Distinct iron isotopic signatures and supply from marine sediment dissolution

Abstract

Iron (Fe) inputs to the surface ocean may stimulate photosynthesis and have an impact on the uptake of carbon dioxide in the ocean on glacial to inter-glacial timescales of climate change1. Global ocean reservoir-flux models2 indicate that 90% of Fe used by marine phytoplankton in the present day surface ocean is supplied from the deep water below, but the sources of dissolved Fe to this deep water are still poorly constrained. Therefore, quantifying and tracking iron supplied to the ocean will provide key information to resolve climate models and sensitivity to the Fe cycle3, 4.

Measurable differences in the isotopic composition of Fe between various sources to the ocean have prompted widespread interest in seawater Fe isotope determintions5, 6, 7, which can potentially be used to track Fe inputs and assess the relative importance of different sources of dissolved Fe to the oceanic reservoir. Microbial sediment respiration supports a major flux of dissolved and isotopically light Fe to the global ocean8, 9, 10, by catalysing the reductive dissolution (RD) of Fe oxyhydroxide minerals during organic matter decomposition11. Reduction of Fe oxyhydroxide enriches soluble Fe(II)(aq) in sediment pore water, which diffuses into bottom water when the oxygenated layer of surface sediment is adequately shallow9, 12, most notably from oxygen-deficient continental margins8, 9, 10. Benthic fluxes of Fe are mixed in bottom waters and can be transported to open ocean and surface waters13, 14, where Fe may control the efficacy of the biological carbon pump15, 16.

Dissolved Fe(II)(aq) produced by RD initially has ?56Fe values 0.5–2.0‰ lighter than the original substrates17, and at isotopic equilibrium, experiments show ?56Fe(II)(aq) is ?1.05 to ?3.99‰ relative to the common reactive Fe oxides haematite17, goethite18 and ferrihydrite17, 19, 20. Similar light ?56Fe values (?1.82 to ?3.45‰) have been observed in both the pore waters21, 22, 23 and overlying seawater9, 24 of river-dominated and dysoxic margins, and light Fe isotopic compositions are recorded in ocean basin sediments coeval with past episodes of ocean oxygen deficiency, consistent with seawater transport of light Fe from ferruginous shelf sediments to ocean basins25. Thus, benthic fluxes of isotopically light Fe appear to be distinguishable from other sources of Fe to the ocean, such as atmospheric dust dissolution (?56Fe=+0.13±0.18‰)26 and river discharge (?56Fe=+0.14±0.28‰)27.

Paradoxically, however, equatorial Pacific seawater originating from the continental margin of New Guinea contains elevated Fe concentrations with heavy Fe isotopic compositions (?56Fe=+0.37±0.15‰)28. These and other seawater isotope measurements have led to the proposition of an additional ‘non-reductive dissolution’ (NRD) mechanism for Fe28, 29, albeit with existing Fe isotope evidence from continental margin sediments indicating otherwise9, 24. These findings coincide with a growing need to evaluate the geographical variability of benthic Fe fluxes to effectively model carbon cycling in the ocean3, 4, where models presently rely on global extrapolations from potentially unrepresentative regions.

Here we characterise the pore water isotopic composition and corresponding flux of dissolved Fe from the Cape margin, South Africa—a semi-arid passive margin derived from deeply weathered saprolite soils and surrounded by oxygenated South Atlantic seawater. These sites are distinct from most previous sites of benthic Fe flux investigation, which have focused on active margins next to areas of rapid uplift with oxygen-deficient shelf waters (Fig. 1). This study reveals that the amount of dissolved Fe released from the Cape margin is less than predicted by benthic Fe flux relationships8 widely used to model ocean Fe–CO2 interaction3, 4. We report solid-phase compositional data that suggests that the small pore water Fe flux reflects geological and hydro-climatic influences on reactive Fe substrate delivery to the shelf. Isotopically heavy Fe present in ‘oxidizing’ pore waters of the Cape margin—a zone previously beyond analytical resolution—provides in situ evidence for the role of ‘NRD’ of Fe proposed by Radic et al.28 These discoveries have implications for past and present oceanic Fe cycles and the parameterization of ocean biogeochemical models.

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Identifiers

ORCID for Rachel A. Mills: orcid.org/0000-0002-9811-246X

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