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Modern thermohaline circulation plays a role in latitudinal heat transport and in deep‐ocean ventilation, yet ocean circulation may have functioned differently during past periods of extreme warmth, such as the Cretaceous. The Late Cretaceous (100–65 Ma) was an important period in the evolution of the North Atlantic Ocean, characterized by opening ocean gateways, long‐term climatic cooling and the cessation of intermittent periods of anoxia (oceanic anoxic events, OAEs). However, how these phenomena relate to deep‐water circulation is unclear. We use a proxy for deep‐water mass composition (neodymium isotopes; ε_Nd) to show that, at North Atlantic ODP Site 1276, deep waters shifted in the early Campanian (∼78–83 Ma) from ε_Nd values of ∼−7 to values of ∼−9, consistent with a change in the style of deep‐ocean circulation but >10 Myr after a change in bottom water oxygenation conditions. A similar …