Climate and ecology in the Rocky Mountain interior after the early Eocene Climatic Optimum
Climate and ecology in the Rocky Mountain interior after the early Eocene Climatic Optimum
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DOI：
10.5194/cp-17-2515-2021
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发表时间：
2021-12
影响因子：
4.3
通讯作者：
Rebekah A. Stein;N. Sheldon;S. E. Allen;Michael E. Smith;Rebecca M. Dzombak;B. Jicha
中科院分区：
文献类型：
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作者：
Rebekah A. Stein;N. Sheldon;S. E. Allen;Michael E. Smith;Rebecca M. Dzombak;B. Jicha
抽象。随着大气中二氧化碳（CO2）和温度随着现代气候变化而增加，古代温室时期成为了解类似条件下生态系统功能的焦点。始新世早期表现出高温，高二氧化碳水平，以及与今天相似的构造板块结构，因此它被用作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色河流域（GGRB）被一个古老的高盐湖（戈休特湖;绿色河组）和相关的河流和洪泛平原系统（瓦萨奇和布里杰组）覆盖。火山岩布里杰组是由内陆三角洲沉积而成的，该三角洲从西北流入淡水湖戈休特，并以其大量的古生物组合而闻名。利用这个保存完好的盆地沉积在一个时期的构造和古气候的兴趣，我们采用多种代理研究的趋势，在整个100万年的物源，母质，风化和气候。Blue Rim悬崖暴露了约100 m的下布里杰组，其中包括植物和哺乳动物化石、单独的古土壤剖面和适合于地球化学分析的有机物遗骸，以及适合于放射性同位素测年的灰层和火山岩砂岩层。来自蓝环陡崖中部和顶部的新的40 Ar/39 Ar年龄将其地层的年龄限制在49.5-48.5百万年前，处于始新世早期气候最佳期的“下降边缘”。我们使用了几种地球化学工具来研究古土壤和相关沉积物中的物源和母质，发现尽管沉积相和有机碳埋藏存在显着变化，但沉积物输入源没有变化。我们还重建了环境条件，包括温度，降水量（均来自古土壤），和大气CO2的同位素组成的植物中发现的植物组合。从古土壤为基础的重建结果进行了比较，使用叶外貌技术和海洋代理人的半同期重建。基于古土壤的降水量（608-1167 mm yr−1）和温度（10.4 - 12.0 ℃）重建（靠近剖面底部）与基于植物群组合的重建误差在范围内，尽管低于植物群组合，植物群组合在剖面中的地层较高，代表了一个高度保存的事件。地球化学和碎屑长石地质年代学表明，蓝环沉积物的来源一致，主要来自爱达荷州古河流，排水活跃的查利斯火山场。因此，由于既没有明显的气候变化，也没有明显的物源变化，沉积相和有机碳埋藏的变化可能反映了局部地貌控制和地下水位的相对高度。生态系统可以被描述为湿润的亚热带森林（即，根据花卉湿度省和Holdridge生活区计划，在整个间隔期间，考虑到蓝缘悬崖的中古纬度位置，这些结果与海洋代用指标相一致，海洋代用指标表明，全球温暖的气候条件持续超过始新世早期气候最佳期的最温暖条件。重建的大气δ 13 C值（−5.3 ‰至−5.8 ‰）与海洋微体化石的独立重建值（−5.4 ‰）非常接近，这为重建提供了信心。同样，重建的同位素组成与地幔最接近（-5.4 ‰），这与其他假设一致，即变暖是由火山释气维持的，而不是同位素耗尽的来源，如甲烷水合物。
Abstract. As atmospheric carbon dioxide (CO2) and temperatures increase with 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 as today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwestern 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. Using this well-preserved basin deposited during a period of tectonic and paleoclimatic interest, we employ multiple proxies to study trends in provenance, parent material, weathering, and climate throughout 1 million years. The Blue Rim escarpment exposes approximately 100 m of the lower Bridger Formation, which includes plant and mammal fossils, solitary paleosol profiles, 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 the age of its strata to ∼ 49.5–48.5 Myr ago during the “falling limb” of the early Eocene Climatic Optimum. We used several geochemical tools to study 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, including temperature, precipitation (both from paleosols), and the isotopic composition of atmospheric CO2 from plants found in the floral assemblages. Results from paleosol-based reconstructions were compared to semi-co-temporal reconstructions 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 and represented a highly preserved event later in time. 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-like forest (i.e., paratropical) 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 the independently reconstructed value from marine microfossils (−5.4 ‰), which provides confidence in this reconstruction. Likewise, the isotopic composition reconstructed matches the mantle most closely (−5.4 ‰), agreeing with other postulations that warming was maintained by volcanic outgassing rather than a much more isotopically depleted source, such as methane hydrates.