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NAME: Coral chemistry and growth patterns
(With contributions from G. Shen and T. Spencer. Revised January 2005)
BRIEF DESCRIPTION: The health, diversity and extent of corals, and the geochemical makeup of their skeletons, record a variety of changes in the surrounding ocean surface water. These include temperature (*18O, Sr/Ca, U/Ca, growth patterns), salinity (*18O, U/Ca), fertility (Ba/Ca, Cd/Ca), insolation (*13C, growth patterns), precipitation (*18O, Sr/Ca, U/Ca), winds (Mn/Ca), sea levels (micro-atoll growth patterns), storm incidence (age of reef of tilted or toppled corals), river runoff (diversity and mortality, fluorescence, trace elements such as Ba), far-travelled atmospheric dust [see dust transport], and human inputs (radionuclides, P, heavy metals such as Pb and Cd). Corals in coastal waters are susceptible to rapid changes in salinity and suspended matter concentrations and may be valuable indicators of the marine dispersion of agricultural, urban, mining and industrial pollutants through river systems, as well as the history of contamination from coastal settlements.
SIGNIFICANCE: There is concern worldwide over the condition of coral reefs, which play an important role in marine ecosystems, and there are many national and international efforts to monitor and preserve their health. Corals themselves can be also used to monitor environmental changes in the oceans and nearby coastal zone: it is this aspect that concerns geoindicators. The combination of abundant geochemical tracers, sub-annual time resolution, near-perfect dating capacity, and applicability to both current and past climatic changes establishes corals as one of the richest natural environmental recorders and archives. A 30 cm-diameter coral colony growing at an average rate of 1 cm/yr will provide 20-25 years of baseline data, whereas massive colonies 3-6 m high may provide historical data, otherwise unavailable, for extensive oceanic areas.
HUMAN OR NATURAL CAUSE: Corals respond to both natural changes in the marine environment and to anthropogenic pollution.
ENVIRONMENT WHERE APPLICABLE: Tropical oceans and coastal regions between latitudes 25°N and 25°S and warmer than 18°C.
TYPES OF MONITORING SITES: Coral reefs, both modern and fossil
SPATIAL SCALE: patch / mesoscale to continental (oceanic)
METHOD OF MEASUREMENT: Rapid (5-15mm annual extension rates) but annually variable coral growth generates skeletal density banding, revealed by X-radiography, analogous to terrestrial tree rings. Analysis of samples from outer (younger) or inner (older) growth layers, though standardized chemical and isotopic, image analyzing, and fluorescence techniques are still being developed.
FREQUENCY OF MEASUREMENT: Generally annually or longer, depending on the record sought, but sub-annual resolution sometimes possible. 14C dating is applicable on long (up to 40 ka) and historic (e.g. 'bomb test' radioisotope horizons of 1950s and 1960s) timescales. Mass spectrometric dating techniques allow precise dating (to ± 1% of actual age) of corals from decades to several hundred thousand years.
LIMITATIONS OF DATA AND MONITORING: The complexity of environmental correlations requires very careful sampling and a high degree of analytical expertise.
APPLICATIONS TO PAST AND FUTURE: Corals are widely-distributed, natural, continuous data-loggers (automatic recording stations) whose records can be read (played back) over centuries, if they are old enough, or over just the last few months. Environmental records from 1587 A.D. in the Galapagos Archipelago, and from 1635 A.D. on the Great Barrier Reef, have been recovered. Variations in skeletal chemistry from corals recovered from uplifted reef terraces allows extension of environmental records to Holocene and Late Quaternary timescales.
POSSIBLE THRESHOLDS: Corals can be stressed to the point of bleaching and/or death when ambient conditions (temperature, salinity, turbidity, predation, etc.) change too quickly, or persist too long. In general, corals bleach at temperatures more than 1°C above summer monthly maximum temperatures, with extensive mortality at excursions over 3°C and/or with prolonged temperatures above the 1°C threshold. However, exact threshold values are difficult to quantify, particularly when more than one property is changing, and may vary from site to site as a consequence of reef adaptation to local conditions. The timing of growth hiatuses within living colonies and of mass mortality events can, however, be useful in inferring past severe and catastrophic disturbances.
Pernetta, J.C. (ed) 1993. Monitoring coral reefs for global change. Cambridge, International Union for the Conservation of Nature.
Barnes , D.J. & J. Lough 1996. Coral skeletons: storage and recovery of environmental information. Global Change Biology 2, 547-558.
Cole, J.E., R.B. Dunbar, T.R. McClanahan & N.A. Muthiga 2000. Tropical Pacific forcing of decadal SST variability in the western Indian Ocean over the past two centuries. Science 287, 617-619.
Dunbar, R.B., G.M. Wellington, M.W. Colgan & P.W. Glynn 1994. Eastern Pacific sea surface temperatures since 1600 A.D.: the δ18O record of climate variability in Galapagos corals. Paleooceanography 9, 291-315.
OTHER SOURCES OF INFORMATION: Global Coral Reef Monitoring Network (http://www.gcrmn.org/), International Society for Reef Studies (http://www.fit.edu/isrs/),. Coral Reef Information System (http://www.coris.noaa.gov/), World Data Center-A for Paleoclimatology
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Corals may provide useful means of monitoring the dispersal of river sediments and pollution in coastal areas. Environmental threats to living coral reefs are widespread, and there is a considerable body of knowledge devoted to monitoring reefs in order to protect and conserve them.
OVERALL ASSESSMENT: Corals constitute a very effective recorder and a rich natural archive of environmental change.
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