Impact of Supraglacial Lakes on Ice-Shelf Stability

冰上湖泊对冰架稳定性的影响

• 批准号: 1443126
• 负责人: Douglas MacAyeal
• 金额: $ 29.54万
• 依托单位: University of Chicago
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-09-01 至 2018-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1443126&HistoricalAwards=false
• 关键词: Impact; Supraglacial; Lakes; Ice; Shelf

Meltwater lakes that sit on top of Antarctica's floating ice shelves have likely contributed to the dramatic changes seen in Antarctica's glacial ice cover over the past two decades. In 2002, the 1,600-square-kilometer Larsen B Ice Shelf located on the Eastern side of the Antarctic Peninsula, for example, broke into thousands of small icebergs, which subsequently floated away as a result of the formation of more than 2,000 meltwater lakes on its surface over the prior decade. Our research project addresses the reasons why surface lakes form on Antarctic ice shelves and how these surface lakes subsequently contribute to the forces that may contribute to ice-shelf breakup like that of the Larsen B. Our project focuses primarily on making precise global positioning system (GPS) measurements of ice-shelf bending in response to the filling and draining of a surface lake on the McMurdo Ice Shelf. The observed vertical displacements (on the order of tens of centimeters) in response to lake filling will be used to calibrate and test computer simulation models that predict the response of ice shelves to surface lakes more generally and in a variety of future climate conditions. Our project will make hourly measurements of both vertical ice-shelf movements (using GPS surveying instruments) and of temperature and sunlight conditions (that drive melting) around a surface lake located close to the McMurdo Station airfield. Following this initial data-gathering effort, computer simulations and other more theoretical analysis will be undertaken to determine the suitability of the chosen McMurdo Ice Shelf surface lake as a field-laboratory for continued study. Ultimately, the research will contribute to understanding of the glaciological processes that link climate change to rising sea level. A successful outcome of the research will allow glaciologists to better assess the processes that promote or erode the influence Antarctic ice shelves have in controlling the transfer of ice from the interior of Antarctica into the ocean. The project will undertake two outreach activities: (1) web-posting of a field-activity journal and (2) establishing an open-access glaciological teaching and outreach web-sharing site for the International Glaciological Society.The proposed project seeks to experimentally verify a theory of ice-shelf instability proposed to explain the explosive break-up of Larsen B Ice Shelf in 2002. This theory holds that the filling and draining of supraglacial lakes on floating ice shelves induces sufficient flexure stress within the ice to (a) induce upward/downward propagating fractures originating at the base/surface of the ice shelf that (b) dissect the ice shelf into fragments that tend to have widths less than about half the ice thickness. The significance of narrow widths is that they promote capsize of the ice-shelf fragments during the break-up process. This capsize releases large amounts of gravitational potential energy (comparable to thousands of kilotons of TNT for the Larsen B Ice Shelf) thereby promoting explosiveness of the Larsen B event. The observational motivation for experimentally verifying the surface-lake mechanism for ice-shelf breakup is based on the fact that 2,000 surface lakes developed on the Larsen B Ice Shelf in the decade prior to its break up, and that these lakes were observed (via satellite imagery) to drain in a coordinated fashion during the day prior to the initiation of the break up.The field-observation component of the project will focus on a supraglacial lake on the McMurdo Ice Shelf where there is persistent summer season surface melting. The lake will be studied during a single provisional field season to determine whether grooming of surrounding surface streams and shorelines with heavy construction equipment will allow surface water to be manually encouraged to fill the lake. If successfully encouraged to develop, the McMurdo Ice Shelf surface lake will allow measurements of key ice-shelf flexure and stress variables needed to develop the theory of ice-shelf surface lakes without having to access the much more logistically demanding surface lakes of ice-shelves located elsewhere in Antarctica. Data to be gathered during the 6-week provisional field season include: energy- and water-balance parameters determining how the surface lake grows and fills, and various global positioning system measurements of the vertical bending of the ice sheet in response to the changing meltwater load contained within the surface lake. These data will be used to (1) constrain a computer model of viscoelastic flexure and possible fracture of the ice shelf in response to the increasing load of meltwater in the lake, and (2) determine whether continued study of the incipient surface-meltwater lake features on the McMurdo Ice Shelf provides a promising avenue for constraining the more-general behavior of surface meltwater lakes on other ice shelves located in warmer parts of Antarctica. Computer models constrained by the observational data obtained from the field project will inform energy- and water-balance models of ice shelves in general, and allow more accurate forecasts of changing ice-shelf conditions surrounding the inland ice of Antarctica. The project will create the first-ever ground-based observations useful for spawning the development of models capable of predicting viscoelastic and fracture behavior of ice shelves in response to supraglacial lake evolution, including slow changes due to energy balance effects, as well as fast changes due to filling and draining.

南极洲漂浮冰架顶部的融水湖可能导致了过去二十年南极洲冰川冰盖的巨大变化。例如，2002年，位于南极半岛东侧1600平方公里的拉森B冰架破裂成数千个小冰山，随后在南极洲形成2000多个融水湖，这些小冰山随后漂走。过去十年的表面。我们的研究项目解决了南极冰架上形成地表湖泊的原因，以及这些地表湖泊随后如何产生可能导致冰架破裂的力量，如拉森 B 那样。我们的项目主要致力于制作精确的全球定位系统（ GPS）测量麦克默多冰架上表面湖泊的充水和排水引起的冰架弯曲。观测到的湖泊充填引起的垂直位移（大约数十厘米）将用于校准和测试计算机模拟模型，该模型可以更普遍地预测冰架对表面湖泊在各种未来气候条件下的响应。我们的项目将每小时测量麦克默多站机场附近的一个表面湖周围的垂直冰架运动（使用 GPS 测量仪器）以及温度和阳光条件（导致融化）。在最初的数据收集工作之后，将进行计算机模拟和其他更多的理论分析，以确定所选的麦克默多冰架表面湖是否适合作为继续研究的现场实验室。最终，这项研究将有助于理解将气候变化与海平面上升联系起来的冰川过程。这项研究的成功结果将使冰川学家能够更好地评估促进或削弱南极冰架对控制冰从南极洲内部向海洋转移的影响的过程。该项目将开展两项外展活动：（1）在网上发布实地活动期刊；（2）为国际冰川学会建立一个开放获取的冰川学教学和外展网络共享网站。拟议项目旨在通过实验验证为解释 2002 年拉森 B 冰架的爆炸性破裂而提出的冰架不稳定理论。该理论认为，漂浮冰架上冰上湖泊的充水和排水会引起足够的弯曲应力(a) 引起源自冰架底部/表面的向上/向下传播的裂缝，(b) 将冰架解剖成宽度往往小于冰厚度约一半的碎片。宽度窄的重要性在于，它们会在破碎过程中促进冰架碎片的倾覆。这次倾覆释放了大量的重力势能（相当于拉森 B 冰架的数千吨 TNT），从而促进了拉森 B 事件的爆炸性。通过实验验证冰架破裂的地表湖泊机制的观测动机是基于这样一个事实：拉森 B 冰架在破裂前的十年里形成了 2,000 个地表湖泊，并且对这些湖泊进行了观测（通过卫星）该项目的实地观测部分将重点关注麦克默多冰架上的一个冰上湖，那里持续存在夏季表面融化。该湖将在一个临时野外季节进行研究，以确定使用重型施工设备对周围地表溪流和海岸线进行整治是否会允许手动鼓励地表水填充湖泊。如果成功鼓励开发，麦克默多冰架表面湖将能够测量开发冰架表面湖理论所需的关键冰架弯曲和应力变量，而无需进入对后勤要求更高的冰架表面湖位于南极洲其他地方。 在为期 6 周的临时野外季节期间收集的数据包括：决定地表湖泊如何生长和填充的能量和水平衡参数，以及冰盖垂直弯曲的各种全球定位系统测量值，以响应融水的变化地表湖中包含的负载。这些数据将用于（1）约束粘弹性弯曲和冰架可能破裂的计算机模型，以响应湖中融水负荷的增加，以及（2）确定是否继续研究早期的地表融水湖麦克默多冰架上的特征为限制位于南极洲较温暖地区的其他冰架上表面融水湖的更普遍行为提供了一个有希望的途径。 受现场项目观测数据约束的计算机模型将为一般冰架的能量和水平衡模型提供信息，并可以更准确地预测南极洲内陆冰周围冰架条件的变化。该项目将创建有史以来第一个地面观测，有助于催生模型的开发，这些模型能够预测冰架的粘弹性和断裂行为，以响应冰上湖泊的演化，包括由于能量平衡效应引起的缓慢变化以及快速变化由于填充和排水。