The current rate of ocean acidification has no precedent in 300 million years of Earth history according to the Yale Environmental Review
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. 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.