Abby's Reef: The earliest convincing evidence of life?
Life on Earth may well have flourished 3.43 billion years ago in an environment not too different to the warm little pond that Darwin imagined: a quiet shallow marine environment sandwiched in time between two active volcanic periods.
Australian Centre for Astrobiology doctoral student Abigail Allwood reveals in a paper in the journal Nature her discovery of a ten kilometre section of an ancient microbial reef system. She says the stromatolite shape associations and other evidence indicate biological influence: that is, no purely physical or chemical processes alone could have produced the entire ecosystem.
The outcrop of the ancient reef is cut off one end by a fault, and at the other it disappears into what would have been deep water, not liked by the microbial communities that created the stromatolite structures, just like in modern reefs.
“If you start at the deep water end and trace it back along the reef system into shallow waters (implying more sunlight), the numbers of stromatolite shapes increase and become more complex and varied, just as occurs in biological reef systems throughout the geologic record,” she says. “It is a classical biological response to the environment.”
The other lines of evidence include the individual structures and the association of morphologies (shapes), and the way they were arranged in the palaeo-environment. Analysis of the rare earth element chemistry (with [Balz Kamber], Laurentian University) confirms that the chert and carbonate (fine-grained sedimentary rocks) that make up the stromatolites were deposited in a marine environment.
“If you take a vertical section through time there is a brief change from the high temperature hydrothermal and volcanic deposition that dominated the Pilbara at the time to a shallow marine environmentt in which life flourishes virtually immediately, “ she remarks. “And then back again to another volcanic and hydrothermal episode, when the stromatolites disappear. This speaks volumes about the conditions that may have nurtured early life”
The Pilbara region of Western Australia contains ancient stromatolite structures up to almost 3.5 billion years old and is a key research area for the Australian Centre for Astrobiology. The stromatolites were first described almost three decades ago, and have been a source of spirited debate. Some think they were formed by primitive microbes, whereas others believe they were formed chemically near hydrothermal vents.
In an attempt to resolve the dispute, Abby and her colleagues studied the 10 kilometre stretch of stromatolite-rich rock, and identified seven differently shaped types of stromatolite, each in their own environmental niche.
Some look like upside-down ice cream cones, others like egg cartons. The complexity of the stromatolite system would require an unbelievable combination of purely chemical or physical processes to have formed without biological input. Viewed as a whole, the stromatolites resemble a reef formation, suggesting the presence of a complex ecosystem.
Abby describes how she made the discovery of the reef system over three field seasons between 2003 and 2005. In the second, in 2004, she and her field assistant Ian Burch, came back to their Pilbara camp one evening after surveying more of the stromatolites, and by the fire tried to figure out how the pieces of what they had seen fitted together.
Together they sketched models of what it would have looked like in the environment around them 3.5 billion years ago. “We suddenly realised this was a reef system,” she said. “So there was a hypothesis, and now it could be tested.”
“What I saw right at that moment just blew me away. It is realising the rocks have a story to tell. You’ve got to give away any preconceived notions give up all those pressures and begin reading what the rocks are telling you.”
Understanding whether the structures were formed by microbial action tells us how soon life got going on Earth. Such knowledge is key to the puzzle of our own origins. but also vital in looking for past life on Mars and other solar system bodies.
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* Abby's work builds on a base of detailed mapping and previous research, particularly by the Geological survey of Western Australia, but also others, as detailed in the paper ‘Stromatolite reef from the Early Archaean Era of Australia’ published in the 8 June 2006 issue of Nature, pp 714-718. [Allwood et al.]