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Stromatolites: What They Are and What They Mean to Us

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Stromatolites: What They Are and What They Mean to Us NATURALIST CORNER Stromatolites: What Th ey Are and What Th ey Mean to Us Text and Photographs by Wayne P. Armstrong elevating the level of free oxygen in the earth’s atmosphere. As one gazes at the spectacular limestone formations in Glacier National Park and the Canadian Rockies, you are awe-struck with the enormous role these ancient cyanobacteria played as they removed carbon dioxide from primeval seas and precipitated it as massive calcium carbonate rocks. Releasing oxygen gas as a metabolic by-product, they must have been a major factor in producing the oxygen-rich atmosphere that allowed the development of other aerobic life forms on earth.In 1990, Dr. Paul Buchheim of the Department of Earth and Biological Sciences at Loma Linda University One billion-year-old stromatolites embedded reported living stromatolites in Carrizo in limestone along Going-To-Th e Sun Road Gorge and San Felipe Creek in Sentenac in Glacier National Park, Montana. Th e Canyon, both within Anza-Borrego Desert concentric rings represent calcareous layers State Park. Previous to this discovery, where ancient colonies of cyanobacteria once modern-day stromatolites were unknown thrived in shallow seas. An ephemeral stream in Grapevine Canyon north of Mexico. Th ese shallow, ephemeral Stromatolites are layered (laminate), of Anza-Borrego Desert State Park. Th e (intermittent) streams have boulders and calcareous structures formed by colonies of submersed boulders and cobbles are encrusted cobbles coated with knobby, calcareous cyanobacteria (formerly called blue-green with living colonies of stromatolite-forming layers and living colonies of mucilaginous, algae). Along road cuts in Glacier National cyanobacteria. fi lamentous cyanobacteria. Several species Park they resemble concentric circles of cyanobacteria grow on the surface embedded in limestone rocks. During of submersed rocks, including tapered countless centuries of time, calcareous fi laments of the stromatolite-forming sediments are trapped in layers of genus Rivularia. fi lamentous colonies of cyanobacteria. Like Filaments of Rivularia are covered with a coral reef, new colonies grow on top of a slimy mucilaginous sheath that is fairly the older ones as the stromatolite increases easy to see under high magnifi cation in height and diameter. Stromatolites (400x) with a compound microscope. were only known as ancient fossils until Calcareous sediments (limestone) and B.W. Logan (1961) fi rst reported living detrital stream particles are trapped stromatolites in warm, intertidal waters in layers of fi lamentous colonies of of Shark Bay in Western Australia. Called the cyanobacteria. Over centuries “cryptozoon stromatolites,” they resemble of time, stromatolites up to an inch reef-like domes composed of concentric thick accumulate on granite rocks and layers where the cyanobacteria colonies boulders. Th e cyanobacteria utilize water, once lived. Th is ancient calcifi cation carbon dioxide and sunlight during process is still going on to this day in photosynthesis. Byproducts of this several remote canyons of Anza-Borrego process are oxygen and calcium carbonate Desert State Park. Magnifi ed view of fi lamentous cyanobacteria (lime). Debris from the water is trapped Th e abundance of stromatolites in resembling Rivularia living on submersed in the slimy mucous coating (sheaths) the fossil record is evidence that boulders in an ephemeral stream. Under of the fi lamentous cyanobacterial mats photosynthetic cyanobacteria were among ideal conditions, colonies of cyanobacteria and is gradually cemented together by the fi rst life forms on earth, dating back similar to these are responsible for the the calcium carbonate. Presumably, more than three billion years. Th ey deposition of calcareous layers known as intervals of creek desiccation facilitate undoubtedly played an enormous role in stromatolites. Magnifi cation 400x. in this cementation process, resulting in A stromatolite-covered boulder in Grapevine Canyon. Th e arrow Th e boulders and cobbles in this image were all submersed during the points to a portion of the stromatolite layer that has broken away, wet seasons of previous years. Th ey are coated with a knobby (bumby) exposing the granite core of the rock. In previous years this boulder calcareous layer composed of microscopic laminae produced by colonies was submersed in calcium-rich, alkaline water and supported dense of cyanobacteria. colonies of cyanobacteria of the genus Rivularia. layers composed of calcite and detrital areas where water fl owed for only a few in their natural habitat. Stromatolites are stream particles. Th e biochemical process months in the spring of each year. Th e important from an evolutionary standpoint of lamina formation can be observed in cyanobacterial mats apparently go into a because they are frequently the subject modern microbial mats cultured in the dormant state when the creek becomes of scientifi c discussions about ancient laboratory. Occasional fl ooding down desiccated. According to Buchheim climates and the origin of life on earth. boulder strewn canyons during wet years (1995), the major factor controlling Th ey can be found in limestone areas undoubtedly scours away developing stromatolite growth in these streams throughout the world that were once stromatolites and controls their thickness. is high solute concentration as well as covered by ancient seas. But to see this Some of the best developed stromatolites calcite supersaturation. Th is explains why calcareous phenomenon actually forming in Carrizo Creek were destroyed by stromatolites grow in some creeks of Anza- in alkaline streams of Anza-Borrego Desert spring fl oods of 1993. According to Dr. Borrego but not in others. is truly remarkable. Buchheim (personal communication, Th ere are other calcareous deposits in For Further Reading 2008), stromatolites have reestablished alkaline desert riverbeds. Tufa deposits in Carrizo Creek, but droughts of the Buchheim, H.P. 1995. “Stromatolites: are common in desert alkaline streams, Living Fossils in Anza-Borrego Desert past several years have hindered their including Sentenac Canyon. Tufa is a development. State Park.” Pages 119-124 in: white, porous calcium carbonate deposit Paleontology and Geology of the Western Stromatolite growth in Anza-Borrego that forms by chemical deposition in Salton Trough Detachment, Anza-Borrego Desert is most abundant in ephermeral water supersaturated with calcium. It coats Desert State Park, California. Volume 1. P. sections of alkaline streams, including solid objects along creeks and the shores Remeika and A. Sturz, Editors. San Diego Carrizo and San Felipe Creeks, where of lake beds. It is easily distinguished Association of Geologists, San Diego, maximum evaporation takes place and from stromatolites which have a layered California. where solutes are the highest. Most of my (laminate) structure. Logan, B.H. 1961. “Cryptozoon and images of stromatolites were taken in an Desert stromatolites are especially ephemeral stream of Grapevine Canyon Associate Stromatolites From the Recent, interesting because they enable scientists Shark Bay, Western Australia.” Journal of that drains into Sentenac Canyon. Th e to study colonies of these “living fossils” best formation of stromatolites occurs in Geology 69: 517-533. .
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