For decades, research into the composition of the Earth’s early atmosphere depended on indirect means. An example is the preservation of water-worn grains of sulphides and uranium oxides in coarse terrestrial sediments older than about 2,200 Ma. Their survival on the continental surface suggested that the atmosphere before then had vanishingly low O₂. Such grains would have otherwise been broken down by oxidation reactions. Younger sediments simply do not contain such detrital grains. This suggested the appearance of an oxidising atmosphere around 2.2 Ga ago: the Great Oxygenation Event. The greenhouse gases – carbon dioxide and methane – are also difficult to estimate directly, especially in the Precambrian. Once plants colonised the land surface, their photosynthesis depended on inhaling and exhaling air through stomata on the surface of leaves (see: Ancient CO₂ estimates worry climatologists; January 2017). The number of stomata per unit area of a leaf surface is expected to increase with lowering of atmospheric CO₂ and vice versa, which has been observed in plants grown in different air compositions. By comparing stomatal density in fossilised leaves of modern plants back to 800 ka allows the change to be calibrated against the record of CO_­2 inside air bubbles trapped in ice-cores. This proxy method has given a guide to CO2 variations through the Cenozoic, Mesozoic and upper Palaeozoic Eras. However, the reliability of extinct plant leaves as proxies is suspect.

Is it possible to find air trapped by other means than in glacial ice? It may be. Tiny pockets of liquid and gas – fluid inclusions – are often found in minerals that crystallised at the Earth’s surface. The most common are crystals of salt (NaCl) and carbonates from ancient lake deposits. A 2019 study revealed that Late Triassic carbonates from Colorado, USA record an increase of atmospheric oxygen levels from 15 to 19% about 215 Ma ago over a period of just 3 million years as dinosaurs first spread into North America, then at equatorial latitudes in the Pangaea supercontinent. This sudden increase in the availability of oxygen may also be linked to the trend towards larger and larger dinosaurs worldwide.  Going further back in time trace-metal chemistry of 1,400 Ma old marine sediments from China indicates oxygenated water that suggests an atmospheric oxygen level greater than 4% of that at present. Small as that might seem, it would have been sufficient to sustain animal respiration about half a billion years before the first evidence for the earliest animals. Further work on ancient salt and carbonate deposits confirms much higher oxygen levels  than geochemists have expected previously.

Source: Voosen, P, 2025. Earth’s rocks hold whiffs of air from billions of years ago. Science, v.387, articlezhst73x; DOI: 10.1126/science.zhst73x