Iron Isotopes Reveal a Benthic Iron Shuttle in the Palaeoproterozoic Zaonega Formation: Basinal Restriction, Euxinia, and the Effect on Global Palaeoredox Proxies

Abstract

The Zaonega Formation in northwest Russia (~2.0 billion years old) is amongst the most complete successions that record the middle of the Palaeoproterozoic era. As such, geochemical data from the formation have played a central role in framing the debate over redox dynamics in the aftermath of the Great Oxidation Event (GOE). However, uncertainty over local redox conditions and the degree of hydrographic restriction in the formation has led to contradictory interpretations regarding global oxygen (O2) fugacity. Here, we provide new iron (Fe) isotope data together with major and trace element concentrations to constrain the local physiochemical conditions. The Zaonega Formation sediments show authigenic Fe accumulation (Fe/Al ≫ 1 wt.%/wt.%) and δ56Fe ranging from −0.58‰ to +0.60‰. Many of the data fall on a negative Fe/Al versus δ56Fe trend, diagnostic of a benthic Fe shuttle, which implies that Zaonega Formation rocks formed in a redox-stratified and semi-restricted basin. However, basin restriction did not coincide with diminished trace metal enrichment, likely due to episodes of deep-water exchange with metal-rich oxygenated seawater, as evidenced by simultaneous authigenic Fe(III) precipitation. If so, the Onega Basin maintained a connection that allowed its sediments to record signals of global ocean chemistry despite significant basinal effects.

Limited expression of the Paleoproterozoic Oklo natural nuclear reactor phenomenon in the aftermath of a widespread deoxygenation event ~2.11–2.06 billion years ago

“…Geochemical models for the Zaonega Formation, based largely on trace metal enrichments in the black shales (e.g., Mänd et al, 2020Mänd et al, , 2021Paiste et al, 2020aPaiste et al, , 2020b, also converge on the view that the Onega basin was likely well-connected to the Paleoproterozoic ocean. In order to provide new insight into whether biogeochemical processes in the Onega and Francevillian basins reflect local processes in restricted marine settings or deposition in marine basins well-connected to the Paleoproterozoic ocean, V/Al vs. Mo/Al and V/Al vs. U/Al cross-plots (Fig.…”

“…Combined biological methane cycling in the Onega basin, high rates of organic carbon loading, enrichment in redoxsensitive elements, and Fe and Mo isotope values indicate deposition of the Zaonega Formation in a redox-stratified basin (cf. Asael et al, 2018;Mänd et al, 2021).…”

Chromium evidence for protracted oxygenation during the Paleoproterozoic

“…There is strong evidence for basin restriction in the Onega Basin that would allow such a scenario-evaporite mineralogy and sedimentary textures indicative of shallow deposition abound in the Tulomozero Formation (Blättler et al, 2018;Melezhik et al, 1999). Furthermore, the sulfur and iron isotope records in the Zaonega Formation have been interpreted to reflect varying levels of restriction (Mänd et al, 2021;Paiste et al, 2020b). In this case, distillation of the dissolved Cr pool by preferential drawdown of 52 Cr (i.e., Rayleigh fractionation) could have driven δ 53 Cr more positive.…”