Seawater chemistry, biomineralization and the fossil record of calcareous organisms

Oscillations in seawater chemistry have characterized at least the last 600 million years of earth history. Three phases of aragonite seas have been separated by two of calcite seas, the former due to high molar ratios of Mg/Ca in seawater and the latter to low ratios. During aragonite seas, inor-ganic marine cements and ooids are aragonitic, whereas during calcite seas they are calcitic. Re-cent interest has focused on the effects of this cyclical change in seawater chemistry on biomineralization, as well as on taphonomy (including diagenesis) and hence fossil preservation. Although there is a need for rigorous testing of hypotheses, data on the geological distributions of aragonite and calcite skeletons among marine invertebrates lends variable support to links between seawater chemistry and: (1) de-novo evolution of calcite vs aragonite skeletons, (2) switchovers in mineralogy during subsequent clade evolution, (3) the relative success of clades using calcite or aragonite as biominerals, and (4) the development of hardgrounds with associated sclerobiont communities