Stromatolites as archives for metal mobilization and early life metabolisms? Uranium and Mo isotope studies of modern and Archean stromatolites and carbonates.

• Applicant: Professor Stefan Weyer, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry ; Palaeontology
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 404682152

In the proposed project, we aim to investigate the suitability of stromatoilites as archive for I) the redox evolution of early oceans and atmosphere and II) for the appearance of microorganism that were capable to reduce metals. In order to reach these aims, we want to investigate the trace element- and Mo- and U-isotope compositions of recent and ancient (Permian, Neoproterozoic and Neo- to Paleoarchean) stromatolites. Many carbonates directly record the Mo and U isotope composition of their water source. However, stromatolites appear to consist of microbial communities, including beside cyanobacteria also a variety of metabolisms that are capable to reduce metals, such as sulfate and iron reducers and potentially also denitrification-performing organisms. Several modern stromatolites show significant enrichment of redox-sensitive metals in the deeper anoxic layers, such as Mo. Thus, in contrast to other carbonates, stromatolites may bear characteristic isotopic signals of Mo and U reduction. Recent experimental studies imply that the characteristically strong U isotope fractionation during U reduction (between UIV and UVI) may be limited to biotic U reduction, which is likely the dominant mechanism in modern near-surface anoxic environments, such as the deep Black Sea and Cariaco Basin. As a result, sediments of such restricted ocean basins frequently show a correlation of Mo and U isotope signatures. The key-questions, related to the investigation of recent stromatolites planned in this proposal are R1) do they record Mo and U isotopic signatures associated with metal reduction or R2) do they rather archive the isotopic signatures of their water source? In case R1) stromatolites may represent a tool to reconstruct the appearance of metal-reducing metabolisms, while in case R2) they may represent recorders for the redox evolution of seawater (and the atmosphere). The key-questions, related to the investigation of the early Proterozoic and Archean stromatolites are: A1) are the isotopic signatures generated during their deposition preserved in these stromatolites and A2) what can they tell us about the redox-evolution of the oceans and/ or the first appearance of metal reducing organisms? Ancient samples are selected in order to represent a variety of early Earth key intervals, but also to investigate effects of diagenetic alteration (incl. dolomitisation and silification) on the Mo and U isotope signatures. Furthermore, the signatures of the stromatilites will be compared with signatures recorded in other carbonates and organic-rich shales from the same period, in order to optimize reconstructions of the paleo-redox evolution.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1833: Building a Habitable Earth
• International Connection: Australia
• Cooperation Partner: Professor Dr. Martin van Kranendonk