Stable Isotopes of Carbon and Oxygen in Modern Sediments of Carbonate Platforms, Barrier Reefs, Atolls and Ramps: Patterns and Implications

Abstract

Stable isotopes of carbon ( 13 C) and oxygen ( 18 O) were determined in surface and subsurface sediments of modern carbonate platform and barrier reef (Belize), atoll (Maldives) and ramp (Kuwait) settings. The  13 C values ranged from 1.6 to +5.3‰ and  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  13 C and  18 O values. The most statistically signifi cant correlations between abundance of carbonate grains, mineralogy and geochemistry occur in the ramp setting, where facies types may be defi ned 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,  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  13 C and  18 O values of samples from Holocene cores also exhibit high total variability ranging from 1.9 to 0.5‰ in  13 C and from 0.2 to +5.2‰ in  18 O. Strong excursions in  13 C and  18 O in individual cores are seen near the base where values become more negative as a consequence of diagenesis, i.e., the infl uence of a subaerial exposure horizon. Above the meteorically infl uenced section, however, variability along individual cores is comparably low (<1.2%). The enormous carbon and oxygen isotopic variability observed in modern shallow-water carbonates as compared with fossil sections, where  13 C is used as stratigraphic marker, is probably a result of as yet poorly defi ned taphonomic and diagenetic fi lters, which signifi cantly reduce isotopic variability.