OCEAN ACIDIFICATION IN DEEP TIME

Is there precedence in Earth history for the rapid release of carbon dioxide (CO2) by fossil fuel burning and its environmental consequences? Proxy evidence indicates that atmospheric CO2 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 (CaCO3) 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. Neverthless, observations and modeling clearly show that during the PETM the deep ocean, at least, became highly corrosive to CaCO3. 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 fossils fuel burning. As Roger Revelle and Hans Seuss pointed out over 50 years ago, humanity is performing the great geophysical experiment by pumping vast quantities of carbon dioxide into the atmosphere (Revelle and Seuss, 1957). One result of this experiment is an atmosphere that already contains more carbon dioxide than at any time in last 800,000 years of Earth history (as indicated from the anlysis of bubbles trapped in Antarctic ice; Luthi et al., 2008) and probably more than has occurred in several tens of millions of years (from various marine and terrestrial proxies; Royer, 2006). As summarized in detail by Feely et al.