Fractionation Behavior of Chromium Isotopes during the Sorption of Cr (VI) on Kaolin and its Implications for Using Black Shales as a Paleoredox Archive

Abstract

AbstractChromium stable isotope variations in marine black shales are increasingly used to constrain redox changes though time. However, how well black shale δ53Cr values represent ambient seawater δ53Cr values is difficult to assess as black shales contain both detrital and authigenic mineral phases, of which only authigenic phases are expected to mirror seawater, while detrital silicate phases are expected to show igneous δ53Cr values. In order to correct for detrital contamination, it is thus imperative to know whether detrital minerals are in fact characterized by igneous δ53Cr values. To investigate this further, three sets of Cr (VI) batch sorption experiments were conducted, reacting kaolin with (1) with a synthetic Cr (VI) solution, (2) with Cr (VI) doped river water, and (3) with Cr (VI) doped seawater. Further, sequential leaches were applied to reacted kaolin to evaluate how well sorbed Cr (VI) can be leached. The experiments revealed that Cr (VI) readily sorbs on kaolin, changing its Cr concentration and isotopic composition. In river water, kaolin sorbed up to 36 μg of Cr (VI) per gram of kaolin and recorded a negative offset in bulk δ53Cr of up to 0.26‰ relative to unreacted kaolin, while no sorption was observed in seawater. Our leaching experiments revealed that already a weak acid leach (0.5M HCl) readily mobilizes some of the sorbed Cr from kaolin, which when applied to black shales will likely contaminate leachates designed to release authigenic Cr. This has major implications for the application of Cr stable isotopes in black shales as marine paleoredox archives.