Oxidative release of chromium from Archean ultramafic rocks, its transport and environmental impact – A Cr isotope perspective on the Sukinda valley ore district (Orissa, India)

Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China

“…Results of these studies allow postulating that isotope effects during riverine transport of Cr to the oceans are not significant, and that the point sources of Cr have a dominant effect on the local Cr isotope signal recorded in seawater. This is consistent with findings for another locality recently reported by Paulukat et al (2015). Ocean Cr isotope signals were also found to correlate strongly with Cr concentration (Scheiderich et al, 2015) and Cr incorporation into synthetic calcite indicated marginal to no effects on Cr isotope fractionation between ambient seawater and marine abiogenic carbonates (Dixon et al, 2013;Rodler et al, 2015).…”

“…The light Cr isotope ( 52 Cr) is assumed to be preferentially retained with Cr(III) within the weathered soil whereas the oxidized and mobilized Cr(VI) is enriched in the heavy 53 Cr isotope (Izbicki et al, 2008;Crowe et al, 2013;Frei and Polat, 2013;Berger and Frei, 2014) and transported -potentially without significant isotope effect during adsorption onto particles (Ellis et al, 2004) -via river water to the open ocean (Bonnand et al, 2013;Frei et al, 2014;Paulukat et al, 2015). There, oxygenated water stabilizes the transported Cr signal (Table 1); the position of samples presented in this figure correlates to their position in the schematic stratigraphic column and to their deposition on the Yangtze Platform (see Fig.…”

“…Planavsky et al, 2014;Reinhard et al, 2014) for example using concentrations of the immobile element Al (e.g. Voegelin et al, 2009;Eroglu et al, 2015;Paulukat et al, 2015), if these are assumed to be detritusrelated. Based on this assumption, detrital contamination can be expressed as a percentage of the analyzed Cr concentration (Table 2) (3)).…”

“…Ocean Cr isotope signals were also found to correlate strongly with Cr concentration (Scheiderich et al, 2015) and Cr incorporation into synthetic calcite indicated marginal to no effects on Cr isotope fractionation between ambient seawater and marine abiogenic carbonates (Dixon et al, 2013;Rodler et al, 2015). However, a possibly more complex internal redox cycling of Cr in the ocean can be assumed based on the variability of Cr isotope signatures in river water (Farkaš et al, 2013;Frei et al, 2014;Paulukat et al, 2015) and the heterogeneity of δ…”

“…This means, that the weathered soil remains isotopically lighter in respect to Cr while oxidative atmospheric weathering enriches the run-off in 53 Cr ( Izbicki et al, 2008;Crowe et al, 2013;Frei and Polat, 2013;Berger and Frei, 2014). The isotopically heavy, mobilized Cr(VI) is then transported by rivers (Farkaš et al, 2013;Frei et al, 2014;Paulukat et al, 2015) possibly affected by signals derived from regional variable Cr isotopic compositions of geologically different drainage systems (Scheiderich et al, 2015) -to the open ocean, where oxygenated water bodies stabilize it. This is perhaps indicated by a comparison of the surface seawater δ 53 Cr signature of the Argentinian Basin (Bonnand et al, 2013), and water samples of the Paraná River estuary and of the Paraná river .…”