Absence of hexavalent chromium in marine carbonates: implications for chromium isotopes as paleoenvironment proxy

Abstract

The oxygenation of Earth's atmosphere is widely regarded to have played an important role in early-life evolution. Chromium (Cr) isotopes recorded in sedimentary rocks have been used to constrain the atmospheric oxygen level (AOL) over geological times based on the fact that a positive Cr isotopic signature is linked to the presence of Cr(VI) as a result of oxidative continental weathering. However, there is no direct evidence of the presence of Cr(VI) in sedimentary rocks yet. Carbonates are most widely distributed over geological times and were thought to have incorporated Cr(VI) directly from seawater. Here, we present results of Cr valence states in carbonates which show Cr(III) is the dominant species in all samples spanning a wide range of geological times. These findings indicate that Cr(VI) in seawater was reduced either before or after carbonate precipitation, which might have caused Cr isotopic fractionation between seawater and carbonates, or marine carbonates preferentially uptake Cr(III) from seawater. As Cr(III) can come from non-redox Cr cycling, which also can cause isotopic fractionation, we suggest that positively fractionated Cr isotopic values do not necessarily correspond to the rise in AOL.

Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil

“…Microbes are particularly efficient in reducing and immobilizing soluble toxic Cr(VI) to less soluble Cr(III) in aquatic settings (Basu et al, 2014;Han et al, 2012;Lovley, 1993;Paulukat et al, 2015;Sikora et al, 2008;Zhang et al, 2018Zhang et al, , 2019. A recent study has shown that Cr(III) may be the dominant species in carbonates regardless of the δ 53 Cr values and the age of sample (Fang et al, 2020). This is a thought-provoking data, and further investigations based on a meticulous petrographic and geochemical characterization of samples and paleoenvironments must be done in order to properly interpret it.…”

Terahertz Dating of Sedimentary Rocks

“…Sedimentary strata provide natural records of geologic history, and the accurate determination of their sequence and age is a prerequisite for studying their paleoenvironment, paleogeography, and tectonic evolution history [1][2][3]. Time, space, materials, and movement are the basic elements used to study geological processes and environmental evolution; and time is the basis for studying geological problems.…”

The changes of chromium speciation in limestone samples during simulated natural attenuation processes

“…The chromium standards, including Cr(VI), Cr 2 O 3 , and Cr(OH) 3 , were also determined. Cr(VI) can be identi ed by its distinct pre-edge peak (~ 5993 eV) in XANES spectra, whereas, the differences in nearedge structures can be used to distinguished the different chemical forms of Cr(III)(Rajapaksha et al 2018;Fang et al 2021). Comparing amorphous Cr(OH) 3 , the spectra of the crystalline Cr 2 O 3 shows more complex structures(Rai et al 2007).…”

“…The Cr content in these maps could be natural occurring Cr(III) minerals, and Cr(III) precipitates, which is the reducing products of Cr(VI). Since the Cr(VI) can be adsorbed by some minerals, such as calcite(Fang et al 2022) in those samples, there was still a small amount of Cr(VI) that could be detected in both sample A and B, even after those samples were washed with DI water to remove soluble Cr(VI) before preparing thin sections. Some high Cr spots were found to be associated with minerals containing sulfur and Fe(II), such as pyrite and chalcopyrite.…”

“…When Fe(II)-containing mineral is oxidized by Cr(VI), it transforms to Fe(III), (Fe(OH) 3 ), and then Cr(III) can substitute for Fe(III) forming mixed Cr(III)-Fe(III) hydroxides(Tang et al 2010). Those precipitates of Cr(III)-Fe(III) hydroxides have greater stability than Cr(III) hydroxides(Sass and Rai 1987), therefore Cr(III)-Fe(III) hydroxides could be the major forms of Cr(III) precipitates, when Fe(II)-containing minerals occurring.Fang et al reported that during co-precipitated with CaCO 3 , most Cr was adsorbed on the crystal surface of calcite or existed as amorphous Cr(III) hydroxide(Fang et al 2022). As a carbonate sedimentary rock, limestone is composed mostly of calcite and aragonite, the chemical form of which is calcium carbonate (CaCO 3…”