How do we know what ocean pH was tens of millions of years ago?
To estimate physical or chemical parameters such as temperature or pH for periods before instruments were available, scientists use so-called proxy parameters or “proxies,” which are measurable parameters that can be related to desired but unobservable parameters. For instance, marine calcifying organisms incorporate many other elements into their hard shells and skeletons besides the calcium, carbon, and oxygen in calcium carbonate. When the hard parts of these organisms that are preserved in sediment are analyzed, the additional elements provide information about environmental conditions during the animal’s lifetime. Historical ocean pH values and changes can be studied using the concentration of the element boron and the ratio of its stable isotopes (δ 10B and δ 11B) in marine carbonates. Additional geochemical evidence and modeling provide strong evidence that the average surface ocean pH has not been much lower than about 8.2 for millions of years.
