Airborne geophysical investigations of conditions at the bed of fast-flowing outlet glaciers of large Canadian Arctic ice caps

最近的研究表明，在评估山地冰川和冰盖对当代全球海平面上升的贡献时，最大的未知因素是北极大冰盖冰山崩解造成的质量损失率（Radic和Hock， 2011，《自然地球科学》）。北极最大的冰帽，实际上是南极和格陵兰冰盖之外最大的冰块，是加拿大北极群岛的冰帽。重要的是，新的研究结果表明，2004-2009年，质量损失率的急剧增加也使加拿大北极群岛成为除格陵兰岛和南极洲外全球海平面上升的最大单一贡献者（Gardner et al., 2011, Nature）。加拿大的每一个大冰盖都被分成一系列的流域，这些流域通过狭窄的、严重裂缝的快速流动的出口冰川流入峡湾，这些冰川分解了岛屿的边缘山脉。因此，科学家和决策者面临的一个主要问题是，这些冰盖将如何继续对21世纪预测的温度上升作出反应，并指出大气环流模型预测北极地区的温度上升幅度将明显大于低纬度地区。然而，由于缺乏对这些出口冰川床的几何形状和性质的了解，大型冰块的数值模拟受到限制。我们将利用机载破冰雷达、激光高度计、重力仪、磁力计和GPS仪器，从加拿大北极地区埃尔斯米尔岛和德文岛的冰帽出口冰川中获取地球物理数据。我们将重点关注每个流域的三个关键区域：严重裂缝的快速流动的出口冰川本身，冰盖内部和狭窄出口冰川之间的上部过渡区；以及加拿大高纬度北极峡湾顶部标志着向浮动冰舌过渡的接地区。我们的科学目标是：(a)确定冰面和冰下床的海拔高度；(b)确定基岩的特征，特别是基岩还是可变形的沉积物；(c)确定冰下融化的分布；(d)揭示内陆冰、出口冰川和接地带过渡带的基本特征变化；(e)在长达十年的时间尺度上对冰川出口崩解通量及其变率提供新的估计。这些信息，结合关于冰川出口表面运动的卫星数据集和我们早期对这些冰盖的区域尺度几何形状的观测，将为这些冰盖的数值模拟提供基本的边界条件，从而为它们如何在未来几十年对大气和海洋变暖做出反应，以及对海平面上升的影响提供依据。

Recent work has shown that the single largest unknown in assessing the contribution of mountain glaciers and ice caps to contemporary global sea-level rise is the rate of mass loss by iceberg calving from large Arctic ice caps (Radic and Hock, 2011, Nature Geoscience). The largest ice caps in the Arctic, and indeed the largest ice masses outside the Antarctic and Greenland ice sheets, are those of the Canadian Arctic islands. Importantly, new findings indicate that, for 2004-2009, a sharp increase in the rate of mass loss also makes the Canadian Arctic Archipelago the single largest contributor to global sea-level rise outside Greenland and Antarctica (Gardner et al., 2011, Nature). Each of these large Canadian ice caps is divided into a series of drainage basins that flow into fjords via narrow, heavily crevassed fast-flowing outlet glaciers which dissect the islands' fringing mountains. A major question for scientists and policymakers is, therefore, how these ice caps will continue to react to the temperature rises that are predicted for the 21st century, noting that Atmospheric General Circulation Models predict that temperature rise will be significantly greater in the Arctic than at lower latitudes. Numerical modelling of large ice masses is constrained, however, by a lack of knowledge of the geometry and nature of the bed of these outlet glaciers. We will acquire geophysical data from ice-cap outlet glaciers draining the large ice caps on Ellesmere and Devon islands in the Canadian Arctic using an airborne ice-penetrating radar, laser altimeter, gravimeter, magnetometer and GPS instruments. We will focus on three key areas of each drainage basin: the heavily crevassed fast-flowing outlet glaciers themselves, an upper transition zone between the ice-cap interior and the narrow outlet glaciers; and the grounding zone marking the transition to floating ice tongues at the head of some Canadian High-Arctic fjords. Our scientific objectives are: (a) to determine ice-surface and subglacial-bed elevation; (b) to characterize the substrate, in particular whether it is bedrock or deformable sediment; (c) to establish the distribution of subglacial melting; (d) to reveal basal character changes at the transition zones between inland ice, outlet glaciers and the grounding zone; (e) to provide new estimates of outlet glacier calving fluxes and their variability on up to decadal timescales. This information, integrated with satellite datasets on outlet-glacier surface motion and our earlier observations of the regional-scale geometry of these ice caps, will provide fundamental boundary conditions for the numerical modelling of these ice caps and, thus, how they may respond to atmospheric and ocean warming over the coming decades, with implications for sea-level rise.