PhD: Tracing Earths Early Oxygenation by Synchrotron Element Mapping of Microbialites

Oxygenation of surface environments during the Archean-Palaeoproterozoic (the Great Oxidation Event, GOE, around 2400 Ma) was a fundamental prerequisite for all later biological evolution on Earth. This key event in Earth history was possible through the development of cyanobacterial oxygenic photosynthesis . However, exactly when oxygenic photosynthesis evolved, and how it shaped the oxygenation of Earth’s surface environments, is an on-going debate. This study uses for the first time modern microbial structures (stromatolites) to determine if they record a trace metal (metallomics) signature of oxygenic photosynthesis. Such a modern baseline signature will then allow a test whether it can survive through geological time and when photosynthesis evolved. Microbial fabrics in rocks such as stromatolites provide a direct record of microbial consortia, including cyanobacteria, and can potentially trace oxygenation. Carbonate precipitation, which stabilizes and promotes lithification of the microbial mat, is a typical by-product of photosynthetic processes , and of heterotrophic degradation of cyanobacterial biomass7 . As these consortia respond to changing oxygen levels, they incorporate vital trace elements, such as metals, into the cell structure and extracellular polymeric substances (EPS). Metals and their degradation potentially provide a tracer of an active photosystem, and this fingerprint can potentially survive over geological time. Colleagues at SEES and Stanford University have developed Synchrotron Rapid Scanning X-Ray Fluorescence (SRS-XRF) methods for non-destructive mapping of the chemical composition within biological structures. This allows high-resolution mapping of element concentrations, their oxidation state and coordination with organic carbon and sulfur. Pilots have demonstrated the technical feasibility of analyzing metal enrichment using SRS-XRF on modern mats and Archean stromatolites.