Zircons Show Earth Absorbed Oxygen 3.4 Billion Years Ago

Takeaway: Small amounts of oxygen could change the mantle without yet changing the whole atmosphere.

How did scientists read a 3.4-billion-year-old signal?

Moreira, the study’s first author, and his colleagues examined zircon crystals from sanukitoids, coarse-grained igneous rocks found in cratons, regions near the centers of many modern continents that geologists treat as their ancient cores. Sanukitoids provide a chemical window into craton formation, and their zircons often trap tiny apatite inclusions.

Apatite is a phosphorus-bearing mineral that can contain traces of sulfur. The sulfur’s oxidation state, a measure of its chemical reaction with oxygen, records the amount of oxygen in the mantle when the apatite formed.

Zircons contain uranium, which decays at a known rate. That clock can date a crystal’s formation to within roughly a million years. Moreira called these minerals time capsules because their trapped inclusions preserve chemical conditions from billions of years ago.

Together, the clocks and chemical records establish both when each sanukitoid formed and the mantle’s oxidation state at that time.

The researchers focused on subarc regions of the upper mantle, areas that once lay beneath active volcanoes. They reasoned that surface oxidation would affect these zones most strongly. The broader finding may extend to the entire mantle because rocks formed in subarc regions should circulate to much deeper levels over geological time.

Takeaway: Two signals in tiny crystals turned an ancient oxygen cycle into a datable record.

How did oxygen change life?

Long before cyanobacteria evolved photosynthesis, most microbes were anaerobic, meaning they did not need oxygen. They obtained energy by consuming iron and other minerals in shallow water and around hydrothermal vents.

Cyanobacteria’s photosynthesis, or sunlight-powered food production, released oxygen as a by-product. The bacteria could form larger colonies wherever water and sunlight were available, freeing life from its earlier dependence on oxygen-free energy systems.

About 2.3 billion years ago, that oxygen accumulated in the atmosphere and triggered the GOE. One of the first consequences was a mass extinction of anaerobic microbes, which could not tolerate oxygen. The oxygen-rich atmosphere also opened the way for complex, oxygen-breathing life.

Takeaway: A faint early oxygen signal preceded the atmospheric revolution that made oxygen-dependent life possible.

What are the study’s main limitations?

Richard Palin, a geologist at the University of Oxford who was not involved in the study, called the method a clever approach to dating the first atmospheric traces from cyanobacteria. He studies Archaean rocks formed between 4 billion and 2.5 billion years ago, before the GOE and within the period examined by the team.

His main reservation was the small sample: 14 sanukitoid rock samples from around the world, yielding 72 zircon crystals and 39 apatite inclusions. Palin said: “The dataset is quite sparse.”

Better coverage remains difficult because sanukitoids are rare, he noted. More abundant rocks would allow a more comprehensive study. The findings therefore offer a dated starting point, not a complete census of oxygen uptake across Earth’s mantle.

Takeaway: The age of the oxygen signal looks persuasive, but the number of rocks available for testing remains small.