Stromatolites form an unusual and special type of reef. Unlike modern coral reefs, stromatolites are formed by microorganisms. These microorganisms trap and bind sand grains together and/or precipitate calcium carbonate to form laminated mounds of limestone. The layered internal structure of these reefs is a characteristic feature and the name “stromatolite” in Greek means “layered rock.” Stromatolites dominated the planet for 80% of Earth history, forming massive reef complexes comparable to any built by corals or other frame-building organisms. The golden age of stromatolite growth was 2.5–1 billion years ago, during Early and Middle Proterozoic. Suffering a major decline with the appearance of eukaryotic plants and animals in the Late Proterozoic, stromatolites were confined to harsh environments lacking eukaryotes for much of the Phanerozic. Living stromatolites were unknown until the 1960s when they were discovered in Shark Bay, a hypersaline lagoon in Western Australia. They have since been found in numerous modern environments including saline, fresh water, and brackish settings. The only known examples of stromatolites forming in open marine environments similar to those of Precambrian platforms are on the margins of Exuma Sound, Bahamas. Exuma stromatolites are ideal model systems for studies of stromatolite morphogenesis. Lamination in these stromatolites results from a cycling of prokaryotic surface communities, with each layer in the subsurface “fossil” part of the stromatolite representing a former surface mat. Lithification of surface mats is syndepositional, and microbially induced. Exuma stromatolites are less than 1,500 years old; they thrive in locations where sediment stress restricts the growth of coral, macroalgae, and other reef-building eukaryotes. Studies of living stromatolites provide models for biogeochemical cycling, population dynamics, and mineral formation in the three billion year old microbial reef ecosystem.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)