Earth became oxygenated earlier than previously thought, study shows

A new analysis of the world’s oldest banded iron formations (BIFs)– marine chemical sediments located in Western Greenland often used as geochemical archives, scientists have discovered that Earth’s atmosphere contained small amounts of oxygen earlier than previously believed.

Determining the point when the planet’s atmosphere became oxygenated is a key component of reconstructing when life originally emerged and began to evolve. Previous research has found that the oxygenation process occurred in two main phases: the first during what is known as the Great Oxidation Event (2.5-2.4 billion years ago), and a second during the Late Neoproterozoic Era (750 to 540 million years ago) that ultimately led to the emergency of animals.

However, in work published in a recent edition of Scientific Reports, Professor Robert Frei of the University of Copenhagen Department of Geoscience and Natural Resource Management and his colleagues conducted a new analysis of BIFs and discovered that Earth’s atmosphere contained a small amount of oxygen 3.8 billion years ago, or at least 700 million years earlier than previously believed.

Findings suggests life may have emerged earlier than previously believed

Picture of Banded Iron Formation

An example of a Banded Iron Formation: Credit of the University of Copenhagen

Banded iron formations (BIFs) are sediment layers originally composed of alternating layers of silica and iron hydroxides, and are able to retain oxygenation and reduction process information in seawater and on the interaction of the planet’s atmosphere and surface, the study authors said in a statement. The Western Greenland BIFs used in this study are the oldest on Earth.

Professor Frei and his colleagues used concentration and isotope compositions, or variations of the same elements with different atomic weights, of the chromium and uranium elements found in the BIFs. These elements were chosen because they weather quickly when exposed to O2 and other types of reactive oxygen species, the researchers explained. Once weathered, they are then transported by river into oceans, where they wind up deposited in chemical sediments.

Since the chromium and uranium elements in the Western Greenland BIF layers show evidence of weathering, it suggests that oxygen was present in the atmosphere earlier than expected, and could indicate that primitive photosynthetic lifeforms may have actually emerged as far back as 3.8 billion years ago.

“It is generally believed that the Early Earth was a completely anoxic,” explained Professor Frei, who was aided on the study by scientists from the US, Canada, Germany and Denmark. “But our study shows that the surface of the Earth was exposed to a low oxygen atmosphere already this time. This has far reaching implications for how we investigate the pace of evolution of life and its biodiversity on our planet.”


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