Ice shelf stress response to large iceberg calving

南极冰盖上的冰流失被大陆大部分边缘的漂浮冰架缓冲。冰架就像天然水坝一样，限制着陆地冰从南极洲输送到南大洋，因此在预测南极冰对海平面上升的贡献时，冰架的稳定性是非常重要的。在大型冰山崩解后的几年里，冰架可能会变得不稳定；例如，1995年1月，拉森B冰架(LBIS)崩解了一座面积为1720平方公里的冰山，并逐渐后退，直到2002年初几周内坍塌；随着LBIS的移除，其支流冰川加速并向海洋排放更多的冰。虽然冰架坍塌的后果得到了很好的认识，但在冰架崩解后从稳定冰架向不稳定冰架转变的过程中，人们对此知之甚少。到目前为止，我们很少有机会研究这样的过程，因为大规模的冰山崩解是罕见的。因此，这项紧急建议试图通过动员一项研究来解决这个问题，该研究是在最近拉森C冰架(LCIS)的一次冰解事件之后进行的。2017年7月12日，LICIS冰山崩解，成为有记录以来最大的冰山之一，命名为A68，面积为5800平方公里(占LICIS冰山面积的12%)，在经历了3.5年的裂谷传播后，与大陆架分离。预测在A68航班失事后剩余的LCIS将如何演变是我们项目的主要动机，该项目是一项关于LCIS的预测数值模拟、卫星遥感和现场地球物理测量的综合活动。许多对冰流的模拟强调了冰川破坏在决定LC IS未来稳定性方面的重要作用：在新的伸展状态下，大陆架现有的弱点(如表面和底部裂缝)往往会打开。然而，冰架结构的局部不均一性可能会使这种反应复杂化，而且冰架反应和稳定的时间尺度也不清楚。为了解决这些含糊不清的问题，我们的目标是捕捉LCIS的早期反应，以解决这些含糊的问题，因此我们申请紧急资金。我们的项目考虑了两个假设：1)对LCIS的压力增加将逐渐提高产冰率，特别是在大陆架已经被裂缝破坏的情况下。2)随着陆架的适应，LCIS的响应会稳定下来，但对长期稳定性的准确预测需要最早响应的约束。我们组建的团队是4种关键方法的专家，集成起来检验我们的假设。我们将使用：i)卫星图像，通过绘制裂缝模式的变化来测量冰架表面的应力演变；ii)将部署在LCIS上的地震测量，以测量冰架内部深处的损害变化；iii)也部署在LCIS上的GPS传感器，以记录冰架运动的短期波动；以及div)数值模拟，以综合所有数据和预测A68冰山崩解之前、之后和之后的损害演变。卫星分析和数值模拟将在10月初项目启动时开始，现场部署将在最早的后勤机会进行(将与英国南极调查局确认，但可能在2017年11月)。这一重要的初步评估将作为2018年前进一步拨款申请的基础，其中将包括部署一套全面的实地工具。我们的项目初步描述了LC IS的新压力状态，为未来的任何研究提供了参考基线。最直接的好处将是对A68产犊事件的具体了解，以及它对其余LC IS稳定性的影响，但我们也提高了对任何同等产犊过程所涉及的机制的理解。

Ice loss from the Antarctic ice sheet is buffered by the floating ice shelves that fringe much of the continent. Acting like natural dams, ice shelves restrict the delivery of terrestrial ice from Antarctica into the southern oceans, therefore the stability of ice shelves is highly important when predicting the contribution of Antarctic ice to sea-level rise. Ice shelves can destabilise in the years following the calving of large icebergs; for example, in January 1995, Larsen B Ice Shelf (LBIS) calved an iceberg 1720 sq.km in area, and progressively retreated until collapsing in a matter of weeks in early 2002; with the removal of LBIS, its tributary glaciers were seen to accelerate and discharge more ice into the ocean. While the consequences of shelf collapse are well-appreciated, the processes involved in the transition from stable to unstable ice shelves following calving are poorly understood. To date, we have had few opportunities to study such processes because large-scale iceberg calving is rare. This urgency proposal therefore seeks to address this issue, by mobilising a study in the aftermath of a recent calving event on Larsen C Ice Shelf (LCIS). On 12th July 2017, LCIS calved one of the largest icebergs ever recorded; termed A68, this iceberg has an area of 5800 sq.km (12% of LCIS) and separated from the shelf following 3.5 years of rift propagation. Predicting how the remaining LCIS will evolve following the loss of A68 is the key motivator for our project, which is an integrated campaign of predictive numerical modelling, satellite remote sensing and in situ geophysical survey on LCIS. Many simulations of ice flow highlight the important role of englacial damage in determining the future stability of LCIS: existing weaknesses (such as surface and basal crevasses) in the shelf would tend to open in the new extensional regime. However, local heterogeneities in the structure of the ice shelf may complicate this response, and the timescale on which the shelf will react and stabilise is also unclear. To resolve these ambiguities, it is our goal to capture the early-stage response of LCIS to resolve these ambiguities hence our application for urgency funding.Our project considers two hypotheses:1) Increased stresses on LCIS will progressively increase calving rates, particularly where the shelf is already damaged by crevasses. 2) The response of LCIS stabilises as the shelf adapts, but accurate forecasts of long-term stability require constraint of the earliest responses.Our assembled team are experts in 4 key methods, integrated to test our hypotheses. We will use:i) satellite imagery, to measure stress evolution at the surface of the ice shelf, by mapping changes in crevasse patterns,ii) seismic surveys, to be deployed on LCIS, to measure variations in damage at depth within the shelf,iii) GPS sensors, also deployed on LCIS, to record short-term fluctuations in the motion of the shelf, andiv) numerical modelling, to integrate all data and predict how damage evolved before, through and after the calving of the A68 iceberg.Satellite analysis and numerical modelling will commence at the initiation of the project in early October, with field deployment taking place at the earliest logistical opportunity (to be confirmed with the British Antarctic Survey, but likely in November 2017). This vital initial appraisal will serve as the basis of further grant applications through 2018, which will include the deployment of a comprehensive suite of field instruments.Our project offers an initial description of the new stress-state for LCIS, providing a reference baseline for any future study. The most immediate benefits will be for the specific understanding of the A68 calving event, and its implications for the stability of the remaining LCIS, but we also improve the understanding of the mechanisms involved with any equivalent calving process.