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.

古新世-始新世热极值是 5600 万年前的气候事件，其特征是碳快速释放和广泛的海洋酸化。然而，我们对酸化和海洋饱和状态演变的理解继续受到相当大的不确定性的阻碍，这主要是由于代理数据的有限可用性。在这种情况下，数据同化允许对在此期间的大气 CO₂ 变化、海洋酸化和碳酸盐饱和状态进行内部一致的评估。在这里，我们通过同化海底沉积物 CaCO₃ 和海面温度代理数据，以及来自地球系统模型（包括一个全面的碳酸盐系统）的模拟，展示了古新世-始新世热极大碳循环扰动的重建。我们的重建表明，大气中 CO₂ 从 890 ppm（95% 可信区间：680-1,170 ppm）大幅增加到 1,980 ppm（1,680-2,280 ppm），同时 pH 值（0.46 个单位，范围从 0.31 到 0.63 个单位）和地表水方解石饱和状态显着下降，从事件前的 10.2 （7.5-12.8） 下降到热最大值期间的 3.8 （2.8-5.1）。在古新世-始新世热盛期期间，高纬度地表水域的碳酸盐不饱和度显著加剧，这与目前由人为 CO₂ 排放驱动的北极文石饱和度下降相呼应。