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Abstract Accurate reconstruction of seasonal atmospheric patterns of the past is essential for reliable prediction of how climate will evolve due to anthropogenic CO 2 forcing. The Eocene ‘hot house’ climate, as the warmest epoch during the Cenozoic, is considered as a potential analogue for ‘high-CO 2 ’ future climate scenarios. In this context, the reconstruction of variations in seasonality are as important as changes in mean annual conditions. Here we combine stable oxygen (δ 18 O) and dual clumped isotope (Δ 47 + Δ 48 ) measurements of a bivalve shell to determine sub-annual variations in sea surface temperatures and oceanic freshening in the Paris Basin during the Mid-Eocene Climate Optimum, 40 million years ago. Our reconstruction indicates to high mean annual temperatures with a small seasonal amplitude (33.3 °C ± 4.4 °C) and an enhanced fresh water input during the summer period. Our results implying a substantially warmer climate state with different hydrological conditions for Western Europe during the Eocene than previously suggested by proxy data or climate modelling.
Kniest et al. (Sat,) studied this question.