Thrombolites (from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

θρόμβος ''thrómbos'' meaning " clot" and λῐ́θος ''líthos'' meaning " stone") are clotted accretionary

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

s formed in shallow water by the trapping, binding, and cementation of sedimentary grains by

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

s of

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s, especially

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

Structures

Thrombolites have a clotted structure without the laminae of

stromatolite Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...

s. Each clot within a thrombolite mound is a separate

colony A colony is a territory subject to a form of foreign rule, which rules the territory and its indigenous peoples separated from the foreign rulers, the colonizer, and their ''metropole'' (or "mother country"). This separated rule was often orga ...

. The clots are on the scale of millimetres to centimetres and may be interspersed with

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

, mud or sparry carbonate. Clots that make up thrombolites are called thromboids to avoid confusion with other clotted textures. The larger clots make up more than 40% of a thrombolite's volume and each clot has a complex internal structure of cells and rimmed lobes resulting primarily from calcification of the cyanobacterial colony. Very little sediment is found within the clots because the main growth method is calcification rather than sediment trapping. There is active debate about the size of thromboids, with some seeing thromboids as a macrostructural feature (domical hemispheroid) and others viewing thromboids as a mesostructural feature (random polylobate and subspherical mesoclots).

Types

There are two main types of thrombolites:

Calcified microbe thrombolites

This type of thrombolites contain clots that are dominantly composed of calcified microfossil components. These clots do not have a fixed form or size and can expand vertically. Furthermore, burrows and trilobite fragments can exist in these thrombolites.

Coarse agglutinated thrombolites

This type of thrombolites is composed of small openings that trap fine-grained

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s. They are also known "thrombolitic-stromatolites" due to their close relation with the same composition of stromatolites. Because they trap sediment, their formation is linked to the rise of algal-cyanobacterial mats.

Differences from stromatolites

Thrombolites can be distinguished from microbialites or stromatolites by their massive size, which is characterized by macroscopic clotted fabric.

Stromatolite Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...

s are similar but consist of layered accretions. Thrombolites appear with random patterns that can be seen by the naked eye, while stromatolites has the texture of built-up layers.

Ancient fossil record

Calcified microbe thrombolites occur in

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s from the shallow water

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

during the

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

and early Palaeozoic.

Locations

Thrombolites are rare on modern Earth, but exist in areas of groundwater discharge with high concentration of nutrients and organic ions, such as shallow seawater, freshwater, and saltwater lakes, and streams. Thrombolites are now found in only a few places in the world, including: * Laguna Negra, Catamarca, Argentina * Basin Lakes and Blue Lake, Australia * Lake Clifton, Australia * Lake Richmond, Australia * Lake Thetis, Australia * Flower's Cove, Canada * Manito Lake and Pavilion Lake, Canada * Lakes Nuoertu and Huhejaran, China * Kiritimati Atoll, Kiribati * Cuatro Ciénegas and Lake Alchichica, Mexico * Ciocaia, Romania * Lake Van and Salda Lake, Turkey * Green Lake, United States * Lake Sarmiento, Chile

References

{{Origin of life Trace fossils Cyanobacteria