Stromatolites
OUR MYSTERIOUS ANCIENT REEFS
by Jon Nelson
Discovery
of ancient stromatolites on Lake Superior's shore astounded the scientific
world, reestablishing the age of the oldest known fossils. These stromatolites
are visible as rings in an outcropping of Gunflint formation chert near
Schreiber, Ontario. 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.
Kona
dolomite at Marquette, Michigan, clearly shows flat layers of small stromatolites
about 1 cm tall. GENE L. LaBERGE
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.
The concentric rings on Lake Superior's Schreiber Channel also vary
in size, with the largest about three feet in diameter. They are apparently
fossilized stromatolites that are strikingly similar to the stromatolites
living today. These fossilized stromatolites are thought to be the petrified
remains of materials left behind by populations of cyanobacteria living
in ancient oceans almost two billion years ago. The rings are the result
of the growth of mats of bacteria. The original dome shape of the stromatolite
has eroded into its current flat form.
The fossilized spheres in the Gunflint formation, including those in
the stromatolites, are also believed to be primarily cyanobacteria, a type
of bacteria that is commonly known as blue-green algae. This misleading
name was originally given to the bacteria because they have a superficial
resemblance to algae and, like algae, they are photosynthetic and give
off oxygen. Since they are thought to be the first photosynthetic organisms,
they played a crucial role in changing the composition of Earth's atmosphere.
Prior to the presence of oxygen-producing cyanobacteria, there was virtually
no oxygen in the atmosphere. The photosynthetic cyanobacteria caused a
slow increase in the levels of atmospheric oxygen, a change that eventually
led to the appearance of organisms that use oxygen for energy.
Cyanobacteria (blue-green algae) living today are exceptionally tolerant
of extreme environments. Some live in hot springs of near boiling water
and others live in very salty water. However, they are also commonly found
in lakes, streams and ponds, especially in the green slime that forms on
ponds. Snails and other organisms feed on cyanobacteria whenever they can.
Where cyanobacteria are concentrated, as in stromatolites, they are a prime
source of food for snails.
A
microscopic cross section of columnar stromatolite at the Mary Ellen Mine,
Biwabik, Minnesota. GENE L. LaBERGE
The concentric rings on Lake Superior's Schreiber Channel also vary
in size, with the largest about three feet in diameter. They are apparently
fossilized stromatolites that are strikingly similar to the stromatolites
living today. These fossilized stromatolites are thought to be the petrified
remains of materials left behind by populations of cyanobacteria living
in ancient oceans almost two billion years ago. The rings are the result
of the growth of mats of bacteria. The original dome shape of the stromatolite
has eroded into its current flat form.
The fossilized spheres in the Gunflint formation, including those in
the stromatolites, are also believed to be primarily cyanobacteria, a type
of bacteria that is commonly known as blue-green algae. This misleading
name was originally given to the bacteria because they have a superficial
resemblance to algae and, like algae, they are photosynthetic and give
off oxygen. Since they are thought to be the first photosynthetic organisms,
they played a crucial role in changing the composition of Earth's atmosphere.
Prior to the presence of oxygen-producing cyanobacteria, there was virtually
no oxygen in the atmosphere. The photosynthetic cyanobacteria caused a
slow increase in the levels of atmospheric oxygen, a change that eventually
led to the appearance of organisms that use oxygen for energy.
Cyanobacteria (blue-green algae) living today are exceptionally tolerant
of extreme environments. Some live in hot springs of near boiling water
and others live in very salty water. However, they are also commonly found
in lakes, streams and ponds, especially in the green slime that forms on
ponds. Snails and other organisms feed on cyanobacteria whenever they can.
Where cyanobacteria are concentrated, as in stromatolites, they are a prime
source of food for snails.
The stromatolites that are living today usually are located in areas
where the water is so salty that snails and other predators can't survive.
The best place to see living stromatolites is a very shallow, salty bay
in western Australia known as Shark Bay. A partially blocked entrance to
a part of the bay, called Hamelin Pool, has caused the water to become
too salty for most organisms other than cyanobacteria. Here rounded pillars
of stromatolites that are similar to the fossilized ones near Schreiber
are still living and growing. The numerous stromatolites form large reefs
that recreate an environment similar to that found 1.9 billion years ago.
A startling discovery was recently made of living stromatolite colonies
in a half dozen lakes in Minnesota. They are coral-like growths up to a
foot in diameter and are usually located in 10 to 20 feet of water. The
names of the lakes are being kept quiet because of the rarity and fragility
of the stromatolites.
When the stromatolites that are now fossilized were flourishing, there
weren't any large predators that could feed on them. From that early time
period, the only known organisms were microscopic bacteria, algae and fungi.
Stromatolites are thought to have been common from 3.5 billion years ago
until multicelled animal predators became common about 600 million years
ago. Fossilized stromatolites, first found in the Gunflint formation, have
subsequently been found in numerous places around the world, including
Steep Rock Lake near Atikokan in northwestern Ontario.
Standing on these unusual rings along the Schreiber Channel, it is
hard to imagine that they were once part of a large reef of stromatolites
that were thriving in a shallow part of the ocean almost two billion years
ago. It also stretches the imagination to think that these large structures
are the result of the growth of microscopic organisms at a time when no
living things were much larger than bacteria. Instead of white pine and
moccasin flowers, whitefish and lake trout, sea gulls and barred owls,
there was only a weird and wonderful variety of microorganisms.
It was then, as it is now, an intriguing place full of fascinating
and interesting creatures. Here existed an ocean without seaweed or fish,
surrounded by a land without plants or animals, under a sky without birds
or even insects. Life in the past was, as a famous biologist once said,
"not only stranger than we imagine, but stranger than we can imagine."
Jon Nelson is a teacher, freelance writer and photographer who makes
his home in Thunder Bay, Ontario. He and his wife worked as park rangers
for 12 years in Quetico Provincial Park and, along with their children,
still avidly canoe wilderness areas.
Gracefully
coiled filaments are easily seen by the naked eye in these 2.1-billion-year-old
fossils from the Upper Peninsula's Empire iron mine. They are possibly
the oldest "megascopic" formations of life forms ever found.
TSU-MING HAN, CLEVELAND CLIFFS INC.
Michigan's Fossil Algae
A recent report of fossils, this time near the south shore of Lake
Superior, has again stirred up controversy and interest among scientists.
These fossils, unlike the microscopic cyanobacteria in the Gunflint formation,
are visible to the naked eye. They were found by Dr. Tsu-Ming Han in the
2.1 billion-year-old Negaunee iron formation at the Empire Mine near Ishpeming,
Michigan. The fossils are coiled forms of marine life that, if unwound,
would stretch up to 90 millimetres (3.54 inches). These fossils are considered
by Dr. Han and Dr. Bruce Runnegar, the co-authors of a scientific article
on the specimens, to be eukaryotic algae.
Eukaryotic cells contain a nucleus and organelles and are considerably
larger and more complex than bacteria cells. Eukaryotic cells are thought
to have evolved from the simpler bacteria cells. Multicellular organisms,
whether plant, fungus or animal, are made up of eukaryotic cells. The Empire
Mine fossils are of great interest because they are apparently the oldest
known remains of megascopic (visible to the naked eye) organisms yet found.
Han and Runnegar believe the fossils are algae because they closely
resemble fossils from Montana that are considered to be photosynthetic
algae. If they are correct and the fossils are eukaryotic algae, then the
origin of eukaryotic cells must be prior to 2.1 billion years ago. This
astonishingly pushes the origin of the large and complex eukaryotic cells
to a date 300 million years older than previously thought. Since photosynthetic
algae, like those found fossilized, both produce and require oxygen to
function, their discovery in 2.1-billion-year-old rock raises questions
about when oxygen became present in the atmosphere in concentrations high
enough to support oxygen-using organisms.