RIFT-TIP: Rates of Ice Fracture and Timing of Tabular Iceberg Production

从冰架上剥离的板状冰山约占每年南极洲失去的冰的一半。当全厚度裂缝(称为裂缝)在冰架上水平传播时，冰山就形成了。由此产生的冰山可能有数千平方公里大小，可能影响野生动物、航运和海洋循环，并可能改变剩余冰架的形状和稳定性，从而影响冰的排放和海平面上升。崩解的时间目前是不可预测的，在一些冰盖模型中只是表面上的，例如，一旦达到一定的厚度，就会移除冰。据观察，裂缝的传播速度非常快，每天可达几公里，或非常缓慢，几年甚至几十年停滞不前。虽然众所周知，冰架坍塌会导致冰川加速，但最近的观察也表明，适度的崩解事件直接和立即影响冰流和基本融化速度，这表明迫切需要限制这一过程的时间以及未来是否会加速。同时，在工程应用的推动下，裂缝近似方法的发展使得用数字表示离散裂缝成为可能，前提是小尺度和大尺度的行为可以用观测来校准。目前缺乏观测限制了这种类型的建模在冰川学中的价值。我们的研究结合了对南极洲布伦特冰架裂隙生长的直接观察和对相同冰样的实验室实验，这些实验结合在一起，在多个尺度上产生了前所未有的断裂过程细节。这一细节水平将应用于断裂问题，使用新的可伸缩相场模型，该模型允许使用裂缝边界处的扩散界面将微观过程映射到低分辨率冰盖尺度网格上。我们将在实验室观察裂缝如何在晶体水平上与冰相互作用，并在现场观察裂缝如何与冰架在千米水平上相互作用，以验证和测试这一模型。这将阐明裂缝增长背后的三维机制，以及控制其速率的物理冰特性。在冰盖模型中表示冰解过程的阶梯变化的改进在整个地球科学界都有好处，从需要估计冰架支撑应力和冰盖崩解对接地线动态的影响的冰盖建模人员，到依赖准确的冰架几何形状来限制淡水通量和海冰形成速度的大型地球系统建模人员。

Calving of tabular icebergs from ice shelves accounts for around half of all the ice lost from Antarctica each year. The icebergs form when full-thickness fractures (known as rifts) propagate horizontally through the ice shelf. The resulting icebergs can be thousands of square kilometres in size, can impact wildlife, shipping and ocean circulation and can modify the shape and stability of the ice shelves which remain, with a subsequent impact on ice discharge and sea level rise. The timing of calving is currently unpredictable and is only included superficially in some ice sheet models, for example by removing ice once a certain thickness is reached. Rifts have been observed to propagate very rapidly, at up to several kilometres per day, or very slowly, stagnating for years or even decades. Whilst it is well established that ice shelf collapse can lead to glacier acceleration, recent observations also show moderate calving events directly and immediately impacting ice flow and basal melt rate, indicating an urgent need to constrain the timing of this process and whether it will accelerate in the future. Simultaneously, developments in fracture approximation methods driven by engineering applications have made it possible to represent discrete fractures numerically, provided the behaviour at the small-scale and large-scale is calibrated with observations. Lack of observations currently limits the value of this type of modelling in glaciology. Our research combines direct observations of rift growth on the Brunt Ice Shelf in Antarctica with laboratory experiments on samples of the same ice, which when linked together produce unprecedented detail on the fracture process across multiple scales. This level of detail will be applied to the fracture problem using a new scalable phase-field model that allows microscale processes to be mapped onto a low-resolution ice-sheet-scale grid using diffuse interfaces at crack boundaries. We will conduct laboratory observations of how cracks interact with ice at the crystal level, and in situ observations of how rifts interact with the ice shelf at the kilometre level to validate and test this model. This will illuminate the three-dimensional mechanism behind rift growth and the physical ice properties that control its rate. A step-change improvement in how the calving process is represented in ice sheet models has benefits across the geoscience community, from ice sheet modellers who need to estimate ice shelf buttressing stress and the impact of calving on grounding line dynamics, to large scale earth-system modellers which rely on accurate ice shelf geometry to constrain freshwater fluxes and rates of sea ice formation.