A whirlwind two-week visit to Western Australia is now over- and I wish we could have spent 4x longer to see things properly! The distances between stops on the few days we spent north of Perth certainly would justify a more leisurely trip. Nevertheless, we tried to see the things that we were most interested in. Oddly enough – it always ends up being something to do with rocks
First up: Stromatolites.
Well, what are Stromatolites exactly?
Most simply put – they are fossilised rocks that have formed by algal activity.
They have been around forever. Almost literally.
The world is about 4.5 billion years old and Stromatolites have been dated to have existed on this planet since around about 3.5 billion years ago. They dominated the scene during the Precambrian Period (4.6 billion years ago – 545 million years ago; that’s 88% of the entire Geological record!) and still grow, albeit less extensively, today – making them the single most prolific life-form ever to have existed! (Take that – dinosaur-aged crocodiles!)
How do they form?
It starts off with one of the earliest forms of biological life: Cyanobacteria. Also known as blue-green algae, Cyanobacteria are still found in every single possible environment that exists on our planet today. By the process of photosynthesis, they use water, carbon dioxide and sunlight to produce energy and oxygen, a waste product.
They also secrete mucus.
This mucus, which contains calcium carbonate, traps small particles and grains of sediment that happen to be drifting by on gentle currents. As the sediment builds up, the hair-like filaments of the algae grow around it, cementing the grains of sand and silt in place. It is only a thin layer of activity, and is confined to the edges. As the Stromatolite grows, the algal activity migrates outwards and the inner parts form limestone. A cross-section of a Stromatolite would reveal concentric rings, similar to the growth rings of trees. Growth is extremely slow – perhaps only as little as 1cm every 30 years!
Different types of Cyanobacteria produce different shaped Stromatolites.
I’ve seen plenty Stromatolites during numerous field trips in my career as a Geologist. All were, most certainly, dead.
Today’s “modern” Stromatolites are found in typically shallow, warm, hyper-saline waters. In these salty waters they flourish, as many of their predators, that would otherwise graze them to extinction, cannot survive in these conditions.
Where can you find them?
Two such locations are found at Lake Thetis and Hamelin Pool, both in Western Australia.
Two weeks ago we, being the Geo-Tourists that we are, checked out the real, live fossils – (and yes, that ought to be an oxymoron) at both locations.
…is 1km from the town of Cervantes, 198km north of Perth. The town is named after the American whaling-ship Cervantes, which was scuttled off the coast back in 1844. The ship itself was named after Miguel de Cervantes, author of Don Quixote.
Cervantes is where we stopped overnight before visiting both Lake Thetis and The Pinnacles of Nambung National Park (full post on this location to come in due course!)
Lake Thetis is a small shallow lagoon – only 1.2km in circumference and is thought to have become separated from the ocean due to receding sea-levels approximately 4000 years ago. It is saline through the process of evaporation, although water levels are replenished via the higher levels of ground water during the rainy season.
Lake Thetis Stromatolites have been dated to be about 3370 years old and consist of both mats and mounds.
An elevated boardwalk has been constructed around the entire lake as well as a small walkway into the water so that you can view the Stromatolites from above.
The boardwalk is there to stop any potential damage to the algal mats. They also have plenty of notices around that ask you not to touch the Stromatolites.
My husband and I were quite chuffed with what we saw at Lake Thetis. Only later that same day, would we come to realise that this was just a taster of things to come. There was a much more spectacular display of Stromatolites at…
Which is 550-600km further north and lies within the Shark Bay World Heritage Site.
A massive seagrass bank called Fauré Sill began to form between 4000 – 6000 years ago, which blocked the tidal flow to Hamelin Pool from the north. Since then, Hamelin Pool has become twice as salty as the open ocean. 3000 years ago, the cyanobacteria began to flourish in this saline environment and Stromatolites began to form in the same way they would have 3.5 billion years ago.
Discovered by scientists in 1956, the Stromatolites at Hamelin Pool represent the most diverse range seen in one location ranging from algal mats in the inter-tidal zone to anvils, columns and mushroom forms in sub-tidal zones.
Sadly, this was also a popular route for the wool traders to transport their wool bales to waiting boats further out in the deeper section of the lagoon. Wheel marks are clearly evident in the algal mats.
The wool trade was in full swing from the late 1800’s to the early 1900’s – long before geologists even knew that living Stromatolites existed and conservation measures were put in place to help prevent the destruction of this incredibly sensitive microbial community.
Sources and Links:
- Department of Conservation and Land Management
- Lake Thetis and Stromatolites
- Stromatolites – Pilbara
- Stromatolites of Shark Bay
- Stromatolites – Wikipedia
- Cyanobacteria – Wikipedia
Lake Thetis & Hamelin Pool, Western Australia – August, 2011