Marine geophysical and geological investigations of past flow and stability of a major Greenland ice stream in the Late Quaternary

最近对格陵兰冰盖周围的观测表明，将冰盖排入海洋的快速流动的冰川——“冰流”正在变薄、加速，在某些情况下，它们的浮动边缘正在解体。目前尚不清楚这是自然循环的一部分，还是对人类引起的气候变化的反应。冰流很重要，因为它们是今天格陵兰冰盖释放的大部分冰的来源，在过去也扮演着类似的重要角色。如果冰流速度增加，更多的冰将被输送到格陵兰岛周围的海洋，结果是全球海平面将上升。冰流是冰山和融水流入海洋的主要机制。因此，它们影响海洋环流和气候。对格陵兰冰盖融化对北大西洋影响的计算机模拟表明，这种融化产生的淡水径流可能严重削弱甚至停止热盐环流，而热盐环流是西欧大部分地区相对温和气候的最终原因。因此，重要的是要了解格陵兰冰盖的这些变化是今天独有的，还是在晚第四纪（过去5万年左右）的“最近”地质历史中，即在任何可能的人类对气候的影响之前，格陵兰冰流发生了类似的变化。了解是什么控制着这种冰流的快速流动，以及什么可能导致它们加速或减速，这一点也很重要。在这个项目中，我们建议解决一个主要冰流的过去行为的问题，以排出现代格陵兰冰盖：雅各布港Isbrae在格陵兰中西部。雅各布港冰流是目前格陵兰冰盖上速度最快的冰流。它每年以几公里的速度流动，约占冰盖总冰量的7%。因此，它对今天的冰盖产生了重大影响，在过去可能也有类似的影响。我们在这个项目中的总体科学目标是重建Jakobshavns Isbrae在晚第四纪时期的行为，特别是自距今约2万年以来。我们将收集Jakobshavns Isbrae近海的海洋地球物理数据和海底沉积物的岩心，以回答有关这个冰流存在多久的问题；在过去的冰河时期，它的大小是多少？它最后一次从格陵兰大陆架撤退的时间和速度；是什么过程导致了冰流的高流速？对冰流位置的主要控制是什么？与Jakobshavns Isbrae相关的沉积性质是什么？在现代之前，它是否经历了大规模的崩溃？该项目的研究结果将显著促进以下领域的科学认识：(1)格陵兰冰盖的主要冰流Jakobshavns Isbrae在最近地质历史（过去5万年）中的行为及其在这一时期整个冰盖演变中的作用。特别是如果雅各布港岛过去曾经历过类似于今天格陵兰岛周围发生的大规模解体事件；(2)冰流快速流动的机制；（三）导致冰流形成和停止的因素；(4)冰流下沉积物的性质及其与冰流高速的关系。这些结果将与冰川学、海洋学和气候模拟等相关领域的工作者高度相关。

Recent observations from around the Greenland Ice Sheet have shown that the fast flowing glaciers - 'ice streams' - which drain the ice sheet into the sea are thinning, speeding up and in some cases their floating margins are disintegrating. It is unknown if this is part of a natural cycle or if it is a response to human-induced climate change. The ice streams are important because they are responsible for most of ice discharged from the Greenland Ice Sheet today and would have played a similar important role in the past. If ice-stream velocities increase, more ice will be delivered to the oceans around Greenland, and as a consequence global sea level will rise. Ice streams are the main mechanism by which icebergs and meltwater are delivered to the ocean. They therefore affect ocean circulation and climate. Computer simulations of the effects of melting of the Greenland Ice Sheet on the North Atlantic have shown that the freshwater runoff produced by this melting could seriously weaken or even stop the thermohaline circulation which is ultimately responsible for the relatively mild climate of much of western Europe. It is therefore important to understand if these changes to the Greenland Ice Sheet are unique to the present day or if similar changes have occurred to Greenland ice streams in the 'recent' geological past of the Late Quaternary (last 50,000 years or so) i.e., prior to any possible human-influence on climate. It is also important to understand what controls the fast flow of such ice streams and what might cause them to speed up or slow down. In this project we propose to address the question of the past behaviour of one of the major ice streams to drain the modern Greenland Ice Sheet: Jakobshavns Isbrae in central west Greenland. Jakobshavns Isbrae is the fastest ice stream in the Greenland Ice Sheet today. It flows at several kilometres per year and accounts for about 7% of the total ice drained from the ice sheet. It therefore exerts a major influence on the ice sheet today and would likely have had a similar influence in the past. Our overall scientific goal in this project is to reconstruct the behaviour of Jakobshavns Isbrae during the Late Quaternary period and particularly since about 20,000 years before present. We will collect marine geophysical data and cores of sea-bed sediments from offshore of Jakobshavns Isbrae in order to answer questions regarding how long this ice stream has been in existence; what were it's dimensions during past glaciations; when and how rapidly did it last retreat from the continental shelf of Greenland; what are the processes responsible for the high flow velocities of this ice stream and what are the major controls on it's location; what is the nature of sedimentation associated with Jakobshavns Isbrae; and has it undergone large-scale collapse prior to modern times. The results from this project will significantly advance scientific understanding in the following areas: (1) The behaviour of Jakobshavns Isbrae - a major ice stream of the Greenland Ice Sheet / in the recent geological past (last 50,000 years) and its role in the evolution of the ice sheet as a whole during this period. In particular if Jakobshavns Isbrae has experienced past episodes of large-scale disintegration analogous to those occurring around Greenland today; (2) The mechanisms by which ice streams flow fast; (3) The factors that cause ice streams to form and to stop; (4) The nature of sediments beneath ice streams and the relationship of these sediments to the high velocities of ice streams. These results will be highly relevant to workers in the related fields of glaciology, oceanography, and climate modelling.