The Great Oxidation Event (GOE) was a major turning point when oxygen began to build up in Earth’s atmosphere and surface environments. To understand what caused this shift, how fast it happened, and how it affected early life and chemical cycles, scientists study elements like nitrogen that change depending on oxygen levels.
Earlier research suggested that the ocean’s nitrogen cycle became more oxygen-rich around 2.33 billion years ago, during the final phase of the GOE. However, recent evidence suggests that the GOE wasn’t a single event; it was more like a series of ups and downs. Plus, existing studies only cover limited areas, so there’s still uncertainty about what the nitrogen cycle looked like before oxygen became a permanent feature in Earth’s atmosphere.
Scientists are eager to learn when and why Earth’s atmosphere began filling with oxygen, because that shift paved the way for complex life, including humans. To solve such a mystery, researchers from Syracuse University and MIT are studying ancient rock layers buried under South Africa. These rocks hold clues about how fast life evolved as oxygen levels rose, shedding light on the long and winding path toward the first eukaryotic cells with a nucleus like ours.
Researchers analyzed ancient sedimentary rock cores from multiple sites in South Africa. These rocks, dated between 2.2 and 2.5 billion years old, are just the right age to preserve signs of Earth’s changing atmosphere.
Oxygen could have been available to life as early as 3.5 billion years ago
By examining the stable isotopes locked inside, researchers uncovered clues pointing to ocean processes that needed nitrate, a chemical that only forms when there’s enough oxygen. This suggests parts of Earth’s oceans were already becoming oxygen-rich during the GOE.
The ancient rocks held only trace amounts of nitrogen. It was far too little to be measured using standard tools. But thanks to cutting-edge tech developed by researcher Chris (one of only a few experts worldwide with such an instrument), the team could measure nitrogen isotope ratios in samples containing 100 to 1,000 times less nitrogen than usual. This breakthrough allowed them to extract valuable information from previously unreadable clues.
To study how nitrogen behaved during the Great Oxidation Event, the research team used a high-tech tool called an Isotope Ratio Mass Spectrometer (IRMS). They started by crushing ancient South African rocks into powder and extracting specific chemical components.
These were then turned into gas and ionized, meaning the particles were given an electric charge. The IRMS sent them through a magnetic field, which sorted the isotopes by weight. By measuring the ratio of nitrogen-15 to nitrogen-14, scientists gained insight into how nitrogen was cycling in Earth’s ancient oceans.
Life on Earth evolved much earlier than previously thought
Microorganisms shape the chemistry of sediments long before they harden into rock, leaving behind subtle clues, like shifts in nitrogen isotopes, that reveal how nitrogen was used in ancient environments. By tracing the ratio of nitrogen-15 to nitrogen-14 over time, scientists can piece together how oxygen levels in Earth’s oceans and atmosphere changed.
One of the study’s most striking insights, according to researcher Uveges, is that oxygen-sensitive nitrogen cycling began about 100 million years earlier than scientists had believed. This suggests that while oxygen was building up in the oceans, it took much longer to saturate the atmosphere, offering a new perspective on the pace and complexity of Earth’s oxygenation.
The study marks a critical tipping point in the nitrogen cycle, when organisms had to update their biochemical machinery to process nitrogen in a more oxidized form that was harder for them to absorb and use. The research also identifies a key biogeochemical milestone that can help scientists model how different forms of life evolved before and after the GOE.
Journal Reference:
- B.T. Uveges, G. Izon, C.K. Junium, S. Ono, & R.E. Summons, Aerobic nitrogen cycle 100 My before permanent atmospheric oxygenation, Proc. Natl. Acad. Sci. U.S.A. 122 (20) e2423481122, DOI: 10.1073/pnas.2423481122 (2025).
