A revised global estimate of dissolved iron fluxes from marine sediments

Abstract

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.

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

“…These observations enable an empirical assessment of the impact of oxygen concentration on the release of Fe(II) from seafloor sediments (Dale et al 2015). Parts of the UK shelf, other than our study site, seasonally undergo modest periods of reduced-oxygen concentration (e.g., 160-200 lmol l -1 , compared to 280-310 lmol l -1 at other times of the year; Greenwood et al 2010).…”

“…To a first approximation, this is a favourable comparison, and it is not unreasonable that individual study sites will have benthic Fe fluxes that deviate from the averaged relationships described by Dale et al (2015). However, it is also clear that well-oxygenated ocean margins have been largely absent from the compiled benthic Fe flux data used to parameterise ocean biogeochemical models to date (Homoky et al 2016), hence there is potential for an underestimated contribution of dFe from oxic ocean margins.…”

“…We can compare our diffusive Fe(II) fluxes from porewaters with those predicted by a recent global assessment of benthic Fe flux measurements from benthic chambers. Dale et al (2015) describes the dependence of benthic Fe flux on the rate of organic carbon oxidation in sediments and bottom water oxygen concentrations based on a compilation and regression of all known determinations.…”

“…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).…”

“…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).…”