Due to the redox-sensitivity of chromium (Cr), ratios of Cr isotopes (δ⁵³Cr) are thought to be useful tracers of changes in the oxygenation state of seawater. Cr isotope ratios in sedimentary rocks thus allow reconstructions of past oceanic (and potentially atmospheric) redox conditions. Despite the extensive use of the Cr isotope system as redox proxy, processes controlling Cr (isotope) cycling are not fully understood.
This thesis investigates modern seawater samples coupled to siliciclastic sediments and biogenic carbonates for their Cr concentrations and δ⁵³Cr values. The collected data contribute to elucidating Cr isotope fractionation effects accompanying Cr uptake into sediments and to better characterising the potential of sediments to record the δ⁵³Cr values of the environment within which they formed. Analysis of modern seawater samples from restricted and open marine conditions reveal that variations in Cr concentrations and isotopic compositions are controlled by local dynamics of biochemical parameters. For example, Cr
fractionation caused by sorption to organic matter or mixing of water-masses with distinct Cr pools seem to have a stronger impact on the Cr isotopic composition of a water sample than the oxygenation state. Still, δ⁵³Cr values of siliciclastic sediments correspond to the oxygenation state of the overlying water column, confirming the redox-sensitivity of Cr isotopes at least under some circumstances. Mollusc shells were found to incorporate heterogeneous
δ⁵³Cr values that are strongly affected by vital effects as well as by salinity and
organic matter. Although Cr accumulation in microbialites is poorly constrained, they seem to incroporate distinct δ⁵³Cr values that may reflect both the conditions of the environment in which they precipitated and the microbial metabolism involved.
Mechanisms other than redox changes (e.g., vital effects) challenge the use of Cr isotopes for redox reconstructions. Analyses of coupled modern seawater and geological samples support that Cr isotopes have the potential to record redox conditions of past environments. However, the limitations of this potential have to be better constrained and archives that may allow reliable interpretations of δ⁵³Cr values have to be further characterised.