Chromium fluxes and speciation in ultramafic catchments and global rivers

McClain, Cynthia N.; Maher, Kate

doi:10.1016/j.chemgeo.2016.01.021

Cited by 94 publications

(71 citation statements)

References 164 publications

(234 reference statements)

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“…A greater abundance of greenhouse gases (CO 2 , CH 4 ) in the Archaean and Proterozoic atmosphere is required to counteract the “faint young sun” and several models have deduced atmospheric CO 2 at least 20 times present‐day atmospheric levels (Kanzaki & Murakami, ; Kasting, ; Sheldon, ; Sleep & Zahnle, ). Higher P CO2 , apart from enhancing weathering rates, could generate a greater abundance of carbonate ions which appear to play a role in stabilising and enhancing the mobility of aqueous Cr(III) (McClain & Maher, ). Lower pH conditions in soils and the hydrosphere, a further potential consequence of higher atmospheric P CO2 , could have also enhanced mineral dissolution and inhibited adsorption of cationic aqueous Cr(III) species.…”

Section: Discussionmentioning

confidence: 99%

Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol

2016

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Fractionation of stable Cr isotopes has been measured in Archaean paleosols and marine sedimentary rocks and interpreted to record the terrestrial oxidation of Cr(III) to Cr(VI), providing possible indirect evidence for the emergence of oxygenic photosynthesis. However, these fractionations occur amidst evidence from other geochemical proxies for a pervasively anoxic atmosphere. This study examined the Cr geochemistry of the ca. 1.85 Ga Flin Flon paleosol, which developed under an atmosphere unambiguously oxidising enough to quantitatively convert Fe(II) to Fe(III) during pedogenesis. The paleosol shows an extreme range in Cr isotope composition of 2.76 ‰ δ Cr. The protolith greenstone (δ Cr: -0.23 ‰), the deepest weathering horizon (δ Cr: -0.15 to -0.23 ‰) and a residual corestone in the upper paleosol (δ Cr: -0.01 ‰) all exhibit Cr isotopic compositions comparable to unaltered igneous rocks. The most significant isotopic fractionation is preserved in the areas influenced by oxidative subaerial weathering (i.e. increase in Fe(III)/Fe(II)) and the greatest loss of mobile elements. The uppermost paleosol horizon is both Cr and Mn depleted and offset to significantly Cr-enriched compositions (δ Cr values between +1.50 and +2.38 ‰), which is not easily modelled with the oxidation of Cr(III) and loss of isotopically heavy Cr(VI). Instead, the currently preferred model for these data invokes the open-system removal of isotopically light aqueous Cr(III) during either pedogenesis or subsequent hydrothermal/metamorphic alteration. The Cr enrichment would then represent the preferential dissolution or complexation of isotopically light aqueous Cr(III) species (enhanced by lower pH conditions and possibly the presence of complexing ligands) and/or the residual signature from preferential adsorption of isotopically heavy Cr(III). Both scenarios would contradict the widely held assumption that only redox reactions of Cr can generate large magnitude isotopic fractionations and, if substantiated, non-redox isotope effects would complicate the conclusive fingerprinting of ancient atmospheric O from Cr isotope data alone.

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Section: Discussionmentioning

confidence: 99%

Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol

2016

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“…In comparison, ground waters are believed to supply only ~0.3% of the riverine Cr flux McClain and Maher, 2016;Qin and Wang, 2017). Both hydrothermal inputs (Sander and Koschinsky, 2000) and benthic fluxes (Jeandel and Minster, 1984) may hold local importance.…”

Section: Marine Biogeochemistry Of Chromiummentioning

confidence: 99%

“…Due to increased flux estimates of the riverine Cr source (McClain and Maher, 2016), the most recent estimate of the Cr ocean residence time is 3000 years (Qin and Wang, 2017;Reinhard et al, 2013).…”

Section: Marine Biogeochemistry Of Chromiummentioning

confidence: 99%

The marine biogeochemistry of chromium isotopes

Moos¹

2018

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“…Both studies imply that formation of inorganic complexes and ion exchange reactions may be important sources of Cr(III) isotopic fractionation in nature. A carbonate complex (

Cr false(III false) OH {({CO}_{3})}_{2}^{2 -}

) may contribute to Cr(III) solubility and attendant isotopic fractionation effects in higher pH waters (McClain & Maher, ; Rai et al., ).…”

Section: Discussionmentioning

confidence: 99%

“…Furthermore, no attempt is made in this paper to determine the more rigorous estimate of a flux‐weighted average δ 53 Cr value for river inputs of Cr to the oceans. This approach is not only hampered by the paucity of δ 53 Cr data on rivers, but by the fact that many rivers are polluted with Cr(VI) by local industry (McClain & Maher, ).…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2019

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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.

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Chromium fluxes and speciation in ultramafic catchments and global rivers

Cited by 94 publications

References 164 publications

Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol

Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol

The marine biogeochemistry of chromium isotopes

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

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