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Modern stromatolites in Shark Bay, Western Australia

Stromatolites (from Greek στρώμα, strōma, mattress, bed, stratum, and λιθος, lithos, rock) are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). They include some of the most ancient records of life on Earth.

Contents

Morphology

A variety of stromatolite morphologies exist including conical, stratiform, branching, domal,[1] and columnar types. Stromatolites occur widely in the fossil record of the Precambrian, but are rare today. Very few ancient stromatolites contain fossilized microbes. While features of some stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-organic) precipitation. Finding reliable ways to distinguish between biologically-formed and abiotic (non-biological) stromatolites is an active area of research in geology.

Fossil record

Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park
Stromatolites (Pika Formation, Middle Cambrian) near Helen Lake, Banff National Park, Canada.
Pre-Cambrian stromatolites (commonly referred to as "Mary Ellen jasper") on display at the North American Museum of Ancient Life. The fossil was originally excavated in the Iron Range of Minnesota.

Stromatolites were much more abundant on the planet in Precambrian times. While older, Archean fossil remains are presumed to be colonies of single-celled blue-green bacteria, younger (that is, Proterozoic) fossils may be primordial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia. The earliest stromatolite of confirmed microbial origin dates to 2,724 million years ago.[2] A recent discovery provides strong evidence of microbial stromatolites extending as far back as 3,450 million years ago.[3]

Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth,[4] with their abundance peaking about 1,250 million years ago. They subsequently declined in abundance and diversity, which by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite builders fell victims to grazing creatures (the Cambrian substrate revolution), implying that sufficiently complex organisms were common over 1 billion years ago.[5][6][7]

The connection between grazer and stromatolite abundance is well documented in the younger Ordovician evolutionary radiation; stromatolite abundance also increased after the end-Ordovician and end-Permian extinctions decimated marine animals, falling back to earlier levels as marine animals recovered.[8]

While prokaryotic cyanobacteria themselves reproduce asexually through cell division, they were instrumental in priming the environment for the evolutionary development of more complex eukaryotic organisms. Cyanobacteria are thought to be largely responsible for increasing the amount of oxygen in the primeval earth's atmosphere through their continuing photosynthesis.

Cyanobacteria use water, carbon dioxide, and sunlight to create their food. The byproducts of this process are oxygen and calcium carbonate (lime). A layer of mucus often forms over mats of cyanobacterial cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone. These laminations can accrete over time, resulting in the banded pattern common to stromatolites. The domal morphology of biological stromatolites is the result of the vertical growth necessary for the continued infiltration of sunlight to the organisms for photosynthesis.

Modern occurrence

Stromatolites at Lake Thetis, Western Australia

Modern stromatolites are mostly found in hypersaline lakes and marine lagoons where extreme conditions due to high saline levels exclude animal grazing. One such location is Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia where excellent specimens are observed today, and another is Lagoa Salgada, state of Rio Grande do Norte, Brazil, where modern stromatolites can be observed as bioherm (domal type) and beds. Inland stromatolites can also be found in saline waters in Cuatro Ciénegas, a unique ecosystem in the Mexican desert, and in Lake Alchichica, a maar lake in Mexico's Oriental Basin. Modern stromatolites are only known to prosper in an open marine environment in the Exuma Cays in the Bahamas (for excellent underwater photos, see # 197-209: http://strata.geol.sc.edu/Bahamas/BahamasGalleryIndex_3.html).

Freshwater stromatolites are found in Lake Salda in southern Turkey. The waters are rich in magnesium and the stromatolite structures are made of hydromagnesite.[9]

Layered spherical growth structures named oncolites are similar to stromatolites, and are also known from the fossil record.

See also

Notes

  1. ^ An unusually complete domed stromatolite with a well-preserved upper surface, over 5 feet in diameter, from the Boxley Blue Ridge Quarry near Roanoke, Virginia, was donated to the Virginia Museum of Natural History ("Two-ton, 500 Million-year-old Fossil Of Stromatolite Discovered In Virginia, U.S."), accessed 4 July 2008.
  2. ^ Lepot, Kevin; Karim Benzerara, Gordon E. Brown, Pascal Philippot (2008). "Microbially influenced formation of 2.7 billion-year-old stromatolites". Nature Geoscience 1: 118–21. doi:10.1038/ngeo107. 
  3. ^ Allwood, Abigail; Grotzinger, Knoll, Burch, Anderson, Coleman, and Kanik (2009). "Controls on development and diversity of Early Archean stromatolites". Proceedings of the National Academy of Sciences. 
  4. ^ Earlier start to life on Earth - Science - Specials - smh.com.au
  5. ^ McNamara, K.J. (20 December 1996). "Dating the Origin of Animals". Science 274 (5295): 1993–1997. doi:10.1126/science.274.5295.1993f. http://www.sciencemag.org/cgi/content/full/274/5295/1993f. Retrieved 2008-06-28. 
  6. ^ Awramik, S.M. (19 November 1971). "Precambrian columnar stromatolite diversity: Reflection of metazoan appearance" (abstract). Science 174 (4011): 825–827. doi:10.1126/science.174.4011.825. PMID 17759393. http://www.sciencemag.org/cgi/content/abstract/174/4011/825. Retrieved 2007-12-01. 
  7. ^ Bengtson, S. (2002), "Origins and early evolution of predation", in Kowalewski, M., and Kelley, P.H. (Free full text), The fossil record of predation. The Paleontological Society Papers 8, The Paleontological Society, pp. 289– 317, http://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf, retrieved 2008-06-28 
  8. ^ Sheehan, P.M., and Harris, M.T. (2004). "Microbialite resurgence after the Late Ordovician extinction". Nature 430: 75–78. doi:10.1038/nature02654. http://www.nature.com/nature/journal/v430/n6995/full/nature02654.html. Retrieved 2007-12-01. 
  9. ^ Braithwaite, C. and Veysel Zedef, Living hydromagnesite stromatolites from Turkey, Sedimentary Geology, Volume 106, Issues 3-4, November 1996, Page 309, DOI: 10.1016/S0037-0738(96)00073-5

References

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