A microatoll is a circular colony of

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

, dead on the top but living around the perimeter. Growth is mainly lateral, as upward growth is limited by exposure to air. Microatolls may be up to in diameter. They are named for their resemblance to island atolls formed during the subsidence of

volcanic island Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...

s, as originally suggested by Darwin (1842). They act as natural recorders of sea level, which allows the monitoring of sea level changes in response to

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

. They have also been used to quantify and date changes in relative sea level in seismically active areas, and to provide information on changes in

sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mas ...

using

oxygen isotope There are three known stable isotopes of oxygen (8O): , , and . Radioactive isotopes ranging from to have also been characterized, all short-lived. The longest-lived radioisotope is with a half-life of , while the shortest-lived isotope is ...

values as a proxy.

Terminology

The term 'microatoll' was first used by Krempf in 1927, although his description lacks a precise definition. Kuenen defined it in 1933 as "a colony of corals" with "a raised rim, more or less completely surrounding a lower, dead surface". This definition has been extended to include similar structures built by non-coral reef-building organisms such as serpulid worms,

pelecypod Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

s and vermetid gastropods.

Occurrence

Microatolls are found only in

s that grow in the lower intertidal zone on shallow

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...

flats. Microatolls are formed by several

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Porites ''Porites'' is a genus of stony coral; they are small polyp stony (SPS) corals. They are characterised by a finger-like morphology. Members of this genus have widely spaced calices, a well-developed wall reticulum and are bilaterally symmetr ...

'', but examples have also been described from '' Acropora'', '' Heliopora'', ''

Favia ''Favia'' is a genus of reef-building stony corals in the family Mussidae. Members of the genus are massive or thickly encrusting colonial corals, either dome-shaped or flat, and a few are foliaceous. There is a great diversity of form even among ...

'', ''

Favites ''Favites'' is a genus of stony corals in the family Merulinidae. Members of this genus are native to the Indo-Pacific region and their ranges extend from the Red Sea through the Indian Ocean and Western Pacific Ocean as far as Japan, the Line Is ...

'', '' Platygyra'', ''

Cyphastrea ''Cyphastrea'' is a genus of massive reef building stony corals in the family Merulinidae Merulinidae is a family of reef-building stony corals. Characteristics All the genera in this family are colonial, reef-building corals. Skeletal struc ...

'' and ''

Goniastrea ''Goniastrea'' is a genus of stony corals in the family Merulinidae. Species belonging to the genus ''Goniastrea'' forms massive colonies, usually spherical or elongate, with well developed paliform lobes. Polyps can be seen only at night. Spe ...

''.

In climate research

Paleogeodesy

The detailed record of

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...

change preserved in

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

microatolls, combined with precise dating of individual annual rings using the Uranium-thorium dating method, allows them to be used to determine past relative sea-level change with uncertainties of about in level and a few years to a few decades in time. They have been used to map the rupture areas of great to giant

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

s and to estimate the recurrence interval of such events before historic records are available.

Sea surface temperatures

Changes in

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...

ratios in fossil microatolls have also been used to provide high-resolution proxy records for

over the last few thousand years.

References