Seismological Investigation of Columbia Glacier Calving Mechanics

山脉冰川和小冰帽共同构成了全球陆地冰的重要组成部分(阿拉斯加和邻近的加拿大冰川占世界冰川面积的13%，不包括南极洲和格陵兰)。尽管物质平衡没有得到很好的解决，但最近的证据表明，阿拉斯加冰川凭借其大的集体面积、快速反应的特点和高质量通量，导致现代海平面上升的速度大约是之前认为的七倍。阿拉斯加冰川的大部分质量损失来自潮水冰川，它们时断时续地进退，长时间的进退和稳定期被快速的、明显不可逆转的退缩所打断。在退缩期间，潮水冰川物质平衡是由鲜为人知的冰山崩解过程主导的，而不是直接的气候强迫，进入海洋的冰通量可能非常大。哥伦比亚冰川目前正以每年约22千米的速度向海洋排放冰山。因此，潮水崩解是现代冰川变化和海平面上升的一个重要因素，但人们对此知之甚少。首席调查人员提出了哥伦比亚冰川的工作计划，其中有两个重点，每个重点都致力于增加对冰川崩解在潮水冰川消退中的作用的理解。首先，他们将继续他们的航空摄影计划，因为冰川末端通过一个主要的海峡狭窄后退。第二，他们将对冰川形成过程进行被动地震调查。这项工作的动机是小型冰川(特别是阿拉斯加潮水冰川)对现代全球冰川质量平衡和海平面变化的重要性，以及需要更好地了解崩解在潮水冰川动力学中的作用。此外，哥伦比亚冰川是阿拉斯加潮水冰川中最后一个退缩的冰川，部分处于退缩阶段，处于其水道几何形状的关键位置。研究潮水崩解的机会在几十年后就会消失，所以现在利用它是很重要的。潮水崩解是冰川动力学和冰/海洋相互作用的重要组成部分，但人们对此仍知之甚少。由于缺乏相关观察和对相关观察应该是什么缺乏了解，阻碍了更好地了解产犊的进展。这项研究将提供一个非常成功的航空摄影观测计划的延续，该计划产生了迄今为止收集到的最长和最详细的潮水冰川运动学观测记录，并构成了目前已知的大部分潮水动力学的基础。首席调查人员将开始对哥伦比亚冰川进行一系列新的地震观测，试图详细发现导致崩解的断裂事件的时间、位置和几何形状。以前对潮水冰川进行了有限的地震调查，包括在哥伦比亚冰川上，但数据采集和处理方面的现代进步将使他们能够更有效地利用地震学，并更详细地观察裂缝和崩解。拟议活动产生的更广泛的影响：除了在现代海平面变化中的作用外，潮水崩解在过去的全球冰盖和气候动态中也发挥了关键作用。来自北大西洋的大量证据表明，被动的间歇性崩解是劳伦蒂德冰盖(LIS)迅速解体的关键，但由于缺乏数学崩解关系，目前的LIS重建并未尝试纳入崩解过程。然而，冰盖模型师渴望得到有关冰解过程的信息，以便纳入LIS模型。此外，对潮水崩解的了解为南极和格陵兰海洋末端冰块提供了有用的交叉知识，因为那里发生崩解的频率要低得多。

Mountain glaciers and small ice caps collectively constitute a significant fraction of global land ice (Alaska and adjacent Canadian glaciers comprise 13% of the world's glacier area, exclusive of Antarctica and Greenland). Despite poorly resolved mass balances, recent evidence shows that Alaskan glaciers, by virtue of their large collective area, rapid response characteristics, and high mass fluxes, contribute to modern sea level rise at a rate approximately seven times greater than previously thought. Much of the mass loss from Alaskan glaciers comes from tidewater glaciers, which advance and retreat episodically, with long periods of advance and stability punctuated by rapid, apparently irreversible retreat. During retreat, tidewater glacier mass balance is dominated by the poorly understood process of iceberg calving, rather than direct climate forcing, and the magnitude of ice flux into the ocean can be very large. Columbia Glacier is presently discharging icebergs into the ocean at a rate of approximately 22 km3 yr-1. Thus, tidewater calving is an important, but poorly understood element of modern glacier change and sea level rise. The Principal Investigators present work plan at Columbia Glacier with two points of focus, each dedicated to an increased understanding the role of calving in tidewater glacier retreat. First, they will continue their program of aerial photography as the glacier terminus retreats through a major channel constriction. Second, they will conduct a passive seismological investigation of calving processes. This work is motivated by the importance of small glaciers (especially Alaskan tidewater glaciers) to modern global glacier mass balance and sea level change and by the need to better understand the role of calving in tidewater glacier dynamics. Additionally, Columbia Glacier is the last of the Alaskan tidewater glaciers to retreat, and is partly through its retreat phase and at a critical location in its channel geometry. This opportunity to study tidewater calving will be gone in a few decades, so it is important to take advantage of it now. Tidewater calving is an important part of glacier dynamics and ice/ocean interaction, but is still poorly understood. Progress toward a better understanding of calving is handicapped by a lack of relevant observations and lack of knowledge of what the relevant observations should be. This research will provide a continuation of a highly successful observational program of aerial photography that has yielded the longest and most detailed observational record of tidewater glacier kinematics ever assembled, and has formed the basis of much of what is presently known about tidewater dynamics. The Principal Investigators will start a new series of seismic observations on Columbia Glacier in an attempt to discover in detail the timing, location, and geometry of fracture events leading to calving. Limited seismic investigations have been previously made on tidewater glaciers, including on Columbia Glacier, but modern advances in data acquisition and processing will allow them to use seismology much more effectively and to look at fracture and calving in much greater detail. Broader impacts resulting from the proposed activity: In addition to its role in modern sea level change, tidewater calving played a critical role in global ice sheet and climate dynamic in the past. Ample evidence from the North Atlantic Ocean suggests that passive episodic calving was essential in the rapid breakup of the Laurentide Ice Sheet (LIS), but current LIS reconstructions do not attempt to incorporate calving processes due to the lack of a mathematical calving relationship. However, ice sheet modelers are eager for input concerning calving processes to incorporate in LIS models. Additionally, an understanding of tidewater calving provides useful crossover knowledge for Antarctic and Greenland marine-ending ice masses where calving occurs much less frequently.