Isotopic disequilibrium in marine calcareous algae

A survey of the delta C-13 and delta O-18 values of CaCO3 precipitated by m arine calcareous algae was conducted to examine 'vital effects' produced by the various styles of calcification of the major algal subdivisions (Codia cea, Dasycladacea, Coralline algae and Calcareous red algae). Algae are cat egorized on the basis of stable isotope ratios and styles of calcification. Styles of calcification and associated processes in these plants are diver se and produce a wide range of delta C-13 values that are both lower and hi gher than predicted equilibrium values (from approximately -6 parts per tho usand to + 8 parts per thousand). In general, delta C-13 values of algal ca rbonate reflect phylogenetic and ontogenetic changes in photosynthesis that produce changes in the delta C-13 value of the calcifying fluid due to mod ification of the photosynthesis/respiration ratio. delta O-18 values are le ss variable, and in many cases, approximate predicted equilibrium values. Kinetic fractionation of carbon and oxygen isotopes associated with hydroxy lation of CO2 is observed in the carbonates of Neogoniolithon sp. and Bossi ella sp., where delta C-13 and delta O-18 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 D asycladacean) are related to the style of each species calcification (i.e., microenvironment of calcification site). Intercellular calcification in Ha limeda sp. and Udotea sp. is characterized by significant metabolic effects in carbonate delta C-13 values (up to 7 parts per thousand variation withi n a specimen). Extracellular calcification in Acetabularia sp. and sheath c alcification in Penicillus sp. have carbonate delta C-13 and delta O-18 val ues that are near predicted equilibrium values (similar to inorganic precip itates). The delta C-13 values of coralline algae are lower than Codiacean green algae by as much as 14 parts per thousand. We suggest that this diffe rence is due largely to the unique ontogeny of coralline algae. Calcificati on in the younger portions of coralline algae occurs under strong influence of respiration. Carbonate from Amphiroa sp. and Galaxaura sp. has relative ly invariant delta O-18 values that may be useful proxies of ambient condit ions. (C) 2001 Elsevier Science B.V. All rights reserved.