ABLATION OF ICE‐CORED MORAINE IN A HUMID,MARITIME CLIMATE: FOX GLACIER,NEW ZEALAND

Depending on thickness, debris‐cover can enhance or reduce ablation, compared to bare‐ice conditions. In the geological record, hummocky moraines often represent the final product of the melt‐out of ice‐cored moraines, and the presence or absence of such moraine deposits can have paleoclimatic implications. To evaluate the effects of varying debris‐cover and climate on ice‐melt in a maritime mid‐latitude setting, an 11‐day ablation stake study was undertaken on ice‐cored moraine at Fox Glacier, on the western flank of the New Zealand Southern Alps. Ablation rates varied from 1.3 to 6.7 cm d^?1, with enhancement of melt‐rate under thin debris‐covers. Highest melt‐rates (effective thickness) occurred under debris‐cover of c. 2 cm, with ～3 cm being the debris thickness at which melt‐rates are equal to adjacent bare‐ice (critical thickness). Air temperature from nearby Franz Josef Glacier allowed for a simple degree‐day approach to ablation calculations, with regression relationships indicating air temperature is the key climatic control on melt. Digital elevation models produced from topographic surveys of the ice‐cored moraine over the following 19 months indicated that ablation rates progressively decreased over time, probably due to melt‐out of englacial debris increasing debris‐cover thickness. The morphology of the sandur appears to be strongly determined by episodic high‐magnitude fluvial flows (jökulhlaups), in conjunction with surface melt. Thus, ‘hummocky’ moraine appears to be a transient landform in this climatic setting.     

深厚覆盖层渗透系数三种测定方法的对比分析--以新疆下坂地水库坝址覆盖层为例

深厚覆盖层结构复杂,其渗透系数的求取是目前勘察工作中一个难以解决的问题.本文通过3种方法的对比分析,并结合地层结构特征进行综合评价,取得了较为合理的渗透参数,为工程设计和施工提供了科学依据.     

Marine Oxygen Isotope Stage (MIS)-6 Glacial Advances on the Tibetan Plateau More Extensive than during MIS-2 due to More Abundant Precipitation

Numerous studies dated glacial deposits within the Himalayan-Tibetan orogen. While most focus on young deposits, i.e., younger than the Last Glacial Maximum (LGM or Marine oxygen Isotope Stage (MIS)-2, ~20 ka), older moraines such as those from MIS-6 (~130–191 ka) are much harder to date and interpret due to the less well-preserved nature of their surfaces and boulders, as well as their scattered and continuous age distribution due to long-lived erosion since deposition. Here, we dated with 10Be, two imbricated moraines near Yadong in southern Tibet, as MIS-2 and 6, showing that the most extensive, smooth surfaces were abandoned during MIS-6. Compiling published data from 54 MIS-6 moraines on the Tibetan Plateau reveals that they exist in most regions, dry or humid. They are particularly well-preserved (sharp crests) in eastern and northern Tibet, while in southern and central Tibet, their crests are rounded to sub-rounded. Because both MIS-2 and 6 were equally cold, and because MIS-6 moraines are much more extensive than those from the LGM, we conclude that MIS-6 glacial advances were controlled by more abundant precipitation than during MIS-2. This would be consistent with the peak in Asian monsoon during MIS-6, revealed by sediments from the South China Sea.     

川藏公路然乌-鲁朗段冰碛高边坡稳定性分析

在帕隆藏布北岸的川藏公路上,有许多冰碛高陡切坡.这些冰碛切坡具有独特的结构力学特征,其几何形态(高度与坡度的组合值)不同于一般经验值的范围.根据其物理力学性质,采用圆弧法对边坡进行稳定性分析,取得了与实际相符的结果.     

ESR dating of glacial moraine deposits: Some insights about the resetting of the germanium (Ge) signal measured in quartz

The electron spin resonance (ESR) dating of tills using germanium-doped (Ge) paramagnetic centers in quartz has advantages over other dating techniques, as quartz is common, processing is easy, and the technique has the potential for dating features several hundreds of thousands years old. ESR dating of moraines is based on the supposition that either subglacial comminution or exposure to sunlight resets the signal. However, actual dating suggests that a signal that is initially present cannot be bleached to zero by grinding alone. We found that grinding coarse samples (0.5–1 mm in diameter) to the mean grain size of fine sand (0.125–0.193 mm) reduced the signal intensity to 53–69% of its original value. From the value of the signal difference, one can devise a correction factor for ESR ages of subglacial sediment. Polymineralic grains are commonly present in till. Exposure of them to sunlight for several days can reduce the signal intensity to 7–8% of its original value within 1–2 mm thick of the sediment surface. However, within 5–8 mm of the sediment surface, exposure to sunlight for over one week only reduced the signal intensity to mean plateau values of 42–50% of the initial value. Mixing upper and lower layers of the samples during exposure to sunlight changed the signal intensity. This suggests that the amount of bleaching varies spatially. Sediments initially deposited at the margins of ice caps or ice sheets and subsequently overridden may have been sufficiently exposed to sunlight to allow ESR dating of moraines. The purity of the quartz and the grain size have significant impacts on signal intensity; intensive purification and the use of a uniform fine sand fraction are thus recommended.     

Beryllium‐10 surface exposure dating of glacial successions in the Central Alaska Range

Glacial landforms and outwash terraces in the Nenana River valley, Reindeer Hills and Monahan Flat in the central Alaska Range were dated with 60 ¹⁰Be exposure ages to determine the timing of Late Pleistocene glaciation. In the Nenana River valley, glaciation occurred at 104–180 ka (Lignite Creek glaciation), ca. 55 ka (Healy glaciation), and ca. 16 ka (Carlo Creek phase); glaciers retreated in the Reindeer Hills and Monahan Flat by ca. 14 ka and ca. 13 ka, respectively. The Carlo Creek moraine is similar in age to at least six other moraines in the Alaska Range, Ahklun Mountains and Brooks Range. The new data suggest that post‐depositional geological processes limit the usefulness of ¹⁰Be methods to the latter part (≤60 ka) of the late Quaternary in central Alaska. Ages on Healy and younger landforms cluster well, with the exception of Riley Creek moraines and Monahan Flat‐west sites, where boulders were likely affected by post‐depositional processes. Copyright © 2010 John Wiley & Sons, Ltd.     

Sedimentary Record of the Younger Saalian Ice Margin Stagnation in Eastern Poland: Development of a Regular Pattern of Glaciolacustrine Kames

Glaciolacustrine kames in the Bielsk Podlaski area (eastern Poland) exhibit a unique regular pattern. Three representative morphological kame types were chosen for detailed sedimentological analyses, specifically: isolated, isometric mounds; isolated, elongated hills; and branching ridges. All types comprised fine‐grained sandy and sandy/silty deposits. Lithofacies analysis resulted in the distinction of several lithofacies associations. Associations dominated by medium‐ or large‐scale, massive or horizontally laminated sands are interpreted as proximal subaqueous fans; associations dominated by medium‐ or small‐scale lithofacies of ripple‐drift cross‐laminated sand are interpreted as distal subaqueous fans; and those dominated by sandy/silty, silty or silty/clayey lithofacies with horizontal lamination are interpreted as lake bottomsets. Rates of sediment accumulation appear to have been fast, resulting in syndepositional and metadepositional deformation structures of two types: water‐escape structures, and slumps on subaqueous slopes. After the ice‐walled lake basins filled with sediment, glaciofluvial erosion and deposition alternated, resulting in erosional channels of up to 1 m deep, later filled with gravel or gravely sand. The results indicate that kames developed in a supraglacial environment within a topography of ice‐cored moraines containing ice‐walled lakes that persisted due to the presence of permafrost. Pauses during retreat of the ice walls resulted in ice‐contact deformations at the edges of the kames. Kame formation is therefore consistent with a continental climate and this may explain the increased abundance of this type of kame system in Eastern Europe.     

Postglacial environmental change in the valley of Malye Chily River (the basin of Lake Teletskoye), northeastern Russian Altai

The evolution and current state of landscapes around Lake Teletskoye have not previously been studied in detail. In the valley of the Malye Chily River, which flows into Lake Teletskoye, the timing of dam failure and draining of two moraine-dammed lakes has been identified. Botanical analysis, ash content determination, and radiocarbon dating of two peat profiles provide insight into postglacial evolution of wetlands related to this landscape. We found clear evidence of the disappearance from the peat of higher vascular species that, today, grows mostly in the plains of Siberia. Correlation of the data obtained with the accepted chronology of the Holocene events in the Russian Altai suggests the following stages of postglacial environmental change in the Malye Chily River valley: (1) the continuation of the Late Würm glaciation degradation (before 7000?cal. yr BP); (2) Holocene Climate Optimum (7000–5000?cal. yr BP); (3) Akkem cooling (5000–4200?cal. yr BP); (4) warm period (4200–3700?cal. yr BP); and (5) Historical cooling (3700–1600?cal. yr BP).     

松辽冰盖与松辽分水岭翘板迁移间的动力关系探讨

广布于松辽平原科尔沁沙地的巨型弧形沙垄的成因一直未能得到很好解释,该项研究采用罗根冰碛变形机理对该区的巨型弧形沙垄、密布的湖泡等特征地貌进行分析,发现其与斯堪的纳维亚冰盖、劳伦泰德冰盖的冰下软基变形地貌特征具有同一性,从而确认巨型弧形沙垄为冰盖冰流形成的巨型流线、罗根冰碛与锅穴构造的遗留,从而佐证了“松辽冰盖”的存在。研究发现松辽平原的巨型弧形流线显示出具有从盆地周围向沉降中心延伸、汇聚的特征,这表明流线的走向受制于地势的控制,而非所谓的风力所为。此外根据巨型冰川流线的展布形态与松辽分水岭之间存在的反常关系,发现在冰盖消融后松辽分水岭发生了北移现象,这一现象的产生可能与冰盖消融后导致的松辽盆地的失压反弹隆升有关。根据松辽分水岭的迁移时间、风沙层序年龄、荷载构造扰动地层年龄的综合约束,初步判断松辽冰盖的最近一次的发生时间在晚更新世早期(MIS4阶段)的东山冰期。     

Deglaciation chronology of the Scandinavian Ice Sheet from the Lake Onega Basin to the Salpausselkä End Moraines

Several long sediment cores (max. 12 m) from various parts (up to 150 km apart) of Lake Onega, Russian Karelia, have been studied for lithology, varve chronology and palaeomagnetism. The two longest varve records from the central basin contain 1300 varves. These indicate the length of the deglaciation period from these localities to the north of Lake Onega, where the drainage of glacial meltwaters was directed towards the White Sea and the deposition of varves in the Onega basin terminated. An estimate of the duration of deglaciation of the whole Onega basin is 1500 years. Natural remanent magnetization (NRM) is strong and stable in these sediments and accurately records changes in the Earth's magnetic field. A distinct change in the magnetic field, when the declination shifted from east to west by at least 60° in 350 varve years, is clearly identifiable in all cores. This palaeomagnetic feature was used for core to core correlation together with other variations in magnetic parameters and widely distributed lithological marker horizons. On the basis of the correlations between the cores and calibration of AMS radiocarbon dates from varves obtained from the northern archipelago of Lake Onega, the age of the westerly declination peak is dated to 13 090 cal. BP and accordingly the deglaciation of the Onega basin took place between 14 250 and 12 750±100 cal. BP. The westerly declination peak was also recognized earlier by Bakhmutov and Zagniy in the Helylä varved clay sequence near Sortavala on the northern shore of Lake Ladoga. Helylä is situated outside the Salpausselkä end moraines and the accumulation of varved clays continued there 1500 years after the declination peak, up until the drainage of the Baltic Ice Lake, which more or less coincides with the ice margin retreat from Salpausselkä II end moraine and the termination of the Younger Dryas event. The date thus arrived at for this event is 11 590±100 cal. BP, close to the recent results from Greenland ice cores and from varved lake sediments and tree rings from Central Europe. It is further suggested that the formation of the major Younger Dryas end moraines, the Finnish Salpausselkä I and Salpausselkä II and their correlatives in Russian Karelia, took place between 12 250 and 11 590 calendar years ago, clearly earlier than earlier estimated through correlation with the Swedish varve chronology.