Identifying vital effects in <i>Halimeda</i> algae with Ca isotopes

Abstract

Abstract. Geochemical records of biogenic carbonates provide some of the most valuable records of the geological past, but are often difficult to interpret without a mechanistic understanding of growth processes. In this experimental study, Halimeda algae are used as a test organism to untangle some of the specific factors that influence their skeletal composition, in particular their Ca-isotope composition. Algae were stimulated to precipitate both calcite and aragonite by growth in artificial Cretaceous seawater. The Ca-isotope fractionation of the algal calcite is much smaller than that for the algal aragonite, similar to the behaviour observed in inorganic precipitates. However, the carbonate from Halimeda is isotopically heavier than inorganic forms, likely due to Rayleigh distillation within the algal intercellular space. In identifying specific vital effects and the magnitude of their influence on Ca-isotope ratios, this study suggests that mineralogy has a first-order control on the Ca-isotope budget of the carbonate sink and the Ca-isotope composition of seawater.