Iron Isotope Constraints on the Archean and Paleoproterozoic Ocean Redox State

Abstract

The response of the ocean redox state to the rise of atmospheric oxygen about 2.3 billion years ago (Ga) is a matter of controversy. Here we provide iron isotope evidence that the change in the ocean iron cycle occurred at the same time as the change in the atmospheric redox state. Variable and negative iron isotope values in pyrites older than about 2.3 Ga suggest that an iron-rich global ocean was strongly affected by the deposition of iron oxides. Between 2.3 and 1.8 Ga, positive iron isotope values of pyrite likely reflect an increase in the precipitation of iron sulfides relative to iron oxides in a redox stratified ocean.

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

“…While igneous rocks and siliciclastic sedimentary rocks with low carbon and sulphur contents have limited Fe isotope variations, diagenetic pyrites in Archaean black shales show highly variable and mostly negative values ranging from -3.5 to +0.5 ‰ (Rouxel et al, 2005;Yamaguchi et al, 2005;Archer and Vance, 2006). The same applies to bulk composition of C-and S-rich shales (Yamaguchi et al, 2005).…”

“…Using reduced isotopic partition function ratios for FeS 2 and Fe(II)-aquachloro complexes determined previously (Polyakov, 1997;Polyakov and Mineev, 2000;Schauble et al, 2001;Polyakov et al, 2007), the isotope fractionation factors  between FeS 2 and FeCl 4 2-or Fe(H 2 O) 6 2+ are ~1.0015 at 350°C. Although these positive fractionation factors are opposite to the kinetic isotope fractionation during FeS precipitation, either from Fe(II) aq solutions at room temperature (Butler et al, 2005) or from silicate melt at magmatic temperatures (Schuessler et al, 2007), it may explain the occurrence of positive A different interpretation involves sulphidisation of Fe-oxide minerals for the origin of these grains, as positive δ 56 Fe values have so far been described mainly from BIFs in the Archaean rock record Dauphas et al, 2004;Rouxel et al, 2005;Whitehouse and Fedo, 2007). Evidence for sulphidisation of Fe and Fe-Ti oxides, such as magnetite and ilmenite, collectively referred as "black sands", as well as BIF clasts has been observed in previous studies (Ramdohr, 1958) and resulted in a long-standing debate on the timing of sulphidisation, with the "placerists" arguing for pre-depositional sulphidisation (Ramdohr, 1958;Reimer and Mossman, 1990) and the "hydrothermalists" arguing for postdepositional sulphidisation of "black sands" (Barnicoat et al, 1997;Law and Phillips, 2006).…”

“…The c. 2.96 and 2.90 Ga magnetite-rich shales of the Witwatersrand Supergroup have also yielded predominantly negative δ 56 Fe values (-1.4 to 0.3 ‰; Yamaguchi et al, 2005). In contrast, Feoxides in Archaean BIFs are frequently characterised by positive δ 56 Fe values up to 1.6 ‰ Dauphas et al, 2004;Rouxel et al, 2005;Whitehouse and Fedo, 2007), although δ 56 Fe values for magnetite-rich bands from the c. 2.48-2.45 Ga Hamersley and Transvaal BIFs of Western Australia and South Africa, respectively, range from -1.0 ‰ to +1.2 ‰, with the average of 0.0 ‰ . Modern seafloor hydrothermal pyrite deposits have variable but mostly negative δ 56 Fe values ranging from -2.1 to -0.1 ‰, a wider range than that of seafloor-hydrothermal fluids defined at around -0.5 ± 0.3 ‰ (Rouxel et al, 2004Sharma et al, 2001;Beard et al, 2003) and, likely, reflecting variable kinetic and/or equilibrium isotope effect during pyrite precipitation pathways in hydrothermal conditions .…”

“…For several large pyrite grains from the Mozaan Contact Reef we analysed handpicked pyrite fragments as well as the bulk pyrite sample. Fe was purified on Bio-Rad AG1X8 anion resin and iron isotope ratios were determined with a Thermo Scientific Neptune multicollector inductively coupled plasma mass-spectrometer (MC-ICP-MS) following previously published methods (Rouxel et al, 2005. Fe isotope values are reported relative to the Fe-isotope standard IRMM-14 using the conventional delta notations.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…6 do not neccessarily support a mixture of hydrothermal and benthic Fe sources. On the other hand, some scatter is expected given the range in δ 56 Fe values for the hydrothermal component (recording differing extents of oxidation), and the range in ε Nd (t) values for the continental endmember, and perhaps the focus of interpretation is the fact that the observed data do not lie near the "L-shaped" trend that is required by a strictly hydrothermal model, such as that proposed by Rouxel et al (2005), who argued that the negative δ 56 Fe values of IFs reflect a residual iron component after extensive oxidation.…”