According to research led by Dr Magali Pujol from the University of Lorraine in France, bubbles of water found in Archaean hydrothermal quartz in Australia hold the key to understanding what caused the First Ice Age.
Quartz crystals (Didier Descouens / CC BY-SA 3.0)
The study, published in the journal Nature, analyzes the amount of ancient atmospheric argon gas (Ar) isotopes dissolved in the bubbles.
“The water samples come from the Pilbara region in north-west Australia and were originally heated during an eruption of pillow basalt lavas, probably in a lake or lagoon environment,” said study co-author Dr Ray Burgess from the University of Manchester.
The findings show that argon levels were very different to those in the air we breathe today.
The new data help explain why Earth didn’t suffer its first Ice Age until 2.5 billion years ago, despite the Sun’s rays being weaker during the early years of Earth’s formation.
“Evidence from the geological record indicates that the first major glaciations on Earth occurred about 2.5 billion years ago, and yet the energy of the Sun was 20 per cent weaker prior to, and during, this period, so all water on Earth should already have been frozen. This is something that has baffled scientists for years but our findings provide a possible explanation.”
The scientists revealed that the ratio of two argon isotopes – 40Ar, formed by the decay of potassium 40K with a half-life of 1.25 billion years, and 36Ar – was much lower than present-day levels. This finding can only be explained by the gradual release of 40Ar from rocks and magma into the atmosphere throughout Earth’s history.
The team used the argon isotope ratio to estimate how the continents have grown over geological time and found that the volume of continental crust 3.5 billion years ago was already well-established being roughly half what it is today.
“High levels of the greenhouse gas carbon dioxide in the early atmosphere – in the order of several percent – which would have helped retain the Sun’s heat, has been suggested as the reason why the Earth did not freeze over sooner, but just how this level was reduced has been unexplained, until now,” Dr Burgess said.
“The continents are a key player in the Earth’s carbon cycle because carbon dioxide in the atmosphere dissolves in water to form acid rain. The carbon dioxide removed from the atmosphere by this process is stabilized in carbonate rocks such as limestone and if a substantial volume of continental crust was established, as revealed by our study, then the acid weathering of this early crust would efficiently reduce the carbon dioxide levels in the atmosphere to lower global temperatures and lead to the first major Ice Age.”
“The signs of the Earth’s evolution in the distant past are extremely tenuous, only fragments of highly weathered and altered rocks exists from this time, and for the most part, the evidence is indirect. To find an actual sample of ancient atmospheric argon is remarkable and represents a breakthrough in understanding environmental conditions on Earth before life existed,” Dr Burgess concluded.
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Bibliographic information: Magali Pujol et al. 2013. Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics. Nature 498, 87–90; doi: 10.1038/nature12152
