Cr(III)-mobility and isotopic effects by adsorption on Mn- and Fe-(hydr-)oxides in the Critical Zone and in marine sediments: Doubts that Mesoarchean Cr isotopic variations directly record free atmospheric oxygen

• Applicant: Professor Dr. Ronny Schönberg, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277010024

Stable chromium isotopic variations recently found in up to 3 billion year old banded iron formations (BIFs; 53Cr enriched) and paleosols (53Cr-depleted) were interpreted to provide evidence for free atmospheric oxygen at that time. This, however, is some 600 million years earlier than the 2.4 Gyr Great Oxidation Event (GOE) defined by the disappearance of mass-independent fractionation of sulfur isotopes (MIF-S) and macroscopic manifestations of oxidative weathering in the rock record. The interpretation of these Cr isotope data is based on the presumption that Cr in soils underwent oxidative weathering forming a soluble 53Cr-enriched Cr(VI)-pool that was transported into the oceans and deposited in BIFs via reductive interaction and adsorption with iron. Meanwhile, oxidative weathering left behind a 53Cr-depleted soil reservoir. New evidence has been put forward, such as heavy stable Cr isotopic compositions of serpentinized ultramafic rocks, the lack of any Cr isotopic variations in Proterozoic near-surface chemical sediments deposited under (low-) O2 conditions or our own 53Cr-enriched values of a 1.85 Gyr old post-GOE paleosol with clear oxidative loss of Cr and retention of Fe, that argue against such an uniformitarian interpretation of stable Cr isotope data in ancient chemical sediments and paleosols. In this study we aim to investigate Cr mobility in reduced aquifers and oceans and stable Cr isotope fractionation associated with adsorption of Cr(III) onto different mineral surfaces. Results from Cr isotope fractionation experiments and Cr speciation and isotope composition of a groundwater profile will be compared to Cr isotopes from a 2.45 Gyr pre-GOE paleosol profile to investigate alternative models for Cr isotope variations in ancient sediments.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1833: Building a Habitable Earth