Carbon and oxygen isotope ratios of marine calcareous algae: Implications for calcification, metabolism and carbon cycling in shallow marine environments

A survey of the delta13C and delta18O values of CaCO3 precipitated by marine calcareous algae was conducted to examine vital effects produced by the various styles of calcification of the major algal subdivisions (Codiacea, Dasycladacea, Coralline algae and Calcareous red algae). Carbon isotope ratios of organic matter were determined for Halimeda and Udotea sp. to examine possible metabolic effects on the carbon isotope ratios of skeletal carbonates. An extensive literature survey was conducted on physiologic and structural aspects of algal calcification. Isotope data were interpreted on the basis of known physiologic and biogeochemical information on the calcification processes of each algal species.;Calcareous organisms (Amphiroa sp., Halimeda sp., Udotea sp.) grown in the Biosphere 2 Ocean were collected to examine the effects of variable environmental factors on calcification and physiology. Weight of organic matter and CaCO3 were measured for Amphiroa sp. grown under distinct light intensity and carbonate chemistry. Data were interpreted on the basis of environmental control on the calcification and the metabolism. Halimeda sp., Amphiroa sp. and Udotea sp. grown under variable pCO2 level were analyzed for the carbon isotope ratios of carbonates and associated organic matter. Effects of ambient pCO2 on the carbon isotope ratios of carbonates and associated organic matter were examined. The importance of Sorites sp. (a large benthic foraminifera) was discussed as a possible proxy indicator of seawater isotope ratios.;Weekly to daily samples of Biosphere 2 seawater were collected for delta 13CDIC measurements during periods of chemical additions from June 1997 to April 1998. Data of SigmaCO2, Total Alkalinity, atmospheric and seawater pCO2 were used to model the delta 13CDIC variations during 1st, 2nd , 6th, and 7th chemical additions. Reef activities estimated from measured carbonate chemistry of Biosphere 2 Ocean and from the model calculation were compared. Results were interpreted on the basis of carbon isotope cycling in a carbonate reef.;Topics on the carbon and carbon isotope cycling in the ocean during glacial and interglacial periods were reviewed in relation to the importance of shallow marine carbonates. Calculations were presented to examine the effects of precipitation and dissolution of shallow marine carbonates on delta13C DIC of seawater, pH and pCO2 variability associated with glacio-eustatic sea level changes.