Doctoral Dissertation Research: High-Resolution, Late Quaternary Climate and Vegetation Record for the American Midwest: Isotope and Pollen Evidence from Stalagmites and Flowstone

来自冰盖和海洋核心的气候指标揭示了过去15万年及以后的重大全球气候变化。然而，人们对这些全球变化在区域和地方层面上的影响知之甚少。这项博士论文研究项目将使用来自三个地点的洞穴代用品，创建过去15万年来南印第安纳州的古环境记录。这一记录将被用来研究在冰川进退期间由于接近冰盖而发生的重大区域气候波动。由于对区域到地方尺度气候变化的因果机制的确切理解对于改进和检查全球气候模型的准确性至关重要，这项重点研究将有助于对长期气候动力学进行更广泛的探讨。这项研究还将对美国中西部地区的气候动力学研究产生积极影响，该地区的经济受到气候的强烈影响，因为该地区目前的全球气候模型难以复制过去气候的一些方面，如冰川晚期风场和花粉记录所指示的非相似气候。由于将在印第安纳州南部进行研究的洞穴靠近印第安纳州中部Laurentide冰盖在冰期极大期间达到其最南端的位置，因此冰缘以南非常陡峭的气候梯度和相关的气候变化应该很容易记录在气候替代物中，从而有助于基本了解冰盖附近的局部气候动态。该项目将提供中西部这一地区目前缺乏的急需的高分辨率古气候数据和现代降水同位素记录，从而有助于更全面地了解印第安纳州气候的自然变化，帮助科学家、政策制定者和公众处理与气候变化有关的问题。研究人员将通过NOAA古气候学数据库免费获取数据，同时将与洞穴所有者、学校和国家组织分享结果，传播给居民，以提高科学意识，传授保护脆弱洞穴环境的价值。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生能够建立强大的独立研究生涯。洞穴(石笋、钟乳石和花石等矿藏)可以保存洞穴上方地区环境变化的记录，特别有价值，因为它们提供了非常准确的铀系年龄，最高可达45万年。通过这个项目的实施，博士生的论文研究将得到该奖项的支持，他们将确定控制美国中西部现代降水同位素(增量-O-18、增量-H-2和d-过剩)值的过程以及每个过程的相对影响。他将通过分析三个洞穴中的石笋来创建古气候记录，利用捕获在花石中的花粉开发植被记录，扩展现有的地区性花粉记录，并将石笋和植被数据整合起来，产生单一的地区性古气候记录。洞穴碳酸盐氧同位素特征的变化将为降雨来源、雨量和季节性的变化提供线索，而对现代降水同位素特征的分析将提供导致这些变化的原因。碳同位素和紫外线激发的洞穴发光分析将反映土壤生产力的变化，而捕获在洞穴中的花粉将提供沉积时当地植被的证据。因此，对印第安纳州南部三个洞穴中的石笋的分析将产生重叠的古气候记录，这些记录将被结合起来确定地区气候模式和历史。组合的洞穴记录将从多个替代物中产生稳健的环境记录，其强度来自于整合空间上不同的样本。

Climate proxies from ice sheets and ocean cores have revealed major global climate changes during the past 150,000 years and beyond. Much less is known about the effects of these global changes at the regional to local level, however. This doctoral dissertation research project will create a southern Indiana paleoenvironmental record for periods of the past 150,000 years using cave-based proxies from three sites. This record will be used to examine the substantial regional climate fluctuations that occurred due to ice sheet proximity during glacial advances and retreats. Because a firm understanding of the causal mechanisms of regional- to local-scale climate variations is crucial to improving and checking the accuracy of global climate models, this focused study will help contribute to a much broader range of inquiry into long-term climate dynamics. This research also will have positive implications for research on climate dynamics in the Midwestern U.S., a region whose economy is strongly influenced by climate, because current global climate models for the region struggle to replicate some past aspects of climate, such as late glacial wind fields and non-analog climates indicated by pollen records. Because the caves to be studies in southern Indiana are close to locations where the Laurentide Ice Sheet reached its southernmost limit in central Indiana during glacial maxima, climate gradients that were very abrupt immediately south of the ice margin and associated climate changes should be readily recorded in climate proxies, thereby contributing to basic understanding about the localized climatic dynamics near ice sheets. This project will provide much-needed high-resolution paleoclimate data and modern precipitation isotope records that currently are lacking for this part of the Midwest, thereby helping to develop a more complete understanding of the natural variations of Indiana's climate to aid scientists, policy makers, and the public in dealing with issues associated with climatic variation and change. Data will be made freely accessible to researchers through the NOAA Paleoclimatology database, while results will be shared with cave owners, schools, and state organizations for dissemination to residents to improve scientific awareness and teach the value of preserving fragile cave environments. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.Cave speleothems (mineral deposits like stalagmites, stalactites, and flowstones) can preserve a record of environmental changes in the area above the cave and are particularly valuable because they offer very accurate uranium-series ages up to 450,000 years. Through the conduct of this project, the doctoral students whose dissertation research will be supported by this award will determine the processes controlling modern precipitation isotope (delta-O-18, delta-H-2, and d-excess) values for the Midwestern U.S. and the relative effect of each process. He will create a paleoclimate record by analyzing stalagmites from three caves, develop a vegetation record from pollen trapped in flowstones that extends existing regional pollen records, and integrate the stalagmite and vegetation data to produce a single regional paleoclimate record. Variations in the oxygen isotope characteristics of speleothem carbonate will offer clues to changes in rainfall source, amount, and seasonality, while analysis of modern day precipitation isotope signatures will provide insight into what caused these changes. Analyses of carbon isotopes and ultraviolet-stimulated speleothem luminescence will reflect variations in soil productivity, while pollen trapped in speleothems will provide evidence of local vegetation at the time of deposition. The analyses of stalagmites from three southern Indiana caves therefore will produce overlapping paleoclimate records that will be combined to determine regional climate patterns and history. The combined speleothem records will produce a robust environmental record from multiple proxies with strength drawn from integrating spatially distinct samples.