Distinct iron isotopic signatures and supply from marine sediment dissolution

Abstract

Oceanic iron inputs must be traced and quantified to learn how they affect primary productivity and climate. Chemical reduction of iron in continental margin sediments provides a substantial dissolved flux to the oceans, which is isotopically lighter than the crust, and so may be distinguished in seawater from other sources, such as wind-blown dust. However, heavy iron isotopes measured in seawater have recently led to the proposition of another source of dissolved iron from ‘non-reductive’ dissolution of continental margins. Here we present the first pore water iron isotope data from a passive-tectonic and semi-arid ocean margin (South Africa), which reveals a smaller and isotopically heavier flux of dissolved iron to seawater than active-tectonic and dysoxic continental margins. These data provide in situ evidence of non-reductive iron dissolution from a continental margin, and further show that geological and hydro-climatic factors may affect the amount and isotopic composition of iron entering the ocean.

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

“…Accordingly, a trend towards lower porewater d 56 Fe is observed from *6 cm depth (* -1.0%) towards the sediment surface (* -3.0%) during both seasons, indicative of oxidative Fe(II) removal, and recycling during DIR (Severmann et al 2006;Homoky et al 2009). A return to higher porewater d 56 Fe in the uppermost sediment layer was observed during late spring and similar trends have been observed in sediment cores collected from shelf-slope sediments in the South East Atlantic (Homoky et al 2013) and in the North Sea (Henkel et al 2016). Henkel et al (2016) reason that oxidative precipitation of Fe preferentially removes light isotopes, as proposed by Staubwasser et al (2013), due to environmental variances in kinetic and equilibrium isotope fractionation processes compared to experiments (e.g.…”

“…The reduction of squared residuals between modelled and observed values was used to optimize the model fit. The approach follows (Berner 1980), in which a single pool of reactive organic C is attributed to oxygen consumption and the influences of bioturbation, seasonal sediment accumulation and porosity structure are ignored, as described elsewhere (e.g., Papadimitriou et al 2004;Homoky et al 2013). …”

“…Welch et al 2003). Whereas Homoky et al (2013) reasoned that a transition to higher d 56 Fe towards the sediment surface resulted from mixing with an isotopically heavier and more stable Fe source, that has a relatively low dFe concentration. In both scenarios, a potential role for organic complexion of Fe exists.…”

“…At each depth, suction was applied and then, after discarding the first 0.5 ml to waste, a porewater sample was collected. This approach allows for the fast extraction and handling of porewater samples (Homoky et al 2013), and effectively isolates redox sensitive trace metals from oxidation artefacts. Subsample aliquots were immediately fixed with the synthetic Fe(II) ligand ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine) for ship-board Fe(II) and total Fe analyses (details below).…”

“…This process is associated with the removal of heavy Fe isotopes leading to further enrichment of light Fe isotopes in the porewater (Severmann et al 2006;Homoky et al 2009). Within more oxidizing sediments, the isotopic signature of dFe is close to crustal compositions, e.g., 0.0-0.2% (Homoky et al 2009(Homoky et al , 2013, and the dFe fraction consists mainly of Fe colloids (0.02-0.2 lm), that are argued to originate from ''non-reductive'' dissolution processes (Radic et al 2011;Homoky et al 2011Homoky et al , 2013. The Fe isotopic composition in the water column may serve as a tool to trace benthic Fe fluxes which originate from different dissolution processes and are transported to the ocean interior (Conway and John 2014).…”