‘Microbial mats’ built 3.4-billion-year-old stromatolites

July 17th, 2009 - 2:23 pm ICT by ANI  

Washington, July 17 (ANI): A team of scientists has provided evidence that 3.4-billion-year-old stromatolites were responsible for building ‘microbial mats’, a finding that may provide insight into the origins of life on Earth.

Stromatolites are dome or column-like sedimentary rock structures that are formed in shallow water, layer by layer, over long periods of geologic time.

Now, researchers from the California Institute of Technology (Caltech) and the Jet Propulsion Laboratory (JPL) have provided evidence that some of the most ancient stromatolites on our planet were built with the help of communities of equally ancient microorganisms.

According to JPL astrobiologist Abigail Allwood, a visitor in geology at Caltech, the finding “adds unexpected depth to our understanding of the earliest record of life on Earth.”

“Stromatolites grow by accreting sediment in shallow water,” said John Grotzinger, the Fletcher Jones Professor of Geology at Caltech.

“They get molded into these wave forms and, over time, the waves turn into discrete columns that propagate upward, like little knobs sticking up,” he added.

Geologists have long known that the large majority of the relatively young stromatolites they study-those half a billion years old or so-have a biological origin; they’re formed with the help of layers of microbes that grow in a thin film on the seafloor.

The microbes’ surface is coated in a mucilaginous substance to which sediment particles rolling past get stuck.

“It has a strong flypaper effect,” said Grotzinger.

In addition, the microbes sprout a tangle of filaments that almost seem to grab the particles as they move along.

“The end result is that wherever the mat is, sediment gets trapped,” said Grotzinger.

Thus, it has become accepted that a dark band in a young stromatolite is indicative of organic material, he adds.

“It’s matter left behind where there once was a mat,” said Grotzinger.

Allwood set about trying to find other types of evidence to test the biological hypothesis. To do so, she looked at what she calls the “microscale textures and fabrics in the rocks, patterns of textural variation through the stromatolites and-importantly-organic layers that looked like actual fossilized organic remnants of microbial mats within the stromatolites.”

What she saw were “discrete, matlike layers of organic material that contoured the stromatolites from edge to edge, following steep slopes and continuing along low areas without thickening.”

She also found pieces of microbial mat incorporated into storm deposits, which disproved the idea that the organic material had been introduced into the rock more recently, rather than being laid down with the original sediment.

“In addition, Raman spectroscopy showed that the organics had been ‘cooked’ to the same burial temperature as the host rock, again indicating the organics are not young contaminants,” Allwood said. (ANI)

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