Oceanographic profiles and ice rafted debris in sediments from the Scoresby Sund, Greenland

Holocene glacimarine sedimentation associated with fast-flowing outlet glaciers draining the Greenland Ice Sheet is investigated using sedimentary and acoustic data from inner Scoresby Sund, East Greenland. Sedimentation in inner Scoresby Sund is dominated by three processes which are influenced by differences in proximity to fast-flowing outlet glaciers, extent of glacier-ice cover and fjord bathymetry: (1) sediment-gravity flow, principally in the form of turbidity currents and debris flows; (2) suspension sedimentation from turbid meltwater plumes; and (3) iceberg rafting. These processes result in texturally and sedimentologically heterogeneous lithofacies. Proportionally, fine-grained muds (laminated, stratified and massive facies) dominate cores recovered from inner Scoresby Sund, accounting for 80% of the total, whereas diamict facies account for only 15%. Abundant fine-grained muds demonstrate that meltwater flux and sedimentation is significant in this high Arctic glacimarine environment, in settings proximal to fast-flowing outlet glaciers. With increasing distance from these glacier termini, muds are replaced progressively by iceberg-rafted, coarse-grained sediment. The dominance of this iceberg-rafted sediment in outer Scoresby Sund reflects both its more distal location from fast-flowing glacier termini, and the high calving flux associated with these ice masses. Laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner Scoresby Sund are similar to lithofacies produced in temperate and subpolar glacimarine systems. This implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically-, glaciologically- (fast-flowing and non fast-flowing ice-masses) and oceanographically-variable glacimarine settings. Recognition of laminated, fine-grained facies in the geological record therefore does not necessarily indicate a temperate palaeo-glacial setting. However, the predominance of iceberg-rafted diamict facies in ice-distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg-sedimentation played a dominant role.本研究采用东格陵兰斯科斯比松（Scoresby Sund）内层区域的沉积与声学数据（sedimentary and acoustic data），对与排泄格陵兰冰盖（Greenland Ice Sheet）的快速流动出口冰川（fast-flowing outlet glaciers）相关的全新世（Holocene）冰川海洋沉积作用（glacimarine sedimentation）展开了系统调查。斯科斯比松内层的沉积作用受三类核心过程主导，这些过程受到与快速流动出口冰川的邻近程度、冰川冰覆盖范围以及峡湾水深的共同影响：（1）以浊流（turbidity currents）和碎屑流（debris flows）为主要形式的沉积物重力流（sediment-gravity flow）；（2）源自浑浊融水羽流（turbid meltwater plumes）的悬浮沉积作用（suspension sedimentation）；（3）冰山筏运（iceberg rafting）沉积作用。上述过程共同形成了结构与沉积学特征均具异质性的岩相（lithofacies）。从占比来看，斯科斯比松内层获取的岩芯中，细粒泥质沉积（fine-grained muds，包括纹层状、层状与块状岩相）占据主导地位，占总沉积量的80%，而混积岩相（diamict facies）仅占15%。丰富的细粒泥质沉积表明，在这片紧邻快速流动出口冰川的高北极（high Arctic）冰川海洋环境中，融水通量与沉积作用均极为显著。随着与这些冰川末端（glacier termini）距离的增加，泥质沉积逐渐被冰山筏运的粗粒沉积物取代。斯科斯比松外层区域以冰山筏运沉积物为主，这既反映了其相较于快速流动冰川末端更偏远的地理位置，也与这些冰体伴随的高崩解通量（calving flux）密切相关。斯科斯比松内层区域由浊流与溢流羽流悬浮沉降形成的纹层状泥质沉积，与温带及亚极地冰川海洋系统（temperate and subpolar glacimarine systems）中形成的岩相特征高度相似。这意味着在气候、冰川学（快速流动与非快速流动冰体）以及海洋学条件各异的广泛冰川海洋环境中，其沉积过程与所形成的岩相均具有相似性。因此，在地质记录中识别出纹层状细粒岩相，并不一定代表其形成于温带古冰川环境（palaeo-glacial setting）。然而，在远离冰川的沉积记录中占主导的冰山筏运混积岩相，则表明当时曾存在相对寒冷的环境，其中冰山沉积作用发挥了主导调控作用。