The current rate of ocean acidification has no precedent in 300 million years of Earth history
Bärbel Hönisch, Andy Ridgwell, Daniela N. Schmidt, Ellen Thomas et al.,"The Geological Record of Ocean," Science 335, no. 6072 (2012): 1058–1063.
A third to a half of all the carbon dioxide that humans have released into the atmosphere has been absorbed by the ocean. Once dissolved in seawater, CO2 forms carbonic acid, which not only lowers the pH of the ocean – that is, makes it more acidic – but also decreases the availability of Carbonate ions (CO32-) to organisms that need it to grow Calcium Carbonate (CaCO3) shells. Laboratory studies have shown that many organisms at the base of the marine food web suffer from reduced shell formation in acidified conditions – conditions that may become typical during this century. In other words, ocean acidification threatens the very foundation of the marine ecosystem.
Scientists have been able to discern past acidification events for some time by noting a decrease in CaCO3 deposition on ancient seafloors. Recent advances in trace element and isotopic chemistry, however, have enabled them to discern past events with greater clarity than before, and to a horizon of about 300 million years. Within that span, scientists are looking for events that are analogous to the one that is happening today.
There aren't many, according to a paper published in the March 2010 issue of Science. In most earlier events, the decrease in ocean pH was not accompanied by decreased availability of CO32-. That is because these past events happened over periods of 100,000 years or more, time during which the natural weathering of terrestrial rock kept the saturation state of CaCO3 in the oceans stable despite the change in pH. Today, acidification is happening too quickly for weathering to counterbalance it, and so this event may be unprecedented in the history of the planet.
Three past acidification events may be partially comparable to what is happening today, but further research is needed to understand their history in more detail. In the meantime, it appears that the current acidification event is driving biogeochemical changes in the oceans that are potentially unparalleled in the history of Earth.