Chromium speciation and its stable isotopic signature in the dolomite-terra rossa weathering system

摘要

We present the results of a study of chromium (Cr) content, Cr speciation and its stable isotope composition in a terra rossa weathering profile, which includes ferromanganese concretions (FMCs) and gibbsite spots, developed on dolomite in the Yunnan-Guizhou Plateau in south central China. Sequential extraction results show that the Cr in all samples is present mainly within the residual fraction and oxidizable fraction, and that the Cr content of the exchangeable fraction and reducible fraction is very low. The significantly high Cr content (mean of 21.0 ppm) of the oxidizable fraction is a potential environmental hazard. The Cr in the terra rossa and gibbsite spots exhibits absolute accumulation throughout the profile; correspondingly, the FMCs are significantly enriched in most trace elements, except for Cr. Re-oxidation of Cr(III) to soluble Cr(VI) by MnO2 is likely responsible for the relative depletion of Cr in the FMCs. In the bulk dolomite, the delta Cr-53 values of the carbonate component and detrital component are 0.447 parts per thousand and - 0.437 parts per thousand, respectively. The delta Cr-53 signal in both fractions is heterogeneous, while both delta Cr-53 signals are significantly different from that of bulk silicate Earth (BSE; -0.124 +/- 0.101 parts per thousand). Our results provide a proof of concept that the allogenic Cr-isotopic composition of terrigenous minerals in carbonate rocks, can provide new insights into paleo-terrestrial oxidative weathering. Overall, the terra rossa is characterized by the enrichment of heavy Cr isotopes compared to the detrital component of dolomite. Hence, the isotopic signature of Cr does not support the idea that the detrital component in dolomite is the sole parent material of the terra rossa. The absolute accumulation of Cr and heavy Cr isotope enrichment in terra rossa indicate that the extraneous input of dissolved Cr(VI) with a heavy isotopic signature occurred during dolomite weathering and terra rossa formation.