microatoll
{{Short description|Circular colony of coral}}
File:Microatoll.jpg form characteristic microatoll formations, with living coral around the perimeter and dead skeleton on the exposed upper surface.van Woesik, R., Golbuu, Y. and Roff, G. (2015) "Keep up or drown: adjustment of western Pacific coral reefs to sea level rise in the 21st century". Royal Society, open science, 2 (7): 150181. {{doi|10.1098/rsos.150181 }}]]
A microatoll is a circular 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 {{convert|6|m|ft|sp=us}} in diameter.{{cite journal|last=Stoddart|first=D.R.|author2=Scoffin T.P.|year=1979|title=Microatolls: review of form, origin and terminology|journal=Atoll Research Bulletin|volume=224|pages=1–17|doi=10.5479/si.00775630.224.1|url=http://www.atollinstitute.org/Selected%20Atoll%20Research%20Bulletins/224%20-%20Microatolls,%20Review%20of%20Form,%20Origin%20and%20Terminology.pdf}} They are named for their resemblance to island atolls formed during the subsidence of volcanic islands, as originally suggested by Darwin (1842).{{cite book|last=Darwin|first=Charles|title=The structure and distribution of coral reefs|publisher=Smith, Elder and Co.|location=London|year=1842|pages=214|url=http://darwin-online.org.uk/content/frameset?itemID=F271&viewtype=text&pageseq=1}}
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,{{cite journal|last=Natawidjaja |first=D. H. |author1-link=Danny Hilman Natawidjaja |author2=Sieh K. |author3=Chlieh M. |author4=Galetzka J. |author5=Suwargadi B.W. |author6=Cheng H. |author7=Edwards R.L. |author8=Avouac J.-P. |author9=Ward S. N. |year=2006 |journal=Journal of Geophysical Research |title=Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls |volume=111 |issue=B06403 |pages=n/a |doi=10.1029/2005JB004025 |bibcode=2006JGRB..111.6403N |hdl=10220/8480 |url=https://authors.library.caltech.edu/20847/1/Natawidjaja_JGR2006.pdf }} and to provide information on changes in sea surface temperature 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 worms, pelecypods and vermetid gastropods.
Occurrence
Microatolls are found only in corals that grow in the lower intertidal zone on shallow reef flats.{{cite book|last=Woodroffe|first=C.|title=Encyclopedia of geomorphology, Volume 2|editor=Goudie A.|publisher=Routledge|year=2004|pages=670–671|isbn= 9780415327381|url=https://books.google.com/books?id=UHRU_6nUSR4C&pg=PA670 |accessdate=2009-10-30}} 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.
In climate research
=Paleogeodesy=
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 {{convert|20|cm|in|sp=us}} 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.{{cite journal|last=Meltzner|first=A.J. |author2=Sieh K. |author3=Chiang H-W. |author4=Philibosian B.E. |author5=Suwargadi B.W. |author6=Natawidjaja D.H. |year=2009|title=Coral microatoll paleogeodesy on Simeulue Island, Sumatra, reveals earthquake clusters and persistent rupture segmentation|journal=2009 Portland GSA Annual Meeting|url=http://gsa.confex.com/gsa/2009AM/finalprogram/abstract_158158.htm|accessdate=2009-10-30}}
=Sea surface temperatures=
Changes in oxygen 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.{{cite journal|last=Woodroffe|first=C.D. |author2=Beech M.R. |author3=Gagan M.K.|year=2003|title=Mid-late Holocene El Niño variability in the equatorial Pacific from coral microatolls|journal=Geophysical Research Letters|volume=30|issue=7|pages=1358|url=http://www.agu.org/pubs/crossref/2003/2002GL015868.shtml|doi=10.1029/2002GL015868|bibcode=2003GeoRL..30.1358W |accessdate=2009-10-30|doi-access=free}}