Fossil‐rich sediments of the Santa Cruz Formation, Patagonia, Argentina, span the initiation of the Miocene Climatic Optimum (MCO), the most recent period of warm and wet conditions in the Cenozoic. These conditions drove the expansion of tropical and subtropical ecosystems to much higher latitudes, with the fossiliferous Santa Cruz Formation recording one of the southernmost examples. We collected new carbon and oxygen isotope compositions of herbivore tooth enamel from fossils ~17.4 to 16.4 Ma in age to investigate ecological and climatic changes across the initiation of the MCO. Enamel δ¹³C values are consistent with a C₃‐dominated ecosystem with moderate precipitation and a mix of wooded and more open areas. Serially sampled teeth reveal little zoning in δ¹³C and δ¹⁸O values, suggesting little seasonal variation in water and plant isotope compositions or seasonal changes in diet. Carbon isotope‐based estimates of mean annual precipitation (MAP) are consistent with aridification, with MAP decreasing from ~1,000 ± 235 mm/yr at 17.4 Ma to ~525 ± 105 mm/yr at the start of the climatic optimum (~16.9 Ma). This decrease corresponds to increasing global temperatures, as indicated by marine proxy records, and was followed by a rebound to ~840 ± 270 mm/yr by ~16.4 Ma. In comparison to a modern mean annual temperature (MAT) in the region of ~8°C, oxygen isotopes indicate high MAT (at least 20°C) at the onset of the MCO at 16.9 Ma and a significant increase in MAT to ~25°C by 16.4 Ma.

中文翻译：

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中新世最佳气候开始时的巴塔哥尼亚干旱化

阿根廷巴塔哥尼亚的圣克鲁斯组富含化石的沉积物跨越了中新世最佳气候（MCO）的开始，这是新生代最近的温暖湿润时期。这些条件将热带和亚热带生态系统扩展到了更高的纬度，化石圣克鲁斯组记录了最南端的例子之一。我们从年龄在17.4至16.4 Ma的化石中收集了食草动物牙釉质的新的碳和氧同位素组成，以研究整个MCO启动期间的生态和气候变化。牙釉质δ ^13个C值是与C一致₃中度降水-dominated生态系统和林地和更加开放的领域的混合。连续采样牙齿揭示δ小区划¹³C和δ ^18个O值，这表明在水中和植物同位素组合物或饮食的季节性变化小的季节变化。基于碳同位素的年平均降水量（MAP）估计与干旱化一致，MAP从气候最佳时期开始时的〜1,000±235 mm / yr在17.4 Ma降低到〜525±105 mm / yr（〜16.9 Ma ）。如海洋代理记录所示，这种下降与全球气温升高相对应，随后回升至〜840±270 mm / yr〜16.4 Ma。与〜8°C左右的现代平均年温度（MAT）相比，氧同位素表明MCO在16.9 Ma发病时的MAT较高（至少20°C），并且MAT显着增加至〜25 °C通过16.4 Ma。