Abstract. The Eocene-Oligocene Transition (EOT) marks the onset of the Antarctic glaciation and the switch from greenhouse to icehouse climates. However, the driving mechanisms and the precise timing of the EOT remain controversial mostly due to the lack of well-dated stratigraphic records, especially in continental environments. Here we present a cyclo-magnetostratigraphic and sedimentological study of a ∼7.6 Myr-long lacustrine record spanning the late Eocene to the earliest Oligocene, from a drill-core in the Rennes Basin (France). Time-series analysis of natural gamma-ray (NGR) log data shows evidence of Milankovitch cycle bands. In particular, the 405 kyr stable eccentricity is expressed with strong amplitudes. Astronomical calibration to this 405 kyr periodicity yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the middle–early Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series and assumptions on their phase relationships, enable to test previously proposed ages for the EO boundary, indicating that 33.71 and 34.10 Ma are the most likely. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr matching the g1-g5 eccentricity term. Such cyclicity has been recorded in other continental records, pointing to its significant expression in continental depositional environments. The record of g1-g5 and sometimes g2-g5 eccentricity terms in previously acquired sedimentary facies proxies in CDB1 core led us to hypothesize that the paleolake level may have behaved as a lowpass filter for orbital forcing. Two prominent changes in the sedimentary facies were detected across the EOT, which are temporally equivalent to the two main climatic steps, EOT-1 and Oi-1. Combined with previously acquired geochemical (δ¹⁵N_org, TOC), mineralogical (Quartz, clays) and pollen assemblage proxies from CDB1, we suggest that these two facies changes reflect the two major Antarctic cooling/glacial phases via the hydrological cycle, as significant shifts to drier and cooler climate conditions, thus supporting the stepwise nature of the EOT. Remarkably, a strongly dominant obliquity expressed in the latest Eocene corresponds in time to the interval from the EOT precursor glacial event till the EOT-1. We interpret the obliquity dominance as reflecting preconditioning phases for the onset of the major Antarctic glaciation, either from its direct impact on the formation/(in)stability of the incipient Antactic Ice Sheet (AIS), or through its modulation of the North Atlantic Deep Water production given the North Atlantic coastal location of the CDB1 site.

中文翻译：

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从湖泊记录的天文学调整开始的始新世-渐新世过渡的年龄和驱动机制（法国，雷恩盆地）

摘要。始新世-渐新世过渡（EOT）标志着南极冰期的开始以及从温室气候向冰室气候的转变。但是，EOT的驱动机制和精确的时间安排仍然引起争议，这主要是由于缺乏良好的地层记录，尤其是在大陆环境中。在这里，我们从雷恩盆地（法国）的一个岩心中获得了一个横跨早始新世至最早渐新世的〜7.6迈尔长湖相记录的环磁地层学和沉积学研究。天然伽马射线（NGR）测井数据的时间序列分析显示了米兰科维奇循环带的证据。尤其是405 kyr的稳定偏心率表现为很强的振幅。对该405千瓦的周期性进行天文校准，可以得出Chrons C12r至C16n.1n的持续时间估算值，在始新世中期尺度上提供了额外的限制。轨道偏心曲线和405 kyr NGR调谐时间序列之间的相关性以及它们之间的相位关系假设，可以测试EO边界先前提出的年龄，表明最可能出现33.71和34.10 Ma的年龄。此外，405 kyr调谐将最明显的NGR周期性校准为与g1-g5偏心项匹配的〜1 Myr周期。这种周期性已在其他大陆记录中得到记录，表明其在大陆沉积环境中的重要表达。在CDB1岩心中先前获得的沉积相代理中g1-g5有时是g2-g5偏心率的记录使我们假设古水平面可能充当了轨道强迫的低通滤波器。在整个EOT期间都发现了沉积相的两个显着变化，这些变化在时间上等同于两个主要的气候步骤EOT-1和Oi-1。与先前获得的地球化学（δ¹⁵ Ñ_有机，TOC），CDB1的矿物学（石英，粘土）和花粉组合代理，我们建议这两个相变反映了通过水文循环的两个主要南极冷却/冰川期，因为向明显更干燥和更凉爽的气候条件转移，从而支持EOT的逐步性质。值得注意的是，在最近的始新世表达的强烈的倾角在时间上对应于从EOT前冰期事件到EOT-1的时间间隔。我们将倾角优势解释为反映了主要南极冰期开始的预适应阶段，这可能是由于其对初始无规冰盖（AIS）的形成/（不稳定性）的直接影响，或者是其对北大西洋深部冰层的调节所致。鉴于CDB1站点位于北大西洋沿海地区，因此产生了水。