Ocean acidification in deep time

Is there precedence in Earth history for the rapid release of carbon dioxide (CO ₂) by fossil fuel burning and its environmental consequences? Proxy evidence indicates that atmospheric CO ₂ concentrations were higher during long warm intervals in the geologic past, and that these conditions did not prevent the precipitation and accumulation of calcium carbonate (CaCO ₃) as limestone; accumulation of alkalinity brought to the ocean by rivers kept surface waters supersaturated. But these were steady states, not perturbations. More rapid additions of carbon dioxide during extreme events in Earth history, including the end-Permian mass extinction (251 million years ago) and the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago) may have driven surface waters to undersaturation, although the evidence supporting this assertion is weak. Nevertheless, observations and modeling clearly show that during the PETM the deep ocean, at least, became highly corrosive to CaCO ₃. These same models applied to modern fossil fuel release project a substantial decline in surface water saturation state in the next century. So, there may be no precedent in Earth history for the type of disruption we might expect from the phenomenally rapid rate of carbon addition associated with fossil fuel burning.