Stromatolite

Stromatolites (from gr στρῶμα stroma, ceiling and λίθος lithos, stone) are biogenic sedimentary rocks that are formed by trapping and binding of sediment particles or precipitation of dissolved substances as a result of the growth and metabolism of microorganisms in a water body. They are usually layered and often consist of very feingeschichtetem limestone. The internal structure of stromatolites is different: flat, even layers, upward curved layers, multiple domed layer packets side by side ( column shape ). Some remember her shell structure from tubers, columns or wavy layers externally to a cauliflower.

The term and the term stromatolite were introduced in 1908 by Ernst Kalkowsky with respect to deposits in the northern German Buntsandstein. The holotype of Kalkowsky used in the initial determination of the rock is now in the Museum of Mineralogy and Geology Dresden, whose director he was.

Education

An essential prerequisite to the formation of stromatolites are biofilms, which consist of micro-organisms ( microbial mats ). In recent stromatolites 1 exist - 10 mm thick biofilms usually consists of a base layer of heterotrophic bacteria, embedded in a layer formed by the microorganisms matrix of mucilage (polysaccharides and other biopolymers ), and an upper layer of predominantly phototrophic microorganisms, associated with a lower Heterotrophic share. The biopolymers are significantly involved in the binding of the particle sediment.

The vast majority of the stromatolites consisting of limestone. This is largely formed by precipitation. Requirement is a typically marine waters with high concentrations of calcium and bicarbonate ions. Through intense microbial assimilation of carbon dioxide ( CO2), the medium is made alkaline and thus shifted the dissociation of carbonic acid in favor of carbonate ions. As a result, the solubility of calcium carbonate is exceeded, so that lime precipitated. Certain biopolymers of biofilms with anionic ( negatively charged ) groups of atoms, to which the positively charged calcium ions bind nuclei dar. In recent stromatolites cause marine especially benthic cyanobacteria ( formerly known as " blue-green algae " or " blue-green algae ") is the lime precipitation by intense CO2 assimilation in relation to photosynthesis. Mainly it is oxygenic photosynthesis, using water as a reducing agent for the reduction of CO2 and formation of oxygen (O2 ), but partly also to anoxygenic photosynthesis with hydrogen sulfide (H2S) as a reducing agent and formation of sulfur (S) or sulfate ( SO42 - ).

Through the accumulation of sediment -bound and / or precipitated lime the microorganisms of the biofilm are included and covered. However, constantly grow through length growth or propagation in the air, so that the biofilm at the base, although is inactive and dies, but continues to grow upward. This has a height growth of stromatolites result. The reasons for the layering are probably numerous and not completely understood. As a cause come into question, among other things: daily sun rhythm, episodic sediment input, seasonal changes in the chemical composition of the water, changes to the biofilm microbiota. About rates of growth, little is known, extant stromatolites in Hamelin Pool ( s u ) grow about 0.3 mm per year.

The formation of stromatolites is encouraged or even made possible when the required for their formation biofilm is not eroded by other organisms. In very ancient stromatolites of the Precambrian was given because there were no such beings. Therefore stromatolites are numerous known from the Precambrian. In younger fossil and recent was or is the case that are unfavorable for other creatures, such as high salinity only in milieus.

Fossils

Stromatolites are regarded as the oldest fossils called and come before in the Precambrian, they have existed for about 3.5 billion years ago (Ga ) ( Warrawoona series found at the North Pole in place Pilbara region in Western Australia, Precambrian, Archaean ). About 3.4 Ga old stromatolites are the Fig Tree Group, Swaziland Supergroup, Barberton Mountain in the country, South Africa. Stromatolites are regarded as the first visible structure built up by organisms.

In some periods of Earth's history - long before there was coral - were the Stromatolithenbildner important Riffbildner. Until a billion years stromatolites were widespread in almost all coastal waters. Before 700 million years the diversity and distribution of stromatolites decreased abruptly strong. Very rarely are stromatolites from 450 million years before present. It is believed that they were grazed by the rise of multicellular eukaryotes masse, thus displacing.

Today's occurrence

General

Recent in education conceived stromatolites are found only in a few ecological niches, which are usually characterized by increased salinities: lagoons, continental salt and soda lakes. However, a conclusion on the environmental conditions of fossil stromatolites or their producers is thus not necessarily the case.

The forming stromatolites arriving in Australia in Shark Bay, in the Pink Lake and Spencer Lake, in Western Australia, in Solar Lake, Egypt, Han - Nan- Iceland in China, the Persian Gulf, in Brazil in Lagoa Salgada in Rio de Janeiro Green Lake in Texas, in Yellowstone National Park in the U.S. and Bermuda and the Gulf of Mexico as well as at a few other places before.

Hamelin Pool Marine Nature Reserve

An example of still growing stromatolites are in Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia. You are a UNESCO World Heritage Site and was discovered in June 1956. The forming them biofilms can survive because of the extreme salinity of the water, so high is about twice as in the open ocean, because the biofilm abweidenden animals are kept (eg snails) by the high salt concentration. The stromatolite biofilms have no natural enemies under these extreme conditions. The stromatolites of Hamelin Pool grow very slowly, more than 1 cm in about 30 years. Structure of about 1 m in height are thus nearly 3000 years old.

The primary production of organic matter by cyanobacteria in the biofilms on these stromatolites corresponds to an assimilation of carbon from carbon dioxide into organic compounds from about 17-113 mg · m -2 · h -1.

In Hamelin Pool Marine Nature Reserve following structures occur:

• The " cauliflower " structures ( cauliflower ) are elderly living stromatolites under water; the oldest are about 1 to 1.5 m tall.
• The so-called " domes with red cap " (red -capped domes ) are flat structures on the beach gray black with rust-red " hood". They heard about 500-1000 years ago to grow, as the water level fell. The cause of the red color is controversial, maybe it's caused by contact with water containing iron or by bacteria.
• The young " bushy mats " ( tufted mats ) form flat black carpets under water that look from a distance like felt mats. Here it is still very young, barely 1 cm tall structures.

Lake Thetis

A second occurrence of stromatolites in Australia is found in the Lake Thetis, near the small town of Cervantes in Western Australia. It is a slightly hypersaline (53 g / l of salt ), alkaline (pH 8.5 to 9 ) salt lake with a depth of about 2-3 m. Herein are dome-shaped stromatolites with diameters from 0.3 to 0.4 m and a height of about 1 m predominantly of aragonite. The upper layer of the biofilms thereon to cyanobacteria contains mainly the local Entophysalis group, the lower layer of the biofilms formed by heterotrophic bacteria.

Stromatolites as indicators of oxygen formation

The occurrence of stromatolites is - together with the appearance of bands iron ores - as an indication of the presence of molecular oxygen ( dioxygen, O2) viewed by oxygenic photosynthesis, ie, by light-driven reduction of carbon dioxide to organic compounds with water as the reducing agent while dioxygen is oxidized. ( Schidlowski, p 528) It is assumed in the context that cyanobacteria have settled as dominant oxygenic phototrophic the stromatolites. This view is justified by the following arguments:

• The start of the Earth's development in the waters only bivalent present (as Fe2 ) ions, iron was oxidized to trivalent iron by dioxygen. Sediments with trivalent iron as hematite and magnetite iron ores containing bands set, requires that dioxygen was present during its formation. ( Schidlowski, p 531 )
• Dioxygen was present (formed by photolysis of water ) at the beginning of the Earth's development at best in low concentrations, which were insufficient for oxidizing ferrous iron ( Schidlowski, p 531).
• Stromatolites are formed biotically. Analogous to the stromatolites tart formed are also the fossil stromatolites of biological communities have been colonized containing oxygen phototrophic cyanobacteria as characteristic components, ie stromatolites show biotic dioxygen education.
• It has been found in ancient fossil stromatolites remains of microorganisms that can be derived from organisms that today's cyanobacteria are similar.

This view, however, was questioned early on, a number of ambiguities were identified. In the course of investigations concrete arguments were raised. Several findings suggest a compelling need of the model is questionable:

• Divalent iron ions are oxidized into trivalent iron even without dioxygen (2 Fe 2 2 H → 2 Fe 3 H 2 ) by exposure to ultraviolet radiation and blue light. Bands of iron ore could therefore also by exposure of Fe2 ions have arisen without dioxygen, dioxygen must at the time not have been present.
• Stromatolites can also be formed abiotically. (Walter, p 290)
• The very ancient stromatolite -like rocks ( Apex Chert, W- Australia, about 3.465 Ga old, and Isua, west Greenland, about 3.8 Ga old) found and interpreted as microorganisms residues structures are considered by some investigators for abiotic. The structures younger than 3.0 Ga stromatolites are found in probably may Abiogen remains of microorganisms found in older stromatolites. (Walter, pp. 291 f )
• Microorganisms remains found in younger stromatolites may also originate from microorganisms other than cyanobacteria according to their shape. Should it, however, be remnants of cyanobacteria, it is therefore still not sure if they have then operated oxygenic photosynthesis, because even some extant cyanobacteria can operate a photosynthesis with sulfide as a reducing agent, so it do not form a dioxygen. This possibility seems to have been realized even in relatively young stromatolites, namely those from the upper Malm, occurring at Thüste (SW Hildesheim ). These stromatolites are probably formed in an anoxic environment with hydrogen sulfide formation and contain iron (II ) disulfide.

The state of knowledge can probably be summarized at the time as follows: It is possible that the first appearance of stromatolites and iron ore bands indicating the presence of dioxygen. However, the evidence for this are not clear proof; Dioxygen could also have been formed later by oxygenic photosynthesis first time.