TRACE-METAL GEOCHEMICAL ASSOCIATION IN SEDIMENTS FROM THE CAIRNS REGION OF THE GREAT-BARRIER-REEF, AUSTRALIA

Some physico-chemical properties and concentrations of the trace metals Cu and Zn have been measured in sediments from the Coral Sea between the coast at Cairns (146-degrees-E 17-degrees-S) and the Great Barrier Reef. Surface sediments along an 18 km transect and a midtransect core were examined. The acid soluble material increased from 20 to 36% and Ca increased from 12 to 47 mg g-1 with increasing distance from shore. Fe passed through a maximum of 32 mg g-1 at about 10 km offshore. The results show the pattern of sedimentation, with Fe representing terrestrial input, and acid soluble material and Ca representing CaCO3. The CaCO3 was mainly calcite with onlv minimal amounts of aragonite (coral). Microscopic examination showed shell material but no coral debris. The Cu concentration in the surface sediments (2.0-17 mug g-1) decreased with increasing distance from Cairns harbour (correlation coefficient Cu:dist -0.793). The Zn concentration (40-53 mug g-1) followed Fe (correlation coefficient Zn:Fe 0.932) with a maximum about 10 km offshore. There was no correlation of Cu and Fe (Cu:Fe 0.041) or Cu and Zn (Cu:Zn 0.178) showing that Cu and Zn came from different sources. The principle source of Cu is in the Cairns harbour region and may be a result of previous use of Cu-based marine antifouling paints. The Zn is dominantlv associated with terrestrial inputs. Within the sediment profile 11 km offshore, the pattern of particle size was consistent with the top 6 cm of sediment having been resuspended during a cyclone. Pb-210 concentrations also showed evidence of sediment mixing. Deeper in the sediment the average sedimentation rate was estimated at about 3.5 mm yr-1. Most of the CaCO3 down the prorile was calcite with no evidence of substantial input of coral material. The different sources of Cu and Zn could not be recognized within the core at this distance offshore.