Doctoral Dissertation Research: Comparing the Effects of Holocene and 20th Century Drought on Minnesota's Lakes

湖泊是独特的地理特征，是地形的决定性组成部分，也是景观变化的记录者。 它们在冰川起源的景观中很常见，例如上中西部，仅在密歇根州，威斯康星州和明尼苏达州就有37，900多个湖泊。 通过量化明尼苏达州湖泊面积和体积随时间的变化，本博士论文研究项目将调查地表水资源对气候变化的敏感性，并将提高对湖泊景观气候驱动动态的基本理解。 一个重要的项目目标是将20世纪30年代“沙尘暴”干旱的影响纳入长期背景。 第二个目标是更彻底地了解湖泊水位变化的沉积记录，因为过去湖泊水位的重建形成了用于验证计算机气候模型的少数几个全球分布的数据集之一。 为了实现这些目标，这项工作将（1）使用航空照片记录湖泊对20世纪30年代干旱的响应的空间变化，（2）获得明尼苏达州过去1,000年和中全新世温暖期干旱的规模和空间范围的证据（6,000年前），（3）比较全新世和世纪干旱的规模，频率和空间格局。 这项研究将以分层的方式进行，首先是对历史航空照片的分析，然后扩展到对湖泊沉积物的研究。 利用地理信息系统分析研究航空照片，博士生将测试有关湖泊对干旱的反应的假设，并创建一个与全新世干旱模式比较的底图。 接下来，将对现代湖泊沉积物进行分析，为随后对过去6,000年湖泊沉积物岩心的调查提供信息。 最后，该项目将结束一个详细的分析全新世沉积地层的一个横断湖泊的状态。 三个湖泊将被选为代表广泛的湖泊在明尼苏达州调查的探地雷达。 将在三个重点湖泊中的每一个湖泊的横断面上收集岩心，以确定地球物理调查所发现的湖泊水位过去变化的日期和地面沉积证据。 通过将沉积物岩心分析与全州范围的地球物理调查相结合，将绘制全新世湖泊地区的精确地图，为评估20世纪和未来干旱的空间范围和规模提供背景。地表水是一种重要但脆弱的资源，特别是湖泊对明尼苏达州来说具有经济和情感价值。 湖泊还充当古代降水量计，提供过去气候变化的记录。 由于区域降水和蒸发水平随时间而变化，湖泊面积和水量对气候变化作出反应，并构成景观对气候变化反应的重要组成部分。 该项目将评估“万湖之地”过去的这种变化范围，并将加强对水资源对气候变化敏感性的基本了解。 利用沉积物证据记录过去6,000年来湖泊面积的变化，为评估20世纪干旱期间明尼苏达州的低湖水位提供了基础。 全新世水位与历史水位之间的比较将检验这样一种观点，即即使是20世纪30年代的沙尘暴干旱也没有不同寻常的规模或持续时间，而且过去发生过更严重的事件。 该项目还将通过使用一系列分析技术的实践研究，为一名青年科学家提供培训。 作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立的研究生涯。

Lakes are unique geographic features, acting as a defining component of the terrain and as a recorder of landscape change. They are common in landscapes of glacial origin, such as the upper Midwest, where more than 37,900 lakes exist in Michigan, Wisconsin, and Minnesota alone. By quantifying changes in lake area and volume in Minnesota through time, this doctoral dissertation research project will investigate the sensitivity of surface water resources to climate change and will improve basic understanding of the climate-driven dynamics of lake-dotted landscapes. One important project goal is to put the effects of the 1930s "Dust Bowl" drought into a long-term context. A second goal is to more thoroughly understand the sedimentary record of lake-level change because reconstructions of past lake levels form one of the few globally distributed datasets used to validate computer climate models. To meet these goals, this work will (1) document the spatial variability of lake responses to the 1930s drought using aerial photos, (2) attain evidence for the magnitude and spatial extent of droughts in Minnesota over the past 1,000 years and during the Mid-Holocene warm-period (6,000 years ago), and (3) compare the magnitudes, frequency, and spatial patterning of droughts during the Holocene and 20th century. This research will be conducted in a hierarchical manner that begins with analyses of historic aerial photos and extends to studies of the sediments deposited in lakes. Using a geographic information system analysis to study air photos, the doctoral candidate will test assumptions about lake responses to drought and create a base map for comparison with Holocene drought patterns. Next, an analysis of modern lake sediments will be conducted to inform the subsequent investigation of lake sediment cores that span the past 6,000 years. Finally, the project will conclude with a detailed analysis of the Holocene sediment stratigraphy of a transect of lakes across the state. Three lakes will be chosen as representative of a broad array of lakes throughout Minnesota surveyed by ground-penetrating radar. Cores will be collected in transects across each of the three focal lakes in order to date and ground-truth sedimentary evidence of past changes in lake levels detected by the geophysical surveys. By combining the analyses of sediment cores with the statewide geophysical survey, accurate maps of Holocene lake areas will be produced that provide a context for evaluating the spatial extent and magnitude of 20th century and future droughts. Surface water is a vital but fragile resource and lakes in particular are economically and emotionally valuable to Minnesota. Lakes also act as ancient precipitation gauges providing a record of past climatic change. As regional precipitation and evaporation levels change over time, lake areas and volumes respond and constitute an important component of landscape response to climate change. This project will evaluate the past range of such changes in "the Land of 10,000 Lakes" and will strengthen basic understanding of the sensitivity of water resources to climate variability. Documenting changes in lake area during the past 6,000 years using sedimentary evidence provides a basis for evaluating the low lake levels in Minnesota during 20th century droughts. A comparison between Holocene and historic water levels will test the idea that even the 1930s Dust Bowl drought was not of unusual magnitude or duration and that even more severe events characterize the past. This project will also provide training to a young scientist through hands-on research using a range of analytical techniques. 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.