How can uranium isotopes help trace the history of the Earth’s oxygenation?

A recent study reveals the existence of a uranium isotopic signature specific to oxygen-depleted environments. This discovery could help to better understand the history of the co-evolution between animal life and the oxygenation of the Earth. For the past 15 years, scientists have been using uranium isotope signatures to study the oxygenation conditions of ancient oceans. Indeed, sediments deposited in oxygenated oceans show a different signature to those formed in oxygen-free oceans. However, the intermediate environments between these two states are not yet well understood. A team of scientists reproduced these intermediate conditions in the laboratory, observing the partial oxidation of a mineral typical of anoxic environments, mackinawite. They discovered a particular isotopic signature in these conditions, opening the way to a better understanding of the environments in which animal life may have emerged, some 600 million years ago.

A new study shows that uranium has a specific isotopic signature in oxygen-depleted environments. Measuring these signatures in ancient rocks could provide insights into the co-evolutionary history of animal life and the oxygenation of the planet.

With a few exceptions, the earth’s surface is now rich in oxygen, from the atmosphere to the seabed, which contains dissolved oxygen. Nevertheless, the marine environment has remained oxygen-free (anoxic) for most of the Earth’s history. As oxygen is necessary for all forms of animal life, its emergence is intimately linked to the oxygenation of the oceans.

For the past fifteen years, uranium isotope signatures (i.e. the distribution of uranium atoms according to whether they are heavy or light) have enabled scientists to reconstruct oxygenation conditions in ancient oceans, which disappeared hundreds of millions of years ago. Indeed, sediments deposited under oxygenated oceans do not have the same isotopic signature as sediments deposited under anoxic oceans. By analyzing the composition of these ancient sedimentary rocks, it is therefore possible to assess the oxygenation conditions under which they were formed.

However, oxygen-depleted zones, i.e. intermediate between oxic and anoxic environments, remain invisible to the methods available for this reconstruction. A team of scientists has experimentally reproduced the intermediate conditions between these two states, by monitoring the partial oxidation of mackinawite, a black mineral with a sulfurous odor, typical of anoxic environments. By analyzing the evolution of uranium isotopic composition during this reaction, the scientists were able to identify an isotopic signature specific to these environments. This study refines our methods for tracing ancient environmental conditions, and could provide a new key to interpreting the conditions under which animal life emerged, some 600 million years ago.

Contact GET: Romain Guilbaud