Stable isotope evidence for long-term stability of large-scale hydroclimate in the Neogene North American Great Plains

IF 3.8 2区 地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY Climate of The Past Pub Date : 2024-04-29 DOI:10.5194/cp-20-1039-2024
Livia Manser, Tyler Kukla, Jeremy K. C. Rugenstein
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Abstract

Abstract. The Great Plains of North America host a stark climatic gradient, separating the humid and well-watered eastern US from the semi-arid and arid western US, and this gradient shapes the region's water availability, its ecosystems, and its economies. This climatic boundary is largely set by the influence of two competing atmospheric circulation systems that meet over the Great Plains – the wintertime westerlies bring dominantly dry air that gives way to moist, southerly air transported by the Great Plains low-level jet in the warmer months. Climate model simulations suggest that, as CO2 rises, this low-level jet will strengthen, leading to greater precipitation in the spring but less in the summer and, thus, no change in mean annual precipitation. Combined with rising temperatures that will increase potential evapotranspiration, semi-arid conditions will shift eastward, with potentially large consequences for the ecosystems and inhabitants of the Great Plains. We examine how hydroclimate in the Great Plains varied in the past in response to warmer global climate by studying the paleoclimate record within the Ogallala Formation, which underlies nearly the entire Great Plains and provides a spatially resolved record of hydroclimate during the globally warmer late Miocene. We use the stable isotopes of oxygen (δ18O) as preserved in authigenic carbonates hosted within the abundant paleosol and fluvial successions that comprise the Ogallala Formation as a record of past hydroclimate. Today, and coincident with the modern aridity gradient, there is a sharp meteoric water δ18O gradient with high (−6 ‰ to 0 ‰) δ18O in the southern Great Plains and low (−12 ‰ to −18 ‰) δ18O in the northern plains. We find that the spatial pattern of reconstructed late Miocene precipitation δ18O is indistinguishable from the spatial pattern of modern meteoric water δ18O. We use a recently developed vapor transport model to demonstrate that this δ18O spatial pattern requires air mass mixing over the Great Plains between dry westerly and moist southerly air masses in the late Miocene – consistent with today. Our results suggest that the spatial extents of these two atmospheric circulation systems have been largely unchanged since the late Miocene and any strengthening of the Great Plains low-level jet in response to warming has been isotopically masked by proportional increases in westerly moisture delivery. Our results hold implications for the sensitivity of Great Plains climate to changes in global temperature and CO2 and also for our understanding of the processes that drove Ogallala Formation deposition in the late Miocene.
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新近纪北美大平原大尺度水文气候长期稳定的稳定同位素证据
摘要北美大平原上存在着明显的气候梯度,将湿润、水源充足的美国东部与半干旱、干旱的美国西部分隔开来,这种梯度影响着该地区的水源供应、生态系统和经济。这一气候分界线在很大程度上是由大平原上空交汇的两个相互竞争的大气环流系统的影响所决定的--冬季西风带来的主要是干燥空气,而在温暖的月份里,大平原低空喷流带来的则是潮湿的偏南空气。气候模型模拟表明,随着二氧化碳的增加,低空喷流将加强,导致春季降水量增加,但夏季降水量减少,因此年平均降水量不会发生变化。气温升高将增加潜在的蒸散量,再加上气温升高,半干旱条件将向东转移,这可能会对大平原的生态系统和居民造成巨大影响。奥加拉拉地层几乎覆盖了整个大平原,提供了全球变暖的中新世晚期水文气候的空间分辨记录。我们利用保存在奥加拉拉地层丰富的古沉积和河流演替中的自生碳酸盐中的氧(δ18O)稳定同位素来记录过去的水文气候。如今,与现代干旱梯度相吻合的是,流星水δ18O 梯度很大,大平原南部的δ18O 较高(-6 ‰至 0 ‰),而平原北部的δ18O 较低(-12 ‰至-18 ‰)。我们发现,重建的晚中新世降水δ18O 的空间模式与现代陨水δ18O 的空间模式没有区别。我们利用最近开发的水汽输送模型证明,这种δ18O 空间模式需要中新世晚期大平原上空干燥的西风气团和潮湿的南风气团之间的混合,这与今天的情况一致。我们的研究结果表明,自中新世晚期以来,这两个大气环流系统的空间范围基本没有变化,大平原低空喷流因气候变暖而加强的任何作用都被西风水汽输送的比例增加所掩盖。我们的研究结果对大平原气候对全球温度和二氧化碳变化的敏感性具有重要意义,同时也有助于我们理解中新世晚期奥加拉拉地层沉积的驱动过程。
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来源期刊
Climate of The Past
Climate of The Past 地学-气象与大气科学
CiteScore
7.40
自引率
14.00%
发文量
120
审稿时长
4-8 weeks
期刊介绍: Climate of the Past (CP) is a not-for-profit international scientific journal dedicated to the publication and discussion of research articles, short communications, and review papers on the climate history of the Earth. CP covers all temporal scales of climate change and variability, from geological time through to multidecadal studies of the last century. Studies focusing mainly on present and future climate are not within scope. The main subject areas are the following: reconstructions of past climate based on instrumental and historical data as well as proxy data from marine and terrestrial (including ice) archives; development and validation of new proxies, improvements of the precision and accuracy of proxy data; theoretical and empirical studies of processes in and feedback mechanisms between all climate system components in relation to past climate change on all space scales and timescales; simulation of past climate and model-based interpretation of palaeoclimate data for a better understanding of present and future climate variability and climate change.
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