First-order calculations suggest that the amount of Fe mobilized within the OMZ and that accumulated at the boundaries are largely balanced. Therefore, benthic Fe fluxes in OMZs should be carefully evaluated prior to incorporation into global models, as much of the initially released Fe may be reprecipitated prior to vertical or offshore transport. First XRF core scanning results for partly laminated piston cores from the OMZ boundaries reveal downcore oscillations in the content of reactive Fe and redox-sensitive trace metals that are attributed to past changes in OMZ extension. Ongoing work on these cores will focus on their dating and the downcore investigation of Fe and trace metal records in order to better understand past Fe cycling within the Peruvian OMZ and potential interactions with climate variability.