Coupled decline in ocean pH and carbonate saturation during the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum, a climate event 56 million years ago, was characterized by rapid carbon release and extensive ocean acidification. However, our understanding of acidification and the evolution of ocean saturation states continues to be hindered by considerable uncertainties, primarily stemming from the limited availability of proxy data. Under such conditions, data assimilation allows for an internally consistent assessment of atmospheric CO₂ changes, ocean acidification and carbonate saturation state during this period. Here, we present a reconstruction of the Palaeocene–Eocene Thermal Maximum carbon cycle perturbation by assimilating seafloor sediment CaCO₃ and sea surface temperature proxy data with simulations from an Earth system model, which includes a comprehensive carbonate system. Our reconstructions indicate a substantial increase in atmospheric CO₂ from 890 ppm (95% credible interval: 680–1,170 ppm) to 1,980 ppm (1,680–2,280 ppm), coupled with a notable decline in pH (0.46 units, ranging from 0.31 to 0.63 units) and surface-water calcite saturation state, decreasing from 10.2 (7.5–12.8) in the pre-event period to 3.8 (2.8–5.1) during the thermal maximum. Carbonate undersaturation intensified substantially in high-latitude surface waters during the Palaeocene–Eocene Thermal Maximum, paralleling the current decline in Arctic aragonite saturation driven by anthropogenic CO₂ emissions.