SEVERAL CENTURIES OF VARIATION IN SKELETAL EXTENSION, DENSITY AND CALCIFICATION IN MASSIVE PORITES COLONIES FROM THE GREAT-BARRIER-REEF - APROXY FOR SEAWATER TEMPERATURE AND A BACKGROUND OF VARIABILITY AGAINST WHICH TO IDENTIFY UNNATURAL CHANGE

Annual variations in skeletal density were measured by gamma densitome try in 35 cores removed from large Porites colonies growing at sites t hroughout the Great Barrier Reef (GBR). Density variations along each core provided data for average annual density and annual extension. Th ese 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 m easurement 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 archi tecture. The period common to the 10 longest cores was 1746-1982, Inst rumental records of sea surface temperature (SST) are available for th e GBR back to 1906. Annual calcification, averaged across these 10 cor es, was significantly related to SST. Thus, average annual calcificati on for these 10 colonies provides a proxy for SST variations on the CB R back to the 18th century. Interpretation of evidence of a recent dec line in calcification of Porites of the GBR is tempered by (1) evidenc e of similar declines and recoveries over the past several centuries a nd (2) evidence that coral calcification on the GBR has: been above th e long-term average for most of this century and the recent decline ma y be a return to more normal conditions. (C) 1997 Elsevier Science B.V .