The Eve of Biomineralization: Seawater and Climate Controls on Skeletal Mineralogy

A quantitative compilation of inorganic and biominerals from the onset of biomineralization (Late Ediacaran (~550 Ma) to the Middle Ordovician (~460 Ma)) infers a correspondence between sea water chemistry and the first adopted mineralogy of skeletal clades. Ediacaran to Tommotian skeletons and inorganic precipitates were composed exclusively of aragonite or high-Mg calcite, but these were replaced by low-Mg calcite mineralogies during the early Atdabanian, inferring the onset of a ‘calcite' sea. This transition is empirically constrained by fluid inclusion data. Late Atbadanian to Botoman inorganic precipitates revert to aragonite, with High-Mg calcite echinoderms and solitary tabulaconids, and aragonitic massive tabulaconids originating during this interval. Mid-Cambrian to Ordovician inorganic precipitates are low-Mg calcite, and the Ordovician radiation in its skeletal expression is due mostly to groups with low-Mg calcite mineralogies. These short-lived transitions can be most parsimoniously explained by either minor oscillations of mMg:Ca around ~2 during this interval, or the progressive onset of greenhouse conditions during the mid-late Cambrian.