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The Earth’s Oldest Fossils

On a splendid white beach in Shark Bay, Western Australia, I finally found the worlds oldest fossils. They are commonly referred to as living rock, and they have ruled the earth for approximately 3.5 billion years, and continue to live today. No other form of life has survived so long.
This article appeared in Vol. 8, No. 2 - 2011

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Photo: Ingvild Ryggen Carstens Since I first read about stromatolites in Gabrielle Walker’s book ‘Snowball Earth’, I have felt a deep desire and pull to one day go and see for myself this ancient and incredible life form at Shark Bay in Western Australia. Their life story and survival have the ability to give any geologist – or geo ‘geek’ – a feeling of awe and amazement. Is it not incredible that they are still around, this organism that spawned complex life?   

It is just before 7am in the morning, and I pull off the main road to Shark Bay and head down a gravel road towards Hamelin Pool. It is still dark outside, but soon the sun will be rising and I want to be there early, to catch the first rays as they strike the primeval living algae I have come all this way to see.  

The ground is covered in dew after a cool night - it is the end of May, and the depth of winter is approaching. I grab my jacket and my camera and as I leave the car park and start walking towards these ancient living creatures, I literally feel like I am stepping back through time.  

The water is tranquil and there is no one down here this early in the morning. Some birds are singing in the treetops as the sun’s first rays strike the bay. I walk along the shore and stare and stare down the beach. A carpet of stromatolites cover the entire inside of the bay – the sight is mesmerizing.   

Living Rock

Source: Nasa Stromatolites are often described as the Earth’s oldest fossils, the earliest proof we have of life’s emergence on Earth. For billions of years the stromatolites ruled the world. They appear in the fossil record in the Precambrian, long before the development of complex life, and for years they puzzled geologists.   

Laminar structures formed of large colonies of cyanobacteria, stromatolites thrive in calm, warm, saline waters, and today we only find them in hypersaline lakes and protected marine bays and lagoons. This hostile environment keeps predators at bay and provides a safe haven for the stromatolites to develop. They form through the process of trapping, binding and cementing sedimentary materials with calcium carbonate produced by the cyanobacteria, which look like sticky slime. Slowly, as sediments and other material are trapped, layers are built up, growing between only 0.5 and 1 mm per year.  

Cyanobacteria, or blue-green algae, are prokaryotes, which is a primitive organism without a cellular nucleus. The cyanobacteria receive their energy through the process of photosynthesis, converting carbon dioxide and water into organic compounds, like sugar and oxygen. During the billions of years that the stromatolites dominated the Earth, they slowly changed the composition of the Earth’s atmosphere into an oxidizing atmosphere. This process is believed to have laid the groundwork for the emergence of complex life during the Cambrian explosion.  

The stromatolite population peaked in numbers and diversity about 1.25 billion years ago, but by the time of the Cambrian explosion their numbers had been reduced to a fifth of this. It is widely believed that the reason for their decline was that the stromatolites became the number one source of energy for more complexly evolved life forms.   

Stromatolites are extremely fragile and vulnerable to external forces. As a result, we only find them at a handful of places throughout the world today – so why here at Hamelin Pool?

Hamelin Pool Marine Nature Reserve

There are three basic types of stromatolites. Sub-tidal stromatolite columns that are always submerged in water take the shape of cones and domes, while inter-tidal stromatolites usually grow in stratiform or branching shapes, like mushrooms or anvils. They are exposed above water during low tide. Stromatolites also grow as mats in inter-tidal environments. Both sub-tidal and inter-tidal stromatolites are found in Hamelin Pool. Photo: Ingvild Ryggen Carstens Hamelin Pool Marine Nature Reserve covers an area of 1,270 km2. The unique conditions here mean that it is one of a handful of places in the world where stromatolites still thrive, and is the location where we find the largest and most diverse population of stromatolites today.  

The hypersaline waters of Hamelin Pool makes life uninhabitable to other marine life and the shallow waters protect the fragile stromatolites from strong currents and violent seas. The water is twice as salty as the water found in the open ocean, and the reason for this is the Faure Sill, a sandbank that has formed over thousands of years through the trapping of sediment and organic materials by sea grass. It inhibits the flow of tidal water between Hamelin Pool and the rest of the bay, creating the hypersaline conditions in Hamelin Pool.

Geologist Philip Playford discovered the stromatolites at Hamelin Pool in 1956. They were the first documented living examples of the structures which had been found fossilized in very old rock from all over the world. Stromatolites were believed to be extinct, wiped out by the competition from more complex life forms. These fossils had puzzled geologists for decades and there had been much disagreement between researchers whether the stromatolites were remnants of living organisms. The discovery of the living stromatolites solved this enigma.

The Magic of Hamelin Pool

During the last 6,000 years, shells have been deposited along the shore of Hamelin Pool and Lharidon Bight. These are remains of Chardiid Cockle, which is another salt-tolerant life-form. The Chardiid Cockle is a brown mollusk living in tiny, bivalve shells, which is more abundant in Shark Bay than anywhere else in the world, as it has been allowed to thrive in this environment without predators.   

The shells have accumulated on the sea floor and are continuously washed ashore. When it rains, calcium carbonates within the shells dissolve, and, when dried, form crystals that cement the shells together to form a soft limestone called coquina. For years, the limestone was quarried and used as building material in several buildings throughout Shark Bay. On a visit to Hamelin Pool, you can still see the old quarries when you are walking down towards the stromatolites. The quarry is no longer in continuous operation. However, limestone is still quarried when the existing coquina limestone buildings need maintenance.  

The sun is shining brightly now, and I think I have seen all there is to see. I dwell a little longer on the walkway, admiring this amazing life form that spawned complex life. A few people have found their way down the trail and onto the walkway now. It is time for me to go, and let others experience the magic of Hamelin Pool. Reluctantly I begin to make my way back to the car. But I am taking a piece of the magic with me – the feeling of having stepped back through time and been allowed to see a little bit of what the world looked like, millions of years ago.

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