Systematic Relationships between Carbon and Oxygen Isotopes in Carbonates Deposited by Modern Corals and Algae

Abstract

Analyses of organic carbonates from Jamaican coral reefs show a positive correlation between the O¹⁸: O¹⁶ ratio and the C¹³ C¹² ratio in some taxonomic groups of corals and algae, but essentially no correlation (nearly constant O¹⁸) in one suborder of reef-building corals. The strontium and magnesium contents apparently are controlled mainly by skeletal mineralogy and show no correlation with carbon or oxygen isotopic composition. The observed positive correlations between C¹³ and O¹⁸ content may be due to calcification processes utilizing carbon-oxygen compounds from two isotopically different sources or utilizing selected portions of a wide spectrum of carbon-oxygen compounds in which there is a positive correlation between C¹³ C¹² and O¹⁸;O¹⁶ ratios. Coral and algal carbonates from Jamaican waters, with an annual temperature range of only about 4°C, exhibit a total δC¹³ range of more than 13 per mil and a σO¹⁸ range of more than 4 per mil. The wide isotopic variability resulting from vital effects of calcifying organisms must be taken into account in applying isotopic analysis to the study of sedimentary carbonate rocks which may include reef-derived carbonates.