Abstract
Many marine Precambrian iron formations (IF) record deep anoxic seawater enriched in Fe(II) (i.e. ferruginous) overlain by mildly oxygenated surface water. This is reflected by iron-rich sediments forming in deep basins, and relatively iron-poor sediments forming in shallow, sunlit waters. Such an iron gradient is often interpreted as a redox interface where dissolved Fe(II) was oxidized and precipitated as Fe(III)-bearing minerals. As such, sedimentary iron enrichments are proxy to the progressive oxidation of the oceans through geological time. However, this interpretation is founded on the assumption that Fe(II) could not persist within an oxygenated water column. Here, we cultivated cyanobacteria in an illuminated column supplied with Fe(II)-rich seawater medium in a laboratory-scale analog of a continental margin supporting IF deposition. We first observed Fe(II) oxidation with oxygen, then biologically-mediated reduction of Fe(III) (oxyhydr)oxides, which maintained a pool of Fe(II) in the presence of oxygen. Such steady-state iron redox cycling may have maintained dissolved, and hence mobile Fe(II) in oxygenated seawater above ferruginous deep basins such as those inferred for many Precambrian IF.
Highlights
Ferruginous) overlain by mildly oxygenated surface water
In order to simulate a Precambrian continental margin from which IF deposited after ferruginous waters upwelled from a deep basin, Fe(II)-containing seawater medium was pumped into the column from ports at the bottom, and exited at the air-water interface (Fig. 1)
The column was designed to simulate the chemical and physical environments available at different depths along an oxygenated Precambrian continental margin subjected to upwelling of Fe(II)-rich water, from which deepwater IF were depositing (Fig. 1)
Summary
Ferruginous) overlain by mildly oxygenated surface water. This is reflected by iron-rich sediments forming in deep basins, and relatively iron-poor sediments forming in shallow, sunlit waters. We first observed Fe(II) oxidation with oxygen, biologically-mediated reduction of Fe(III) (oxyhydr)oxides, which maintained a pool of Fe(II) in the presence of oxygen Such steady-state iron redox cycling may have maintained dissolved, and mobile Fe(II) in oxygenated seawater above ferruginous deep basins such as those inferred for many. Due to the low redox potential of Fe2+/Fe3+ (E°′ = +0.36 V), non-diagenetic sedimentary enrichments indicative of Fe(II) in seawater should indicate the absence of oxygen in that same water This delineation between oxic and ferruginous water masses manifests itself as a sharp redoxcline between Fe(II) and oxygen-containing layers of water in modern anoxic basins and lakes, or in sediment cores, such that Fe(II) and oxygen are generally not observed to coexist in stratified systems[5,6]. Processes for Fe(III) reduction may have been favored by the higher concentrations of organic material available to complex and solubilize Fe(III) in the upper water column of Precambrian seawater[15]
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