ARAGONITE/CALCITE SEAS AND THE EVOLUTION OF BIOCALCIFICATION

The aragonite/calcite sea hypothesis postulates that throughout the Phanerozoic the inorganic phase of CaCO₃ in oceans shifted five times between low-magnesium calcite (LMC) and a combination of aragonite + high-magnesium calcite (HMC). These aragonite/calcite seas represent an influential theoretical framework in which to interpret extrinsic influences on the evolution of biocalcification. However, interpreting the rock record in the context of aragonite/calcite seas faces two main challenges: (1) the unstable nature aragonite and HMC through geological time, and (2) an inconsistent characterization of aragonite/calcite seas with respect to the known physical and chemical controls of aragonite vs. calcite formation. Recent analytical advances that focused on microscopic relic inclusions of aragonite extended the known range of preserved aragonite in the rock record by around 130 million years and thus offer a new opportunity to assess the early Paleozoic rock record for original aragonite. The chemical controls of aragonite vs. calcite formation in the context of aragonite/calcite seas have largely focused on the role of pCO₂ and the ratio of Mg/Ca. However, a variety of other parameters are known to significantly influence the CaCO₃ mineral system. The conventional view that aragonite/calcite seas reflect globally homogenous conditions is challenged by the fact that Mg/Ca controls aragonite and calcite formation as a function of temperature. Based on existing laboratory experiments and models of Mg/Ca through time, it should be expected that warm and shallow tropical surface waters above 23 °C permanently maintained aragonite sea conditions throughout the Phanerozoic with the temperature-dependent boundary to calcite sea conditions waxing and waning latitudinal with fluctuating ocean chemistry. Combining a thorough understanding of the physical and chemical controls of the CaCO₃ system with the distribution of aragonite and calcite through time is essential to assess the role of the aragonite/calcite sea hypothesis on the evolution of biomineralization.