Collaborative Research: Absolute-dated records of Late Quaternary paleohydrology in the Bonneville Basin, western U.S., from novel cave archives

美国大盆地的水平衡在很大程度上反映了西风风暴路径的位置和强度，这表明过去的水文变化记录为了解风暴路径在不同气候状态下的行为提供了重要的见解。然而，目前现有的过去水平衡记录的测年精度和样本分辨率有限，使得科学家无法解决过去水平衡变化的准确时间和速率以及它们与其他古气候档案中记录的突变事件的关系。此外，很少有年代准确的记录延续到最后一次冰期之后，这使得人们对以前的间冰期和冰川终止几乎没有了解。该项目将使用洞穴沉积来重建博内维尔盆地过去突然的水平衡变化，博内维尔盆地是大盆地上一次冰川时期最大的古湖泊所在地。这项工作既利用了湖水淹没的一系列洞穴中形成的湖泊洞穴沉积，也利用了盆地西南边缘雷曼洞穴的洞穴洞穴。利用稳定的同位素测量以及U-Th和14C测年的组合，该小组将能够产生具有前所未有的测年精度和样品分辨率的记录，涵盖最近的冰川终止以及之前的两次间冰期和终止。一个新的古水文档案--湖泊洞穴沉积的数据表明，它们形成于近30KYR时期，当时湖面高于给定的洞穴？S高程，水文闭合。因此，这些沉积物记录了湖泊通过每个洞穴的高程以及盆地溢出的开始和停止；它们还保存了湖泊氧同位素组成和化学的高分辨率记录。重要的是，这些沉积物可以通过U-Th方法精确测年，提供了博纳维尔湖水平衡变化的第一个高精度、绝对日期记录。标定的~(14)C年龄与湖相洞穴沉积的U-Th年龄相吻合，表明湖相洞穴沉积对~(14)C的储集作用很小。作为该项目的一部分，将使用一系列成对的U-Th和14C年龄来改进14C校准数据集，重点放在19至25个日历Ka之间。雷曼洞穴是对湖泊洞穴碳酸盐的重要补充，可以追溯到过去300 KYR的大部分地区。来自洞穴石笋的稳定同位素记录，由U-Th年代锚定，将提供前两次间冰期和冰川终止期间的一些第一批准确的区域水文记录。雷曼洞穴和湖泊洞穴记录结合在一起，为了解大盆地东部在过去三个冰川周期中对突然的和轨道规模的气候变化的反应提供了关键的见解。非技术摘要模型一贯预测，随着温室气体变暖，世界上的旱地平均将变得更加干燥，但尚不清楚这种干燥将如何影响特定地区。在美国，人们尤其感兴趣的是了解大盆地未来干旱的可能性，这是一个干旱多发的地区，在过去20年里经历了大幅的人口增长。过去可用水的记录为预测大盆地未来的变化提供了一个关键的起点，它向我们展示了该地区对各种气候的反应。在过去的3万年里，美国西部内陆大盆地的降水和蒸发平衡发生了巨大变化，留下了今天冰川时代存在大量湖泊的干燥湖床和高盐度湖泊。该地区的水资源供应在很大程度上取决于冬季风暴路径带来的降雨量，这表明过去水文变化的记录为了解风暴路径在不同气候状态下的行为提供了重要的见解。然而，目前记录的有限的测年精度和样本分辨率，让我们只能对过去的水文变化有一个大致的了解，目前还没有准确的记录可以延伸到过去的间冰期暖期。该项目将使用洞穴沉积来重建犹他州北部的博纳维尔盆地的过去的水平衡变化，该盆地是大盆地最大的古湖泊的所在地。这些沉积物记录了过去300,000年来盆地降水的变化，并能够准确地测定日期，为我们提供了构建该地区过去气候变化的详细和广泛图景的机会。重要的是，来自盆地西缘雷曼洞穴的石笋记录了过去两个间冰期暖期，使我们有机会研究该地区在与现在一样温暖或略高于现在的时期的水文平均状态和变异性。这项工作对我们理解过去的气候变化做出了重要贡献，并将有助于校准试图重现过去变化的模型，以便更好地预测未来。此外，这些记录将有助于我们解释该盆地过去的生态变化和该地区早期人类占领的历史。

TECHNICAL ABSTRACTWater balance in the U.S. Great Basin largely reflects the position and intensity of the westerly storm track, suggesting that records of past hydrologic changes offer important insights into the behavior of the storm track in different climate states. The limited dating precision and sample resolution of presently available records of past water balance, however, leave scientists unable to address questions about the precise timing and rates of past water balance changes and their relation to abrupt events recorded in other paleoclimate archives. Further, few well-dated records extend past the last glacial period, allowing little understanding of previous interglacials and glacial terminations. This project will use cave deposits to reconstruct past abrupt water balance changes in the Bonneville Basin, the site of the largest of the Great Basin's paleolakes during the last glacial period. The work draws upon both a) lacustrine cave deposits formed in a series of caves flooded by lake waters and b) speleothems from Lehman Cave on the southwestern edge of the basin. Using stable isotope measurements and a combination of U-Th and 14C dating, this group will be able to produce records with unprecedented dating precision and sample resolution covering the most recent glacial termination as well as the previous two interglacials and terminations. Data from lacustrine cave deposits, a novel paleohydrologic archive, indicate that they formed during periods of the last 30 kyr when the lake was both above a given cave?s elevation and hydrologically closed. The deposits thus record the passage of the lake through each cave's elevation and the onset and cessation of basin overflow; they also preserve high-resolution records of the lake's oxygen isotope composition and chemistry. Importantly, the deposits can be precisely dated by U-Th methods, providing the first high-precision, absolute-dated records of Lake Bonneville's water balance changes. Calibrated 14C ages match U-Th ages in lacustrine cave deposits, indicating that 14C reservoir effects were minimal in the lake. As a part of the project, a series of paired U-Th and 14C ages will be used to improve the 14C calibration dataset, focusing on the period between 19 and 25 calendar ka.Lehman Cave speleothems comprise an important complement to the lacustrine cave carbonates, extending back through substantial portions of the last 300 kyr. Stable isotope records from the cave's stalagmites, anchored by U-Th dates, will provide some of the first well-dated archives of regional hydrology during the previous two interglacials and glacial terminations. Together, Lehman Cave and lacustrine cave records promise key insights into the eastern Great Basin's response to both abrupt and orbital-scale climate changes during the last three glacial cycles.NON-TECHNICAL ABSTRACTModels consistently predict that the world's drylands will become drier on average in response to greenhouse warming, but it is unclear how this drying will affect specific regions. In the U.S., there is particular interest in understanding the potential for future drying in the Great Basin, a drought-prone region that has experienced substantial population growth over the last two decades. Records of past water availability provide a key starting point for forecasting future changes in the Great Basin by showing us the region's responses to a wide variety of climates.The balance of precipitation and evaporation in the Great Basin in the western interior of the U.S.A. has changed dramatically over the past 30,000 years, leaving dry lake beds and hypersaline lakes today where extensive lakes existed during glacial times. Water availability in the region largely depends upon precipitation brought by the winter storm track, suggesting that records of past hydrologic changes offer important insights into the behavior of the storm track in different climate states. The limited dating precision and sample resolution of present records, however, leave us with only a general sense of past hydrologic changes, and no well-dated records currently extend to past interglacial warm periods.This project will use cave deposits to reconstruct past water balance changes in northern Utah's Bonneville Basin, the site of the largest of the Great Basin's paleolakes. These deposits record changes in precipitation in the basin over much of the past 300,000 years and are able to be precisely dated, offering us the opportunity to construct a detailed and extended picture of past climate changes in the region. Importantly, stalagmites from Lehman Cave on the western edge of the basin record the last two interglacial warm periods, allowing us the opportunity to study the mean state and variability of the region's hydrology during times when the region was as warm as or slightly warmer than at present. This work represents an important contribution to our understanding of past climate changes and will help calibrate models seeking to reproduce past changes in order to better forecast the future. In addition, these records will assist us in interpreting past ecological changes in the basin and the history of early human occupation in the region.