September 17, 2014
We owe our oxygen-fueled existence to Earth's many photosynthetic plants and bacteria. Oxygen exists presently in Earth's atmosphere at a partial pressure of about 0.21 atmospheres, but there was a time when the pressure was nearly zero. It's understandable that scientists are interested in when our atmosphere started to become oxygen-enriched, since that would mark the time when such life first evolved on our planet. The history of Earth's oxygen is contained in its geological record.
As I wrote in a previous article (Ordovician Carbon Sinks, February 28, 2012), oxygen-producing organisms weren't present on land until about 450 million years ago, during the Ordovician period. These early land plants were primitive, non-vascular mosses. They likely evolved from green algae in the oceans, but the oceans were teeming with oxygen-producing prokaryote cyanobacteria long before then.
Cyanobacteria are thought to have appeared about 2.5 billion years ago, but the oxygen that they produced chemically combined with iron and organic compounds. Since the seas that time were acidic, there would have been large quantities of dissolved iron, and some nickel. Oxygen produced by the cyanobacteria would have combined with iron to form iron oxide. One record of this is the appearance of iron bands in Archean sedimentary rocks (see photograph).
When such oceanic oxygen sinks became depleted, oxygen appeared in Earth's atmosphere in the aptly-termed Great Oxygenation Event, about 2.3 billion years ago (see figure). All this oxygen remade Earth's crust by producing a diversity of minerals. These minerals were hydrates and oxides that couldn't form without sufficient oxygen, and the Great Oxygenation Event doubled the number of mineral types on Earth. Interestingly, oxygen concentration was much higher about 300 million years ago, at the end of the Carboniferous period, than today. The partial pressure of oxygen may have been as high as 0.35 atmospheres at that time.
Now, a team of geologists from the Presidency University (Kolkata, India), the Indian Atomic Minerals Directorate (Jharkhand, India), and Trinity College Dublin (Dublin, Ireland) have found evidence that there was a transient atmospheric oxygenation event that occurred at least 600 million years before the Great Oxygenation Event, and about 60 million years before a previously documented Archean oxygenation event. Not only that, but they propose that there were several such short oxygenation events prior to the Great Oxygenation Event.[1-2]
For this to have happened, oxygen-producing life forms must have been present 60 million years earlier than previously thought. The evidence is chemical weathering of rocks caused by oxygen that produced ancient soil called palaeosol. Such soil was found near the town of Keonjhar, in the Singhbhum Craton of Odisha.[1-2] Quentin Crowley, Assistant Professor and Ussher Lecturer at Trinity and senior author of the paper in Geology that describes this research, says
|Estimated oxygen content of Earth's atmosphere over the ages.|
Earth's oxygen partial pressure is now about 0.21 atmospheres.
(Figure by Heinrich D. Holland, modified, via Wikimedia Commons.)
"This is a very exciting finding, which helps to fill a gap in our knowledge about the evolution of the early Earth. This paleosol from India is telling us that there was a short-lived pulse of atmospheric oxygenation and this occurred considerably earlier than previously envisaged."
Very few of these paleosols have an age prior to 2.5 billion years ago. To date the soil, the geologists used uranium-lead isotope dating of contained zircons. They found that the Paleosols were at least 3.02 billion years old, and they may be as old as 3.29 billion years.[1-2] There is also some evidence from South African paleosols of increased atmospheric oxygen about 2.96 billion years.
- Joydip Mukhopadhyay, Quentin G. Crowley, Sampa Ghosh, Gautam Ghosh, Kalyan Chakrabarti, Brundaban Misra, Kyle Heron, and Sankar Bose, "Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India," Geology, vol. 42, no. 10 (October 2014), pp. 1–4.
- Trinity geologists re-write Earth's evolutionary history books, Trinity College Dublin Press Release, September 4, 2014.
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