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 (