This process generates the characteristiclamination of stromatolites, a feature that is hard to interpret, in terms of its temporal and environmental significance.[7][8] Different styles of stromatolite lamination have been described,[9][10] which can be studied through microscopic and mathematical methods.[10] A stromatolite may grow to a meter or more.[11][12] Fossilized stromatolites provide important records of some of the most ancient life. As of theHolocene, living forms are rare.
Paleoproterozoic oncoids from the Franceville Basin, Gabon, Central Africa. Oncoids are unfixed stromatolites ranging in size from a few millimeters to a few centimeters
Stromatolites are layered, biochemical, accretionary structures formed in shallow water by the trapping, binding andcementation of sedimentary grains inbiofilms (specifically microbial mats), through the action of certain microbial lifeforms, especiallycyanobacteria.[12]
Fossilized stromatolites exhibit a variety of forms and structures, or morphologies, including conical, stratiform, domal, columnar,[13] and branching types.[14] Stromatolites occur widely in the fossil record of thePrecambrian but are rare today.[15] Very fewArchean stromatolites contain fossilized microbes, but fossilized microbes are sometimes abundant inProterozoic stromatolites.[16]
While features of some ancient apparent stromatolites are suggestive of biological activity, others possess features that are more consistent withabiotic (non-biological) precipitation.[17] Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology.[18][19] Multiple morphologies of stromatolites may exist in a single local or geological stratum, depending on specific conditions at the time of their formation, such as water depth.[20]
Most stromatolites arespongiostromate in texture, having no recognisable microstructure or cellular remains. A minority areporostromate, having recognisable microstructure; these are mostly unknown from the Precambrian but persist throughout thePalaeozoic andMesozoic. Since theEocene, porostromate stromatolites are known only from freshwater settings.[21]
Some Archean rock formations show macroscopic similarity to modern microbial structures, leading to the inference that these structures represent evidence of ancient life, namely stromatolites. However, others regard these patterns as being the result of natural materialdeposition or some other abiogenic mechanism. Scientists have argued for a biological origin of stromatolites due to the presence of organic globule clusters within the thin layers of the stromatolites, ofaragonite nanocrystals (both features of current stromatolites),[18] and of other microstructures in older stromatolites that parallel those in younger stromatolites that show strong indications of biological origin.[22][23]
Stromatolites are a major constituent of the fossil record of thefirst forms of life on Earth.[24] They peaked about 1.25 billion years ago (Ga)[22] and subsequently declined in abundance and diversity,[25] so that by the start of the Cambrian they had fallen to 20% of their peak. The most widely supported explanation is that stromatolite builders fell victim to grazing creatures (theCambrian substrate revolution); this theory implies that sufficiently complex organisms were common around 1 Ga.[26][27][28] Another hypothesis is thatprotozoa such asforaminifera were responsible for the decline, favoring formation ofthrombolites over stromatolites through microscopicbioturbation.[29]
The connection between grazer and stromatolite abundance is well documented in the youngerOrdovicianevolutionary radiation; stromatolite abundance also increased after theLate Ordovician mass extinction andPermian–Triassic extinction event decimated marine animals, falling back to earlier levels as marine animals recovered.[30] Fluctuations inmetazoan population and diversity may not have been the only factor in the reduction in stromatolite abundance. Factors such as the chemistry of the environment may have been responsible for changes.[31][15]
Whileprokaryotic cyanobacteria reproduce asexually through cell division, they were instrumental in priming the environment for theevolutionary development of more complex eukaryotic organisms.[24] They are thought to be largely responsible for increasing the amount of oxygen in the primeval Earth's atmosphere through their continuing photosynthesis (seeGreat Oxygenation Event). They use water, carbon dioxide, and sunlight to create their food. A layer ofpolysaccharides often forms over mats of cyanobacterial cells.[32] In modern microbial mats, debris from the surrounding habitat can become trapped within the polysaccharide layer, which can be cemented together by the calcium carbonate to grow thin laminations oflimestone. 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. Layered spherical growth structures termedoncolites are similar to stromatolites and are also known from the fossil record. Thrombolites are poorly laminated or non-laminated clotted structures formed by cyanobacteria, common in the fossil record and in modern sediments.[18] There is evidence that thrombolites form in preference to stromatolites whenforaminifera are part of the biological community.[33]
The Zebra River Canyon area of the Kubis platform in the deeply dissected Zaris Mountains of southwesternNamibia provides a well-exposed example of the thrombolite-stromatolite-metazoan reefs that developed during the Proterozoic period, the stromatolites here being better developed in updip locations under conditions of higher current velocities and greater sediment influx.[34]
Time lapse photography of modern microbial mat formation in a laboratory setting gives some revealing clues to the behavior of cyanobacteria in stromatolites. Biddandaet al. (2015) found that cyanobacteria exposed to localized beams of light moved towards the light, or expressedphototaxis, and increased theirphotosynthetic yield, which is necessary for survival.[35] In a novel experiment, the scientists projected a school logo onto a petri dish containing the organisms, which accreted beneath the lighted region, forming the logo in bacteria.[35] The authors speculate that such motility allows the cyanobacteria to seek light sources to support the colony.[35]
In both light and dark conditions, the cyanobacteria form clumps that then expand outwards, with individual members remaining connected to the colony via long tendrils. In harsh environments where mechanical forces may tear apart the microbial mats, these substructures may provide evolutionary benefit to the colony, affording it at least some measure of shelter and protection.
Lichen stromatolites are a proposed mechanism of formation of some kinds of layered rock structure that are formed above water, where rock meets air, by repeated colonization of the rock byendolithic lichens.[36][37]
Inland stromatolites can be found in saline waters inCuatro Ciénegas Basin, a unique ecosystem in the Mexican desert.Alchichica Lake inPuebla, Mexico has two distinct morphologic generations of stromatolites: columnar-dome like structures, rich inaragonite, forming near the shore line, dated back to 1,100years before present (ybp) and spongy-cauliflower like thrombolytic structures that dominate the lake from top to the bottom, mainly composed ofhydromagnesite,huntite,calcite and dated back to 2,800 ybp.[44] The only open marine environment where modern stromatolites are known to prosper is theExuma Cays in the Bahamas.[45][46]
Laguna de Bacalar in Mexico's southernYucatán Peninsula has an extensive formation of living giantmicrobialites (that is, stromatolites or thrombolites). The microbialite bed is over 10 km (6.2 mi) long with a vertical rise of several meters in some areas. These may be the largest sized living freshwater microbialites, or any organism, on Earth.[47]
A 1.5 km stretch of reef-forming stromatolites (primarily of the genusScytonema) occurs inChetumal Bay inBelize, just south of the mouth of theRio Hondo and the Mexican border.[48] Large microbialite towers up to 40 m high were discovered in the largestsoda lake on Earth,Lake Van in eastern Turkey. They are composed ofaragonite and grow by precipitation ofcalcite from sub-lacustrine karst-water.[49] Freshwater stromatolites are found inLake Salda in southern Turkey. The waters are rich inmagnesium and the stromatolite structures are made ofhydromagnesite.[50]
Two instances of freshwater stromatolites are found in Canada, atPavilion Lake andKelly Lake inBritish Columbia. Pavilion Lake has the largest known freshwater stromatolites, andNASA has conductedxenobiology research there,[51] called the "Pavilion Lake Research Project." The goal of the project is to better understand what conditions would likely harbor life on other planets.[52][53]
Microbialites have been discovered in an open pit pond at an abandoned asbestos mine nearClinton Creek,Yukon,Canada.[54] These microbialites are extremely young and presumably began forming soon after the mine closed in 1978. The combination of a low sedimentation rate, high calcification rate, and low microbial growth rate appears to result in the formation of these microbialites. Microbialites at an historic mine site demonstrates that an anthropogenically constructed environment can foster microbial carbonate formation. This has implications for creating artificial environments for building modern microbialites including stromatolites.
A very rare type of non-lake dwelling stromatolite lives in the Nettle Cave atJenolan Caves,NSW, Australia.[55] The cyanobacteria live on the surface of the limestone and are sustained by the calcium-rich dripping water, which allows them to grow toward the two open ends of the cave which provide light.[56]
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