PhD defence: Alexandra Rodler

Alexandra Rodler defends her thesis,

Chromium isotope uptake in carbonates - An experimental and application-based evaluation of the potential use of chromium isotopes as a paleo-climatic tracer in the marine environment

Supervisor:
Professor Robert Frei, IGN

Assessment committee:
Associate Professor Caroline Peacock, University of Leeds - UK
Professor Ronny Schönberg, Universität Tübingen - Germany
Associate Professor Jens Konnerup-Madsen (chairman), IGN

Summary:
Chromium (Cr) is a redox sensitive element potentially capable of tracing fine-scale fluctuations of the oxygenation of Earth’s early surface environments and seawater. The Cr isotope composition of carbonates could perhaps be used as paleo-redox proxy to elucidate changes in the geological past related to the rise of oxygen and the evolution of the biosphere. However, before the Cr isotope system can be applied to faithfully delineate paleo-environmental changes, careful assessment of the signal robustness and a thorough understanding of the Cr cycle in Earth system processes is necessary. Processes that potentially fractionate Cr isotopes, perhaps during deposition, burial and alteration need to be constrained.
Marine carbonates are ubiquitous throughout Earth’s rock record rendering them a particularly interesting archive for constraining past changes in ocean chemistry. This thesis includes an investigation of the fractionation behavior of Cr isotopes during coprecipitation with calcium carbonate. These results indicate that at lower Cr concentrations typical for seawater, marginal to no Cr isotope fractionation can be expected, which is a first step towards enabling a reliable application of Cr isotopes recorded in ancient carbonates and towards constraining the environmental information they can provide. The Cr isotope proxy was then applied to natural carbonates from several contemporaneous sections from today’s northern Namibia and southern China, to decipher redox changes during build-up and retreat of one of the major late Neoproterozoic glaciations (Marinoan). These carbonate δ⁵³Cr signatures were sensitive to changes in terrestrial weathering balanced between detrital contamination and oxidative weathering as well as diagenetic alteration.
The redox changes of past surface environments can potentially be explored using the Cr isotope composition of ancient marine carbonates, where a marginal offset compared to contemporaneous seawater δ⁵³Cr is expected and the degree of contamination and later diagenetic alteration needs to be evaluated. An improved understanding of processes that affect the Cr isotope composition during deposition and burial of the archive they are recorded in will perhaps improve our understanding of redox processes of Earth’s surface environments in the geological past.

The thesis is available from the PhD administration office, 04.1.417