Stromatolite

Stromatolites ( stroh-MAT-ə-lytes, strə-) or stromatoliths (from Ancient Greek στρῶμα (strôma), GEN στρώματος (strṓmatos) 'layer, stratum' and λίθος (líthos) 'rock') are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineralized "microbial mats". In turn, these mats build up layer by layer, growing gradually over time. This process generates the characteristic lamination of stromatolites, a feature that is hard to interpret, in terms of its temporal and environmental significance. Different styles of stromatolite lamination have been described, which can be studied through microscopic and mathematical methods. A stromatolite may grow to a meter or more. Fossilized stromatolites provide important records of some of the most ancient life. As of the Holocene, living forms are rare.

Definition

Stromatolites are layered, biochemical, accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains in biofilms (specifically microbial mats), through the action of certain microbial lifeforms, especially cyanobacteria. The layered nature or laminae of stromatolites distinguishes them from thrombolites which are defined by their clotted, non-layered internal texture. The term "stromatolite" is sometimes confused with the misspelled word "strombolite"—a probable conflation of the closely-related geological terms: stromatolites and thrombolites.

Ancient stromatolites

Morphology Fossilized stromatolites exhibit a variety of forms and structures, or morphologies, including conical, stratiform, domal, columnar, and branching types. Stromatolites occur widely in the fossil record of the Precambrian but are rare today. Very few Archean stromatolites contain fossilized microbes, but fossilized microbes are sometimes abundant in Proterozoic stromatolites. While features of some ancient apparent stromatolites are suggestive of biological activity, others possess features that are more consistent with abiotic (non-biological) precipitation. Finding reliable ways to distinguish between biologically formed and abiotic stromatolites is an active area of research in geology. 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. Most stromatolites are spongiostromate in texture, having no recognisable microstructure or cellular remains. A minority are porostromate, having recognisable microstructure; these are mostly unknown from the Precambrian but persist throughout the Palaeozoic and Mesozoic. Since the Eocene, porostromate stromatolites are known only from freshwater settings.

Fossil record 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 material deposition 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, of aragonite nanocrystals (both features of current stromatolites), and of other microstructures in older stromatolites that parallel those in younger stromatolites that show strong indications of biological origin.

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