Oyster shells record seasonal climate variability in the middle Eocene Paris Basin under higher-than-modern temperatures and seasonal rainfall patterns

The Eocene experienced pronounced temporal changes in temperature and atmospheric pCO₂, with multiple warming phases from the early to late middle Eocene. High-resolution, sub-annual palaeoclimate reconstructions are essential to evaluate the impact of elevated pCO₂ on seasonal climate dynamics, providing critical insights for mitigating future climate crises. Middle Eocene Climatic Optimum (MECO), the Lutetian–Bartonian boundary warming event (~41 Ma) is particularly relevant, as current pCO₂ levels are rising rapidly and could reach similar concentrations within a century.

Bivalvia shells, growing incrementally, record seasonal to even sub-daily climatic and environmental fluctuations throughout their life. Shells of the oyster Cubitostrea cubitus, a shallow-to-marginal marine cementing bivalve from the Sables du Guépelle Formation (~41 Ma) of the Paris Basin (~41° N palaeolatitude), contain very low Mn and Fe concentrations (<250 µg/g), indicating their pristinity. These shells are used as a palaeoclimate archive in a multiproxy approach that combines LA-ICP-MS trace element analyses and clumped isotope thermometry (Δ₄₇), integrated with simulations from the Community Earth System Model (CESM). Sub-annual periodic variations in trace elements to Ca ratios along the oyster hinge indicate an oyster lifespan of ~16 months when aligned with monthly temperature variability from CESM simulations. Clumped isotope thermometry (Δ₄₇-T) records a seasonal sea surface temperature (SST) amplitude of ~8 °C, where the summer temperature reaching 28.3 ± 4.4 °C (68% CI) and winter temperatures of  19.6± 3.5 °C. Summer δ¹⁸O_w (-1.1± 0.9  ‰), consistent with Bartonian seawater compositions (-0.5 to -1.0 ‰), indicate a strong seasonal marine influence in early Bartonian Paris Basin. In contrast, significantly lower winter δ¹⁸O_w values (-2.9± 0.7 ‰) reflect enhanced freshwater input, which is further supported by relatively lower Sr/Ca profile, a salinity indicator consistent with increased winter rainfall predicted by CESM simulations.

In summary, our preliminary results indicate that during the MECO, the Paris Basin experienced seasonal sea-surface temperature variability comparable to that of modern shallow waters along the French North Sea coast, but with higher temperatures of approximately 10 °C throughout the year. In contrast to the modern climate (in the region of : 0–5° E, 46–50° N), where annual precipitation is relatively evenly distributed, rainfall during the MECO appears to have been strongly seasonal.