Abstract. Paleoclimate reconstructions of the Early Eocene provide important data constraints on the climate and hydrologic cycle under extreme warm conditions. Available terrestrial water isotope records have been primarily interpreted to signal an enhanced hydrologic cycle in the Early Eocene associated with large-scale warming induced by high atmospheric CO2. However, orbital-scale variations in these isotope records have been difficult to quantify and largely overlooked, even though orbitally driven changes in solar irradiance can impact temperature and the hydrologic cycle. In this study, we fill this gap using water isotope–climate simulations to investigate the orbital sensitivity of Earth's hydrologic cycle under different CO2 background states. We analyze the relative difference between climatic changes resulting from CO2 and orbital changes and find that the seasonal climate responses to orbital changes are larger than CO2-driven changes in several regions. Using terrestrial δ18O and δ2H records from the Paleocene–Eocene Thermal Maximum (PETM), we compare our modeled isotopic seasonal range to fossil evidence and find approximate agreement between empirical and simulated isotopic compositions. The limitations surrounding the equilibrated snapshot simulations of this transient event and empirical data include timing and time interval discrepancies between model and data, the preservation state of the proxy, analytical uncertainty, the relationship between δ18O or δ2H and environmental context, and vegetation uncertainties within the simulations. In spite of the limitations, this study illustrates the utility of fully coupled, isotope-enabled climate models when comparing climatic changes and interpreting proxy records in times of extreme warmth.

中文翻译：

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早始新世二氧化碳驱动和轨道驱动的氧同位素变异

摘要。始新世早期的古气候重建为极端温暖条件下的气候和水文循环提供了重要的数据约束。现有的陆地水同位素记录主要被解释为始新世早期水文循环增强的信号，这与大气中高二氧化碳引起的大规模变暖有关。然而，这些同位素记录的轨道尺度变化很难量化，并且在很大程度上被忽视，尽管轨道驱动的太阳辐照度变化会影响温度和水文循环。在这项研究中，我们利用水同位素-气候模拟来研究不同二氧化碳背景状态下地球水文循环的轨道敏感性，从而填补了这一空白。我们分析了二氧化碳引起的气候变化和轨道变化之间的相对差异，发现在一些地区，季节性气候对轨道变化的响应大于二氧化碳驱动的变化。利用古新世-始新世热最大值 (PETM) 的陆地 δ18O 和 δ2H 记录，我们将模拟的同位素季节范围与化石证据进行比较，并发现经验和模拟同位素组成之间大致一致。该瞬态事件和经验数据的平衡快照模拟的局限性包括模型和数据之间的时间和时间间隔差异、代理的保存状态、分析不确定性、δ18O 或 δ2H 与环境背景之间的关系以及植被范围内的不确定性。模拟。尽管存在局限性，这项研究还是说明了完全耦合、同位素支持的气候模型在比较气候变化和解释极端温暖时期的代理记录时的实用性。