NSFGEO-NERC:Collaborative Research: A New Mechanistic Framework for Modeling Rift Processes in Antarctic Ice Shelves Validated through Improved Strain-rate and Seismic Observations

板状冰山的崩解占南极冰盖冰块质量损失的很大一部分。除了这种直接的质量损失之外，崩解还可以通过减少冰架提供的阻力来进一步加速地面冰的向海流动，冰架是冰盖的浮动延伸部分。板状冰山比它们的厚度长得多，也宽得多，当被称为裂缝的全厚度裂缝与冰架边缘相交时，它们就形成了。冰架裂解的过程和驱动因素既不能很好地理解，也不能在冰流模型中得到准确的描述。因此，目前还不可能预测板状冰山何时可能形成，它们将有多大，以及随着气候变化可能会发生多大的崩解。这种预测能力的缺乏有两个重要的后果。首先，不可能有把握地预测从浮动货架上崩解造成的质量损失率。其次，不可能有把握地预测表状冰山的崩解将如何影响地面冰盖的质量损失。第二个结果是预测海平面随人类时间变化的不确定性的一个主要来源。在这个项目中，该团队旨在通过结合详细的观察和一套日益复杂的模型来研究冰架裂解的过程和驱动因素，以提高对列表冰山崩解的理解。这项工作将集中在一个自然实验室：由东南极布伦特冰架和斯坦科姆-威尔斯冰川舌形成的冰架系统，该冰架系统是南极洲拥有最长和最详细的观测记录之一。截至2020年初，两个活跃的裂谷系统正在布伦特冰架上传播，其中一个应该很快就会形成一个巨大的板状冰山。因此，有一个难得的机会来观察多个活跃的裂谷。该项目将利用这一情况，利用从各种空间仪器收集的遥感观测，对整个冰架，特别是活动裂谷尖端附近的速度和应变率场进行随时间变化的详细测量。在这些尖端，由于裂缝的存在，预计压力将加剧。这些观测将为一套冰流和破裂模型提供信息，这些模型将被开发并用于更好地理解裂缝如何传播，以及如何最好地在大规模冰流模型中表示裂缝传播。建模目标遵循的是一条发展道路，旨在产生一个能够模拟裂谷传播的社区冰流模型，并已与观测结果进行了测试。已经在活动裂谷附近收集的地震数据将提供有关裂谷过程的详细知识，并将补充遥感观测，并为建模工作提供信息。这一多方面观测和模型的组合旨在阐明冰架裂谷的基本过程，从而有助于提供必要的知识，以对冰盖演化和海平面上升做出可靠预测。该项目由国家科学基金会地球科学局(NSF/GEO)和英国国家环境研究委员会(UKRI/NERC)通过NSF/GEO-NERC牵头机构协议共同资助。该协定允许美国和英国提交一个单一的联合提案，并由该机构进行同行审查，该机构的调查员在预算中所占比例最大。在成功地共同确定奖项后，每个机构将为预算和调查人员的比例提供资金，这些资金与其自身的调查人员和工作的组成部分有关。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Calving of tabular icebergs accounts for a significant fraction of ice mass loss from the Antarctic Ice Sheet. In addition to this direct mass loss, calving may further accelerate the seaward flow of grounded ice by reducing resistive stresses provided by ice shelves, the floating extensions of the ice sheet. Tabular icebergs are much longer and wider than they are thick and form when full-thickness fractures known as rifts intersect the edges of an ice shelf. The processes and drivers of ice-shelf rifting are neither well understood nor accurately represented in ice-flow models. As a result, it is not currently possible to predict when tabular icebergs may form, how large they will be, and how calving may evolve as the climate changes. This lack of predictive capability has two important consequences. First, it is not possible to confidently project rates of mass loss due to calving from floating shelves. Second, it is not possible to confidently project how tabular iceberg calving will influence the mass loss from the grounded ice sheet. This second consequence is a major source of uncertainty in projections of sea-level rise over human timescales.In this project, the team aims to improve understanding of tabular iceberg calving using a combination of detailed observations and a suite of increasingly sophisticated models to study the processes and drivers of rifting in ice shelves. The work will focus on a natural laboratory: the ice-shelf system formed by the Brunt Ice Shelf and Stancomb-Wills Glacier Tongue, East Antarctica, which has one of the longest and most detailed observational records in Antarctica. As of early 2020, two active rift systems are propagating across the Brunt Ice Shelf, one of which should soon form a large tabular iceberg. Thus, there is a rare opportunity to observe multiple active rifts. The project will take advantage of this situation by employing remote-sensing observations collected from a variety of spaceborne instruments to make detailed time-dependent measurements of velocity and strain-rate fields across the ice shelf and, notably, in the vicinity of the active rift tips. At these tips, stresses are expected to intensify due to the presence of the rift. The observations will inform a suite of ice-flow-and-fracture models that will be developed and used to better understand how rifts propagate and how best to represent rift propagation in large scale ice-flow models. The modeling objective follows a development path that aims to yield a community ice-flow model capable of simulating rift propagation, and that has been tested against observations. Seismic data already collected in the vicinity of an active rift will provide detailed knowledge of rifting processes and will complement the remote sensing observations and inform the modeling efforts. This combination of multi-faceted observations and models aims to illuminate the fundamental processes of ice-shelf rifting, thereby contributing to the knowledge necessary to make reliable projections of ice-sheet evolution and sea-level rise.This project is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (UKRI/NERC) of the United Kingdom (UK) through the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own ivestigators and component of the work.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.