Xenon trapped in rock provides insight into the evolution of life

Maintaining the balance needed to make our planet habitable

Volatile elements such as water, carbon dioxide, halogens, noble gases, etc., trapped inside the Earth are injected into the atmosphere during volcanic eruptions. The accumulation of these gases over geological time has been crucial to the formation and evolution of the Earth’s atmosphere and oceans, enabling the development of life. The flow of gases from these elements in the Earth’s mantle towards the surface is offset by their return at depth during subduction of the oceanic crust. The balance between outflows and inflows has made it possible to maintain the conditions necessary for the habitability of our planet. However, this balance is sometimes upset when large quantities of gas are released during extreme volcanic phenomena such as eruptions. Volcanic gas flows can be observed in real time, but past flows are much more difficult to analyse.

A rare gas stored in rocks as a witness to the effect on the atmosphere

This new study proposes to analyse the composition of the ancient atmosphere, trapped in rocks and minerals formed several billion years ago. To do this, the CRPG researchers analysed data obtained from a high-precision tracer trapped in ancient rocks and minerals. Xenon, written “Xe” on Mendeleïev’s famous table, is made up of 9 forms, each with different physical properties, known as “isotopes”. Of these, xenon-129 is enriched in the Earth’s mantle compared with the atmosphere. Rock analysis shows that samples of ancient atmosphere
contain less xenon-129, because more recent volcanic outgassing has enriched the atmosphere in this isotope over time. However, the xenon-129 deficit observed in ancient samples requires past volcanic outgassing rates to be much higher, by one to two orders of magnitude compared with the present. In addition, the flux and therefore the xenon-129 deficit was more or less constant between 3.3 and 2.7 billion years ago, then increased rapidly around 2.4 billion years ago (the transition period between the Archean and Proterozoic eras).

The impact of volcanic evolution on greenhouse gas emissions

The flows caused by plate tectonic movements are not sufficient to cause extreme degassing over such a short period of time, and the heat cannot be dissipated quickly enough. The scientific hypothesis is to envisage a transitional thermal regime during which the mantle evacuates the excess heat via intense magmatism (and outgassing), then cools to return to the plate tectonic regime in which we live today. By analysing these past flows, the study shows that they were stronger than they are today, and this transition may have triggered a major environmental change, the oxygenation of the atmosphere, or ‘Great Oxygenation Event’.

The impact of human activity on our atmosphere

This study highlights the impact of global volcanism on the atmosphere and the environment. It could help us to better understand the effects of massive injections of greenhouse gases into the atmosphere, such as those currently being emitted. In fact, the quantity of greenhouse gases we are emitting is comparable to that of 2.5 billion years ago, a period of major environmental change.

> see the press release published on the Université de Lorraine website.