Late Quaternary West Antarctic Ice Sheet Dynamics: Remote Sensing and Substrate Studies of Palaeo-Ice Sheet Beds on the Amundsen Sea Shelf
Late Quaternary West Antarctic Ice Sheet Dynamics: Remote Sensing and Substrate Studies of Palaeo-Ice Sheet Beds on the Amundsen Sea Shelf
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晚第四纪西南极冰盖动力学：阿蒙森海架古冰盖床的遥感和底质研究
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发表时间：
2014
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通讯作者：
J. Klages
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作者：
J. Klages
以海洋为主的南极西部冰盖的大部分地区目前正受到全球冰冻圈最迅速变化的影响。近几十年来，从冰盖内部流入阿蒙森海的冰流受到的影响最为严重。随着上升流、相对温暖的环极深水流向大陆架，流向大陆架深处的内腔，它从下面融化冰架，使其变薄，并导致冰流加速，接地线因支撑减少而后退。由于冰流以海洋为基础，位于显著加深内陆的斜坡上，它们很容易受到未来冰质量损失增加的影响，直接导致海平面上升3.4米，假设所有以海洋为基础的南极西部冰部分都将解体和融化（Fretwell et al. 2013）。然而，旨在阐明这些未来情景的“最先进的”冰盖模型仅使用覆盖过去30-40年的观测数据进行初始化（例如Favier et al. 2014），因此排除了跨越末次极大期和随后的去冰期的冰盖变化的长期经验数据。为了检验预测的未来情景的可靠性，有必要根据现代南极大陆架上的古冰盖床的经验数据所证实的过去的冰盖结构来验证模式。重现LGM冰盖的尝试（Golledge et al. 2013）显示，模式模拟与经验数据之间存在相当大的不匹配。这种差异主要归因于缺乏全面的古冰川学数据，特别是来自外部大陆架和大型古冰流沟槽之间区域（称为冰间流脊的区域）的古冰川学数据，这些数据将更精确地揭示南大洋阿蒙森海地区西南极冰盖的时空变化。
Extensive parts of the largely marine-based West Antarctic Ice Sheet are currently subject to the most rapid changes in the global cryosphere. In recent decades, ice streams that drain >35% from the ice sheet’s interior into the Amundsen Sea have been affected most dramatically. As upwelling, relatively warm Circumpolar Deep Water flows onto the continental shelf towards the deep inner shelf cavities, it melts the ice shelves from below, thins them, and causes ice stream acceleration and grounding line retreat in response to decreased buttressing. Since ice streams are marine-based on slopes that significantly deepen inland, they are susceptible to increased future ice mass loss, directly resulting in a sea-level rise of up to 3.4 meters, assuming that all marine-based West Antarctic ice portions would disintegrate and melt (Fretwell et al. 2013). However, ‘state-of-the-art’ ice sheet models that aim to elucidate these future scenarios are only initialised with observational data covering the past 30-40 years (e.g. Favier et al. 2014), thus excluding long-term empirical data of ice sheet change spanning the Last Glacial Maximum and the subsequent deglacial period. To test the reliability of predicted future scenarios it is essential that models are validated against past ice sheet configurations confirmed by empirical data from palaeo-ice sheet beds on modern Antarctic continental shelves. Attempts at reproducing the LGM ice sheet (Golledge et al. 2013), have revealed considerable mismatches between model simulations and empirical data. Such disparities have been attributed principally, to a lack of comprehensive palaeo-glaciological data particularly from outer continental shelves and regions in between the large palaeo-ice stream troughs, regions known as inter-ice stream ridges, that would reveal the spatial and temporal variations of the West Antarctic Ice Sheet in the Amundsen Sea sector of the Southern Ocean more precisely.
This thesis presents the mapping and detailed analysis of new marine geophysical and geological data from three formerly unstudied regions on the Amundsen Sea shelf that significantly improve our understanding of Antarctic palaeo-ice sheet dynamics. In Chapter 2 I will present the first sea-floor geomorphological record of former basal ice conditions on an inter-ice stream ridge that entirely differ from those commonly investigated in the nearby palaeo-ice stream troughs. From these data, an improved temporal and spatial reconstruction of flow conditions of the former ice sheet in inter-ice stream areas of the eastern Amundsen Sea Embayment is revealed. Age constraints aiming to reveal the minimum grounding line retreat from the ridge broadly correspond with those from the nearby Pine Island Trough. Palaeo-ice sheet dynamics as inferred from the glacial landform and sediment record on the inter-ice stream ridge are well complemented by a large-scale multibeam bathymetry survey of the middle and outer shelf, north of the inter-ice stream area, presented in Chapter 3. This new dataset compiles bathymetry from 11 separate research cruises to the region, and is supplemented by the analysis of new sedimentological and seismic data. From comprehensive landform mapping, the detailed palaeo-flow pathways of the WAIS in the northern and easternmost Amundsen Sea Embayment is revealed. Furthermore, geomorphological analysis of the bathymetry data allows thermal regimes at the palaeo-ice sheet bed to be defined in unprecedented detail, showing the complex relation of trough geometries and the subglacial geology to palaeo-ice flow behaviour. In combination with findings from Chapter 2, a coherent pattern of episodic post-Last Glacial Maximum retreat between the Pine Island and Abbot glacial troughs across the inter-ice stream ridge is revealed by the landform information, from which uniform retreat across the entire eastern Amundsen Sea Embayment is inferred. The same episodic style of retreat is also evident for a formerly unstudied part of the Amundsen Sea shelf offshore the Hobbs Coast presented in Chapter 4, as here the analysis and interpretation of marine geophysical and geological data reveal a large grounding-zone wedge recording a prolonged grounding line stabilization phase after the West Antarctic Ice Sheet reached the continental shelf edge during the last maximum extent. The initial retreat here is demonstrated to have been initiated at a pre- or early Last Glacial Maximum stage with deglaciation of inner shelf regions completed by ~12.9 cal. ka BP. Set in the context of other studies, a diachronous initial retreat of West Antarctic Ice Sheet grounding lines is indicated, which is discussed as a possible response to different local settings.
This thesis will ultimately help to better understand West Antarctic Ice Sheet dynamics during and since the Last Glacial Maximum. The new information will significantly add to a hitherto sparse database of previous work, helping to test, validate, and improve ice sheet models in the vital region of the Amundsen Sea. Only by enhancing their ability to simulate past ice sheet configurations more accurately will more reliable predictions of the future evolution of these dramatically changing parts of the West Antarctic Ice Sheet be possible.