Oceanic anoxic events (OAEs) are historical intervals of global-scale ocean deoxygenation associated with hyperthermal climate states and biological crises^1,2. Massive volcanic carbon dioxide (CO₂) emissions frequently associated with these events are thought to be a common driver of ocean deoxygenation through several climate-warming-related mechanisms^1,3,4. The Early Cretaceous OAE1a is one of the most intense ocean deoxygenation events, persisting for more than 1 Myr (refs. ^5,6). However, existing records of marine chemistry and climate across OAE1a are insufficient to fully resolve the timing and dynamics of the underlying processes, thus obscuring cause-and-effect relationships between climate forcing and ocean oxygenation states. Here we show that rapid ocean deoxygenation during OAE1a is linked to volcanic CO₂ emissions and the crossing of an associated climate threshold, after which the sluggish pace of the silicate-weathering feedback and climate recovery delayed reoxygenation for >1 Myr. At the end of OAE1a, recrossing this threshold allowed for ocean reoxygenation. Following OAE1a, however, the Earth system remained sufficiently warm such that orbitally forced climate dynamics led to continued cyclic ocean deoxygenation on approximately 100-kyr timescales for another 1 Myr. Our results thus imply a tight coupling between volcanism, weathering and ocean oxygen content that is characterized by a climate threshold.

海洋缺氧事件 (OAE) 是与高温气候状态和生物危机相关的全球范围海洋脱氧的历史间隔^1,2 。经常与这些事件相关的大量火山二氧化碳 (CO ₂ ) 排放被认为是通过多种气候变暖相关机制导致海洋脱氧的常见驱动因素^1,3,4 。早白垩世 OAE1a 是最强烈的海洋脱氧事件之一，持续时间超过 1 Myr（参考文献^5,6 ）。然而，OAE1a 的现有海洋化学和气候记录不足以完全解析潜在过程的时间和动态，从而模糊了气候强迫和海洋氧合状态之间的因果关系。在这里，我们表明OAE1a期间海洋快速脱氧与火山CO ₂排放和相关气候阈值的跨越有关，此后硅酸盐风化反馈和气候恢复的缓慢步伐延迟了>1 Myr的再氧化。在 OAE1a 结束时，重新跨越这个阈值允许海洋重新充氧。然而，在 OAE1a 之后，地球系统仍然保持足够的温暖，使得轨道强迫气候动力学导致海洋持续循环脱氧，在大约 100 kyr 的时间尺度上又持续了 1 Myr。因此，我们的结果意味着火山活动、风化和海洋含氧量之间存在紧密联系，其特征是气候阈值。