Earth's Oxidation Tracked with Chromium Isotopes

One of the basic facts of life is clearly the knowledge that life on our planet cannot survive without oxygen. When the Earth first formed, there was a very small concentration of the gas, maybe less than one percent of the total atmosphere. However, two big oxidation events were recorded over the eons, both of which eventually determined the appearance of complex life. Now, new studies of these events, conducted based on chromium-isotope analysis, reveal more puzzling questions about the early days.

The Great Oxidation Event (GOE) was the first instance in which the oxygen concentration in our planet's atmosphere increased noticeably since the Earth solidified from a ball of hot magma. Until now, experts believed that it took place some 2.45 to 2.2 billion years ago, based on evidence collected from analyzing molybdenum and rhenium isotopes, among isotopes of other metals. The second steep rise was estimated to have taken place some 750 million years ago, when the conditions again became appropriate for the development of more complex organisms.

By analyzing samples of banded iron formations – an iron-rich sedimentary rock – dating from around and in between the two main periods of intense oxygen increases, experts from the University of Copenhagen, in Denmark, led by researcher Robert Frei, have recently concluded that oxygen, in fact, started making its way into the world's oceans some 2.8 to 2.6 billion years ago. This means that the first process at least began 200 million years before the time inferred from studying other metallic isotopes.

But the investigation also revealed a very weird fact – some 1.9 billion years ago, oxygen concentrations again dropped to their one-percent threshold, contrary to what one might expect after a period of intense oxidation. Details of the find appear in the latest issue of the respected scientific journal Nature. Experts from the University of Southern Denmark, in Odense, have also participated in the investigation, Nature News reports.

The recent study relied on the properties of chromium isotopes. When there is no oxygen around, the chemical is locked in rocks, in a form in which all atoms have three electrons less than the elemental form of the metal (+3 oxidation). When oxygen appears, the isotopes “steal” electrons from landlocked chromium, and morph into an oxidized, +6 form. This form is more likely to be washed by water into the oceans, where it reacts with iron metal, and is stored into layers of the stuff. By analyzing these layers, the team was able to track the evolution of oxygen concentrations in the early atmosphere.