Oxygen is the third most abundant element in the universe and makes up almost 21% of Earth’s atmosphere. It’s difficult to envisage a world without it, but early Earth was very different to how it is now. Until around 2.45 billion years ago, the atmosphere was devoid of oxygen. However, researchers strongly believed that the biological production of oxygen began long before that, some 2.7 to 2.8 billion years ago, meaning that there was a significant gap between the first oxygen-producing organisms and the “Great Oxidation Event” that flooded the oceans and air with oxygen.
Prior to the evolution of photosynthetic organisms, primitive microbes were anaerobic, meaning that they thrived in this absence of oxygen. However, there was evidence to suggest that before oxygen became abundant, there were small “oases” of oxygen-rich water, driven by early microbes.
This idea was fuelled by the discovery of ancient rocks, dating between 4 to 2.5 billion years ago, that demonstrated bands of iron-rich minerals. It is thought that these may have formed when microorganisms began pumping out oxygen which reacted with dissolved iron in seawater, producing particles that were then deposited on the seabed.
Scientists therefore proposed that oxygen may have first started accumulating in this hostile world in isolated pockets. “The idea of oxygen oases in ancient seas has been around for a long time, but no one was able to pinpoint a specific example of such an oasis,” Robert Riding told New Scientist. Now, Riding and colleagues believe that they have identified an early example of a marine oxygen oasis, representing the first convincing evidence of these ancient pockets. The study has been published in Precambrian Research.
For the study, the team collected 2.8 billion year old shallow-marine limestones and deep-water iron-rich sediments from Steep Rock Lake, Canada. According to Riding, these rocks are amongst the oldest and best preserved on Earth and consequently have not changed much since they were laid down. The rocks in this location also contain stromatolites which are layered structures of sedimentary rocks formed by mats of microbes, in particular cyanobacteria.
The scientists examined the composition of the rocks using rare earth element analyses and found that the seawater, from which the shallow-water limestones precipitated, was oxygenated. In contrast, the nearby deeper waters that were home to iron-rich sediments were not oxygenated.
These findings were critical pieces of evidence given the fact that the main ingredient of limestone, calcium carbonate, can only form in water devoid of dissolved iron. The team therefore suggest that the production of limestone was facilitated by microbes pumping out oxygen, which when reacted with the iron in the seawater.
The oasis only lasted for around 5 million years, however, because rising sea levels flooded the area with a fresh supply of iron, bringing oxygen levels back down once again.
Given its highly reactive nature, the oxygen in the oases would have actually been harmful to the anaerobic microbes. This would have therefore driven the evolution of defense mechanisms that eventually gave rise to organisms that could not only survive in oxygen rich environments, but also harness it to produce energy, making a major transition in life on Earth.
[Via New Scientist]
[Header image, "Limestone Quarry Cave & Lake #3" by Brent Moore, via Flickr, used in accordance with CC BY-NC 2.0]