Astrobiologists supported in part by the Exobiology and Evolutionary Biology program have discovered that the biological oxygen cycle of the early Earth may have been more dynamic than previously thought. Complex life on Earth needs oxygen to survive. However, it wasn’t until the “Great Oxidation Event (GOE),” some 2.4 billion years ago that oxygen became a significant component of our planet’s atmosphere.

Previously, geologists had used the presence of sulfur isotopes in the rock record to determine when the GOE occurred. The isotopes form when solar energy interacts with sulfur dioxide in low-oxygen conditions. Their presence in the rock record is used a geological “fingerprint” indicating that oxygen levels were still low.

However, when studying how sulfur and its isotopes cycle through Earth’s crust, the team found that sulfur signals could have persisted in ocean sediments long after oxygen levels in that atmosphere increased. Weathering rocks from the contents transferred the sulfur signals into the oceans where they became incorporated in ocean sediments long after the isotopes were actually created in the atmosphere.
The lag between isotope formation and incorporation into the rock record could challenge geologists’ efforts to determine when and how the GEO occurred.

Funding for the research came from an O.K. Earl Postdoctoral Fellowship at Caltech, a National Science Foundation postdoctoral fellowship, and a NASA Exobiology grant. The paper “Long-term sedimentary recycling of rare sulfur isotope anomalies,” was recently published in the journal Nature under lead author Christopher T. Reinhard.