News
07/02/2013

Researchers unearth secrets of ancient climate

What was Earth’s climate like almost four billion years ago? Given that the Sun was 30% cooler, was the Earth chilly or did an atmosphere much richer in greenhouse gases keep it warm?

By studying the world’s oldest sedimentary rocks from Greenland, these are among Earth’s secrets that are being answered by research led by UOW’s Dr Allen Nutman and Dr Vickie Bennett from the Australian National University.

The research aims to solve problems surrounding Earth’s early climate almost four billion years ago, and to understand the part that life might have already played in regulating it.

Dr Nutman, from the School of Earth and Environmental Sciences, says that the research will give the “deepest time perspective on Earth’s changing climate feedback loops”.

“The research requires mapping and sampling the oldest geological record preserved in the extremely rare, most ancient sedimentary rocks found in small areas of Greenland”, he said.

“These ancient samples are then studied in the laboratories of UOW and ANU using new analytical approaches to reveal the details of early atmosphere and environmental conditions.”

Dr Nutman said that the team’s first finding was that Earth’s climate almost four billion years ago was mild, with liquid water present at the Earth’s surface. The evidence for this comes from some 3.7 billion year old sedimentary rocks, within which evidence can be seen that sediment layers were disrupted and jumbled by storm wave action. Storm waves would not be present on a frozen planet with oceans capped by ice.

The second finding was that CO2 (carbon dioxide) was an important atmospheric gas, because the oldest rocks include up to 3.9 billion years old carbonate sediments. On the present day Earth, atmospheric CO2 is sequestered into carbonate sediments – such as corals being turned into limestone. High CO2 concentrations in the atmosphere could have been why Earth did not freeze under the early cool Sun.

How did Earth’s oldest carbonate rocks form? From already published research led by Allen Nutman and by Nicolas Dauphas in the US, their chemical composition argues that life played a part in their deposition – indicating the great antiquity of life on Earth.

However, are there more direct signs of early life activity in the form of fossils? Hummocky layering preserved in these rocks at one key outcrop resembles microbial mat structures that can be seen forming in some present-day environments.

Dr Nutman says “if the current research by UOW and ANU confirms the microbial origin for these structures, they will be by far the oldest fossils ever found”.

This research is supported by an Australian Research Council Discovery Project Grant, UOW’s School of Earth & Environmental Sciences, the GeoQuEST Research Centre and ANU’s Research School of Earth Sciences.

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  • The oldest rocks from Greenland: The carbon cycle, life and climate linkages almost four billion years ago.

  • The Greenland field team (left to right): Dr Clark Friend (UK, freelance), Dr Vickie Bennett (the Australian National University), Ms Laure Gauthiez-Putallaz (ANU PhD student) and project leader Dr Allen Nutman (School of Earth & Environmental Science, University of Wollongong). Photo: Laure Gauthiez-Putallaz.

  • Reconnaissance geology at the edge of Greenland’s icecap, in areas of the oldest rocks not studied previously. Photo: Laure Gauthiez-Putallaz.

  • Allen Nutman and Vickie Bennett sampling a transect through layered carbonate rocks that might preserve the oldest rock fossils. Photo: Laure Gauthiez-Putallaz.

  • Ancient sedimentary rocks deposited in shallow water showing fragmentation of layers by storm wave action. Storm wave action indicates that seas at that time were not ice covered – thus Earth was not an icehouse despite the thirty per cent cooler sun at those times.

  • The single outcrop in the world where carbonate sedimentary rocks might preserve mesoscopic fossil evidence for life in the form of hummocky microbial mats (tip of pen for scale – top right).