Oxygen-producing life may have developed much earlier on Earth than initially suspected, based upon investigation of ancient soil markers in South Africa. This, according to the research group involved, could raise interesting new insight into the evolution of early life on our planet.
The study was conducted by researchers working for the University of Copenhagen and the University of British Columbia, and was published in the journal Nature.
Investigating some of the most ancient soil samples found on Earth – some three billion years old – the team unexpectedly discovered low concentrations of oxygen. This startling finding conflicts with prior research, which suggested that oxygen first began filling the atmosphere 2.3 billion years ago, representing a 700 million year discrepancy.
The Great Oxygenation Event
Conventional theory suggests that molecular oxygen was first pumped into the atmosphere 200 million years after the appearance of cyanobacteria. Cyanobacteria uses carbon dioxide and water, during photosynthesis, to convert light energy into chemical energy; during this reaction, oxygen is released.
According to a study produced by Heinrich D. Holland, of the Department of Earth and Planetary Sciences, Harvard University, several stages explained the transition from an oxygen-free atmosphere to the Great Oxygenation Event. Oxygen was initially released, but was then absorbed by the vast oceans and seabed (stage 2). Subsequently, oxygen was released from the oceans, but was partly absorbed by the Earth’s land surfaces (stage 3). Ultimately, during the final stages, under a billion years ago, the oxygen sinks became filled and the gas began to accumulate within the atmosphere.
This event may have been catastrophic to obligate anaerobes, who would have been unable to survive the ever-increasing oxygen levels. On the other hand, the increased concentration of the gas created a boom in the evolution of complex organisms.
In addition, scientists have postulated that these photosynthesizing bacteria were responsible for one of the longest ice ages recorded, called the Huronian glaciation event. Molecular oxygen reacted with the greenhouse gas, methane, and temperatures began to plummet as a result.
In light of recent findings, however, scientists are now questioning whether such oxygenation events may have transpired much earlier.
The Ancient Soil Study
The research team analyzed the ratios of different chromium isotopes in the ancient soil sediments in Kwazulu-Natal Province, situated on the east coast of South Africa.
Isotopes on an element are essentially atoms with the same number of protons, but a different number of neutrons. The mass number (no. of protons and neutrons), therefore, differs between isotopes; the atomic number (no. of protons) remains the same.
The scientists looked specifically at chromium-53 and chromium-52 isotopes. Chromium-53 is the heavier isotope, and demonstrates greater solubility than chromium-52 when oxidized. Consequently, soils that have been exposed to molecular oxygen will possess little chromium-53, as this isotope’s solubility resulted in it being washed away by rain water. Meanwhile, chromium-53 will be present in much greater concentrations in sea sediment, following the afore-mentioned weathering process.
This process was noted in the ancient Kwazulu-Natal soils and sea sediment. Based upon the researchers’ mathematical models, they estimated that low concentrations of molecular oxygen was present within the atmosphere – approximately 0.03%.
According to BBC News, Professor Michael Bau, of Jacobs University in Germany, explains that this is much higher than had been previously estimated.
Oxygen Production “almost certainly biological”
Dr. Sean Crowe, the study’s co-author and assistant professor at the University of British Columbia, says that scientists have always been confident that aerobic life resulted from photosynthesis-based oxygenation of the planet. He reasons that greater accuracy can now be obtained, as the sensitivity of new techniques continues to improve.
In addition, Crowe states that oxygen production was “almost certainly biological,” stating that atmospheric levels of oxygen, three billion years ago, were around 100,000 times greater than what could be explained by mere chemical reaction occurring within the Earth’s atmosphere.
In a press statement, Crowe went on to discuss the implications of his study. He explains that his study’s results hint to the conclusion that oxygenation of the planet began much earlier on in Earth’s history, inferring “greater antiquity” of oxygen-producers and aerobic life.
The team hope to perform similar investigations into material in Australia and Greenland. Locating rocks that are three billion years old remains a difficult task. Although, the difficulty in finding these samples would be well worth the effort; Crowe indicates that rocks even older than this could reveal whether oxygen production dates back even further.
Although Earth had oxygen much earlier than suspected, Lasse Dossing, another of the study’s lead scientists, indicates that it took a long period of time for “… geological and biological processes to conspire…” in yielding today’s oxygen-rich atmosphere. Maybe further groundbreaking discoveries in the ancient soils of South Africa, and the rest of the world, are yet to be made.
By: James Fenner