Middle Eocene hyperthermal seasonality from Paris Basin marine mollusks

The Earth has experienced hyperthermal events in the past, characterized by maximum durations of hundreds thousand years, significant magnitude, global extent, and drivers associated with increases in greenhouse gas concentrations, therefore making them potential analogues for current climate change. The Middle Eocene Climatic Optimum (MECO) that occurred 40 Ma ago, is marked by a CO 2 -driven global warming of +4 to +6 °C, affecting global temperatures. Here, we present a detailed reconstruction of seasonal fluctuations in seawater temperatures during this warming event in littoral environment, based on geochemical analyses (δ 18 O and ∆ 47 ) of shallow-marine mollusks from the Paris Basin. Our data show a stability in mean winter temperatures compared to pre-MECO conditions, but a marked warming of +10 °C in maximum estuarine water temperatures, with a seasonal temperature range increasing from 12 °C before the MECO to 22 °C at the climax of the event. We demonstrate that at mid-latitudes, annual maximum shallow-water temperatures increased from 30 ± 2 °C before the event to a maximum of 41 ± 4 °C at the warming peak. This pattern is associated with a seasonal regime characterized by dry summers and wet winters, implying that the Paris Basin experienced a super-hot summer Mediterranean climate during the MECO. • Identification of the MECO in the shallow-marine deposits of the Paris Basin. • Seasonal temperature range in Paris Basin rose to ~22 °C during the MECO. • Summer seawater peaked at ~41 °C, while winter temperatures stayed stable. • δ 18 O and Δ₄₇ analyses from mollusks enable high-resolution climate reconstructions. • Heterogeneous hydrologic trend with rainy winters and dry summers during the MECO.