Microatoll
Circular coral colonies that record sea level and temperature changes.
A microatoll is a circular colony of coral, dead on the top but living around the perimeter, with growth mainly lateral due to upward growth being limited by exposure to air. They are notable for acting as natural recorders of sea level, allowing monitoring of sea level changes in response to global warming, and for providing information on changes in sea surface temperature using oxygen isotope values as a proxy.
- Maximum diameter
- 6 meters (20 ft)
- First use of term
- 1927
- First definer
- Krempf (1927)
- Refined definer
- Kuenen (1933)
- Dating method
- Uranium-thorium dating
- Uncertainty in level
- about 20 centimeters (7.9 in)
- Uncertainty in time
- a few years to a few decades
Lore & Background
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, pelecypods and vermetid gastropods. Microatolls are named for their resemblance to island atolls formed during the subsidence of volcanic islands, as originally suggested by Darwin (1842).
Microatolls are found only in corals that grow in the lower intertidal zone on shallow reef flats. They are formed by several species of the genus Porites, but examples have also been described from Acropora, Heliopora, Favia, Favites, Platygyra, Cyphastrea and Goniastrea. Growth is mainly lateral, as upward growth is limited by exposure to air, and they may be up to 6 meters (20 ft) in diameter.
Reader's Guide
Microatolls have significant scientific value 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. The detailed record of sea level 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 centimeters (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 earthquakes and to estimate the recurrence interval of such events before historic records are available. Additionally, changes in oxygen isotope ratios in fossil microatolls have been used to provide high-resolution proxy records for sea surface temperature over the last few thousand years. This dual capability—recording both sea level and temperature—makes microatolls a key tool in paleoclimate and paleogeodesy research.
Did You Know?
- They are named for their resemblance to island atolls formed during subsidence of volcanic islands, as suggested by Darwin (1842).
- Changes in oxygen isotope ratios in fossil microatolls provide proxy records for sea surface temperature.
Frequently Asked Questions
What is a microatoll?
A microatoll is a ring-shaped coral colony whose upper surface is dead while the living tissue continues around the outer edge. Because upward growth is constrained by air exposure, the colony expands mainly sideways, producing a distinctive flat-topped disc.
Why are microatolls important for climate research?
They serve as natural archives of sea-level history, letting scientists track how ocean levels have shifted in response to global warming. Oxygen isotope values preserved in their skeletons also provide a proxy for past sea-surface temperature changes.
Who first identified and named microatolls?
The term was introduced by Krempf in 1927, and Kuenen later refined the definition in 1933 to clarify the structural characteristics that distinguish these colonies from other coral forms.
How do scientists date microatolls?
Uranium-thorium radiometric dating is applied to the coral skeleton to determine its age. The resulting sea-level estimates carry an uncertainty of roughly 20 centimeters (about 7.9 inches).
How large can a microatoll grow?
The maximum recorded diameter is about 6 meters (20 feet), after which the lateral expansion that defines the form typically tapers off.
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