PhD defence: Cora Paulukat

Cora Paulukat defends her thesis,

Chromium stable isotope fractionation in modern biogeochemical cycling: Insights from continental weathering flux to the ocean, and chromium incorporation into carbonate shells.

Supervisor
Professor Robert Frei, IGN

Assessment committee
Professor Tod Waight (chairman), IGN
Professor Rachael James, University of Southampton - UK
Professor Thomas Nägler, University of Bern - Switzerland

Abstract
Chromium (Cr) is, due to its redox-sensitive properties, a powerful tracer for redox processes in environmental studies. Changes in its preferred oxidation state (III and VI) are accompanied by Crisotope fractionation. The Cr-isotope system is a promising tool to reconstruct the evolution of free oxygen in the Earth’s atmosphere.
Oxidative rock weathering on land induces oxidation of immobile Cr(III) to mobile Cr(VI). Isotopically relatively heavy Cr(VI) is released to runoff, and transported by rivers to the oceans, where it is incorporated into chemical sediments and carbonate shells.
In the present study, the isotopic composition of Cr is traced along this pathway to fill gaps in the understanding of the Cr-isotope system in natural environments. The thesis can be divided into three parts:
The first part focuses on Cr release during oxidative weathering. Isotopically light Cr in modern laterite soils from India, formed on ultramafic rocks, indicates extensive leaching of isotopically heavy Cr(VI). Transferring this knowledge to ancient weathering profiles, negatively fractionated Cr is clear evidence for the presence of free oxygen in the atmosphere.
The second part demonstrates that the positively fractionated Cr from the catchment area is preserved during its riverine transport to the sea. A global compilation of surface seawater Cr-isotope compositions shows a heterogeneous Cr-isotope distribution in the oceans, indicating that the signal is influenced by local factors as reduction processes (biological productivity).
In the third part the potential use of the Cr-isotope system in the marine environment is discussed. Incorporation into biogenic carbonates (bivalves, gastropods, corals) is accompanied by Cr-isotope fractionation, causing negative seawater-shell offsets. None of the herein studied species show a defined δ⁵³Cr_seawater-δ⁵³Cr_sample offset.

The thesis is available from PhD administration office 04.1.417