Here, I used the novel, redox-sensitive chromium isotope system to trace the evolution of an oxygenated atmosphere. This was investigated by laboratory experiments and ancient carbonates using a multi-proxy approach.

In my dissertation research, I propose that only marginal Cr isotope fractionation can be expected during co-precipitation with calcium carbonate phases [1]. This indicates that Cr stable isotope signals (denoted as δ53Cr) of ancient carbonate rocks might reflect the coeval seawater composition and might thus be useful to infer paleo-redox conditions. Additional co-precipitation experiments of chromate with calcium carbonate also indicated kinetic isotope fractionation potentially due to sorption effects on mineral surfaces (in preparation).

Assuming carbonates can retain an original δ53Cr signal, this has a strong potential to provide important constraints on past atmospheric oxygen levels. Following this experimental approach to validate the Cr stable isotope system for paleo-environmental reconstructions, I used this proxy in tandem with other paleo-proxies to evaluate the environmental conditions following two major glaciation events (‘Snowball Earth’), the Sturtian (ca. 715 million years ago) and the Marinoan (635 million years ago), recorded by Neoproterozoic carbonate sequences from China (Yangtze Platform) and Namibia (Otavi and Witvlei Groups) [2, 3, 4].

Left to right: Keilberg section, Otavi Group, Namibia, dropstone in Neoproterozoic carbonate sediments, calcium carbonate/SEM image (photographs by A.S. Rodler)

This project was based at the Geocenter Denmark, DCIG, IGN/Section for Geology, University of Copenhagen, as part of the research project “The development and application of non-traditional isotope tracers to past climate change” and was funded by the Danish Agency for Science, Technology and Innovation (grant #31512), the Nordic Center for Earth Evolution (NordCEE/Copenhagen node) and the Danish National Research Foundation (grant #DNRF53).