Abstract. As increasing atmospheric carbon dioxide (CO₂) and temperatures accompany modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO₂ levels, and similar tectonic plate configuration to today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwest Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger Formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. The Blue Rim escarpment exposes approximately 100 meters of the lower Bridger Formation, which includes plant and mammal fossils, paleosols and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New ⁴⁰Ar/³⁹Ar ages from the middle and top of the Blue Rim escarpment constrain age of its strata to ~49.5–48.5 Ma ago, during the “falling limb” of the early Eocene climatic optimum. Using several geochemical tools, we reconstructed provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions at the time, including temperature and precipitation (from paleosols) and the isotopic composition of CO₂ from plants found in the floral assemblages, before comparing them to reconstructions for the same time made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608–1167 mm yr⁻¹) and temperature (10.4 to 12.0 °C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls, and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical forest throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim Escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene climatic optimum. The reconstructed atmospheric δ¹³C value (−5.3 to −5.8 ‰) closely matches both the independently reconstructed value from marine microfossils (−5.4 ‰), as well as the isotopic composition of the mantle (−5.4 ‰), suggesting that the warm conditions were maintained by volcanic outgassing.

中文翻译：

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始新世气候最优化后的落基山脉内部气候与生态

摘要。随着现代气候变化伴随着大气二氧化碳（CO ₂）和温度的升高，古代温室时代已成为了解相似条件下生态系统功能的重点。始新世早期表现出高温，高CO ₂级别，以及与今天相似的构造板块构造，因此已被引用为现代气候变化的模拟。在始新世早期，怀俄明州西南部的大绿河盆地（GGRB）被一个古老的高盐湖（Gosiute湖；绿河形成）和相关的河流和洪泛区系统（Wasatch和Bridger形成）覆盖。火山碎屑的布里杰组是由一个内陆三角洲沉积的，该三角洲从西北方向排入淡水的Gosiute湖，并以其庞大的古生物学组合而闻名。蓝缘陡坡暴露了下布里杰地层约100米，包括适于地球化学分析的植物和哺乳动物化石，古土壤和有机物，以及适合放射性同位素测年的灰烬层和火山碎屑砂岩层。新的从蓝缘悬崖中部和顶部开始的⁴⁰ Ar / ³⁹ Ar年龄将其地层年龄限制在约49.5–48.5 Ma之前，这是始于始新世气候最佳时期的“下肢”。使用几种地球化学工具，我们重建了古土壤和相关沉积物中的物源和母体材料，尽管沉积相和有机碳埋藏发生了显着变化，但沉积物输入源没有变化。我们还重建了当时的环境条件，包括温度和降水（来自古土壤）以及CO ₂的同位素组成在将它们与使用叶生理学技术和海洋代理进行的相同时间的重建物进行比较之前，将它们从花卉组合中发现的植物中提取出来。降水（608-1167 mm yr ^-1）的古土壤重建（在剖面底部附近））和温度（10.4至12.0°C）在误差范围内，尽管低于基于花卉组合的误差，该组合在地层中较高。地球化学和碎屑长石的地球年代学表明，蓝缘沉积物具有一致的出处，主要来自爱达荷古河床，这消耗了活跃的查理斯火山场。因此，由于既没有明显的气候变化也没有明显的物源变化，因此沉积相和有机碳埋藏的变化很可能反映了局部地貌控制和地下水位的相对高度。根据花卉湿度省份和Holdridge生活区方案，整个时间间隔内，该生态系统都可以描述为湿润的亚热带森林。鉴于蓝缘悬崖峭壁的中古位置，这些结果与海洋代理相吻合，后者表明全球温暖的气候条件继续超出了始新世早期气候最佳状态的峰值温暖条件。重建大气δ¹³ C值（-5.3到-5.8‰）与海洋微化石的独立重建值（-5.4‰）以及地幔的同位素组成（-5.4‰）非常匹配，表明温暖的条件得以保持。火山喷发。