Thursday, September 4, 2014

Dublin's Trinity geologists re-write Earth's evolutionary history books

The study site landscape is shown with boulders of the paleosol in the foreground. 

Credit: Quentin Crowley

Geologists from Trinity College Dublin have rewritten the evolutionary history books by finding that oxygen-producing life forms were present on Earth some 3 billion years ago, a full 60 million years earlier than previously thought.

These life forms were responsible for adding oxygen (O2) to our atmosphere, which laid the foundations for more complex life to evolve and proliferate.

Working with Professors Joydip Mukhopadhyay and Gautam Ghosh and other colleagues from the Presidency University in Kolkata, India, the geologists found evidence for chemical weathering of rocks leading to soil formation that occurred in the presence of O2.

Using the naturally occurring uranium-lead isotope decay system, which is used for age determinations on geological time-scales, the authors deduced that these events took place at least 3.02 billion years ago.

The ancient soil (or paleosol) came from the Singhbhum Craton of Odisha, and was named the 'Keonjhar Paleosol' after the nearest local town.

The pattern of chemical weathering preserved in the paleosol is compatible with elevated atmospheric O2 levels at that time.

Such substantial levels of oxygen could only have been produced by organisms converting light energy and carbon dioxide to O2 and water.

This process, known as photosynthesis, is used by millions of different plant and bacteria species today.

It was the proliferation of such oxygen-producing species throughout Earth's evolutionary trajectory that changed the composition of our atmosphere, adding much more O2, which was as important for the development of ancient multi-cellular life as it is for us today.

Quentin Crowley, Ussher Assistant Professor in Isotope Analysis and the Environment in the School of Natural Sciences at Trinity, is senior author of the journal article that describes this research which has just been published online in Geology, the world's top-ranked Geology journal.

He said: "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."

Credit: Quentin Crowley

The early Earth was very different to what we see today. Our planet's early atmosphere was rich in methane and carbon dioxide and had only very low levels of O2.

The widely accepted model for evolution of the atmosphere states that O2 levels did not appreciably rise until about 2.4 billion years ago.

This 'Great Oxidation Event' event enriched the atmosphere and oceans with O2, and heralded one of the biggest shifts in evolutionary history.

More Information: "Oxygenation of the Archean atmosphere: New paleosol constraints from eastern India" Joydip Mukhopadhyay1, Quentin G. Crowley: First published online August 28, 2014, doi: 10.1130/G36091.1

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