Deformation and recrystallisation at the firn-ice transition - Micro structural simulations

雪冰过渡可能是极地冰盖中最重要的区域之一，同时也是最不了解的区域。在覆盖层的压力下，雪压缩成雪，然后在大约80-100米深的地方结冰。在此过渡处，封闭的空气之间失去连通性，封闭的空气被困在孤立的气泡中。被困的空气提供了宝贵的古气候记录，在极地深钻项目中可以测量。在气泡中夹带空气之前，空气与柱中较高水平处的空气开放连通，因此可以交换和平衡其气候特征。因此，冰和滞留空气之间存在年龄差异。正确理解孔隙闭合的物理过程以及由此产生的雪冰过渡对于正确解释极地冰盖的气候记录至关重要。通过开发先进的显微结构分析工具，阿尔弗雷德·韦格纳研究所（AWI）的研究人员已经能够证明，雪冰过渡的压实过程比迄今为止所认为的要复杂得多，因为动态再结晶是一级过程。与此同时，图宾根和巴塞罗那的工作人员开发了数值代码，使用全场方法模拟再结晶和高度异质晶体塑性变形的复杂相互作用。拟议的项目结合了AWI，蒂宾根和巴塞罗那的专业知识（分析和建模工具），对复杂的复合冰与空气的变形和再结晶进行数值模拟。数值模拟结合对天然冰样的分析，将有可能约束的相对贡献的众多操作过程及其对冰和积雪的微观结构和力学性能的影响。反过来，这将提供更好的洞察力，在冰中的空气，这是古大气记录在极地冰盖。

The firn-ice transition is probably one of the most important and, at the same time, least understood regions in polar ice sheets. Under pressure of overlying layers, snow compacts to firn and then to ice at about 80-100 m depth. At this transition connectivity is lost between enclosed air, which becomes trapped in isolated bubbles. Trapped air provides an invaluable paleoclimate record, measurable in polar deep-drilling projects. Before entrapment of air in bubbles, air is in open communication with air at higher levels in the column and can thus exchange and equilibrate its climate signature. There is thus a difference in age between ice and trapped air. Correct understanding of the physical process of porosity closure and, hence, the firn-ice transition is imperative for the correct interpretation of the climate record in polar ice sheets.By developing advanced microstructural analysis tools, researchers of the Alfred Wegener Institute (AWI) have been able to show that the compaction process at the firn-ice transition is more complex than hitherto assumed, as dynamic recrystallisation is a first-order process. In the meantime, workers in Tübingen and Barcelona have developed numerical code to simulate the complex interplay of recrystallisation and highly heterogeneous crystal-plastic deformation, using the full-field approach. The proposed project combines the expertise (in both analytical and modelling tools) of the AWI, Tübingen and Barcelona to numerically model deformation and recrystallisation of the complex composite ice with air. The numerical simulations, in combination with analyses on natural ice sample, will make it possible to constrain the relative contributions of the numerous operating processes and their impact on microstructure and mechanical properties of ice and firn. This, in turn, will provide better insight in the entrapment of air in ice, which is where palaeo-atmosphere is recorded in polar ice sheets.