Australia: The Land Where Time Began
The name stromatolite derives from a Greek word meaning "stony cushion". A stromatolite is comprised mostly of layers of limestone and the remains of the cyanobacteria and the silt they trapped from the water. They are found in a number of other places in Australia and other parts of the world, but those in Hamlin Pool in Shark Bay are the largest collection of the different types of stromatolite, all types being represented in Hamlin Pool.
At 3.5 billion years old, the fossil stromatolites at North Pole in Western Australia are among the oldest record of life on Earth. There are a number of other fossil stromatolite sites in Western Australia.
Until 500 million years ago they were very common but since then they have been declining. It is thought that the decline was caused by the appearance of algae-grazing organisms such as gastropods. There are several factors which determine the environments where they can survive. Being photosynthetic cyanobacteria require the water to be shallow enough to reach the bottom of the water column and they require water currents that bring the right amount of silt so they cement the silt with the limestone they extract from the water, without being smothered by the silt. The known living stromatolites also occur only in places of high temperature or high salinity. It was believed that these conditions may be required to preclude the grazing of gastropods, etc., or at least to reduce it to a rate which is less than the rate of formation of the mat, but they have now been found in places that don't have these "required" conditions. See below. The high temperature environments are found in places such as Yellowstone National Park, which have algal mats comprised of similar microbial populations as the stromatolites at Shark Bay. The high saline environment is found in places like Hamlin Pool. The cyanobacteria are phototropic, they can move towards light, that allows them to avoid being buried by the accumulating silt as long as the silt deposition rate is low enough.
The Hamlin Pool stromatolites began to grow about 4500 years ago when the high saline conditions began to develop. The exchange of water between the open ocean and the bay is limited and at low tide the water in the area known as Hamlin Pool the high evaporation rate increases the salinity of the water until it is nearly twice that of normal sea water. The stromatolites in Shark Bay grow so slowly that some of them are believed to be more than 1000 years old.
All 7 known varieties of stromatolite are found in the tidal and sub-tidal parts of Hamlin Pool.
The stromatolites are thought to have dominated the seas for about 3 billion years. The reef like structures began to decline about 1 billion years ago, the decline accelerating towards the end of the Precambrian. It is believed the decline may have resulted from the increases in nutrient levels in the oceans with the rise of the Metazoans that culminated in the Cambrian explosion that ushered in the modern type animals (at least those that were more solid than the Ediacaran Fauna) including grazing marine invertebrates, about 600 Ma. An example of early life, a stromatolite in the Pilbara, the cyanobacteria that built the stromatolites were morphologically similar to the present day cyanobacteria, but it is believed that would have utilised an Archaean type of metabolism, that synthesised carbohydrates, with sulphur as a by-product, from hydrogen sulphide, that was common at that time when there was widespread volcanic activity. It is believed the type of metabolism of modern cyanobacteria, photosynthesis producing carbohydrate by using CO2 and water, and releasing oxygen as a waste product, evolved about 1 billion years later.
For some time after the production of oxygen began the oxygen would not have accumulated in the atmosphere as it was used up in the process of oxidising the world's oceans, the seas were rusted, the origin of the banded ironstone deposits found around the world, especially at Hamersley. Between the 3.5 billion years ago, the period of the North Pole Stromatolites, and the 2.8 billion years ago period of the stromatolites on what is now the bank of the Nullagine River about 80 km south-east of North Pole, there had been a huge increase in the area of continental crust. Erosion would have begun as soon as the rocks were exposed to the atmosphere and one of the consequences was that huge amounts of soluble iron had been leached from the emerging rocks and was accumulating in the oceans. Prior to large-scale production of oxygen by photosynthetic organisms using processes which produced oxygen as a by-product, the iron remained in solution, but as soon as dissolved oxygen became available the iron began to rust to iron oxide. So much of it was produced that at least parts the oceans probably turned red. The rust began to settle to the sea floor and the result was that by 2.5 billion years ago, at places like the present-day Hamersley Range, huge deposits of iron oxide began to accumulate about 300 million years after the proliferation of stromatolites around 2.8 Ga, and accumulated to the point where they are now mined as iron ore. The amount of iron deposited here is so great that after the sediments were compressed to rock, then raised above the sea surface, and after many millions of years of erosion, the ore deposits are still 2.5 km thick.
Eventually enough oxygen was produced that it began accumulating in the atmosphere once most of the iron had been removed from solution in the oceans. The increasing oxygen concentrations in the atmosphere allowed the increasingly complex organisms that were evolving in the oceans to begin to form hard parts by the time of the Burgess Shale stage, an example of which is found at Emu Bay on Kangaroo Island, as well as at the earlier stage of the Ediacaran Fauna, at which point it is believed that the atmospheric oxygen reached a high enough level to allow the formation of collagen, at about 3 %.
An unexpected find is that stromatolites exist in places where they shouldn't, according to the older ideas of required conditions for their survival. They have now been found in Antarctica in glacial lakes, as well as at the other extreme, in volcanic lakes in Yellowstone National Park. They have also been found in places where the water varies from brackish to fresh. But the most exotic location to have been suggested has to be in a crater on Mars. In the area called Terra Sabaea, within the rim of a crater that hasn't been named, near the Martian equator, there is an outcrop that has an area of about 200 km2, that has been suggested to be of stromatolitic origin (White, 1).
The stromatolites are of several types, having differing internal structure, depending on their location in relation to the shore and the species involved in their formation. In the intertidal zone, they are of pustular mat type, in which the internal structure is poorly defined with no layering. This type are produced mainly by a coccoidal cyanobacterium, Entophysalis. In the zone between the intertidal zone and the subtidal zone, there are 'smooth mat' stromatolites. Their internal structure is well defined and the outer surface is smooth. The main organism involved in their construction is the filamentous cyanobacterium, Schizothrix. Their internal structure is well-defined and the outer surface is smooth. The stromatolites occurring at greater depth of about 3.5 m, are of a coarse structure that is poorly laminated that grow to about 1 m in height. The organisms involved in their construction are a mixture of microbes. The cyanobacteria are from 2 genera, Microcoleus and Phormidium. There are also a number of protists, mostly mucus-secreting diatoms, the mucus trapping coarse sediment.
The Hamlin Pool embayment was once thought to eliminate the grazing invertebrates, which would allow the stromatolites to grow unhindered by predation. The proliferation of the stromatolites is now believed to result from the low nutrient status of the water.
In lakes such as Lake Clifton, Lake Richmond, Government House Lake on Rottnest Island, and Pink Lake near Esperance, stromatolites have been found that are constructed by the precipitation of calcium carbonate, without the need for sediment. These are sometimes called thrombolites. The water in these lakes where stromatolites have been found can range from saline to fresh.
When the water in Lake Clifton drops in summer, a reef is exposed near the northern end of the lake that is about 5 km long and about 30 m wide. Most of the stromatolites composing the reef are less than 50 cm across, though their diameters can be up to 1 m. The main organism involved in their building is the cyanobacterium Scytonema, a filamentous form. Groundwater seepage provides the carbonate and bicarbonate for use in the construction of the stromatolites, thus the location of the seep determines the location of the stromatolitic reef.
The lake has a diverse invertebrate fauna that includes isopods and amphipods (crustaceans), both grazers, that feed on the stromatolites. There are also molluscs that include the stromatolites in their diet. This existence of the stromatolites near the seep has been suggested as proof that if the balance is right between the grazers and the conditions for growth stromatolites can survive in the presence of grazers.
A better understanding of stromatolites has been gained as the result of a research project at Lake Clifton. Based on the difference between stromatolites, that are laminated, and thrombolites, that are not laminated, it has been suggested that the name stromatolite be changed to microbialite. It was previously believed that the non-layered internal structure of thrombolites resulted from damage by metazoan grazers and borers. As the Lake Clifton thrombolites have been found to have a consistent internal morphology with no signs of disorganised laminations, it appears that the different internal morphology of thrombolites probably results from the microenvironmental-chemical changes produced by the mainly filamentous cyanobacteria, the resulting structure being composed of aragonite, a crystalline form of calcium carbonate.
Most of the microbialites in Lake Clifton are thrombolites that are on, or partially buried in the sediment. The oldest dated to about 1950 BP, with growth rates of about 10 cm/100 years. The outward form of the thrombolites varies, depending on their location in the lake. Some laminated small stromatolites have also been found in the lake. The numbers of stromatolites declined near the end of the Proterozoic, the thrombolites remaining fairly common until after the Cambrian when they too declined.
In Nettle, one of the Jenolan Caves system, New South Wales, an active stromatolite has been found.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|