Sediment Transport by the Laurentide Ice Sheet: Testing the Bed-Deformation Hypothesis Using Till Microstructural Characteristics

劳伦太德冰盖的沉积物输送：利用 Till 微观结构特征检验河床变形假说

• 批准号: 0444921
• 负责人: Neal Iverson
• 金额: --
• 依托单位: Iowa State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-06-01 至 2010-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0444921&HistoricalAwards=false
• 关键词: Sediment; Transport; Laurentide; Ice; Sheet

AbstractVast sheets of till were deposited at the bases of Pleistocene ice sheets, but the mode of transport of this sediment is uncertain. A leading hypothesis attributes sediment transport to shear deformation of subglacial till, resulting in high till fluxes and landforms emblematic of glaciation, such as drumlins and moraines. Shearing of weak, subglacial till may have also helped instigate fast flow of ice sheets and consequent climate change. Although till microstructural characteristics provide a high-resolution, temporally-integrated record of deformation, their relationship to strain magnitude has been essentially unknown. The result is that magnitudes and patterns of deformation necessary to test the bed-deformation hypothesis have not been extracted from the geologic record.We have studied the evolution of till microstructural characteristics as a function of shear-strain magnitude with a ring-shear device that shears a large till specimen to high strains ( 100). Results indicate several quantitative indices of shear strain: particle-fabric strength and microshear orientation measured optically, clay-mineral fabric strength measured with x-ray texture goniometry, and fabric strength defined by orientations of maximum magnetic susceptibility. Calibrations of these indices to strain magnitude provide tools previously unavailable for interpreting origins of basal tills.Support is requested to apply our calibrations to three tills that we have studied experimentally and that are thought to have sheared beneath the Laurentide Ice Sheet: the Batestown Till of the Lake Michigan Lobe, the Horicon Till of the Green Bay Lobe, and the Douglas Till of the Lake Superior Lobe. The first of these has been modeled as an archetype for the bed-deformation hypothesis; the second has been sculpted into drumlins.frequently attributed to bed deformation; the third has an unusual microfabric signature. These tills will be densely sampled along multiple vertical profiles. Microstructural characteristics will be measured to determine magnitudes and patterns of deformation. Three criteria will be met if the bed sheared pervasively to at least moderate strains: the maximum shear strain detectable with microstructural characteristics (20-40) should be indicated over most of the till thickness; micro-shears should be visible (after etching of carbonate) indicative of subglacial rather than englacial deformation, and till surrounding large clasts should contain microstructural elements deflected symmetrically by clast rotation. These data will provide the most complete geologic test, to date, of the bed-deformation hypothesis.Broader Impacts: This research will benefit other disciplines and provide a vehicle for education and outreach. Owing to the potential influence of bed deformation on fast glacier flow, this project will impact efforts to model ice sheet-climate interactions. It could also benefit other fields, in which anisotropy of sheared granular materials is commonly a central issue, including structural geology, petroleum geology, and geotechnical engineering. Also, this research will support the education of three graduate students: 84% of the project.s direct costs will provide stipends, tuition, travel, and research support for these students. Results of the work will be integrated into undergraduate teaching. In addition, as we have demonstrated with our research beneath the Svartisen Ice Cap, this project can serve as a platform for emphasizing to nonscientists the role of glaciers in Earth.s past and modern environment.

摘要更新世冰盖底部沉积了大量冰碛物，但这些沉积物的运输方式尚不确定。一个主要的假说将沉积物的搬运归因于冰下冰碛物的剪切变形，从而导致冰碛物的高通量和冰川作用的地貌，如鼓丘和冰碛。冰下冰碛物的剪切也可能促使冰盖快速流动，从而导致气候变化。虽然直到显微结构特征提供了一个高分辨率，时间积分记录的变形，它们的关系，应变幅度一直是基本未知的。其结果是，从地质记录中还没有提取出检验河床变形假说所必需的变形量和变形模式。我们研究了冰碛物微结构特征随剪切-应变量的变化，所用的环剪装置将一个大冰碛物试样剪切到高应变（100）。结果表明几个定量指标的剪切应变：颗粒织物强度和微剪切取向光学测量，粘土矿物织物强度与X射线纹理测角法测量，和织物强度定义的最大磁化率的方向。这些指标的应变幅度校准提供了以前无法解释的起源basal tills.Support的支持，要求我们的校准应用到三个tills，我们已经研究了实验，被认为有剪切下的劳伦提斯冰盖：密歇根湖叶的Batestown冰碛，霍里孔冰碛的格林湾叶，和道格拉斯冰碛的上级叶。其中第一个已被建模为床变形假说的原型;第二个已被雕刻成鼓，经常归因于床变形;第三个有一个不寻常的微组构签名。将沿沿着多个垂直剖面对这些层积进行密集采样。将测量微观结构特征，以确定变形的幅度和模式。如果岩层被普遍剪切到至少中等应变，则将满足三个标准：在大部分冰碛厚度上应指示出可通过显微结构特征（20-40）检测到的最大剪切应变;显微剪应可见（在碳酸盐侵蚀后）表明冰下而不是冰内变形，而在大碎屑周围的冰碛物应包含因碎屑旋转而对称偏转的微结构元素。这些数据将提供迄今为止对河床变形假设最完整的地质测试。更广泛的影响：这项研究将使其他学科受益，并为教育和外展提供工具。由于河床变形对快速冰川流动的潜在影响，该项目将影响建立冰盖-气候相互作用模型的努力。它也可以有益于其他领域，其中剪切颗粒材料的各向异性通常是一个中心问题，包括构造地质学，石油地质学和岩土工程。此外，这项研究将支持三名研究生的教育：84%的项目的直接成本将为这些学生提供津贴，学费，旅行和研究支持。工作成果将纳入本科教学。此外，正如我们在斯瓦蒂森冰帽下的研究所证明的那样，这个项目可以作为一个平台，向非科学家强调冰川在地球过去和现代环境中的作用。