Stromatolites Can Form By Non-Biological Processes
Exclusive Stromatolites have been Exhibit A for stories of the rise of life on the early earth. These column-shaped rocks found in Precambrian strata are usually assumed to be evidence of microbial mats that grew upward as sediment slowly accumulated on top of them. Scene 1 is usually Shark’s Bay in Australia, where stromatolites form in shallow coastal lagoons. Scene 2 might be a place like Transvaal Supergroup in South Africa, where fossil stromatolites are assumed to preserve a record of the earliest life on earth.
Scientists at Caltech decided to investigate the origin of stromatolites. Dr. John Grotzinger1 gave a presentation at the Jet Propulsion Laboratory on March 21 in which he shared some surprising findings. When the Opportunity rover on Mars found a structure resembling a stromatolite, he was not ready to jump to the conclusion it was evidence for life. He and his colleagues decided to take a neutral stance on whether they are biogenic, and find mechanisms that might produce these structures under inorganic conditions. (A number of Mars rover scientists were present in the audience.)
The problem with the biogenic theory, he said, is that there is no way to demonstrate it. Plus, in the classic field cases that compare Shark’s Bay with Transvaal, the cross-sections of these structures are completely different. Grotzinger and his team used both theory and experiment to show how stromatolites can arise by chemical and geological processes alone. Crystals growing upward from regularly-spaced starting points, for instance, will eventually interfere and form convex tops. As sediments become entrained between the fronds of the crystal, new lenses of crystal and sediment will continue to grow upward, resulting in side-by-side columns. Occasionally, higher fluxes of sediment will flatten the upper surface, and the process can begin again. This is apparently what happened at the Transvaal site.
Employing an original mathematical model, Grotzinger showed how stromatolites can originate on a flat surface. If crystals begin growing upward, any points slightly higher will attract more sediment, while the sides will interfere with nearby crystals. The growing points will amplify the column height. There are probably many circumstances where this can happen – life or no life. In his opinion, the type sections for stromatolites are not microbial mats, but travertine springs or playa lakes.
By contrast, he showed areas where current microbial mats possessing what would seem ideal conditions for stromatolite growth are not producing stromatolites. He said a researcher is in a “fool’s paradise” to just observe the morphology of these structures to understand them. “Don’t start with biology,” he said; “start with the rock.” Understand its diagenetic history, then reconstruct the primary texture, then evaluate the sediment accretion process, and consider the biological contribution last.
In the rock record, therefore, do radical changes in morphology of the rock necessarily indicate radical changes in biology? No; he preferred to call these unusual structures “environmental dipsticks” rather than “evolutionary mileposts.”
1See an earlier publication of this work at:
Grotzinger, J.P., and Knoll, A.H., 1999, Stromatolites in Precambrian carbonates: Evolutionary mileposts or environmental dipsticks?: Annual Reviews of Earth and Planetary Science, v. 27, p. 313-358.
One of the lessons from this talk was how assumptions can subtly influence the scientist’s approach. Dr. Grotzinger showed conflicting definitions of stromatolites, one that began, “organogenic structures….” – in other words, there was a biological bias built into the very definition of the word. Interestingly, the NASA Astrobiology Student Focus website says that stromatolites were formerly defined as “laminated organo-sedimentary structures formed by the trapping and binding, and/or precipitation of minerals by microorganisms,” but then does not provide a more neutral definition, and goes on to portray stromatolites as evidences for life. Biased definitions like these could send a graduate student off in a prejudiced direction to assume they were made biologically, and just as easily influence a TV producer working on a script about the early earth.
Dr. Grotzinger and his team are to be commended for shaking off this bias and trying to look at the data objectively. Unfortunately, in other parts of his talk, he accepted other aspects of the geo-biological evolution story without question: the geological column, Milankovich cycles, dating methods, etc. At least this talk indicated progress away from one evolutionary assumption. Now, how do we clean up the textbooks, museums and documentaries?