ISOTOPIC DISEQUILIBRIUM IN MARINE CALCAREOUS ALGAE

Abstract

A survey of the δ13C and δ18O 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). Algae are categorized on the basis of stable isotope ratios and styles of calcification. Styles of calcification and associated processes in these plants are diverse and produce a wide range of δ13C values that are both lower and higher than predicted equilibrium values (from approximately -6%% to +8%%). In general, δ13C values of algal carbonate reflect phylogenetic and ontogenetic changes in photosynthesis that produce changes in the δ13C value of the calcifying fluid due to modification of the photosynthesis/respiration ratio. δ18O values are less variable, and in many cases, approximate predicted equilibrium values.Kinetic fractionation of carbon and oxygen isotopes associated with hydroxylation of CO2 is observed in the carbonates of Neogoniolithon sp. and Bossiella sp., where δ13C and δ18O values are positively correlated. This positive correlation is similar to trends observed in many carbonate-secreting organisms. Stable isotope data from green algae (Codiaceans and Dasycladacean) are related to the style of each species calcification (i.e., microenvironment of calcification site). Intercellular calcification in Halimeda sp. and Udotea sp. is characterized by significant metabolic effects in carbonate δ13C values (up to 7%% variation within a specimen). Extracellular calcification in Acetabularia sp. and sheath calcification in Penicillus sp. have carbonate δ13C and δ18O values that are near predicted equilibrium values (similar to inorganic precipitates). The δ13C values of coralline algae are lower than Codiacean green algae by as much as 14%%. We suggest that this difference is due largely to the unique ontogeny of coralline algae. Calcification in the younger portions of coralline algae occurs under strong influence of respiration. Carbonate from Amphiroa sp. and Galaxaura sp. has relatively invariant δ18O values that may be useful proxies of ambient conditions.