Doctoral Dissertation Research: Determining the Phasing of Southern Laurentide Ice-Sheet Retreat and Deglacial Climate Forcings using Cosmogenic Radionuclide Dating

本研究将通过测量终末冰和退行冰边缘沉积物中冰川沉积巨石上的宇宙成因核素，改进美国威斯康星州南劳伦泰德冰盖（LIS）的退缩年代学。采样地点将跨越一系列气候环境和冰川床条件。拟议的研究将解决两个假设。(1)威斯康辛州初始冰缘退缩与美国南部其他地区同步。(2)北方夏季日晒的增加迫使LIS消冰。确定冰盖消退的阶段相对于日照、大气温室气体和全球气候的变化，对于理解导致冰川消融的机制非常重要。关于强迫冰川消融的主要假设是，北半球夏季日晒的增加引发了冰盖退缩和冰川消融。另外，热带海面温度和大气二氧化碳可能导致部分冰盖退缩，暗示它们是冰川消融的驱动因素。辨别这些冰川消融强迫假说的关键是精确的北半球冰盖边缘年代学。在北半球冰原中，南部冰原下降到最低纬度，应该具有最高的气候敏感性。因此，它是研究初始冰退缩机制的理想地点。然而，尽管有超过40年的年代学研究，南LIS年代学仍然不完整。在爱荷华州、伊利诺斯州、印第安纳州、俄亥俄州和新英格兰地区，美国冰川消退年代学得到了很好的限制，但在威斯康星州，末次盛冰期的永久冻土条件使传统的放射性碳测年方法无法使用，因此在初始冰川消退年代学上存在很大的差距。这个新的数据集将确定美国从威斯康星州终末冰碛撤退的时间，并填补美国南部去冰年代学的空白。将把这些数据与南大陆架边缘其他部分的年代学、冰川消融气候强迫和气候模式模拟进行比较，以确定北半球冰川消融的主要触发因素。更广泛的影响包括支持威斯康星大学麦迪逊分校博士候选人大卫·乌尔曼的博士研究和教育，以及对该领域和实验室本科生的教育，向他们介绍冰川地貌学、古气候学、地质年代学、古地理学和地球化学等领域。这项研究的结果和总体概念将通过威斯康辛州的冰河时代步道联盟（IATA）向公众展示，以改善该组织的教育重点，并在IATA步道沿线和各个游客中心发布更新的标志、标语牌和海报。威斯康星大学麦迪逊分校地质博物馆也将扩大威斯康星冰川消融历史的展览。

This study will improve the retreat chronology of the southern Laurentide Ice Sheet (LIS) in Wisconsin by measuring in-situ cosmogenic nuclides on glacially deposited boulders from terminal and recessional ice marginal deposits. Sample locations will span a range of climate settings and glacial bed conditions. The proposed research will address two hypotheses. (1) Initial ice margin retreat in Wisconsin was synchronous with the rest of the southern LIS. (2) Increasing boreal summer insolation forced LIS deglaciation. Determining the phasing of ice-sheet retreat relative to changes in insolation, atmospheric greenhouse gases, and global climate is important to understanding the mechanisms that give rise to deglaciations. The leading hypothesis for the forcing of deglaciations is that increasing Northern Hemisphere summer insolation initiates ice-sheet retreat and deglaciation. Alternatively, tropical sea surface temperatures and atmospheric carbon dioxide may lead portions of ice-sheet retreat, implicating them as the driver of deglaciation. Key to discerning between these deglacial-forcing hypotheses is a precise Northern Hemisphere ice-sheet margin chronology. Of the Northern Hemisphere ice sheets, the southern LIS descended to the lowest latitude and should have had the highest climate sensitivity. It is thus an ideal location to study the mechanisms driving initial ice retreat. Despite over 40 years of chronologic research, however, the southern LIS chronology remains incomplete. While the LIS retreat chronology is fairly well constrained in Iowa, Illinois, Indiana, Ohio, and New England, a substantial gap in the chronology of initial deglaciation exists in Wisconsin where permafrost conditions at the Last Glacial Maximum preclude the use of traditional radiocarbon dating methods. This new dataset will identify the timing of LIS retreat from terminal moraines in Wisconsin and fill a gap in the southern LIS deglacial chronology. Dates will be compared with other parts of the southern LIS margin chronology, deglacial climate forcings and climate model simulations to determine the major triggers of Northern Hemisphere deglaciation. Broader impacts include support of the doctoral research and education of Ph.D. candidate David Ullman at the University of Wisconsin-Madison along with the education of undergraduate students in the field and the laboratory, introducing them to the fields of glacial geomorphology, paleoclimatology, geochronology, paleogeography and geochemistry. Results and overall concepts from this research will be presented to the public via the Ice Age Trail Alliance (IATA) in Wisconsin to improve the educational focus of the organization with updated signs, placards and posters along the IATA trails and at various visitor centers. The University of Wisconsin-Madison Geology Museum exhibit on the deglacial history of Wisconsin will also be expanded.