UBC Theses and Dissertations
Chromium isotopes : fractionation from continental weathering through to deposition in marine sediments Davidson, Ashley Breanne
Variation in the composition of chromium (Cr) stable isotopes has emerged as a powerful tracer for environmental processes as a result of the Cr isotope fractionation induced during the reduction of Cr(VI) to Cr(III). Comprehensive characterization of the geochemical processes that act on and can change Cr speciation, and of all processes that result in Cr isotope fractionation, is needed in order to make robust inferences based on the variation in Cr isotopic compositions. Our knowledge of Cr geochemistry in the natural environment is limited, with lab-based experiments often not capturing the natural complexity of the environment, in part due to Cr speciation being driven by kinetic factors, resulting in disequilibrium, as well as limited speciation data, partially due to a lack of reproducible methods. This dissertation presents a new speciation method and a wealth of new Cr speciation data resulting from the application of this method, including the first species-specific Cr isotope compositions measured in seawater across an oxic-anoxic boundary. These results provide clear evidence of positive and negative excursions in water column δ53Cr(III) in response to oxygenation and deoxygenation events, respectively, which are likely to be reflected in the authigenic Cr sedimentary record. Modeling of water column data also highlights the role of reservoir effects in muting Cr isotope fractionation relative to the intrinsic fractionation factor associated with a given process, like Cr(VI) reduction. We find non-redox processes such as water mass mixing to be a key process in Saanich Inlet, while in treated stream waters we find evidence of substantial oxidative remobilization of Cr(VI) due to reaction between reactive Cr(III) precipitates and manganese (Mn) oxides. Together, the work in this dissertation identifies the local key processes controlling Cr speciation and isotopic composition in these studied environments, with water mass mixing and in-situ Cr(III) oxidation in particular having a strong effect on the resulting Cr isotope composition, both of which are underrepresented processes in current studies. By contributing this new knowledge on Cr behaviour in the modern environment, we help to facilitate more robust inferences of Cr cycling in past environments.
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