Noah Planavsky
MGG Student Seminar Abstract
Steep Rock Stromatolites: The Earliest Record of Oxygenic Photosynthesis?
One of the most dramatic changes in Earth's history is the shift from a reducing to an oxidizing atmosphere. There are multiple lines of evidence that the atmosphere first became oxygenated around 2.4 billion years ago. The evolution of oxygenic photosynthesis is ultimately responsible for the Earths oxygenation. The timing of the emergence of the earliest oxygenic photosynthetic organisms, likely cyanobacteria, is still unclear. Since the first discovery of early Precambrian microfossils, the dominant view has been that cyanobacteria evolved well before the atmosphere became oxygenated. This model was supported by classical interpretations of stromatolites and organic biomarker work. More recently there has been questioning of the theoretical and empirical evidence for an early emergence of cyanobacteria. Numerous models have directly linked the evolution of oxygenic photosynthesis with the collapse of a methane rich atmosphere and the shift to oxidizing conditions. I will demonstrate that the 2.9Ga Steep Rock carbonate succession contains microbial influenced stromatolites and carbonate precipitates with trace element signatures of oxygenated environments. The trace element systematics, stable isotopes, and carbonate textures indicate exceptional preservation and rule out the possibility of diagenetic overprint. Therefore, the Steep Rock carbonates provide strong evidence for at least localized oxidizing conditions in Early Precambrian and for the evolution of oxygenic photosynthesis well before the shift to an oxidizing atmosphere.