This study will quantitatively reconstruct deep ocean carbonate chemistry changes across the Paleocene-Eocene thermal maximum to determine the degree of ocean acidification during a past period of high CO2 and extreme warming. The PIs will generate records from multiple sites in different water depths and different oceans using palaeontological, geochemical, and modeling techniques. There will be particular focus on the application of two Boron-based proxies for ocean pH. Ocean acidification is a topic of great societal concern because of its impact on marine organisms and on the rate at which the ocean can absorb additional CO2 from the atmosphere. The research will involve graduate and undergraduate students at the four participating institutions.