PhD defence: Anja Birte Frank

Anja Birte Frank defends her thesis,

Chromium Isotope Variations in Black Shales
Evaluating the Applicability of Chromium Stable Isotope signatures of Ancient Black Shales as a Paleoredox Archive

Professor Robert Frei, IGN

Assessment Committee:
Professor Caroline L. Peacock, University of Leeds - UK
Professor Bernd Lehmann, Technical University of Clausthal - Germany
Assistant Professor Nina Søager (chair), IGN

Chromium (Cr) stable isotopes in marine sediments, such as banded iron formations, have the potential to better our understanding of atmospheric oxygenation as Cr isotope fractionation is redox sensitive. Recent studies have focussed increasingly on Cr stable isotope compositions of black shales to constrain changes in surface oxygen levels. However, how representative of ancient seawater measured δ53Cr values of black shales are, is difficult to assess as black shales typically contain both detrital and authigenic mineral phases, as well as organic matter. Cr isotope studies aimed at reconstructing ancient marine δ53Cr values target the authigenic phases (e.g. carbonates and phosphates) as they are expected to mirror ancient seawater best, while detrital silicate-phases (e.g. feldspars and clays) are expected to show unfractionated δ53Cr values typical for continental igneous rocks. In order to best possible characterise the authigenic component of such sediments, it is imperative to know whether or not the detrital component is in fact characterised by igneous Cr isotope values, as bulk isotope analyses of black shales need to be corrected for the relative proportion and isotope composition of such detrital components.
With this thesis, I aim to investigate and constrain this further using laboratory experiments on ancient black shales and clay minerals and by combining δ53Cr data of Ediacaran-Cambrian drill core samples with other geochemical redox proxies. A four-step sequential leach method was applied on ancient black shale, chert and phosphorite samples, which revealed that the majority of Cr is hosted in likely detrital silicates, while generally only ≤10% of the total Cr is hosted by authigenic carbonates/phosphates phases. Further, no difference in degree of fractionation was observed between the leachates of the different samples compared to their respective bulk, which suggests 1) that the detrital component of the investigated samples were positively fractionated compared to typical igneous δ53Cr values and 2) that the Cr isotope composition of the samples was homogenised between the different Cr hosting phases. Cr(VI) batch adsorption experiments on kaolin (as a proxy for the clay mineral fraction of black shales) revealed that Cr(VI) can potentially be sorbed onto clay minerals during riverine transport, recording the δ53Cr value of ambient river water to some extent. Sorbed Cr was readily remobilised from the reacted kaolin using already weak acids, suggesting that weak leaches designed to selectively attack the authigenic phases of black shales are likely contaminated by sorbed Cr from the clay mineral fraction.
The laboratory experiments demonstrate that the δ53Cr values of detrital and authigenic mineral phases in black shales are dependent on more processes than just those induced by oxidative weathering. Combined with other geochemical redox tracers, however, δ53Cr data of black shales was successfully applied to constrain redox conditions during the Early Cambrian. This supports that Cr stable isotopes in black shales are a valuable paleoredox archive, as long as their limitations are properly addressed and accounted for.

The thesis is available for inspection at the PhD administration office 04.1.413