During the middle Eocene to early Oligocene Earth transitioned from a greenhouse to an icehouse climate state. The interval comprises the Middle Eocene Climatic Optimum (MECO; ~40 Ma) and a subsequent long-term cooling trend that culminated in the Eocene-Oligocene transition (EOT; ~34 Ma) with the Oi-1 glaciation. Here, we present a refined calcareous nannofossil biostratigraphy and an orbitally tuned age model for the Monte Cagnero (MCA) section spanning the middle Eocene to the early Oligocene (~41 to ~33 Ma). Spectral analysis of magnetic susceptibility (MS) data displays strong cyclicities in the orbital frequency band allowing us to tune the identified 405 kyr eccentricity minima in the MS record to their equivalents in the astronomical solution. Our orbitally tuned age model allows us to estimate the position and duration of polarity chrons (C18 to C13) and compare them with other standard and orbitally tuned ages. We were also able to constrain the timing and duration of the MECO event, which coincides with a minimum in the 2.4 Myr and 405 kyr eccentricity cycles. Our study corroborates the previous estimated age for the base of the Rupelian stage (33.9 Ma) and estimates the base of the Priabonian stage in the MCA section to be at 37.4 Ma. Finally, calcareous nannofossils with known paleoenvironmental preferences suggest a gradual shift from oligotrophic to meso-eutrophic conditions with an abrupt change at ~36.8 Ma. Besides, nannofossil assemblages suggest that enhanced nutrient availability preceded water cooling at the late Eocene. Altogether, this evidence points to a poorly developed water column stratification prior to the cooling trend.

在始新世中期到渐新世早期，地球从温室转变为冰库气候状态。该区间包括中始新世气候最适 (MECO; ~40 Ma) 和随后的长期降温趋势，随着 Oi-1 冰川作用在始新世-渐新世过渡 (EOT; ~34 Ma) 中达到顶峰。在这里，我们提出了一个精细的钙质纳米化石生物地层学和一个轨道调谐年龄模型，用于跨越中始新世到早渐新世（~41 至~33 Ma）的 Monte Cagnero (MCA) 剖面。磁化率 (MS) 数据的光谱分析显示轨道频带中的强周期性，使我们能够将 MS 记录中确定的 405 kyr 偏心率最小值调整为天文解中的等效值。我们的轨道调谐年龄模型使我们能够估计极性年代（C18 到 C13）的位置和持续时间，并将它们与其他标准和轨道调谐年龄进行比较。我们还能够限制 MECO 事件的时间和持续时间，这与 2.4 Myr 和 405 kyr 偏心率循环中的最小值相吻合。我们的研究证实了先前估计的 Rupelian 阶段底部的年龄（33.9 Ma），并估计 MCA 部分的 Priabonian 阶段的底部为 37.4 Ma。最后，具有已知古环境偏好的钙质纳米化石表明，从贫营养状态逐渐转变为中富营养状态，并在~36.8 Ma 发生突然变化。此外，纳米化石组合表明，在始新世晚期，在水冷却之前，营养物质的可用性增强。共，