Sub-equilibrium oxygen-18 and carbon-13 levels in biological carbonates: carbonate and kinetic models

Many biological carbonates contain less oxygen-18 and carbon-13 than expected for isotopic equilibrium with ambient waters. 'Carbonate' explanations for the oxygen-18 deficiencies draw inspiration from McCrea's (1950) observation that dissolved inorganic carbonates (DIC), in isotopic equilibrium with water, lose oxygen-18 in proportion to the abundance of the carbonate ion. Spero et al. (1997) therefore suggested that high carbonate levels might cause foraminifera to produce isotopically lighter shells at elevated pH. Adkins et al. (2003) extended this idea to the (presumably) alkaline internal calcification sites of deep-sea corals, and related skeletal carbon-13 content to the use of molecular carbon dioxide in calcification. This review compares these 'carbonate' ideas with an updated 'kinetic' model, which attributes isotopic enlightenment (for carbon-13 and oxygen-18) to carbon-dioxide-based calcification, and incomplete isotopic equilibration between DIC and water.