Microatoll
Encyclopedia
A microatoll is a discoid colony of coral
, 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 6 metres (19.7 ft) in diameter. They are named for their resemblance to island
atoll
s formed during the subsidence of volcanic islands, 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
. 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
using oxygen isotope values as a proxy.
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 worm
s, pelecypods and vermetid
gastropods.
s that grow in the lower intertidal zone
on shallow reef
flats. Microatolls are formed by several species
of the genus
Porites
, but examples have also been described from Acropora
, Heliopora, Favia, Favites, Platygyra, Cyphastrea and Goniastrea.
change preserved in fossil
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 20 centimetres (7.9 in) 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
s and to estimate the recurrence interval of such events before historic records are available.
isotope
ratios in fossil microatolls have also been used to provide high-resolution proxy records for sea surface temperature
over the last few thousand years.
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
, 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 6 metres (19.7 ft) in diameter. They are named for their resemblance to island
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
atoll
Atoll
An atoll is a coral island that encircles a lagoon partially or completely.- Usage :The word atoll comes from the Dhivehi word atholhu OED...
s formed during the subsidence of volcanic islands, 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
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
. 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
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
using oxygen isotope 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 worm
Serpulidae
Serpulidae is a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from the sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes when they withdraw into the tubes. In addition, serpulids secrete...
s, pelecypods and vermetid
Vermetidae
Vermetidae, common name the worm snails or worm shells, is a taxonomic family of small to medium-sized sea snails, marine gastropod molluscs in the clade Littorinimorpha....
gastropods.
Occurrence
Microatolls are found only in coralCoral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s that grow in the lower intertidal zone
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
on shallow reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
flats. Microatolls are formed by several species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Porites
Porites
Porites is a genus of scleractinian coral which is characterised by a finger-like morphology. Members of this genus have widely spaced calices, a well-developed wall reticulum and are bilaterally symmetrical...
, but examples have also been described from Acropora
Acropora
Acropora is a genus of scleractinian coral in the Phylum Cnidaria. Some of its species are known as table coral, elkhorn coral and staghorn coral. There are currently 149 described species...
, Heliopora, Favia, Favites, Platygyra, Cyphastrea and Goniastrea.
Paleogeodesy
The detailed record of sea levelSea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
change preserved in fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
microatolls, combined with precise dating of individual annual rings using the Uranium-thorium dating
Uranium-thorium dating
Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique commonly used to determine the age of calcium carbonate materials such as speleothem or coral...
method, allows them to be used to determine past relative sea-level change with uncertainties of about 20 centimetres (7.9 in) 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
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s and to estimate the recurrence interval of such events before historic records are available.
Sea surface temperatures
Changes in oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
ratios in fossil microatolls have also been used to provide high-resolution proxy records for sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
over the last few thousand years.