Gastropod carbonate δ¹⁸O and δ¹³C values record strong seasonal productivity and stratification shifts during the late Eocene in England

High-resolution δ¹⁸O and δ¹³C measurements obtained from the gastropod Clavilithes macrospira provide the most detailed record of seasonal change in temperature (range ∼14 °C), seawater chemistry, and bottom-water ventilation yet obtained from late Eocene strata (type Bartonian, ca. 40 Ma). The δ¹⁸O oscillations suggest a linear rate of long-term growth and rapid growth in spring–early summer. Coherence analysis shows that δ¹³C patterns vary from being closely in phase with respect to annual δ¹⁸O oscillations, to being antiphase, with a phase shift toward a spring, or more rarely, an autumnal δ¹³C peak. These patterns are attributed to unstable seasonal productivity and/or the development of a seston-rich bottom layer, in which the δ¹³C gradients arising from seasonal eutrophication were perturbed by spring storms. Such episodes may have led to the decline and extinction of oligotrophic marine biota with photosymbionts, including Nummulites, in the late Eocene.