Seawater pH and isotopic paleotemperatures of Cretaceous oceans

Stable oxygen isotope ratios (δ18O) of foraminifera are widely used to reconstruct the climatic history of Earth. It is well known that temperature reconstructions based on δ18O are complicated by factors such as the unknown isotopic composition of the ocean. In addition, recent experimental and theoretical work has shown that the seawater pH has a marked effect on the δ18O of foraminifera. Here I employ this effect to demonstrate that reconstructions based on δ18O of foraminifera may underestimate sea surface temperatures in the geological past. Ocean surface temperatures for the mid-Cretaceous are estimated to have been ∼2–3.5°C higher than previously thought.