The Onset of Recrystallization in Polar Firn, Summit, Greenland, June 2017

Constraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13m in a Summit, Greenland (7235'N, 3825'W) firn core were subjected to creep tests at -14ºC and 0.21MPa compressive stress to strains of 7, 12, 18, and 29%. Microstructural analyses using thin-section imaging and microcomputed x-ray tomography (micro-CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain-induced boundary migration (SIBM) and nucleation and growth starting at ~7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth.约束动态再结晶（dynamic recrystallization，DRX）的起始过程及其对粒雪（firn）力学性能的影响，对于粒雪致密化模拟至关重要。为此，我们取自格陵兰萨米特（Summit）地区（北纬72°35′，西经38°25′）一处13米深度的粒雪冰芯样品，在-14℃、0.21MPa压应力条件下开展蠕变试验，将样品应变分别控制至7%、12%、18%与29%。通过薄片成像与显微X射线计算机断层扫描（microcomputed x-ray tomography，micro-CT）开展的微观结构分析显示：随着应变升高，粒雪的晶粒尺寸减小、比表面积与连通性降低，同时孔隙率下降伴随密度升高。上述结果表明，蠕变条件下的粒雪中确实发生了动态再结晶，且应变诱导晶界迁移（strain-induced boundary migration，SIBM）以及形核与长大过程在应变约7%时即已启动。动态再结晶会导致晶粒被拉长、晶粒尺寸减小，并形成择优晶体取向，这表明本次观测到的动态再结晶同时通过应变诱导晶界迁移与形核长大两种机制发生。