The rise and fall of Archean atmospheric oxygen - Did temporary carbon burial as Fe(ox)-DOM complexes play a modulating role?

• Applicant: Professor Christian Hallmann, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 404679450

Access to molecular oxygen is still considered the primary driver for the evolution of increasingly complex life on Earth. In this respect the emergence of oxygenic photosynthesis, and the subsequent Great Oxidation Event (GOE) represent two pivotal incidents in Earth history. Details of the protracted rise of atmospheric oxygen are however still sparse. For example, we know that the initial, substantial rise of atmospheric O2 during the GOE was immediately followed by a renewed drawdown. Only much later, during the Neoproterozoic, did a renewed rise of atmospheric O2 occur - this time to stable near-modern levels. What caused the initial overshoot and renewed drawdown? The oxidizing power released during oxygenic photosynthesis corresponds stoichiometrically to the amount of carbon that is fixed into biomass from CO2. Remineralization of the biomass causes renewed O2 drawdown and hence the oxidation of Earth’s early atmosphere would not have been possible without carbon burial and the initial establishment of a crustal carbon reservoir. Once biomass reaches the sediment it is gradually converted to stable kerogen, and typically locked up in the geological portion of the carbon cycle on time scales >10^6 years. But this could have been very different for more labile forms of buried carbon. We know that the Archean marine pool of dissolved organic matter (DOM) must have been magnitudes larger than at present, and that enormous volumes of banded iron formations (BIF) were deposited during the Archean. The BIF-precursor mineral ferrihydrite is prone to adsorbing large amounts of DOM, but due to a low stability it tends to recrystallize already during earliest stages of metamorphosis. In this project we will test the hypothesis that DOM-ferrihydrite complexes could have temporarily buried sufficient organic matter to cause a substantial rise in atmospheric oxygen. In contrast to sedimentary kerogen, virtually nothing is known about the geological fate of DOM. The possibility exists that it would have been lost from immature BIF sediments upon recrystallization of the ferrihydrite during earliest metamorphism. Heterotrophic reworking of the released DOM would subsequently down-regulate the previously released oxygen. We will create complexes of ferrihydrite and pure cyanobacterial DOM, which will be subjected to simulated regional metamorphism in an autoclave to reveal the stability and behavior of DOM under geological conditions of catagenesis and early metamorphism. Eventually we will evaluate the possibility of temporary carbon storage in BIFs as a modulator for Archean atmospheric oxygen levels - both the overshoot and the renewed drawdown - thereby essentially evaluating the likelihood of tectonic control on the protracted rise of oxygen and the evolution of organismic complexity.

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