【作者】 唐学远；

【导师】 张占海；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2010， 博士

【摘要】 南极冰盖作为地球系统的一个重要组成部分,在重建地球气候演化历史和调查海洋环流变异及海平面变化的过程中扮演着关键角色。位于东南极冰盖中心的冰穹-Dome A(昆仑站)由于其独特的环境要素,是一个有助于深入理解南极冰盖演化、稳定性和找寻地球气候久远记录的理想地点。目前,Dome A作为可能存在超过百万年古气候记录的理想深冰芯钻探位置,成为南极研究的焦点之一。本文从南极冰盖气候演化历史研究的最新进展入手,深入讨论在Dome A开展深冰芯钻探的可能性和战略,通过对东南极Dome A中心区域的中国第21次和第24次地面雷达数据以及最近AGAP (Antarctica’s Gamburstev Province project)计划和德国AWI的机载雷达数据(DoCo)的综合分析,结合Vostok、Dome C、Dome Fuji冰芯数据和Dome A气象数据,揭示典型区域的冰流动力学特征;构建冰盖数值模式尝试理解东南极中心区域的冰盖演化与冰流特征。我们得到如下结果:1、模拟出冰层等时线上的水平平均积累率,反演出过去至少8.5万年的古积累率分布;2、模拟出冰盖雷达断面上的积累率空间分布,揭示Dome A的长时间尺度演化特征;3、描述等时线空间分布,揭示冰盖内部冰流动力学机制;4、模拟冰盖雷达断面上的深度-年代关系,为深冰芯钻探可能地点的选取提供理论依据。通过中国第21次(CHINARE 21,2004/05)南极科学考察期间在Dome A中心区域及其外围200km内获得的一条冰雷达典型断面,得到Dome A附近冰盖内部的雷达等时层分布。分析表明:在冰盖浅层(0-500m)内部等时层局部出现向斜层(syncline)和背斜层(anticline),总体比较平坦。在冰盖深部(500-2000m),内部等时层水平分布很不规则,在冰下地形波长小于冰厚或与冰厚可比较时,分布呈现显著的褶皱现象,追踪并趋向于平行冰下地形;而当其冰下地形波长为长波并大于冰厚时,内部等时层覆盖了冰下地形,并不随着冰下地形的起伏而起伏;在冰下局部出现单个或两个地形剧烈变化的山峰附近的内部等时层被冰下地形强烈扰动。Dome A中心区域深部的雷达等时层存在不连续性以及褶层曲率较大的现象,但在小尺度内抑制了内部等时层的剧烈形变。另外,结合冰盖厚度数据,找出了3个供选择构建预测冰芯钻探点年代-深度关系的地点。通过对雷达数据的分析,揭示出Dome A的三维冰下和表面特征,发现其下方存在一个东北向的马鞍形槽谷。最大的冰厚已经超过了3000m(昆仑站的下方),最高处的表面高程达到4092m。典型向斜层和背斜层的出现表示那里的表面物质积累存在空间分布的不均匀性。内部等时层存在局部的断裂,冰盖底部的内部等时层有时由于反射信号的衰减或者雷达波不能穿透至底部而出现信号盲区。冰盖内部的等时层结构和浅层积累率是数值模拟末次冰期冰盖演化的必要参数和边界条件之一。通过对2004/05年度中国21次南极考察队在Dome A获取的高频雷达数据的分析,示踪出三条内部等时层,利用穿越Dome A内部等时层与Vostok冰芯钻取位置的交定年,得到层上相应的深度-年代关系。将深度-年代关系作为输入,使用一个一维冰流模式计算了Dome A过去8.46万年以来三个不同阶段的古积累率,揭示了Dome A地区在相应历史时期的古积累时间与空间变化。结果显示Dome A核心区域的古积累率有着一致的分布特征,且在过去32.6kyr,44.6kyr, 84.6kyr这三个晚更新世的不同时期, Dome A核心区域的平均积累率较低,分别为0.020m/yr, 0.023m/yr, 0.018m/yr,发现在距今3.5-4.5万年间存在一个相对较为湿润的时期;此外,给出了沿着Dome A中心区域向其外围积累率的空间变化。本文也介绍冰盖和冰川的动力学和热力学基本理论,推导了浅冰近似下的冰盖演化方程组。同时,介绍三维的冰盖耦合模式-GLIMMER模式并在EISMINT下做了固定边界和移动边界的两个实验,得到了反应冰盖物质平衡和能量平衡的一系列温度、速度和物质通量特征曲线。同时为了描述冰穹位置的物理特征,以及为了检验Dansgaard-Johnsen冰流模式,我们做了该模式的二维冰流情形的简化和改进。实验的结果与冰盖基本理论是吻合的。更多还原

【Abstract】 The Antarctic ice sheet is an important part in the earth system, and plays a prominent status in reconstructing the history of climate evolution, investigating the variability of ocean circulation and sea level change.But the Antarctic contitent is least accessible , extremely complicated and poorest known. Dome A(Kunlun Station), the summit of the Antarctic ice sheet,is considered a likely place for finding an ice core record reaching back to one million years. During the 21st and 24th Chinese National Antarctic Research Expedition (CHINARE 21, 2004/05;CHINARE 21, 2007/08), two datasets of Ice radio-echo sounding based ground in Dome A,the central region of East Antarctica, were made. And the other datasets from airborne radar systems was collected by AGAP(Antarctica’s Gamburstev Province project)and Alfred Wagner Institute(DOCO plans,Dome Connection East Antarctica)。Using the radar datasets and numerical modelling,some results are shown here. The aim of the thesis is to understand the flow and form of the ice sheet around the Dome A region of central East Antarctica. The thesis has the following objectives:first,dealing with and analyzing the radar datasets of linking Dome A and Vostok,where exsists the location of ice core;second,to determine the depth-age relationship of ice around Dome A and to reconstruct the accumulation of the past hundred thousand years by numerical model;third, to establish the ice thickness and the bed topography of Dome A,and to reveal the long-term glacial history of Dome A.Putting the results together would maximise our knowledge of Dome A,even of the Antarctic ice sheet and the global change.The internal isochronous layering of the Antarctic Ice Sheet, revealed by ice radar, is a prerequisite for selecting sites for deep ice core drilling that can be used for studying the paleoclimatic record. In 2004/2005, during the 21st Chinese National Antarctic Research Expedition (CHINARE 21), a 200 km long, continuous radar profile was obtained across Dome A. The internal layers along the profile were derived from the stratigraphy detected by the radar. The morphology of the isochronous layers shows that: (1) The internal layers in the shallow ice sheet (0–500 m) are generally flat, with no more than 50 m of layer intervals, and have typical synclines and anticlines in some localized regions; (2) At 500–2000 m below the surface of the ice sheet, the layers appear as“bright layers”, and the width of the layer intervals expands to 50–100 m; (3) When the basal topographic wavelengths are approximate to the thickness of the ice (3 km), the traced internal layers, with localized bumps or concave folds, are asymptotic parallel to the subglacial topography. For the longer topographic wavelengths (-20 km) wider than the thickness of the ice, the layers do not rise and fall with the basal topography. The internal layers surrounding some mountain peaks representing the most extreme variation in the terrain are sharply disturbed by the subglacial topography; (4) Layer discontinuity and fracture were detected in the basal ice sheet. Finally, by combining this new information with that derived from existing data regarding ice thickness , we were able to select three potential sites for reconstructing the age-depth relationship of the ice core.The radar profiles were done providing a full information of centre of the Dome A region. The RES data were acquired using a high-frequency monopulse ice-penetrating radar with multi-frequency of 60MHz and 179MHz . The datasets show arches and troughs in isochronous ice layers. In some sites the inclined layers and distortion of isochronous layers in ice revealed by ice-penetrating radar. We also find there is no radar signal in some sites and some sections exhibit instable sedimentation and discontinuation of the internal reflections. At Dome A, the changes of surface elevation is not obvious, the highest is 4092m. The data of ice-penetrating radar revealed flat isochronous layers. In addition, bed topography appears to be the result of a saddle-backed basin. Also,we find a north-east deep channel according to 3D RES exploration result in Dome A, its ice thickness exceeds 3000m.By the disscusions, the identification of the depth-age relationship of ice shows that there is the best site to drill the deep ice core around the Dome A. The datasets also show some arches and troughs in isochronous ice layers. There exist some typical synclines and anticlines in ice revealed by ice-penetrating radar in some local regions.A 1-D model is used to calculate the past rates of ice accumulation by internal layering. An approximate mean accumulation from 0.018 to 0.027m/yr over the past 34.6k years, along the RES profiles, was estimated. Also, we have taken the accumulation was 0.02-0.045m/yr in the past 34.6-44.6kyr , and 0.01-0.023m/yr in the past 44.6-84.6kyr along the profile. The variability of accumulation in time -space around the ice divide also was given.We also introduce the 3-dimensional coupled model for Antarctic ice sheet– GLIMMER model, and make an idealized ice sheet simulation test under EISMINT-1 benchmark,gets various evolution curves expressing conservation of mass, momentum and energy of ice sheet, and indicates that under fixed boundary condition and moving condition, the physical characteristic curves when ice sheet is in a steady state are in accordance with our current knowledge on Antarctic ice sheet on the whole. In order to consider further simulation of the relation of‘‘depth-age-accumulation’’at Dome A region, we simplify the GLIMMER model into a 2-dimensional ice flow model with coupled temperature field and test it under some brief assumptions. The change of elevation,temperature field and velocity field of ice flow profile in evolution steady state of ice sheet are obtained quantitatively.The results are similar to the calculation results of GLIMMER model.更多还原