Dictionary:
stro·mat·o·lite (strō-măt'l-īt')  |
[Late Latin strōma, strōmat-, covering; see stroma + -LITE.]
stromatolitic stro·mat'o·lit'ic (-măt'l-ĭt'ĭk) adj.| 5min Related Video: stromatolite |
| Britannica Concise Encyclopedia: stromatolite |
For more information on stromatolite, visit Britannica.com.
| Sci-Tech Encyclopedia: Stromatolite |
A laminated, microbial structure in carbonate rocks (limestone and dolomite). Stromatolites are the oldest macroscopic evidence of life on Earth, at least 2.5 billion years old, and they are still forming in the seas. During the 1.5 billion years of Earth history before marine invertebrates appeared, stromatolites were the most obvious evidence of life, and they occur sporadically throughout the remainder of the geologic record. In Missouri and Africa, stromatolite reefs have major accumulations of lead, zinc, or copper; and in Montana, New Mexico, and Oman, stromatolites occur within oil and gas reservoirs. For geologists, the shapes of stromatolites are useful indications of their environmental conditions, and variations in form and microstructure of the laminations may be age-diagnostic in those most ancient sedimentary rocks that lack invertebrate fossils. See also Dolomite; Limestone; Reef.
Stromatolites are readily recognizable in outcrops by their characteristic convex-upward laminated structure. Individual, crescent-shaped laminations, which are generally about a millimeter thick, are grouped together to produce an enormous range of shapes and sizes.
The tiny, filamentous cyanobacteria (blue-green algae) that make present-day stromatolites, and similar filaments associated with the oldest stromatolites known, are considered one of the most successful organisms on Earth. Living stromatolites in the Bahamas and Western Australia possess laminations that record the episodic trapping and binding of sediment particles by the microbial mat. In the modern oceans, stromatolites develop almost exclusively in extreme marine conditions that exclude or deter browsing invertebrates and fish from destroying the microbial mats and inhibit colonization by competing algae. See also Algae; Cyanobacteria.
| Wikipedia: Stromatolite |
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.
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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.
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] A recent discovery provides strong evidence of microbial stromatolites extending as far back as .[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[verification needed] 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).[verification needed] 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 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 de Janeiro, 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.
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Stromatolite". Read more |
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