Properties of a marine ice layer under the Amery Ice Shelf, East Antarctica
Properties of a marine ice layer under the Amery Ice Shelf, East Antarctica
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DOI：
10.3189/002214309789470941
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发表时间：
2009-01-01
影响因子：
3.4
通讯作者：
Warner, Roland
中科院分区：
文献类型：
--
作者：
Craven, Mike;Allison, Ian;Warner, Roland
南极洲东部的Amery冰架在其接地线的南部边界附近经历了很高的基础融化速率，在那里80%的冰在漂浮240公里内融化。其中相当一部分后来以海冰的形式在下游重新冻结。这就在冰架的西北部分形成了一层厚达200米的海洋冰层，集中在一对纵向带中，一直延伸到裂冰锋约200公里。我们在同一条管线上相隔70公里的两个地点钻穿了东部的海洋冰带。我们确定，在参考密度为920 kg m（-3）的钻孔点之间，海冰的平均吸积率为1.1 +/- 0.2 ma(-1)，并推断上游的平均吸积率为1.3 +/- 0.2 ma（-1）。较深的海冰具有足够的渗透性，因此当钻头仍在冰架基地上方70-100米时，就可以进行水力连接。在这个海洋封闭深度以下，钻孔视频图像显示，可渗透的冰充满了水的空洞，单个的冰片融合在一起，而上层的海洋冰则是不可渗透的，有小的盐水细胞包裹体。我们推断，随着时间的推移，随着更多的海冰在冰架底部积聚，可渗透冰的最上层变得不可渗透。我们估计钻孔点之间的平均闭合速率为0.3 ma (-1)；上游的平均闭合速度更快，为0.9 m a（-1）。我们估计整个海洋冰层的平均孔隙率为14-20%，因此，更深的冰必须有更高的数值。高渗透性意味着海水可以相对自由地在材料中移动，我们提出，在这种海冰存在的地方，这使得冰架的深层部分特别容易受到海洋性质变化的影响。
The Amery Ice Shelf, East Antarctica, undergoes high basal melt rates near the southern limit of its grounding line where 80% of the ice melts within 240 km of becoming afloat. A considerable portion of this later refreezes downstream as marine ice. This produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front. We drilled through the eastern marine ice band at two locations 70 km apart on the same flowline. We determine an average accretion rate of marine ice of 1.1 +/- 0.2 m a(-1), at a reference density of 920 kg m(-3) between borehole sites, and infer a similar average rate of 1.3 +/- 0.2 m a(-1) upstream. The deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70-100 m above the ice-shelf base. Below this marine close-off depth, borehole video imagery showed permeable ice with water-filled cavities and individual ice platelets fused together, while the upper marine ice was impermeable with small brine-cell inclusions. We infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf. We estimate an average closure rate of 0.3 m a(-1) between the borehole sites; upstream the average closure rate is faster at 0.9 m a(-1). We estimate an average porosity of the total marine ice layer of 14-20%, such that the deeper ice must have even higher values. High permeability implies that sea water can move relatively freely through the material, and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties.