The rocks in the hyper-saline waters of Lake Thetis, about 120 km north of Perth in Western Australia, are not quite what they seem. They are actually living things. Stromatolites are the oldest living life-forms on our planet.

They are formed through the activity of primitive unicellular organisms: cyanobacteria (blue-green algae) and other algae. These grow through sediment and sand, binding the sedimentary particles together, resulting in successive layers which, over a long period of time, harden to form rock. For at least three-quarters of the Earth's history stromatolites were the main reef building organisms, constructing large masses of calcium carbonate.

However their most important role in the history of the Earth has been that of contributing oxygen to the Earth's atmosphere. When stromatolites first appeared on Earth about 3,5 billion years ago there was little or no oxygen in the atmosphere. The organisms which construct stromatolites are photosynthetic.  They take in carbon dioxide and water to produce carbohydrates, and in doing this they liberate oxygen into the atmosphere. It has been speculated that bacteria found in these organisms were responsible for increasing the level of oxygen in the atmosphere on Earth from much less than 1% to the present day level of 21%. lt was through the oxygen generating activity of stromatolites that other animal life on Earth was able to develop. These amazingly persistent living fossils form complex microbial communities. Conversely, it is believed that the decline in numbers of stromatolites is related to the evolution of animals that consumed cyanobacteria and algae. However, most living animals, which feed on the bacteria and algae of which stromatolites are composed, cannot tolerate the extremely saline conditions of places such as Lake Thetis, and as a result stromatolites can grow here successfully, undisturbed.

Western Australia has perhaps the best stromatolite fossils, giving a record through the eons of time. Fossils of the earliest known stromatolites, about 3,5 billion years old, are to be found near Marble Bar which lies on part of the ancient Pilbara Craton.

Stromatolites represent what is seen to be the biggest continuous biological lineage known in the world. The evolutionary events of the last 600 million years were in the time when most of the major groups of animals and plants on Earth evolved. If we look very closely they are primitive-celled organisms; these organisms have remained virtually unchanged during the comings and goings of all the animals and plants that have ever lived. Not only have they been found in some of the oldest rocks on Earth, but they have persisted with no other life forms for company. The existence of these ancient rocks extends three-quarters of the way back to the origins of the Solar System. These amazingly persistent living fossils form complex microbial communities. The long period of time over which these fossils have survived is amazing and these simple organisms have no peers.

The organisms’ existence is preserved in rocks by their fossilised remains, but also more commonly by the structures they created, domes or columns of sediment called stromatolites. They come in many shapes and sizes. It is extremely remarkable that the living stromatolites are only in a small number of places throughout the world. It is known that in the period from one billion to three billion years ago, stromatolites were prevalent on the shores of lakes and seas around the world. Such structures are still being formed today. Cervantes is one of several such sites in Western Australia where you can view them easily. Stromatolites grow as layers of sediment that have been trapped. These layers or mats slowly build on top of each other over many years with each stromatolite formation only growing at a rate of 5 cm in 100 years! They need light so are limited to shallow water where the sunlight can penetrate.

In South Africa the Malmani dolomite near Sudwala (and its equivalent Cambell Rand dolomite in the Northern Cape) are part of the Transvaal Supergroup that was deposited in a vast inland sea on the Kaapvaal Craton. Here too algal mats formed about 2,5 million years ago. On the upper part of the Sudwala Pass, there are small to large rounded or elongated dome-like structures visible in rocks alongside the road. The Barberton Greenstone Belt of eastern South Africa contains some of the most widely accepted fossil evidence for Archean life. These cell-sized prokaryote fossils are seen in rocks as old as 3,5 billion years. The Barberton Greenstone Belt is an excellent place to study the Archean Earth due to exposed sedimentary and metasedimentary rocks. The Barberton Greenstone Belt is located on the Kaapvaal craton, which covers much of the south eastern part of Africa, and was formed by the emplacement of granitoid batholiths. The Kaapvaal craton was once part of a supercontinent geologists term Vaalbara that also included the Pilbara craton of Western Australia. Though the exact timing is still debated, it is likely that Vaalbara existed from approximately 3,6 to 2,2 billion years ago, and then split into two different continents. Only in Australia though are modern-day algal mats or living stromatolites still to be seen.  JW

Lake Thetis Information pamphlet - DEC
Geological Journeys – Nick Norman & Gavin Whitfield

All Photos are of Lake Thetis and were taken by Jo Wicht