Testing the instability theory of subglacial bedform production

当冰原流过大地时，往往会形成特殊的地貌，其中最著名的就是鼓形地貌。它们是细长的流线型山丘，长数公里，高数十米。超大尺度冰川线（MSGL）类似于鼓纹，但存在于巨大的尺度（长达100公里）。它们记录了冰盖形成时的冰流方向，我们的观点是，如果我们知道它们是如何形成的，那么我们就能更多地了解冰盖是如何流动的。了解冰下地层的形成，如鼓状地层和MSGL，突然变得非常重要，因为社会关注南极洲的冰盖如何应对全球变暖。最近，科学家们发现，非常快的冰流集中在相对狭窄的走廊上，被称为冰流。这些冰主导着冰向海岸线的运输，最终在那里以冰山的形式排出（影响海平面）。因此，冰盖对全球变暖的反应在很大程度上是由冰流决定的；它们移动的速度有多快，是在扩大还是在缩小。了解冰流的最大问题是很难测量冰底部的过程，但这正是快速流动发生的地方。我们所知道的是，通过观察曾经存在冰盖的地区，它们产生了冰下床状。事实上，在过去的10年里，来自英国和海外的研究人员一直在研究南极洲周围的海底，并在冰流前面发现了鼓状物和超大规模的冰川线，这些冰流曾经延伸到更远的海洋。最近，利用复杂的地球物理设备，人们甚至在一条冰流下面发现了一个鼓声。因此，无论什么过程使冰流流得如此之快，也会产生鼓丘和超大规模的冰川线。或者，从另一个角度来看，如果我们能找出是什么形成了这些地貌；我们将更多地了解是什么使冰流如此之快。我们将采用的方法是基于一个简单的想法，这个想法来自大自然如何产生其他（非冰川）类型的模式。一个经典的例子是云中的波浪状图案。这种重复的模式是由不稳定机制形成的。当非常小的不规则（比如云中的空气流动）自发地增长并经常产生规则模式时，系统就会发生不稳定性。我们认为冰下床形是重复的模式，并提出某种形式的不稳定性产生了它们。事实上，我们认为我们已经找到了答案，但为了让我们自己和其他科学家满意，他们完全有理由持怀疑态度，还需要做更多的工作。通过数学分析和计算机模拟，我们已经确定冰和下面沉积物的相互作用会产生不稳定性。但这还不足以让我们相信这就是正确答案。我们现在需要确定不稳定性是否会产生地形，比如我们在现实生活中可以观察到的鼓点。例如，如果不稳定性只产生5毫米或1000公里大小的地形，或者形状错误，那么我们就知道我们错了。我们将绘制和测量存在于加拿大，英国和爱尔兰以及南极洲周围海底的鼓鱼和超大规模的线条（大约60,000个例子），以了解它们的大小和特征。这不能通过实地工作来实现，因此我们将使用卫星图像和地球物理。这些数据将用于测试模型。如果模型被证明是正确的，那么结果将成为理解冰流的关键，并将有助于预测它们未来的行为。此外，我们预计南极冰盖下的整个地貌组合将很快被发现。我们的研究结果将为解释它们告诉我们的东西提供坚实的物理基础。

When ice sheets flow across the landscape they often produce particular landforms, the best known of which are drumlins. These are elongate streamlined hills, kilometres in length and tens of metres in height. Mega-scale glacial lineations (MSGL) are similar to drumlins but exist at huge scales (up to 100 km in length). They record the ice flow direction at the time at which they were formed, and our contention is that if we know how they formed, then we would learn much more about how ice sheets flow. Understanding the formation of subglacial bedforms, such as drumlins and MSGL has suddenly become very important because society is concerned about how the ice sheets in Antarctica might respond to global warming. Recently, scientists have found that very fast ice flow is concentrated into relatively narrow corridors known as ice streams. These dominate the transport of ice to the coastline, where it is eventually discharged as icebergs (which influences sea level). Therefore, the response of ice sheets to global warming is largely determined by the ice streams; how fast they are moving and whether they are widening or shrinking. The biggest problem for understanding ice streams is that it is difficult to measure processes at the base of the ice, but this is where the rapid flow occurs. What we do know, from looking at areas where ice sheets used to be, is that they produce subglacial bedforms. Indeed, over the last 10 years, researchers from Britain and overseas have been examining the sea-floor around Antarctica and have found drumlins and mega-scale glacial lineations right in front of the ice streams, where they once extended further out to sea. A drumlin has even been detected beneath an ice stream recently using sophisticated geophysical equipment. Thus, whatever process makes ice streams flow so fast, also produces drumlins and mega-scale glacial lineations. Or, looking at it the other way around, if we can find out what produces these landforms; we will know a lot more about what makes ice streams flow so fast. The approach that we will take is based on a simple idea drawn from how nature works to produce other (non-glacial) types of patterns. A classic example is wave-like patterns in clouds. Such repetitive patterns are known to form by instability mechanisms. Instability is said to occur in a system when very small irregularities (say in air flow in a cloud) spontaneously grow and often produce regular patterns. We view subglacial bedforms as repetitive patterns and propose that some form of instability produced them. In fact we think we have found the answer, but in order to satisfy ourselves and other scientists, who quite rightly should be sceptical, requires more work. By mathematical analysis and computer-modelling we have already established that the interacting flow of ice and the underlying sediment can produce instabilities. But this is not enough to convince us that it is the right answer. We now need to establish if instabilities grow to produce landforms, such as the drumlins, which we can observe in real life. If for example the instability only produces landforms 5 mm in size or 1000 km in size, or of the wrong shape, then we know we have got it wrong. We will map and measure drumlins and mega-scale lineations (around 60,000 examples) that exist in Canada, UK, and Ireland and on the seafloor around Antarctica to see what their size and characteristics are. This cannot be achieved by fieldwork and so we will use satellite images and geophysics. The data will be used to test the modelling. If the model turns out to be correct then the results become critical for understanding ice streams and will help predict their future behaviour. Also, we anticipate that whole assemblages of landforms beneath the Antarctic Ice Sheet will soon be found. Our results will provide a sound physical basis for interpreting what they tell us.