Dating and modelling fast ice-sheet grounding-line retreat over the last 4000 years in the SW Weddell Sea, Antarctica

南极西部冰盖对海平面构成的威胁主要是通过接地线的退缩，这条线将浮冰和接地冰分开，浮冰对海平面的影响可以忽略不计，而接地冰会影响海平面。一个令人担忧的问题是，理论模型表明，接地线的后退可能是不稳定的。南极冰架下的接地线后退的地质证据很少，因为难以接近。南极冰架是发生最大规模后退的地方。我们将使用一种新技术，根据罗讷冰架内小冰升的地层异常来确定冰流变化的年代。这将提供有关那里的接地线撤退的信息。首先，将应用一种着眼于背斜几何的既定技术。一个新的特点是应用了一种技术（bas开发的pRES）来直接测量冰的变形，这将使我们能够定量地限制背斜的发展，而不必对冰的粘度做出假设。因此，我们的项目将调查南极冰盖的退缩和流变（流动）。我们将访问Ronne西南地区的几个冰分水岭——Korff、Henry、Skytrain和Fowler——应用这些技术，并确定与接地线经过冰升相关的最近流量变化的日期。这些资料将提供伯克纳岛和埃尔斯沃思山脉之间的罗纳接地线撤退的信息。有许多迹象表明，这种情况发生在过去的4000年里，代表了南极冰盖对末次盛冰期后冰川消融的一些最新反应。冰流变学的测量与冰盖建模具有普遍的相关性。冰从大冰原流入海洋的速度直接影响海平面。驱动这种流动的力是由众所周知的冰盖几何形状控制的，但对流动的阻力取决于冰的粘性（流变）特性。冰的粘度取决于冰的变形速率。这种灵敏度通常用格伦指数来描述。格伦指数可以在实验室或现场测量。实验室测量与现场测量不同，并且很难在现场观察到的低应变率下进行。在许多现场测量中，很难很好地表征应力以量化粘度。通过划分地点，在这些地方，应力场可以被表征，冰的来源很好地限制，我们将能够测量格伦指数。该技术基于这样一个事实，即雷达层在冰内提供标记，并且可以使用干涉相敏雷达技术测量相对短时间内的垂直位移。这些装置提供了冰原上部三分之一到一半的瞬时应变率场。这项技术已经在格陵兰的Summit得到了验证。一旦我们获得了数据，我们就会知道接地线撤退的速度有多快。然后我们将运行冰盖模型，看看什么样的情况可能导致这种情况。接地线后退率最终由几个变量确定；海洋是如何将温暖的水泵到冰架下的，海平面是如何上升的，陆地表面是如何随着冰的卸载而恢复的，以及冰是如何随着变暖而软化的。数学模型将能够评估所有这些因素的影响。我们的团队还收集了许多关于该地区冰盖范围的其他数据，我们将能够看到模型是否可以预测与所有这些相匹配的冰盖几何形状。在项目结束时，根据我们新的搁浅线后退日期和代表过去的模型，我们将更清楚地了解冰盖是如何以及为什么像威德尔海地区那样后退的。

The threat the West Antarctic ice-sheet poses to sea-level is largely through retreat of the grounding line, the line that separates floating ice, whose effects on sea-level are negligible, and grounded ice, which can affect sea-level. A concern is that theoretical models suggest grounding-line retreat may be unstable.Geological evidence for the grounding line retreat under the Antarctic ice shelves, where the most substantial retreat took place, is scarce owing to poor accessibility. We will use a new technique that dates ice-flow changes from stratigraphic anomalies in the small ice-rises lying within the Ronne ice shelf. This will provide information about retreat of the grounding line there. Firstly, an established technique which looks at anticline geometry will be applied. A new feature is the application of a technique (the BAS-developed pRES) for the direct measurement of deformation in the ice, which will allow us to quantitatively constrain the development of the anticlines, rather than having to make assumptions about ice viscosity. Our project will therefore investigate both the retreat and rheology (flow) of the Antarctic ice-sheet. We will visit several ice divides in the SW Ronne area - the Korff, Henry, Skytrain and Fowler - to apply these techniques and date the most recent flow changes associated with the passage of the grounding line past the ice-rise. These will give information about the retreat of the Ronne grounding line between Berkner Island and the Ellsworth Mountains. There are many indications that this has happened in the past 4000 years, representing some of the most recent response of the Antarctic ice-sheet to deglaciation following the last glacial maximum.Measurements of ice rheology have general relevance to ice-sheet modelling. The rate at which ice flows into the sea from the large ice-sheets directly affects sea-level. The forces which drive this flow are controlled by the largely well-known geometry of the the ice-sheets, but the resistance to flow depends upon the viscous (rheological) properties of ice. Ice has the property that the the viscosity depends upon the rate at which the ice is deforming. This sensitivity is usually described with the Glen index. The Glen index can be measured in the laboratory or the field. Laboratory measurements diverge from field measurements, and are very difficult to make at the low strain-rates observed in the field. In many field measurements it is difficult to characterise the stresses sufficiently well to quantify the viscosity.By going to divide locations, where the stress field can be characterised and ice provenance is well constrained, we will be able to measure the Glen index. The technique is based on the fact that radar layers provide markers within the ice, and their vertical displacement over relatively short time periods can be measured using interferometric phase-sensitive radar techniques. These provide instantaneous strain-rate fields in the upper third to a half of the ice field. The technique has been proven at Summit in Greenland.Once we have obtained the data, we will have an idea of how quickly the grounding line retreated. We will then run ice-sheet models, to see what kind of situations could have caused this. Grounding line retreat rates are ultimately fixed by several variables; how the ocean pumps warm water under shelves, how sea-level is rising, how the land surface recovers as ice unloads, and how ice softens as it warms. The mathematical model will be able to assess the influence of all of these factors. Our team has also collected a lot of other data about ice-sheet extent in the area, and we will be able to see if the model can predict ice-sheet geometry that matches all of these.At the end of the project, with our new dates for grounding line retreat and our model that represents the past, we will have a much clearer idea of how and why the ice-sheet retreated as it did in the Weddell Sea area.