CARBONATE SKELETAL MINERALOGY REFLECTS CHEMISTRY OF OCEAN IN WHICH SKELETON FIRST EVOLVED

Over the past >600 Ma, changes in the Mg/Ca ratio of seawater have driven oscillations between ‘aragonite' and ‘calcite seas'. The carbonate mineralogy of dominant reef-building and sediment-producing taxa appear to be controlled by these oscillations, but it is thought that more ‘sophisticated' biomineralizers are immune to these effects. Using microstructural, phylogenetic, and petrographic data, I estimated original mineralogy for animal clades that independently evolved carbonate biomineralization during Neoproterozoic through Ordovician time. Fluid inclusion data indicate that an aragonite-to-calcite transition occurred during this interval, sometime during the Nemakit-Daldynian Stage (~542-525 Ma). Significantly, of those taxa whose mineralogy could be well constrained, all (N=8) that evolved mineralization during Ediaracan or Nemakit-Daldynian time were aragonitic, whereas all (N=13) that evolved mineralization during the Tommotian (~525-521 Ma) through Ordovician were calcitic. This suggests that original carbonate skeletal mineralogy is determined by the Mg/Ca ratio of seawater at the time the skeleton first evolved, consistent with the idea that taxa initially ‘choose' their mineralogy based on materials most easily available. (Once a polymorph has been chosen, however, it becomes difficult to change.) It has long been suggested that changes in ocean chemistry triggered the Cambrian biomineralization event; the discovery of an almost three-fold increase in calcium ion concentrations between Neoproterozoic and late Early Cambrian seawater has been proposed as just such a trigger. However, increased calcium concentrations are associated with the transition to a calcite sea. Thus if, as suggested here, several taxa began mineralizing before this transition, the calcium rise postdates the start of the Cambrian biomineralization event and cannot be its trigger. Other factors, such as the onset of predation, are more likely the cause.