FACTORS AFFECTING CARBON AND OXYGEN ISOTOPIC COMPOSITION OF MARINE CARBONATE SEDIMENTS .2. HERON ISLAND GREAT BARRIER REEF AUSTRALIA

Carbonate sediments from off-shore, fore-reef slope, reef crest and margin, lagoonal and beach environments were collected at measured depths, subdivided into six size fractions, and analyzed for mineralogie, isotopic, and faunal composition. The goal of this investigation is an understanding of 13C and 18O distribution in sediments associated with coral reefs, and elucidation of the factors responsible for variations in the stable isotope ratios of carbon and oxygen. Maxima, minima, or changes in the slope of the curve expressing the concentration of 13C as a function of environment are closely correlated with the nature of the environment. Evidence obtained from analysis of within-sample variation, mineralogy, biogenic derivation of the carbonate components, and vital effects on isotope fractionation by calcareous organisms is used to account for the carbon stable isotope distribution pattern. Carbon isotope ratios also provide information concerning wind direction and sediment transport from one environment to another across the reef. One of the more interesting discoveries is the ability of carbon isotope ratios to differentiate accumulations of allochthonous, lagoon-derived carbonate deposited in deeper water on the leeward reef slope from autochthonous carbonate with about the same mineralogical composition. The results are compared with those for Bermuda reefs and the application of stable isotope geochemistry to ancient reef-associated carbonate rocks is discussed. © 1969.