Stromatolites: Our Mysterious Ancient Reefs

By Jon Nelson

Hiking along the shore of Lake Superior's Schreiber Channel between Schreiber and Rossport, Ontario, one comes across a group of unusual concentric rings embedded in the rock. The rings vary in color and size, with the largest more than three feet in diameter. They are clearly visible over a couple hundred square yards of bedrock that slopes down to the lake. Undoubtedly, there are many other rock rings in the vicinity that are covered with water, soil and a variety of vegetation, but these, washed by waves along the shore, are clearly visible.

It is not readily apparent what caused these intriguing circles in the rock. They are known as stromatolites, a word derived from the Greek which means "stony carpet." Originally, many geologists thought the rings were the result of mud swirling in currents or eddies in the bottom of the seas that covered this area millions of years ago. Other scientists, however, thought that the concentric circles might have been made by living organisms.

These stromatolites are located in rocks of the Gunflint formation, a 1.9 billion-year-old narrow band of generally iron-rich and cherty rocks that extends from Gunflint Lake on the Minnesota-Ontario border to near Schreiber, Ontario. Cherts are glassy rocks with high silica content that are ideal for producing sparks. Flints for early muzzle loaders were made from Gunflint formation cherts and that is the source of the name for the Gunflint formation.

Prior to finding the stromatolites, scientists had discovered unusual, tiny forms in the Gunflint formation cherts. Rocks from numerous locations, including one in the city of Thunder Bay, were analyzed. They cut the cherts and ground them until they were paper thin. The thin-sections were then carefully examined through microscopes. Unusual spheres and chainlike threads unlike anything that had been seen before in rocks were observed.

Stanley Tyler from the University of Wisconsin and Elso Barghoorn from Harvard University first described these unusual forms in a 1957 article. They proposed that the forms in the cherts were fossilized microorganisms, primarily bacteria. They were the first fossils of microorganisms ever found and were thought to be the same age as the rock itself ­1.9 billion years old. This caused a major stir in the scientific community because, not only were they the first microfossils, their proposed age also made them the world's oldest known fossils.

Most of the fossils they described had shapes like contemporary bacteria. Many had a striking similarity to a type of bacterium known as cyanobacteria. However, some had unusual shapes different from any known organism living today, including a strange umbrella-shaped form that, because of its age, was given the name Eoastrion or "dawn star."

The fossilized spheres that Tyler and Barghoorn were convinced were microorganisms were greeted with skepticism by many other scientists. Some thought they were geofacts ­ rock particles that merely happen to look like bacteria. There was also resistance to the idea that microorganisms such as bacteria could be preserved in rock. Usually only the hard parts of an organism, such as teeth or bone, are fossilized. It still is not known how something as small and soft as bacteria can turn to stone while maintaining its shape.

In the intervening years, the fossils have slowly gained almost universal acceptance among scientists. Their dramatic discovery also stimulated searches throughout the world for other locations where microorganisms may have been fossilized. Ancient rocks from every continent were examined in the hope of finding fossilized microorganisms. Many confirming locations were found in the Americas and in other parts of the world. Some of these contain bacteria that are much older than those in the Gunflint formation. Presently, the oldest known sites are in Africa and Australia, where 3.5 billion-year-old fossilized bacteria are the oldest evidence for life yet found.

The Gunflint formation fossils are not as well known as other North American fossils, such as the dinosaur bones from the western United States and Canada, but like dinosaurs they are impressive because of their age and size. Comparatively, dinosaurs are from the recent past; they became extinct just 65 million years ago. The fossils along Lake Superior are almost two billion years old. These fossils are also of great interest because of their size ­ instead of gigantic dinosaurs they are minute bacteria.

Since fossilized bacteria had already been found in Gunflint formation rocks, the stromatolites along the Schreiber Channel were also examined for fossils. It was hoped that a detailed study of the stromatolite rings would shed some light on their origin. Sections of the rings were cut and ground, and thin-sections were viewed through a microscope. Embedded in the rock were spheres and chainlike threads that were virtually identical to those that scientists had previously seen in other Gunflint formation rocks. It now seemed logical to speculate that the stromatolites were somehow formed by these bacteria. However, the idea that bacteria created large structures composed of concentric rings seemed even more absurd than finding fossilized bacteria in rock.

The origin of the fossil stromatolites became clearer when living stromatolites were found in western Australia, Florida and elsewhere. Stromatolites are almost unique in that their fossil forms were found before living ones were discovered. Those living today are the result of the growth of tightly packed layers of bacteria, mainly cyanobacteria, growing on top of each other. As these colonies of bacteria grow and eventually die, they leave behind a hard material composed of tiny bits of rock and sand that gets trapped in their slimy mats. The layers of rock under the living mat of bacteria keep increasing as long as the bacteria continue living, reproducing and growing on top of previous populations. The stromatolites that form are dome-shaped and vary in size, with large ones more than a yard high and close to four feet in diameter.