About 2.5 billion years ago, Earth’s atmosphere began to accumulate free oxygen, leading to the formation of complex life. This period, known as the Great Oxidation Event or GOE, lasted for at least 200 million years and was crucial in shaping our planet’s history. However, the initial rise of oxygen was not a straightforward process, as research led by University of Utah geochemist Chadlin Ostrander reveals. Analyzing marine shales from South Africa’s Transvaal Supergroup, Ostrander and his team found evidence of fluctuations in ocean oxygen levels that coincided with changes in atmospheric oxygen.
The presence of rare, mass-independent sulfur isotope signatures in sedimentary records before the GOE serves as evidence of an anoxic atmosphere. This evidence suggests that for the first half of Earth’s existence, the atmosphere and oceans were devoid of oxygen. Cyanobacteria in the ocean were producing oxygen, but it was being rapidly destroyed by reactions with minerals and volcanic gases. Ostrander and his colleagues discovered that multiple rises and falls of oxygen occurred during the GOE, indicating a dynamic process rather than a single event.
Understanding the dynamics of oxygen levels in Earth’s oceans during the GOE is essential for unraveling the evolution of early life on our planet. Ostrander’s research team focused on stable thallium isotopes in marine shales to map oxygen levels in the ocean. Thallium isotopes are sensitive to manganese oxide burial on the seafloor, a process that requires oxygen in seawater. By analyzing thallium isotopes in samples that also tracked atmospheric oxygen fluctuations, the team found enrichments in the lighter-mass thallium isotope during periods of oxygen accumulation in seawater.
The findings of Ostrander’s research shed light on the co-evolution of Earth’s atmosphere and oceans during the GOE. When sulfur isotopes indicated an oxygenated atmosphere, thallium isotopes revealed oxygenation in the oceans as well. This synchronization between atmospheric and oceanic oxygenation is a key aspect of the study, providing new insights into the complex processes that shaped Earth’s oxygen levels during this critical period. The research not only validates existing hypotheses but also extends the understanding of oxygen dynamics to the ocean environment where early life likely originated and evolved.
By extending the understanding of how oxygen levels fluctuated during the GOE, Ostrander and his team have contributed valuable insights into the evolution of life on Earth. The research highlights the importance of studying both atmospheric and oceanic oxygenation processes in determining the conditions that allowed for the rise of complex life forms. Ostrander’s expertise in stable thallium isotopes has enabled a detailed analysis of ancient oxygen levels in the oceans, providing a more comprehensive picture of the environmental changes that paved the way for life as we know it. This new information is crucial for advancing our knowledge of early Earth and the processes that led to the development of complex life forms.