Several centuries of variation in skeletal extension, density and calcification in massive Porites colonies from the Great Barrier Reef: A proxy for seawater temperature and a background of variability against which to identify unnatural change

Abstract

Annual variations in skeletal density were measured by gamma densitometry in 35 cores removed from large Porites colonies growing at sites throughout the Great Barrier Reef (GBR). Density variations along each core provided data for average annual density and annual extension. These were used to estimate average annual calcification. Records ranged from 49 to 507 years in length. The period common to all colonies was 1934–1982. Annual growth data were averaged over periods of at least 5 years to avoid problems associated with dating of records and with measurement techniques. This also made some allowance for distortion of environmental information during coral growth and for intrusion into the environmental signal of information associated with skeletal architecture. The period common to the 10 longest cores was 1746–1982. Instrumental records of sea surface temperature (SST) are available for the GBR back to 1906. Annual calcification, averaged across these 10 cores, was significantly related to SST. Thus, average annual calcification for these 10 colonies provides a proxy for SST variations on the GBR back to the 18th century. Interpretation of evidence of a recent decline in calcification of Porites of the GBR is tempered by (1) evidence of similar declines and recoveries over the past several centuries and (2) evidence that coral calcification on the GBR has been above the long-term average for most of this century and the recent decline may be a return to more normal conditions.