Evidence of dynamic recrystallization in polar firn
Evidence of dynamic recrystallization in polar firn
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DOI：
10.1029/2008jb005583
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发表时间：
2009-05
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通讯作者：
S. Kipfstuhl;S. H. Faria;N. Azuma;J. Freitag;I. Hamann;P. Kaufmann;H. Miller;Karin Weiler;F. Wilhelms
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作者：
S. Kipfstuhl;S. H. Faria;N. Azuma;J. Freitag;I. Hamann;P. Kaufmann;H. Miller;Karin Weiler;F. Wilhelms
[1]对南极洲德龙宁毛德地欧洲冰芯取芯计划钻探点的极地积雪进行了微结构分析。从微观分辨率的积雪结构的图像中得出的结果表明，动态重结晶是活跃的积雪在所有深度，它主导的微观结构的演变时，积雪密度超过730 kg/m3的临界值（覆盖雪荷载<$0.2 MPa）。在雪冰过渡（密度约820 kg/m3）的微观结构的特点是许多小颗粒和凸起或不规则形状的晶界。超过一半的晶粒显示出亚晶界。因此，应变引起的晶界迁移是描述不规则晶粒结构的基本特征。与以前的研究一致，已观察到显着的晶粒生长与深度的样品中最大的晶粒。然而，我们的微观分析表明，如果考虑所有直径大于65 μ m的晶粒，则晶粒随深度的生长实际上消失。这一结果反映了这样一个事实：最大颗粒的生长被旧颗粒的收缩和细分以及新颗粒的产生所导致的颗粒尺寸减小所抵消。因此，以前的结论，在极性积雪中的晶粒生长基本上是类似于正常的金属和陶瓷烧结体中的晶粒生长和存储的应变能是小的晶界能量相比，可以不再支持。此外，我们的观察表明，在极地冰盖的动态重结晶的开端是不敏感的温度到目前为止，假设。由于测量技术的平均晶粒尺寸的变化的讨论是必要的。
[1] Microstructural analyses have been performed on polar firn from the European Project for Ice Coring in Antarctica drilling site in Dronning Maud Land, Antarctica. The results derived from images of the firn structure in microscopic resolution indicate that dynamic recrystallization is active in firn at all depths, and it dominates the evolution of the microstructure when the firn density exceeds a critical value of 730 kg/m3 (overburden snow load ∼0.2 MPa). At the firn-ice transition (density ∼820 kg/m3) the microstructure is characterized by many small grains and bulged or irregularly shaped grain boundaries. More than half of all grains show subgrain boundaries. Thus, strain-induced boundary migration is an essential feature to describe the irregular grain structure. In agreement with previous studies, significant grain growth has been observed with depth for the largest grains in the samples. However, our microscopic analysis reveals that the grain growth with depth in fact vanishes if all grains larger than 65 μm in diameter are taken into account. This result reflects the fact that the growth of the largest grains is counteracted by grain size reduction by shrinking and subdivision of old grains, as well as production of new grains. Consequently, previous conclusions that grain growth in polar firn is essentially analogous to normal grain growth in metallic and ceramic sinters and that the stored strain energy is small in comparison with grain boundary energy can no longer be supported. Additionally, our observations show that the incipience of dynamic recrystallization in polar ice sheets is not as sensitive to temperature as supposed so far. A discussion of the change of the mean grain size due to the measuring technique is imperative.