Stromatolites and Suppositions

We know some things about stromatolites, but we don’t know what we don’t know.

In “Unlocking the secrets of Shark Bay’s stromatolites” on PhysOrg, Samille Mitchell introduces the world’s most famous examples of stromatolites. These are columnar rock formations caused by bacterial mats that grow upward, depositing sand grains and other geological material with their biological material (the microbes subsequently vanish, leaving just the columns). The word stromatolite simply means “layered rock” in Greek without any necessary biological association. We can measure their growth rates in the few known active biogenic stromatolites, such as the ones at Shark Bay, Australia. Why the fuss about stromatolites?

They appear as strange-shaped columns of rock—a mere oddity. Yet this mundane appearance belies a fascinating structure whose fossilised remains hold records to the earliest life on Earth.

Mitchell mixes confidence with diffidence in his treatment of the subject. At one moment, he speaks like a prophet about how they “provide a picture into life billions of years ago” and how their microbes “generated the oxygen that went on to help make the planet habitable to human life” (but see Evolution News & Views). At the next moment he is confessing, “Despite their importance to our very existence, and many years of research, much about stromatolites remains disputed.” Reading on, we find some factors that could lead a neutral observer to question Mitchell’s confidence.

1. Recent research “has transformed the way we understand the modern stromatolites at Shark Bay,” implying that previous orthodoxy was misinformed.
2. “Understanding how the local environment affects stromatolite morphology will help scientists interpret the ancient stromatolite structures so prevalent throughout the rock record,” a researcher says, implying that assumptions that do not take into account local environment effects could lead to misinterpretations.
3. The article mentions prior doubts whether fossil stromatolites are analogous to current ones. The new research seems to conclude they share characteristics, but to what degree? Which factors are shared, and which are not?

In Astrobiology Magazine, Charles Q. Choi takes the reader below Antarctic ice where a team is investigating microbial mats for “Clues To Life on Early Earth.” Spectacular and diverse stromatolites in this remote environment are actively growing.

“Microbes very rarely fossilize well, and even when they do, the shapes of the microbes don’t necessarily tell us much about the lifestyle of the organisms,” Mackey said. “The overall shape of microbial mats, however, is much more likely to be preserved.”

One unique characteristic of these Antarctic stromatolites is their branching structures – a feature scientists from UC Davis attribute to changing environments.

The shape of stromatolites is intimately linked to environmental and microbial activity. These shapes varied over time, potentially shedding light on how the ancient world changed over the course of millions of years.

During the Proterozoic — the eon 2.5 billion to 540 million years ago before multicellular life appeared — some stromatolites grew branched formations rather than simple flat layers. The branches varied in shape, the degree of divergence from the underlying stromatolite, and whether the branches themselves branched.

But questions come to mind once we scrape away the confident assertions.

• What about the lack of multicellular life would cause stromatolites to branch out instead of grow flat?
• If “Branches also varied in abundance over time, potentially serving as markers of long-term environmental changes and microbial evolution,” how does one distinguish causation from correlation? “However, the environmental and biological processes responsible for different stromatolite shapes are difficult to deduce from fossil stromatolites.“
• “Studies of modern microbial mats could yield insights into the factors that cause specific variations in stromatolite shape,” the article says, but conditions today are very different than the presumed past, especially regarding the presence of oxygen, multicellular life, a more complex ecology, and other unknowns such as ocean temperature, minerals present, and more. So even if a cause were found for a branching shape in a growing stromatolite today, that does not necessarily confirm the same factor(s) caused the shape changes in the past.
• “There are so many observations that could be made, samples that could be collected,” one scientist comments, implying possible selection bias. “We need to maintain a balance between collecting samples that can test a hypothesis and being open to the unexpected.”
• Some of the specimens collected had branches, and some did not, but they were collected in the same environment at the same time. What conclusions could be drawn, therefore, about fossil stromatolites over assumed vast ages of time? They found one microbe responsible for branching structures that seemed to prefer a certain depth, but can that supposition be extrapolated back to conditions hundreds of millions of years ago?

Yet these scientists hope to correlate their suppositions with the search for life on Mars, where no microbes have ever been proved to exist.

“If we were to find some ambiguous squiggly layers in sedimentary rocks on Mars, we would have a very high burden of proof to determine their origin, much less to actually say whether these were due to microbial communities. Microbial mat growth models like the one we have outlined here are a necessary step in developing interpretations of microbial community activity from the sedimentary record.”

It appears there’s a long way to go before such conclusions could be drawn, and even then, they would be interpretations, not proofs. Indeed, some geologists have argued for abiotic formation of stromatolite-like structures on Mars and in the fossil record on Earth. See our 3/22/06 where a leading JPL planetary scientist questioned the assumption that fossil stromatolites are biogenic and provided a plausible geochemical model for their formation.

We want to make clear that we at CEH support good basic research on natural phenomena. Studying and measuring stromatolites is great; the details will go into the corpus of observational literature. These scientists did right to point out the “very high burden of proof” necessary to determine the origin of stromatolites (or possible stromatolites in the fossil record and on Mars). They were also right to point out the difficulties in interpretation, and the potential for the unexpected. And undoubtedly, they had fun scuba diving and taking pretty pictures. Nothing wrong with any of that.

It’s the interpretation of the data that is problematic. You probably noticed the going-in assumptions about the ancient age of the earth, the vast time scales, the evolution of life, and major geological changes; none of those were factored into their interpretations. What if conditions were very different in the recent past, causing stromatolites to grow rapidly? No scientist has observed millions of years. No human could.

Creationists are also studying stromatolites. Some scientists at the Geoscience Research Institute at Loma Linda University are working on them. ICR has posted several articles about stromatolites. Creationists also freely admit difficulties with interpretation when causes are not clear. Everybody has puzzles and mysteries about this subject. What is not needed is overconfidence, boasting and bluffing about them in order to avoid running afoul of the Darwin Sharia enforcers.

Exercise: list some factors that might influence the structure and formation rates of stromatolites. For instance, would algal and bacterial stromatolites form at the same rate at the same water depth, water temperature, sunlight and oxygenation? How different would they look? How could one test such factors?