Palaeoecological evidence for early aragonite dissolution in ancient calcite seas

It has been proposed that shallow carbonate seas of Upper Ordovician and Jurassic ages precipitated calcite as the primary calcium carbonate phase 1–3, unlike such settings today. It is expected that such seas were undersaturated with respect to the more soluble aragonite, so that aragonitic invertebrate skeletons would have tended to dissolve. Here we describe examples in which this seems to have been the case. Our evidence consists of submarine-cemented horizons in which aragonite fossils have been dissolved out to leave open moulds. The walls of the moulds are encrusted by contemporaneous organisms, confirming the early timing of the aragonite dissolution.