We often take for granted that oxygen is readily available and expect to find it everywhere, after all, it makes up to 21% of the modern atmosphere. However, free O2 (di-oxygen), which is the root of respiration processes, was scarce during the first half of the Earth’s history. Our good old Earth is 4.5 billion years old and evidences for a permanent rise of O2 in the atmosphere are aged at 2.4 to 2.1 billion years.
The accumulation of O2 in the atmosphere is a result of the evolution of life, specifically the process of photosynthesis which converts carbon dioxide into oxygen. As oxygen levels increased, the ozone (O3) layer formed and protects the Earth's surface from harmful ultraviolet radiation from the sun. This protection is essential for the development and maintenance of life as we know it today.
The most comprehensive record of ancient climate conditions is the air trapped in ice cores from Antarctica (Figure 1), although the oldest sample of trapped air is around 800,000 years old. So, studying the atmospheric composition of 2.4 billion years is challenging as there is no direct record of such an old atmosphere. The only material available from this age on Earth is rocks. And these rocks bear some elements that help us to decipher the atmospheric composition at that time.
Figure 1: Air bubbles in an ice core from the Antarctic. Credit: Bernhard Bereiter, Scripps Institution of Oceanography, University of Bern.
For example, the sulfur has a specific chemical signature if it formed under ultraviolet radiations. So, there is a specific sulfur signature in rocks that stops around 2.4 billion years ago, when the ozone layer formed. It is important to note that this observation is a consequence of the ozone formation and can give us information on the atmospheric conditions at that time but does not help our understanding of the atmospheric processes which trigger the O2 accumulation.
Figure 2: Pictures from the quartz samples I crushed to release the old Xe trapped in the fluid inclusions.
Xenon, like sulfur, has a distinctive chemical signature that was present in the atmosphere between 4.5 and 2 billion years ago (Figure 3). However, unlike sulfur, xenon is not directly influenced by ultraviolet radiation, meaning that its chemical signature is a result of another process that occurred prior to the accumulation of oxygen.
Figure 3: Xe chemical signature evolution through time.
Our new measurements are the colorful diamonds. The red shape represents the sulfur chemical signature evolution that stops between 2.4 and 2.1 Ga. Both signals are normalized to the present-day atmospheric composition.The blue shape represents the evolution of partial pressure O2 into the Earth’s atmosphere.
Our new measurements clearly demonstrate a temporal connection between the sulfur and xenon records. The changing chemical composition of xenon over time suggests the existence of an atmospheric process that may have contributed to the accumulation of O2 in the atmosphere, independently from the O2 production by life that occurred prior to the permanent build-up of oxygen in the atmosphere.
Interested in the full details on how it is done? Check out the complete paper here!
Written by Lisa Ardoin