Over 200 modern sediment samples were obtained from nearshore stations in Bowling Green Bay and stations located across the inner and middle shelf regions of the central Great Barrier Reef. Surface sediments (0-1 cm) were collected using a frame-supported Van Veen grab sampler, then transferred to glass jars and stored frozen. Vibrocore samples were collected in August 1984 from the lagoon of Davies Reef. Samples of different species of the calcareous algae Halimeda were collected from the reef front at Davies Reef. All frozen samples were later freeze dried and homogenized in an agate ring mill. Halimeda samples were ground using a mortar and pestle.Measurements were conducted using a Beckman Tocamaster Model 915B total carbon analyser which simultaneously measures dissolved and particulate organic carbon. The samples were syringe-injected into a stream of oxygen and swept into a catalytic combustion tube held at 950°C. All carbonaceous material is oxidized to CO2, which was then swept into a nondispersive infrared analyzer sensitized to measure CO2. The transient CO2 present was directly proportional to total carbon in sample.