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.