1 Stable isotopes of carbon and oxygen in modern sediments of carbonate platforms, barrier reefs, atolls and ramps: patterns and implications EBERHARD GISCHLER 1 , PETER K. SWART 2 AND ANTHONY J. LOMANDO 3 1 Johann Wolfgang Goethe-Universität, Geowissenschaften, 60325 Frankfurt am Main, Germany 2 University of Miami, RSMAS-MGG, Miami, Florida 33149, USA 3 ChevronTexaco, Bahrain Pouch, San Ramon, California 94583, USA ABSTRACT Stable isotopes of carbon (δ 13 C) and oxygen (δ 18 O) were analyzed in surface and subsurface sediments of modern carbonate platform and barrier reef (Belize), atoll (Maldives) and ramp (Kuwait) settings. The δ 13 C values ranged from -1.6 to +5.3‰ and δ 18 O values from -3.2 to +2.3‰. Among individual particle types, non-skeletal grains such as peloids and ooids exhibit highest values whereas coral fragments display relatively low δ 13 C and δ 18 O values. The highest statistically significant correlations between abundance of carbonate grains, mineralogy and geochemistry occur in the ramp setting, where facies types may be defined using carbon and oxygen isotopic composition. In the platform, barrier and atoll examples, correlations are poor and there are no clear spatial trends in the carbon and oxygen isotopic values of sediments or facies. In locations with comparably high amounts of non-skeletal grains, δ 13 C values correlate with the aragonite content of the samples. The difference between the ramp and other examples is probably a consequence of the continuous depositional energy gradient, which appears to be the major controlling factor of facies distribution in the ramp setting. In the atoll, barrier reef and platform examples, depositional energy gradients are more variable due to higher morphological and environmental variability. The δ 13 C and δ 18 O values of samples from Holocene cores also exhibit high total variability ranging from -1.9 to -0.5‰ in δ 13 C and from -0.2 to +5.2‰ in δ 18 O. Strong excursions in δ 13 C and δ 18 O in individual cores are seen near the base where values became