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Past lives: Chronicles of Canadian Paleontology Pethei stromatolites
Stromatolites are the largest and most impressive biologically-made
structures in Precambrian rocks. If they are fossils, they have the
longest stratigraphic record of any organism; reaching back 3.5 billion
years
Living stromatolites at low tide at Shark Bay, western Australia.
(Photo by Paul Hoffman (c)) |
Shark Bay is a large embayment off the Indian Ocean located a long
day's drive north of Perth along the North West Coastal Highway that
crosses arid Western Australia. This bay has become somewhat of a sanctum
sanctorum for sedimentary geologists because it is the only
place in the world where the living algal mounds called stromatolites
approach the dimensions and the variety of shapes they achieved during
their heyday in the Precambrian. First discovered here in the late 1950s,
the best stromatolites in Shark Bay are found in the intertidal zone along
an inner lagoon called Hamelin Pool. They flourish here because the high
salinity -- twice that of normal sea water -- keeps out algal grazers such
as chitons and snails. The shape and size of the Hamelin Pool
stromatolites are directly controlled by the immediate environment. Large
mushroom-shaped stromatolites grow at headlands where waves and tide scour
approach from different directions. Loaf-shaped stromatolites occur in
protected bights perpendicular to the shore.
Stromatolites are solid cauliflower-shaped structures built by
consortia of microbes. A thin sticky carpet of oxygen-producing
photosynthetic cyanobacteria coats the sunlit top surface while microbes
that depend on fermentation survive in the dark recesses just below. The
cyanobacterial carpet traps sediment and also promotes the precipitation
of carbonate to build up solid cauliflower-shaped structures. Because
their growth is controlled by tides, temperature and sunlight,
stromatolites are rhythmically laminated on the sub-millimetre scale.
Exhumed stromatolites 1.8 billion years old from the East Arm of Great Slave Lake.
(Photo by Paul Hoffman (c)) |
Cross-section of Great Slave Lake stromatolites.
(Photo by Paul Hoffman (c)) |
Paul Hoffman was one of the geologists who travelled to Shark Bay in
the 1960s to take a close look at these living stromatolites. Hoffman, a
Canadian, was pursuing a Ph.D. thesis at Johns Hopkins University in
Baltimore on the stratigraphy of late Precambrian rocks exposed in the
East Arm area of Great Slave Lake. The East Arm reaches 300 kilometres
northeastward from Great Slave Lake into the granite heart of the Canadian
Shield. The islands and the long finger-like peninsulas that choke the
East Arm are largely made of sedimentary rocks with superbly preserved and
exposed stromatolites of the Pethei Group and of volcanic rocks. Despite
their great age (about 1.8 Ga), these rocks are well preserved and they
probably have never been deeply buried.
The shallow-water stromatolites of the Pethei are strikingly similar to
the living shallow-water stromatolites in Shark Bay. The loaf-shaped
fossil forms, arrayed perpendicular to the shoreline, were clearly
controlled by the same environmental forces as the living forms. Hoffman
showed that quite different stromatolites grew in deep water settings of
the Pethei. These are minuscule cones and stunted columns that stood only
a couple of centimetres high. Living in perennial darkness, these dwarf
stromatolites could not have been made by photosynthetic cyanobacteria.
They were probably formed by microbes sustained by fermentation.
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