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.
Highlights
The Proterozoic eon (2500–541 Ma) hosted some of the most fundamental global environmental transitions in Earth’s history, including the multi-step rise of oxygen in the atmosphere [1]
The most pronounced amongst these are the Great Oxidation Event (~2502–2320 Ma) [2,3,4], the Lomagundi Event, which has been proposed to coincide with an “O2 overshoot” (~2200–2060 Ma) [5,6,7], and the terminal oxygenation of the atmosphere in the Neoproterozoic Oxidation Event ~800–550 Ma [8]
The Palaeoproterozoic Zaonega Formation comprises a sedimentary succession that records the end of the Lomagundi Event and the O2 overshoot [19]
Summary
The Proterozoic eon (2500–541 Ma) hosted some of the most fundamental global environmental transitions in Earth’s history, including the multi-step rise of oxygen in the atmosphere [1]. The Palaeoproterozoic Zaonega Formation comprises a sedimentary succession that records the end of the Lomagundi Event and the O2 overshoot [19] As such, it has been the target of a number of attempts to assess global redox conditions using a variety of geochemical proxies, such as carbon and nitrogen isotopes [10,12,20], rare earth element patterns [12,21], sulphur isotopes [17], selenium isotopes [22,23], and the abundances and isotopes of several redox-sensitive metals [13,17,24,25,26,27]. This assumption is being increasingly challenged on the basis that the Zaonega Formation was influenced by significant syn-depositional magmatic activity resulting in hydrothermal circulation, hydrocarbon generation and seepage, basinal reconfiguration and restriction, as well as local modulation of both the sulphur and carbon cycles [12,15,16,20,21,28,29]
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