The continental shelf benthic iron flux and its isotope composition

Severmann, Silke; McManus, James; Berelson, William M.; Hammond, Douglas E.

doi:10.1016/j.gca.2010.04.022

Cited by 260 publications

(316 citation statements)

References 107 publications

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“…Porewater dFe maxima are isotopically light (d 56 Fe -1.7 to -0.9%), consistent with supply of dFe(II) from the dissimilatory reduction of Fe(III) oxide by bacteria (Severmann et al 2006;Homoky et al 2009;Henkel et al 2016). In addition, our observations of dFe, d 56 dFe, dMn, nitrate and POC content in the Celtic Sea are similar to observed ranges in sediments from low-oxygen California-Oregon shelves (Severmann et al 2006;Homoky et al 2009Homoky et al , 2012 where benthic fluxes of isotopically light dFe to the ocean are observed (Severmann et al 2010). For the first time we have combined physicochemical Fe observations in porewater with speciation measurements.…”

Section: Discussionsupporting

confidence: 83%

“…Albeit, our reported values remain lower than surface sediments underlying near-anoxic waters (e.g. *100 lM dFe(II), (Severmann et al 2006(Severmann et al , 2010. Dissolved Fe concentrations in overlying bottom waters (20 nM, with 15 nM as dFe(II); Table S4) were 1-5 orders of magnitude lower than the upper centimetre of porewater, and even lower in bottom water 10 m above the seafloor (5-10 nM; Fig.…”

Section: Discussionmentioning

confidence: 60%

“…Atmospheric inputs are most significant where dust inputs are large, and are directly delivered to the photic zone (de Jong et al 2007). However, sediments underlying shelf and slope waters are argued to be a major source of Fe that can ultimately be transferred to the ocean (Johnson et al 1999;Elrod et al 2004;Severmann et al 2010;Homoky et al 2012;Conway and John 2014;Dale et al 2015).…”

Section: Introductionmentioning

confidence: 99%

“…These Fe(II) maxima carry diagnostically light isotopic signals, typically -2.0 to -1.0% (Severmann et al 2006(Severmann et al , 2010Homoky et al 2009). It is widely accepted that towards the oxic sediment-water boundary Fe(II) is oxidised to Fe(III), and lost to insoluble Fe(III) oxides.…”

Section: Introductionmentioning

confidence: 99%

“…Benthic chamber measurements have shown that the largest fluxes of dFe are released from continental margin sediments with high inventories of organic carbon and reactive iron minerals underlying oxygen-depleted waters (Severmann et al 2010;Dale et al 2015). However, few studies have examined the Fe cycle in oxic shelf seas, even though they represent a large fraction of the ocean's continental boundaries (Homoky et al 2016).…”

Section: Introductionmentioning

confidence: 99%

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Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

et al. 2017

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Shelf sediments underlying temperate and oxic waters of the Celtic Sea (NW European Shelf) were found to have shallow oxygen penetrations depths from late spring to late summer (2.2-5.8 mm below seafloor) with the shallowest during/after the spring-bloom (mid-April to mid-May) when the organic carbon content was highest. Sediment porewater dissolved iron (dFe,\0.15 lm) mainly ([85%) consisted of Fe(II) and gradually increased from 0.4 to 15 lM at the sediment surface to *100-170 lM at about 6 cm depth. During the late spring this Fe(II) was found to be mainly present as soluble Fe(II) ([85% sFe, \0.02 lm). Sub-surface dFe(II) maxima were enriched in light isotopes (d 56 Fe -2.0 to -1.5%), which is attributed to dissimilatory iron reduction (DIR) during the bacterial decomposition of organic matter. As porewater Fe(II) was oxidised to insoluble Fe(III) in the surface sediment layer, residual Fe(II) was further enriched in light isotopes (down to -3.0%). Ferrozine-reactive Fe(II) was found in surface porewaters and in overlying core top waters, and was highest in the late spring period. Shipboard experiments showed that depletion of bottom water Responsible Editor: Martin Solan.Electronic supplementary material The online version of this article (doi:10.1007/s10533-017-0309-x) contains supplementary material, which is available to authorized users. 49-67 DOI 10.1007/s10533-017-0309-x oxygen in late spring can lead to a substantial release of Fe(II). Reoxygenation of bottom water caused this Fe(II) to be rapidly lost from solution, but residual dFe(II) and dFe(III) remained (12 and 33 nM) after [7 h. Iron(II) oxidation experiments in core top and bottom waters also showed removal from solution but at rates up to 5-times slower than predicted from theoretical reaction kinetics. These data imply the presence of ligands capable of complexing Fe(II) and supressing oxidation. The lower oxidation rate allows more time for the diffusion of Fe(II) from the sediments into the overlying water column. Modelling indicates significant diffusive fluxes of Fe(II) (on the order of 23-31 lmol m -2 day -1 ) are possible during late spring when oxygen penetration depths are shallow, and pore water Fe(II) concentrations are highest. In the water column this stabilised Fe(II) will gradually be oxidised and become part of the dFe(III) pool. Thus oxic continental shelves can supply dFe to the water column, which is enhanced during a small period of the year after phytoplankton bloom events when organic matter is transferred to the seafloor. This input is based on conservative assumptions for solute exchange (diffusion-reaction), whereas (bio)physical advection and resuspension events are likely to accelerate these solute exchanges in shelf-seas.

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Section: Discussionsupporting

confidence: 83%

Section: Discussionmentioning

confidence: 60%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

et al. 2017

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Iron sources and dissolved‐particulate interactions in the seawater of the Western Equatorial Pacific, iron isotope perspectives

Labatut

Lacan

Pradoux

et al. 2014

Global Biogeochemical Cycles

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This work presents iron isotope data in the western equatorial Pacific. Marine aerosols and top core margin sediments display a slightly heavy Fe isotopic composition (δ 56 Fe) of 0.33 ± 0.11‰ (2SD) and 0.14 ± 0.07‰, respectively. Samples reflecting the influence of Papua New Guinea runoff (Sepik River and Rabaul volcano water) are characterized by crustal values. In seawater, Fe is mainly supplied in the particulate form and is found with a δ 56Fe between À0.49 and 0.34 ± 0.07‰. The particulate Fe seems to be brought mainly by runoff and transported across continental shelves and slopes. Aerosols are suspected to enrich the surface Vitiaz Strait waters, while hydrothermal activity likely enriched New Ireland waters. Dissolved Fe isotopic ratios are found between À0.03 and 0.53 ± 0.07‰. They are almost systematically heavier than the corresponding particulate Fe, and the difference between the signature of both phases is similar for most samples with Δ 56 Fe DFe -PFe = +0.27 ± 0.25‰ (2SD). This is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases. The dissolved phase being heavier than the particles suggests that the exchanges result in a net nonreductive release of dissolved Fe. This process seems to be locally significantly more intense than Fe reductive dissolution documented along reducing margins. It may therefore constitute a very significant iron source to the ocean, thereby influencing the actual estimation of the iron residence time and sinks. The underlying processes could also apply to other elements.

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A revised global estimate of dissolved iron fluxes from marine sediments

Dale

Nickelsen

Scholz

et al. 2015

Global Biogeochemical Cycles

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Literature data on benthic dissolved iron (DFe) fluxes (μmol m À2 d À1 ), bottom water oxygen concentrations (O 2BW , μM), and sedimentary carbon oxidation rates (C OX , mmol m À2 d À1 ) from water depths ranging from 80 to 3700 m were assembled. The data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic DFe fluxes: DFe flux ¼ γ Á tanh COX O2BW where γ (= 170 μmol m À2 d À1 ) is the maximum flux for sediments at steady state located away from river mouths. This simple function unifies previous observations that C OX and O 2BW are important controls on DFe fluxes. Upscaling predicts a global DFe flux from continental margin sediments of 109 ± 55 Gmol yr À1 , of which 72 Gmol yr À1 is contributed by the shelf (<200 m) and 37 Gmol yr À1 by slope sediments (200-2000 m). The predicted deep-sea flux (>2000 m) of 41 ± 21 Gmol yr À1 is unsupported by empirical data. Previous estimates of benthic DFe fluxes derived using global iron models are far lower (approximately 10-30 Gmol yr À1 ). This can be attributed to (i) inadequate treatment of the role of oxygen on benthic DFe fluxes and (ii) improper consideration of continental shelf processes due to coarse spatial resolution. Globally averaged DFe concentrations in surface waters simulated with the intermediate-complexity University of Victoria Earth System Climate Model were a factor of 2 higher with the new function. We conclude that (i) the DFe flux from marginal sediments has been underestimated in the marine iron cycle and (ii) iron scavenging in the water column is more intense than currently presumed.

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The continental shelf benthic iron flux and its isotope composition

Cited by 260 publications

References 107 publications

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

Iron sources and dissolved‐particulate interactions in the seawater of the Western Equatorial Pacific, iron isotope perspectives

A revised global estimate of dissolved iron fluxes from marine sediments

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