Life's ancient cooker

Raman spectroscopy has shown that organic material may have been 'cooked' in rocky mounds known as stromatolites that have been dated to 3.45 billion years ago.

Stromatolites are sedimentary rock structures formed in shallow water, layer by layer, over long periods of geological time, they usually take on a dome- or column-like shape. Some of them are truly ancient and geologists hope that their analysis could unlock the secrets of the early Earth. The findings may even hint at ways of finding the remnants of life on other planets including Mars.

Now, researchers at California Institute of Technology (Caltech) and the Jet Propulsion Laboratory (JPL) have unearthed evidence that some of the oldest stromatolites on our planet were built with the help of communities of truly ancient microbes. JPL astrobiologist Abigail Allwood, currently a visitor in geology at Caltech says the discovery "adds unexpected depth to our understanding of the earliest record of life on Earth."

"One of my motivations for understanding stromatolites," Allwood says, "is the knowledge that if microbial communities once flourished on Mars, of all the traces they might leave in the rock record for us to discover, stromatolite and microbial reefs are arguably the most easily preserved and readily detected. Moreover, they're particularly likely to form in evaporative, mineral-precipitating settings such as those that have been identified on Mars. But to be able to interpret stromatolitic structures, we need a much more detailed understanding of how they form."

Geologists have known for some time that most young stromatolites, as in half a billion years old or thereabouts, have a biological origin. They form layer by layer as microbes accrete sediment on the floor of shallow seas.

Caltech's John Grotzinger explains, that "they get moulded into these wave forms and, over time, the waves turn into discrete columns that propagate upward, like little knobs sticking up."

How this happens is down to the surface of the microbial film which is covered with a mucous-like layer to which sediment particles floating in the water's readily stick. Moreover, microbes often sprout filamentous tangles that essentially grab on to particles as they float by. "The end result," adds Grotzinger, "is that wherever the microbial mat is, sediment gets trapped." A dark band in a young stromatolite is usually indicative of long-dead organic matter from such a microbial mat.

However, the stromatolite picture from 3.45 billion years ago, the early Archean period of geological history, is not quite so clear-cut, as a matter of fact. Such truly ancient stromatolites have undergone geological processing to a much greater degree, they are pushed deeper towards the centre of the Earth and exposed to increasing, temperatures. Uncovering the biological origins of these cooked stromatolites is a problem because any organic matter in the form of hydrocarbons will have long since been driven off by the heat, the residue will be nothing but carbon. It's very difficult to prove one way or another whether a particular carbon deposit in such stromatolites is microbial in origin or just good old-fashioned elemental carbon.

"When the rocks are old and have been heated up and beaten up," says Grotzinger, "all you have to look at is their texture and morphology." This is where samples gathered at the Strelley Pool stromatolite formation in Western Australia come into their own when exposed to Raman spectroscopy. These well preserved stromatolites have dark lines of putatively organic matter which is clearly associated with the layering of the type seen in younger microbially derived stromatolites.

Aside from microscopic examination of the samples revealing new information, Allwood notes that Raman spectroscopy showed that the organic matter had been 'cooked' to the same burial temperature as the host rock. This suggests that the carbon-containing layer is not a young contaminant of these stromatolites.

Allwood asserts that it's no longer any "great leap" to accept these stromatolites as biological in origin. "I think the more we dig at these stromatolites, the more evidence we'll find of Early Archean life and the nature of Earth's early ecosystems," she says.