Ancient burrows?: Various views of ambient inclusion trails in minerals from Western Australia.
Credit: David Wacey
SYDNEY: Nanoscale views of ancient rocks might help detect traces of Earth's earliest life, say geologists. They looked in great detail at rocks thought to hold Earth's oldest fossils, and found tiny trails likely made by ancient microbes.
The researchers analysed samples of sandstone taken from the Strelley Pool Formation in the Pilbara of Western Australia. The formation is famously home of fossilised stromatolites, ancient algae beds that first appeared on Earth 3.4 billion years ago and are the earliest known fossils.
The research, led by field geologist David Wacey from the University of Oxford in the U.K., was presented last week at the Australian Earth Sciences Convention 2008 in Perth.
Wiggly trails
Wacey's team looked at probable small, wriggly burrows called ambient inclusion trails, found in mineral grains within the rocks. To view the samples in detail, they used a nanoscale imaging technique called NanoSIMS (Nano Secondary Ion Mass Spectrometry). The technique looks at the composition of very small samples by scattering an ion beam off the surface and analysing the particles scattered by the beam.
Within the trails there are a suite of elements intimately linked to life, such as carbon and nitrogen. The nature of these elements and the ratio of different isotopes of carbon show that the material is organic and in the right range for biological carbon fixation, the researchers said.
"Each of these pieces of evidence alone would not be enough to claim that these AITs are biological, but when you put it all together it becomes very difficult to argue for a non-biological mechanism," said Wacey.
"Early life would have been very simple and extremely small, leaving only the slightest clues about its existence. This means that you need very high spatial resolution technology to get at these small clues," he said.
The Strelley Pool Formation has been the focus of geological research since the description of Earth's oldest fossils in the June 2006 issue of the journal Nature.
Biodiverse bacteria
Martin Van Kranendonk, a field geologist from the Geological Survey of Western Australia in Perth, who has worked extensively on the Strelley Pool Formation, said it was important to differentiate between signs of early life and those made by physical processes. This new technique could not only pinpoint traces of Earth's earliest life but could also be used in the search for fossil life on Mars, he said.
"It's extremely difficult to prove or disprove the existence of life. If I look for a microfossil in the Strelley Pool Chert, it's a needle in a million haystacks," he said. "Strelley Pool Chert has the best record for early life in the world from a variety of different pieces of evidence. This research represents another way of looking for traces of life in the rocks."
Van Krandendonk also presented his research on the Strelley Pool Formation at the conference, describing four very different environments in which cyanobacteria, the microorganisms that construct stromatolites, are found. Even at this early stage in Earth's history cyanobacteria were living in very diverse environments; in shallow seas, at the mouths of low temperature hydrothermal vents and at the margin of undersea lava flows.
"It proves that life is opportunistic, dynamic and able to inhabit different environments at a very early stage of Earth's history," he said.
