[meteorite-list] Caltech, JPL Researchers Provide Evidence that Microbial Mats Helped Build 3.4-Billion-Year-Old Stromatolites

[Ron Baalke]

Caltech News Release
For Immediate Release
July 16, 2009

Caltech, JPL Researchers Provide Evidence that Microbial Mats Helped 
Build 3.4-Billion-Year-Old Stromatolites

Findings may provide insight into the origins of life on Earth, and 
even the search for life on Mars

PASADENA, Calif. - 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, a finding that "adds unexpected 
depth to our understanding of the earliest record of life on Earth," 
notes JPL astrobiologist Abigail Allwood, a visitor in geology at 
Caltech.

Their research, published in a recent issue of the Proceedings of the 
National Academy of Sciences (PNAS), might also provide a new avenue 
for exploration in the search for signs of life on Mars.

"Stromatolites grow by accreting sediment in shallow water," says 
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."

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.

How? The microbes' surface is coated in a mucilaginous substance to 
which sediment particles rolling past get stuck. "It has a strong 
flypaper effect," says Grotzinger. In addition, the microbes sprout a 
tangle of filaments that almost seem to grab the particles as they 
move along.

"The end result," says Grotzinger, "is that wherever the mat is, 
sediment gets trapped."

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."

But when you look back 3.45 billion years, to the early Archean 
period of geologic history, things aren't quite so simple.

"Because stromatolites from this period of time have been around 
longer, more geologic processing has happened," Grotzinger says. 
Pushed deeper toward the center of Earth as time went by, these 
stromatolites were exposed to increasing, unrelenting heat. This is a 
problem when it comes to examining the stromatolites' potential 
biological beginnings, he explains, because heat degrades organic 
matter. "The hydrocarbons are driven off," he says. "What's left 
behind is a residue of nothing but carbon."

This is why there has been an ongoing debate among geologists as to 
whether or not the carbon found in these ancient rocks is diagnostic 
of life or not.

Proving the existence of life in younger rocks is fairly simple - all 
you have to do is extract the organic matter, and show that it came 
from the microorganisms. But there's no such cut-and-dried method for 
analyzing the older stromatolites. "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."

Which is exactly what Allwood and Grotzinger did with samples 
gathered at the Strelley Pool stromatolite formation in Western 
Australia. The samples, says Grotzinger, were "incredibly well 
preserved." Dark lines of what was potentially organic matter were 
"clearly associated with the lamination, just like we see in younger 
rocks. That sort of relationship would be hard to explain without a 
biological mechanism."

"We already knew from our earlier work that we had an assemblage of 
stromatolites that was most plausibly interpreted as a microbial reef 
built by Early Archean microorganisms," adds Allwood, "but direct 
evidence of actual microorganisms was lacking in these ancient, 
altered rocks. There were no microfossils, no organic material, not 
even any of the microtextural hallmarks typically associated with 
microbially mediated sedimentary rocks."

So 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, mat-like 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," Allwood notes, "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 says she, Grotzinger, and their team have collected enough 
evidence 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.

That's no small feat, since it's been difficult to prove that life 
existed at all that far back in the geologic record. "Recently there 
has been increasing but still indirect evidence suggesting life 
existed back then, but direct evidence of microorganisms, at the 
microscale, remained elusive due to poor preservation of the rocks," 
Allwood notes. "I think most people probably thought that these Early 
Archean rocks were too poorly preserved to yield such information."

The implications of the findings don't stop at 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."

The other authors on the paper, "Controls on development and 
diversity of Early Archean stromatolites," are Mark Anderson, Max 
Coleman, and Isik Kanik from JPL; Andrew Knoll, the Fisher Professor 
of Natural History at Harvard University; and Ian Burch from the 
University of New South Wales in Australia.

The research described was supported in part by the Agouron 
Institute; Allwood was supported by the National Aeronautics and 
Space Administration Postdoctoral Program.

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Contact:            Lori Oliwenstein
                         (626) 395-3631
                         lorio at caltech.edu