Modern and ancient stromatolites
A stromatolite is a microbially mediated rock – so it is both biological and geological. A community of microbes forms mats and secretes a mucus that traps sedimentary grains, cementing them into layers. Because the upper part of the mat requires sunlight for photosynthesis, the mat migrates upwards, leaving the layers that are seen in both modern and ancient stromatolites.
The microbial communities that made stromatolites younger than three billion years may have been instrumental in providing oxygen to the early atmosphere, which many scientists believe was mostly carbon dioxide and nitrogren before then. The microbial communities that seem to have formed the Pilbara stromatolites might not have included oxygen-bearing organisms. There is no generally accepted direct evidence of microfossils in stromalites until around about 2.8 billion years ago. Therefore the comparison with the modern stromatolites of Shark Bay extends only to the morphology (shape) of the dome structures with those of 3.5 billion years ago.
Modern stromatolites are made mainly by layers of prokaryote microbial communities especially cyanobacteria. The latter uses sunlight to convert carbon dioxide and water into food, one waste product being oxygen. Below the cyanobacteria are other communities, the bottom-most part often being the anaerobic purple bacteria, to which oxygen is poisonous. The microbial communities live in a symbiosis. To understand that more clearly take a voyage in the Stromatolite Explorer.
Stromatolites come in a number of shapes – domical (dome like), conical or coniform (cone like), columnar, wrinkly, and branching. The varieties extend to around 1,000 types at their most prolific between 2.5 billion years ago and 700 million years ago. But this contrasts with only about 20 documented varieties for the period 2.8 billion years to 3.5 billion years, according to Prof Malcolm Walter, Director of the Australian Centre for Astrobiology at Macquarie University, Sydney.
With the appearance of complex life around 600 million years ago came animals that fed on the mats or disturbed their habitats, so the very slow growing stromatolites became less prevalent. Now they are generally found in hot or hyper-saline environments because there are few grazing or burrowing animals.
The mats are big house builders. In Shark Bay it is estimated there are three billion individual individuals per square meter of mat, yet they build homes up to 1.5 meters high – 10 million times their size. Hamelin Pool along with the rest of Shark Bay is a World Heritage site. To understand why Hamelin Pool is so salty, click here to watch a short explanatory slide show provided by the West Australian government.
Some scientists think that the 3.5 billion year old stromatolites formed chemically in unusual conditions on Early Earth. Other geologists argue there is no difference in shape, context and isotopic evidence to those 2.8 billion year old stromatolites that are generally accepted as being biological. Another argument advanced is that in some places, the 3.4 to 3.5 billion year old stromatolites are so ornate that it is hard to explain them non-biologically. Using Occam’s Razor, the simplest explanation is that they are of biological origin.
For greater detail on this, here is a chapter written by one of the scientists who thinks that the biological explanation is more likely. Dr Bill Schopf, of the University of California Los Angeles (UCLA) goes into depth on the geology of the Warrawoona Group in he Pilbara, where both the ancient stromatolite bearings areas of the Dresser Formation and the Strelley Pool Chert are located.
Walter, M.R. (1999) The Search for Life on Mars, Allen and Unwin, Australia
Schopf, J.W. (1999) Cradle of Life, Princeton University Press, USA