Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol, Northwest Territories, Canada

Abstract

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox‐sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle‐reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ53Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady‐state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ53Cr values, and possibly lower than the igneous inventory (–0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H2O2, but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.

A paleosol record of the evolution of Cr redox cycling and evidence for an increase in atmospheric oxygen during the Neoproterozoic

“…A vague picture of the temporally and/or spatially confined nature of these pulses may be drawn from a cluster of ca. 1.9–1.8 Ga paleosols and marine sediments from Canada, which show quite different ranges of δ 53 Cr (Figure ), as well as variable Fe mobility (Babechuk et al, ; Bellefroid et al, ; Frei & Polat, ; Toma, Holmden, Shakotko, Pan, & Ootes, ). The redox conditions that prevailed in any one of these paleosols appear to have been localized in time and/or space, as the majority of Proterozoic paleosols as well as marine records indicate that the background state was one of limited (<1% PAL) atmospheric oxygen.…”

“…Where error bars do not appear, uncertainty is smaller than symbol size. The range of paleosol δ 53 Cr data from several Paleoproterozoic paleosols are also plotted as gray bars: 2.45 Ga Cooper Lake (Babechuk et al, 2019), 1.9 Ga Beaverlodge Lake (Toma et al, 2019), and 1.85 Ga Flin Flon (Babechuk et al, 2017). We omit the Schreiber Beach paleosol (Frei & Polat, 2013) owing to the evidence for trace and radiogenic element mobility (see discussion within that paper) and questions about its geologic context (i.e., whether it is a typical paleosol; Fralick et al, 2017).…”

Chromium evidence for protracted oxygenation during the Paleoproterozoic

“…Furthermore, δ 53 Cr values of Rhyacian iron formations are even less fractionated (up to +0.03‰) (26). The anomalous feature of the later Paleoproterozoic δ 53 Cr record is high positive fractionations (up to +2.34‰) in the ~1.9 Ga Flin Flon and Beaverlodge Lake paleosols (89,90), which constitute the opposite of the expected isotope effect of oxic Cr weathering and have, thus, not been linked to oxidative Cr cycling.…”

“…Collectively, these data have been used to suggest a late Paleoproterozoic to Mesoproterozoic Cr cycle highly distinct from the modern-variable atmospheric pO 2 produced, potentially in an alternating fashion, localized positively fractionated Cr runoff related to Mn(IV) oxide cycling (87,88) or negatively fractionated runoff related to ligandor acid-based solubilization (89,90). The oceans, meanwhile, are implied to have remained predominantly anoxic, with relatively minor fractionated Cr runoff diluted within an unfractionated marine reservoir.…”

Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK

“…However, the mean value for river water (0.49 ± 0.45‰) is higher than the mean value for seawater (1.07 ± 0.35‰), which implies that δ 53 Cr values may be altered after oxidative weathering occurs, for example by redox reactions of Cr in surface ocean waters (Scheiderich et al, 2015;Goring-Harford et al, 2018;Bruggmann et al, 2019). Although other potential mechanisms for Cr isotope fractionation in natural waters have yet to be investigated in detail, it is likely that refinements to the model proposed by Frei et al 2009 are required to properly employ Cr as a proxy for atmospheric oxygenation (see also Toma et al 2019).…”