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Oxygen, trace metals, and macronutrient density distributions. Data from all stations are shown: the gray and black dots indicate trace metal or macronutrient concentrations at offshore (bottom depth > 2,000 m) and onshore (bottom depth < 300 m) stations, respectively. The lines indicate oxygen concentrations from Transect 1 (red) and Transects 2–4 (blue). Highest Fe concentrations at onshore stations for Transects 1–3 exceed the scale (up to 124‐nM reactive pFe and 30.4‐nM dFe).
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Some trace metals, including iron, are essential micronutrients for phytoplankton growth. However, the solubility of iron is very low under oxygenated conditions. Consequently, restricted iron availability in oxygen‐rich seawater can limit phytoplankton growth in the ocean, including in the Eastern Tropical South Pacific. Und...
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... Benthic Fe that escapes phytoplankton uptake and scavenging on the shelf can be transported to the open ocean by subsurface currents (Siedlecki et al., 2012). In the presence of upwelling and vertical mixing, this Fe can fertilize phytoplankton in remote ocean regions, linking the cycles of carbon, O 2 , and Fe (Johnson et al., 1999;Rapp et al., 2020;Wallmann et al., 2022). ...
Release of iron (Fe) from continental shelves is a major source of this limiting nutrient for phytoplankton in the open ocean, including productive Eastern Boundary Upwelling Systems. The mechanisms governing the transport and fate of Fe along continental margins remain poorly understood, reflecting interaction of physical and biogeochemical processes that are crudely represented by global ocean biogeochemical models. Here, we use a submesoscale‐permitting physical‐biogeochemical model to investigate processes governing the delivery of shelf‐derived Fe to the open ocean along the northern U.S. West Coast. We find that a significant fraction (∼20%) of the Fe released by sediments on the shelf is transported offshore, fertilizing the broader Northeast Pacific Ocean. This transport is governed by two main pathways that reflect interaction between the wind‐driven ocean circulation and Fe release by low‐oxygen sediments: the first in the surface boundary layer during upwelling events; the second in the bottom boundary layer, associated with pervasive interactions of the poleward California Undercurrent with bottom topography. In the water column interior, transient and standing eddies strengthen offshore transport, counteracting the onshore pull of the mean upwelling circulation. Several hot‐spots of intense Fe delivery to the open ocean are maintained by standing meanders in the mean current and enhanced by transient eddies and seasonal oxygen depletion. Our results highlight the importance of fine‐scale dynamics for the transport of Fe and shelf‐derived elements from continental margins to the open ocean, and the need to improve representation of these processes in biogeochemical models used for climate studies.
... We apply a numerical reaction-transport model to reproduce the observed iodine distribution in the pore water and sediment and to examine the mechanistic and quantitative constraints on benthic I − fluxes and I:C org . Importantly, the Peruvian margin is also the area of two previous water column iodine speciation surveys (Cutter et al., 2018;Rapp et al., 2020), and an Ocean Drilling Program (ODP) transect (Martin et al., 1993). Our study of shallow sediments thus links these environments and allows for the opportunity to evaluate iodine cycling from the water column to early diagenesis and deep burial. ...
... Organic carbon degradation in the anoxic water column is coupled to denitrification Lam et al., 2009), whereas in the underlying sediments sulfate reduction is the dominant organic carbon degradation pathway (Bohlen et al., 2011). Nitrogenous conditions in the water column, as indicated by the presence of a secondary nitrite maximum, are accompanied by elevated concentrations of I − (Cutter et al., 2018;Rapp et al., 2020). Surface sediments underneath the anoxic water column are mostly ferruginous (presence of Fe 2+ ) and occasionally sulfidic (Plass et al., 2020;Scholz et al., 2011Scholz et al., , 2016. ...
... Water column Fe anomalies are accompanied by the presence of excess dissolved iodine suggesting that anoxic shelf sediments represent a source of I − to the water column (Cutter et al., 2018). Lower benthic Fe fluxes as well as iron and excess dissolved iodine concentrations in the water column have been observed after a period of shelf oxygenation (Noffke et al., 2012;Rapp et al., 2020). ...
Iodine cycling in the ocean is closely linked to productivity, organic carbon export, and oxygenation. However, iodine sources and sinks at the seafloor are poorly constrained, which limits the applicability of iodine as a biogeochemical tracer. We present pore water and solid phase iodine data for sediment cores from the Peruvian continental margin, which cover a range of bottom water oxygen concentrations, organic carbon rain rates and sedimentation rates. By applying a numerical reaction‐transport model, we evaluate how these parameters determine benthic iodine fluxes and sedimentary iodine‐to‐organic carbon ratios (I:Corg) in the paleo‐record. Iodine is delivered to the sediment with organic material and released into the pore water as iodide (I⁻) during early diagenesis. Under anoxic conditions in the bottom water, most of the iodine delivered is recycled, which can explain the presence of excess dissolved iodine in near‐shore anoxic seawater. According to our model, the benthic I⁻ efflux in anoxic areas is mainly determined by the organic carbon rain rate. Under oxic conditions, pore water dissolved I⁻ is oxidized and precipitated at the sediment surface. Much of the precipitated iodine re‐dissolves during early diagenesis and only a fraction is buried. Particulate iodine burial efficiency and I:Corg burial ratios do increase with bottom water oxygen. However, multiple combinations of bottom water oxygen, organic carbon rain rate and sedimentation rate can lead to identical I:Corg, which limits the utility of I:Corg as a quantitative oxygenation proxy. Our findings may help to better constrain the ocean's iodine mass balance, both today and in the geological past.
... Past and future changes to the size, biogeochemistry and productivity of the Peruvian OMZ are therefore of significant societal relevance (Christensen et al., 2014;Salvatteci et al., 2019). On interannual timescales, El Niño-Southern Oscillation (ENSO) dynamics are known to strongly affect the size, intensity and productivity of the Peruvian OMZ (Barber & Chavez, 1983;Chavez et al., 2008;José et al., 2019) and have also been proposed to affect dFe dynamics through the interlinked relationships between upwelling, productivity and benthic dFe effluxes (Browning et al., 2018;Rapp et al., 2020;Scholz et al., 2011). Generally, El Niño (La Niña) are classified when 3-month running-mean Niño1+2 sea surface temperature (SST) index is above (lower than) 0.4°C (-1.0°C) for at least three consecutive months (L'Heureux et al., 2017). ...
... During the warm phase of ENSO, the thermocline, nutricline and oxycline deepen, the lateral subtropical-sourced oxygen supply enhances (José et al., 2019), and the upwelling source water nutrient concentrations decrease (Espinoza -Morriber ó n et al., 2017). Consequently, warm, oxygen-replete, macronutrientdepleted water is upwelled to the surface layer, resulting in a reduction of productivity and Chl-a concentrations (Rapp et al., 2020;Stramma et al., 2016). A model simulation indicates that whilst the upwelling-favorable wind increases during El Niño, the coastal upwelling weakens in winter and spring due to a compensating effect from an onshore geostrophic flow (Espinoza-Morriberó n et al., 2017). ...
... The cruise SO243 was specifically designed to investigate the development of dFe concentrations along cross-shelf transects and found evidence that oxygenation of the water column under El Niño conditions reduced the water column inventory of dFe and other redox-sensitive species (Rapp et al., 2020). Two other cruises have a similar distribution of cross-shelf profiles for dFe in the same region ( Fig. S3) and can be compared to further test whether or not the same link between O 2 and dFe can be reproduced more widely. ...
... Under oxic conditions, iodate is the dominant species in the water column (Truesdale et al., 2000). Elevated iodide and low iodate are observed in low oxygen regions, such as oxygen minimum zones (OMZs) and deep anoxic basins (Wong and Brewer, 1977;Cutter et al., 2018;Rapp et al., 2020). The asymmetric reaction rates of iodide oxidation and iodate reduction complicate the relationship between iodine and oxygen, decoupling the relative concentrations in the subsurface ocean (Hardisty et al., 2021). ...
... Concentrations of seawater iodate and iodide data were taken from published sources (Wong and Brewer, 1977;Elderfield and Truesdale, 1980;Bluhm et al., 2011;Cutter et al., 2018;Moriyasu et al., 2020;Rapp et al., 2020). Station sites were selected in the closest proximity to the coral site, and the representative value was interpolated from the two closest available measurement depths. ...
... (A) Dissolved oxygen concentration versus dissolved iodate concentration in seawater. Proximal seawater iodate/iodide data were taken from published sources (Wong and Brewer, 1977;Elderfield and Truesdale, 1980;Bluhm et al., 2011;Cutter et al., 2018;Rapp et al., 2020). mmol/kg (Thiagarajan et al., 2013). ...
The distribution of dissolved iodine in seawater is sensitive to multiple biogeochemical cycles, including those of nitrogen and oxygen. The iodine-to-calcium ratio (I/Ca) of marine carbonates, such as bulk carbonate or foraminifera, has emerged as a potential proxy for changes in past seawater oxygenation. However, the utility of the I/Ca proxy in deep-sea corals, natural archives of seawater chemistry with wide spatial coverage and radiometric dating potential, remains unexplored. Here, we present the first I/Ca data obtained from modern deep-sea corals, specifically scleractinian and bamboo corals, collected from the Atlantic, Eastern Pacific, and Southern Oceans, encompassing a wide range of seawater oxygen concentrations (10–280 μmol/kg). In contrast to thermodynamic predictions, we observe higher I/Ca ratios in aragonitic corals (scleractinian) compared to calcitic corals (bamboo). This observation suggests a strong biological control during iodate incorporation into deep-sea coral skeletons. For the majority of scleractinian corals, I/Ca exhibits a covariation with local seawater iodate concentrations, which is closely related to seawater oxygen content. Scleractinian corals also exhibit notably lower I/Ca below a seawater oxygen threshold of approximately 160 μmol/kg. In contrast, no significant differences in I/Ca are found among bamboo corals across the range of oxygen concentrations encountered (15–240 μmol/kg). In the North Atlantic, several hydrographic factors, such as temperature and/or salinity, may additionally affect coral I/Ca. Our results highlight the potential of I/Ca ratios in deep-sea scleractinian corals to serve as an indicator of past seawater iodate concentrations, providing valuable insights into historical seawater oxygen levels.
... 3-fold higher than dFe below 4,000 m (Figure 3, panel c and f). Observed dFe concentrations were elevated near the coast as a result of sedimentary inputs on the Peruvian shelf (Cutter et al., 2018;Heller et al., 2017;Rapp et al., 2020). Elevated dFe concentrations were also observed at mid depths (∼2,500 m) to the west of the east Pacific Rise, and extended to the western end of the transect as a result of Fe inputs by hydrothermal vents (Fitzsimmons et al., 2017;Resing et al., 2015). ...
... 3-fold higher than dFe concentrations in the water columns of the Atlantic and Pacific Oceans (Figure 3). The similarities and differences between dFe and SFe(III) app in both ocean basins largely reflected the strong influence of point sources on the dFe distribution (e.g., shelf, atmospheric and hydrothermal inputs) (Rapp et al., 2020;Resing et al., 2015;Rijkenberg et al., 2014) or removal process (e.g., scavenging or biological uptake) (Boyd et al., 2005;Tagliabue et al., 2019). However, the distributions of dFe showed a correspondence to the changes in SFe(III) app , particularly when considering the vertical distributions. ...
An insufficient supply of the micronutrient iron (Fe) limits phytoplankton growth across large parts of the ocean. Ambient Fe speciation and solubility are largely dependent on seawater physico‐chemical properties. We calculated the apparent Fe solubility (SFe(III)app) at equilibrium for ambient conditions, where SFe(III)app is defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to organic matter formed at a free Fe³⁺ concentration equal to the solubility of Fe hydroxide. We compared the SFe(III)app to measured dissolved Fe (dFe) in the Atlantic and Pacific Oceans. The SFe(III)app was overall ∼2–4‐fold higher than observed dFe at depths less than 1,000 m, ∼2‐fold higher than the dFe between 1,000 and 4,000 m and ∼3‐fold higher than dFe below 4,000 m. Within the range of used parameters, our results showed that there was a similar trend in the vertical distributions of horizontally averaged SFe(III)app and dFe. Our results suggest that vertical dFe distributions are underpinned by changes in SFe(III)app, which are driven by relative changes in ambient pH and temperature. Since both pH and temperature are essential parameters controlling ambient Fe speciation, these should be accounted for in investigations of changing Fe dynamics, particularly in the context of ocean acidification and warming.
... The mass balance results in this study indicate the probable influence of atmospheric deposition of desert varnish and anthropogenic Mn, which can substantially alter the estimated mean annual soluble Mn deposition fluxes (and their uncertainty). Furthermore, this and earlier studies highlight the importance of diffusive dMn input from the reducing shelf/slope sediments in controlling the dMn inventory in the water column of the AS (Floback and Moffett, 2021;Lewis and Luther, 2000;Saager et al., 1989) and the global ocean (Morton et al., 2019;Rapp et al., 2020;Vieira et al., 2019). However, these fluxes remain poorly estimated. ...
... nM Co, comparable to previous observations of Fe (7.9 nM) and Co (0.2 nM) for the same shelf area (Noble et al., 2012). Extremely high Fe concentrations in our study were similar to observations from the OMZ on the Peruvian shelf where bottom Fe concentrations over 30 nM have been observed (Bruland et al., 2005;Rapp et al., 2020). Enhanced concentrations of dissolved Mn were observed in near-bottom waters, but the maximum Mn concentration (16.2 nM) was observed in surface waters likely due to photo-reduction of Mn(IV) oxides to Mn(II) by sunlight coupled to an increased stability of Mn(II) (Sunda et al., 1983). ...
Upwelling of subsurface waters injects macronutrients (fixed N, P, and Si) and micronutrient trace metals (TMs) into surface waters supporting elevated primary production in Eastern Boundary Upwelling Regions. The eastern South Atlantic features a highly productive shelf sea transitioning to a low productivity N‐Fe (co)limited open ocean. Whilst a gradient in most TM concentrations is expected in any off‐shelf transect, the factors controlling the magnitude of cross‐shelf TM fluxes are poorly constrained. Here, we present dissolved TM concentrations of Fe, Co, Mn, Cd, Ni, and Cu within the Benguela Upwelling System from the coastal section of the GEOTRACES GA08 cruise. Elevated dissolved Fe, Co, Mn, Cd, Ni, Cu and macronutrient concentrations were observed near shelf sediments. Benthic sources supplied 2.22 ± 0.99 μmol Fe m⁻² day⁻¹, 0.05 ± 0.03 μmol Co m⁻² day⁻¹, 0.28 ± 0.11 μmol Mn m⁻² day⁻¹ and were found to be the dominant source to shallow shelf waters compared to atmospheric depositions. Similarly, off‐shelf transfer was a more important source of TMs to the eastern South Atlantic Ocean compared to atmospheric deposition. Assessment of surface (shelf, upper 200 m) and subsurface (shelf edge, 200–500 m) fluxes of Fe and Co indicated TM fluxes from subsurface were 2–5 times larger than those from surface into the eastern South Atlantic Ocean. Under future conditions of increasing ocean deoxygenation, these fluxes may increase further, potentially contributing to a shift toward more extensive regional limitation of primary production by fixed N availability.
... Pour la Polynésie française, ces manifestations se traduisent par une baisse de pression atmosphérique, des précipitations plus abondantes et une eau plus chaude que les normales saisonnières (Merle & Tourre, 1984 ;Hopuare, 2014). Cette dernière est de fait moins chargée en nutriments et autres sels dissous, ce qui y limite par conséquent la croissance d'organismes planctoniques (Rapp et al., 2020). Par la suite, une anomalie La Niña fut avérée à partir de septembre 2020, avec un retour à des conditions « El Niño Southern Oscillation » (ENSO) neutres rapporté à la fin du mois de mai 2021 (WMO, 2021). ...
L’éponge marine Dactylospongia metachromia est l’objet d’études chimiques visant la valorisation pharmaceutique des métabolites secondaires qu’elle produit, notamment l’ilimaquinone et la 5-épiilimaquinone. Elle a été cultivée pendant 18 mois sur des structures immergées, à la fois en milieu récifal où elle est naturellement présente et également dans le lagon de Rangiroa, le plus vaste atoll de l’archipel des Tuamotu en Polynésie française. Les résultats obtenus montrent que le système de production aquacole retenu est à la fois durable, rapide d’installation et d’entretien de même qu’économiquement compétitif. Le suivi des nouvelles générations d’explants issus de spécimens sauvages a démontré une très forte variabilité intraspécifique au regard des performances de culture (i.e. croissance et survie) pour cette éponge, de même que certaines disparités temporelles et spatiales. Les analyses spatio-temporelles de plusieurs paramètres environnementaux ont fait ressortir leur implication, parfois conjointe, à des échelles locales, régionales et internationales. Les deux molécules ciblées pour l’extraction se sont révélés être présentent en ratios très hétérogènes selon les explants, mais en quantités non dissemblables en fonction du temps de mise en culture. Enfin, une forte activité bactéricide a pu être démontrée pour ces deux composés contre une espèce de pathogène marin néfaste au développement de la pisciculture en Polynésie française.
... It is located in the Eastern Tropical Pacific and marked by low oxygen subsurface waters extending from the Peruvian coast into the open ocean. Marine productivity in this region is constrained by light availability and limited by iron that is released from continental shelf and slope sediments and transported into surface waters by upwelling and vertical mixing Chever et al. 2015;DiTullio et al. 2005;Hutchins et al. 2002;John et al. 2018;Noffke et al. 2012;Plass et al. 2020;Rapp et al. 2020;Schlosser et al. 2018b;Scholz et al. 2016). Dissolved phosphate and nitrate are supplied by the nutrient-rich Peru-Chile Undercurrent which is the main source for the coastal upwelling off Peru (Montes et al. 2014). ...
... 12. Net export of dissolved Fe from the model across the outer model boundaries. Dissolved ferric iron is bound by organic ligands (L) and occurs as organic complex (FeL) for iron concentrations ≤ 0.8 nM Most of the dissolved iron present in the Peruvian OMZ is released from shelf and slope sediments that are covered by oxygen-depleted bottom waters (2) as previously shown by a range of measurements conducted in the Peruvian OMZ Chever et al. 2015;DiTullio et al. 2005;Hutchins et al. 2002;John et al. 2018;Noffke et al. 2012;Plass et al. 2020;Rapp et al. 2020;Schlosser et al. 2018b;Scholz et al. 2016). The deposition of terrigenous iron at the seabed (1) is thus the ultimate source of new dissolved iron in the Peruvian system. ...
A new box model is employed to simulate the oxygen-dependent cycling of nutrients in the Peruvian oxygen minimum zone (OMZ). Model results and data for the present state of the OMZ indicate that dissolved iron is the limiting nutrient for primary production and is provided by the release of dissolved ferrous iron from shelf and slope sediments. Most of the removal of reactive nitrogen occurs by anaerobic oxidation of ammonium where ammonium is delivered by aerobic organic nitrogen degradation. Model experiments simulating the effects of ocean deoxygenation and warming show that the productivity of the Peruvian OMZ will increase due to the enhanced release of dissolved iron from shelf and slope sediments. A positive feedback loop rooted in the oxygen-dependent benthic iron release amplifies, both, the productivity rise and oxygen decline in ambient bottom waters. Hence, a 1% decline in oxygen supply reduces oxygen concentrations in sub-surface waters of the continental margin by 22%. The trend towards enhanced productivity and amplified deoxygenation will continue until further phytoplankton growth is limited by the loss of reactive nitrogen. Under nitrogen-limitation, the redox state of the OMZ is stabilized by negative feedbacks. A further increase in productivity and transition to sulfidic conditions is only possible if the rate of nitrogen fixation increases drastically under anoxic conditions. Such a transition would lead to a wide-spread accumulation of toxic sulfide with detrimental consequences for fishery yields in the Peruvian OMZ that currently provides a significant fraction of the global fish catch.
... In contrast, during the March campaign, no hydrogen sulfide was present at the sediment surface and H 2 S concentrations remained below 1 mM within the first few centimeters of the sediment core. As the mobility and cycling of TMs is dependent on redox conditions (Sundby et al. 1986;Morford and Emerson 1999;Tribovillard et al. 2006;Scholz and Neumann 2007;Scholz et al. 2011;Rigaud et al. 2013;Rapp et al. 2020), the differing redox-conditions during our test deployments are expected to affect TM gradients close to the seafloor. ...
The benthic boundary layer plays a crucial role in the exchange of trace metals between surface sediments and the water column. So far it has been difficult to study dissolved–particulate interactions of trace metals in this highly reactive interface layer due to the lack of suitable sampling methods. We developed a new device, called Benthic Trace Profiler (BTP), which enables simultaneous sampling of near‐bottom water and suspended particles in high depth resolution within the first 3 m above the seafloor. The device was tested successfully in the Baltic Sea. The concentrations of several trace metals (Co, Ni, Cu, Zn, and Cd) in the collected bottom waters overlapped with concentrations in water column samples above collected with conventional methods. This observation indicates that the sampling device and method is trace metal clean. The trace metals Fe and Mn showed concentration gradients within the benthic boundary layer indicating an upward diffusive flux. This observation is consistent with a diffusive benthic flux of these trace metals across the sediment–water interface, which was independently verified using pore‐water profiles. Suspended particles can be used to study precipitation processes and to determine the carrier phases of trace metals. The BTP fulfilled all the intended requirements as it allowed a simultaneous, uncontaminating and oxygen‐free sampling of seawater and suspended particles to gather high‐resolution profiles of dissolved and particulate trace metal concentrations above the seafloor. The device closes the gap between water column and sediment sampling and helps researchers to better understand trace metal exchange processes across the ocean's lower boundary.
