Thursday, 22 January 2015

The Earth's Earliest Atmosphere

Our planet's atmosphere has an intimate connection with life. 2,6 billion years ago, when oxygen first became a significant component, it sparked the first mass extinction in evolutionary history. Some organisms, however, developed the ability to use oxygen as part of the their metabolic processes and gained a crucial energetic advantage over their anaerobic relatives. Today all complex cells respire aerobically. Fluctuations in oxygen levels have since had a profound effect on the planet's biotic composition. Low oxygen has caused extinction crises, high oxygen has played a role in sudden and rapid diversification.

Thanks to chemical proxies in the geological record, we have been able to reconstruct the composition of the atmosphere with a fair degree of accuracy. However, the older the time period being reconstructed, the less accurate the reconstructions become. Ice cores from the past few hundred thousand years actually contain samples of the atmosphere present when they formed. There is an abundance of strata from the past few hundred million years which provide enough data to reliably reconstruct the atmosphere in older geological periods. Yet as we start to look at the atmosphere billions of years ago, available rock samples become scare. Many have also been metamorphosed, rendering their geochemical composition useless.

As a result, the earliest atmosphere has been a highly contentious subject, based primarily on the atmospheres of other planets and a fair degree of speculation. Yet now a study of the most ancient rocks on the planet has given scientists an indication of the atmosphere from an incredible 4.3 billion years ago, just 200 million years after the formation of the planet. The Nuvvuagittuq Greenstone belt is a mix of volcanic and sedimentary rocks in northern Canada dating from the Hadean eon. Analysis of samples taken from the greenstone belt was conducted by researchers from McGill's Earth and Planetary Sciences Department, Canada. They used mass spectrometry to measure the cyclical changes of concentrations of sulphur isotopes across time, finding that the atmosphere 4.3 billion years ago extremely oxygen-poor compared to today, with potentially more methane and carbon dioxide.

The Nuvvuaqittuq Greenstone Belt in northern Canada contains
the oldest rocks on the planet at 4.28 billion years old
What is intriguing is that this atmospheric cycling is very similar to rocks that are a billion to two billion years younger, when life had a firm hold on the planet. 'Those younger rocks contain clear signs of microbial life and there are a couple of possible interpretations of our results,' said Boswell Wing, an associate professor at McGill and co-author of the new study. 'One interpretation is that biology controlled the composition of the atmosphere on early Earth, with similar microbial biospheres producing the same atmospheric gases from Earth's infancy to adolescence. We can't rule out, however, the possibility that the biosphere was decoupled from the atmosphere. In this case geology could have been the major player in setting the composition of ancient air, with massive volcanic eruptions producing gases that recurrently swamped out weak biological gas production.'

The research team's next product is to determine whether the cycling was as a result of biological or geological processes. The results of this future study should prove invaluable as there is an on-going debate as to whether the early Earth could have supported life. The Nuvvuaqittuq greenstone belts indicate that temperatures may have been cool enough to sustain liquid water. Conversely, the Hadean Earth may have been too tectonically unstable to support cells. By understanding the interplay between biological, atmospheric and geochemical processes we will be able to ascertain when life first appeared on our planet.