Open ocean vs. continentally-derived iron cycles along the Neoarchean Campbellrand-Malmani Carbonate platform, South Africa

Abstract

The deposition of large amounts of mixed-valence Fe minerals in iron formations during the Archean and Paleoproterozoic indicates that the Fe(II)aq (aqueous) content of coeval anoxic seawater was likely several hundred μM, compared to ca. 1 to 20 nM of the modern oxygenated ocean. It has been suggested that oxygen production along shallow marine continental shelves, which probably started several hundred million years before the rise of atmospheric oxygen, effectively oxidized Fe(II)aq from deeper seawater and removed it as Fe(III)ppt (poorly soluble precipitates). However, the reconstruction of the marine Fe cycle during the Archean is still incomplete, partly because of diagenetic redox processes that challenge the interpretation of Fe concentration and isotope signatures of sedimentary archives. In this study, we present new Fe concentrations and isotope compositions of carbonate and mudrock samples from the Neoarchean Campbellrand-Malmani carbonate platform (CMCP) in South Africa. These samples are from the shelf facies of the CMCP and in combination with previously published data of Czaja and others (2012) from carbonates and mudrocks of the slope facies, we show that different depositional settings and conditions resulted in different data distributions. Coupled δ56Fe values (−3.685 to +0.083 ‰) and iron concentrations (861–27672 μg g−1) of pure carbonates deposited during open marine conditions, can be explained by partial Fe(II) oxidation between ferruginous deeper water and oxygenated shallow water, leaving the residual Fe(II)aq pool isotopically light, although Fe(II) oxidation by anoxygenic phototrophy cannot be ruled out. Pure carbonates deposited in a peritidal setting, with less exposure to open ocean water, show a smaller Fe isotope variability with δ56Fe values of −1.207 to −0.204 permil and Fe concentration range from 388 to 5413 μg g−1, respectively. We propose that the Fe systematics of peritidal carbonates were dominated by early diagenetic Fe cycling between carbonates and adjacent mudrocks. Synchrotron based X-ray adsorption spectroscopy reveals a change in Fe speciation, where Fe(II)-bearing ankerite and Fe-sulfide dominate the carbonates in the lower part of the CMCP, whereas carbonates of the upper part of the CMCP mainly contain Fe(III)-(oxyhydr)oxides. The fact that Fe(III) phases are still preserved argues for a higher oxidation state on the shelf of the upper CMCP. This is likely because of a lower content of reductants in those settings, in particular organic carbon, sulfide species, as well as restricted influx of reducing species from the anoxic open ocean due to the formation of a rimmed margin. Nevertheless, more studies of similar carbonate settings are necessary to verify our model. We propose that unfractionated Fe(II)aq in seawater was about two to three times lower on the shelf (30–310 μM) than along the slope (61–928 μM), which implies that Fe(II)aq was removed from the water column closer to the continent, likely by oxidation and precipitation. Overall, the Fe isotope composition and Fe speciation of CMCP sediments support the presence of molecular oxygen in the shallow-marine system and emphasize the utility of Ca-Mg carbonates as proxies for iron cycling in the aqueous environment.