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.