Mass flows and river response in rapid uplifting regions – A case of lower Yarlung Tsangpo basin,southeast Tibet,China

The fluvial geomorphology in tectonically active (particularly rapid uplift) regions often undergoes continuous change. The rapid uplift is coincident with high erosion rates; consequently, incised valleys are formed. Mass flows (for example, avalanches, landslides, and debris flows) in incised valleys can markedly influence fluvial processes and even reshape valley geomorphology. However, these processes and long-term evolution corresponding to mass flows require further clarification. Field campaigns were carried out in the region near the Yigong Tsangpo and Palong Tsangpo Rivers (hereafter the Yigong and Palong Rivers), the two largest tributaries of the lower Yarlung Tsangpo River, to examine the feedback between fluvial processes and mass flows. Remote sensing images from recent decades were used to compare the channel morphology before and after typical mass flows (particularly catastrophic ones). The morphology of the lower Yigong River has evidently been impacted by landslides, while that of the Palong River has mainly been shaped by glacial processes and debris flows. At present, the morphology of the latter consists of alternating sections of gorges and wide valleys, with a staircase-like longitudinal profile. The gorge sections exhibit single and deeply incised channels with a high-gradient channel bed and terraces. In contrast, the wide valley sections consist of lakes, braided or anabranching channels, gentle bed gradients, and thick alluvial deposits. Debris flows occur more frequently in gullies in the reaches of the gorge sections and rarely in gullies along the wide valley sections. The occurrence of mass flow events has resulted in an imbalance of the previous (quasi-)equilibrium in the river morphology; however, this has triggered negative feedback that is driving the transient river morphology to a new state of (quasi-)equilibrium.     

地震时间序列的周期图分析方法和极大熵谱分析方法

利用华北地震区的地震资料（1000～1977）,对地震时间序列进行了周期图分析和极大熵谱分析,提取了隐含在随机噪声中的隐含周期。结果表明,这两种方法所得结果很接近,地震应变能√Ei;释放的隐含周期利用周期图分析提取的主要周期是30年、245年和305年,极大熵谱分析方法提取的主要周期是30年、240年和300年。这三个主要周期存在“倍30”的关系,240年和300年周期是30年周期的整数倍。30年周期反映了各活跃幕中的幕式活动规律,300年周期同各活跃期高潮时段之间的时间差的平均值相符,华北自1000年以来的四个活跃期高潮时段之间的时间差平均值是300年。     

Last Glacial snowlines in the Tibetan Plateau: an argument against an extensive coalescing icesheet

Two opposing theories are circulating with regard to the extent of the Last Glacial ice cover in the Tibetan Plateau. One says that only less than 20% of plateau was covered with ice, and another insists that the plateau be completely covered with an extensive coalescing icesheet. The extent of the ice cover is thought to be significant in shaping global climatic systems, and a further discussion on this issue may help to understand the earth's surface feedback mechanisms to the global climates. This paper focuses on the Last Glacial snowline reconstruction and uses the reconstructed snowline to argue against the existence of an extensive coalescing icesheet. The reconstructed Last Glacial snowlines suggest that the snowlines dropped 500–700 m in the western and northern marginal mountains and about 1000 m in the southern and eastern marginal mountains of the Tibetan Plateau. However, the magnitude of the snowline dropping decreases dramatically towards the interior of the plateau, from 300–400 m in those mountains adjacent to the marginal mountains to about 100 m in the driest area in the interior. This means that the snowlines were too high and associated glaciers were too limited to extend to the vast intermountainous basins. To be blamed are weakened summer monsoons and lowered condensation elevations, both of which were probably responsible for not bringing in an adequate amount of precipitation into the interior for developing an extensive coalescing icesheet. The relatively high radiation in these relatively low latitudes could be a major negative force to prevent the snow and ice from forming a coalescing icesheet. In contrast, the enhanced plateau blockade to the monsoons may have helped to significantly lower the snowlines and expand the glaciers in the outer slopes of the southern and eastern marginal mountains. The westerlies may have greatly helped those glaciers in the western and northern marginal mountains. This revised version was published online in July 2006 with corrections to the Cover Date.     

江苏省近百年汛期旱涝变化的诊断分析

本文应用历史气候资料和现代降水记录,对近百年来汛期江苏省各区域各年代进行旱涝诊断分析,采用滑动平均方法探讨其变化趋势,并用最大搞谱方法提取显著周期。得出以下三个比较有意义的结论：（1）近百年来,汛期全省较易发生旱的灾害,20－30年代为旱灾濒发期;（2）淮北地区近年有向早年发展的趋势,江淮之间及苏南地区进入90年代以来向旱年发展的趋势则愈来愈明显;（3）全省具有2－3年、5－6年的旱涝周期。     

中国最高、最低温度及日较差在海拔高度上变化的初步分析

本文利用中国740个气象台站1963~2012年均一化逐日最高温度和最低温度资料,分析了中国地区最高、最低气温和日较差变化趋势的区域特征及其与海拔高度的关系。结果表明:近50年气温的变化趋势无论是年或季节变化,最低温度的增温幅度都高于最高温度,且其增温显著区域都对应我国高海拔地区。除了春季,其他季节最高、最低温度及日较差的升温幅度随着海拔高度的升高而增大,其中最高温度的变化趋势与海拔高度的相关性最好。同一海拔高度上,最高、最低温度在不同年代的增幅具有不一致性:20世纪80年代,二者变化幅度最小;20世纪90年代,二者增幅最大,尤以低海拔地区最为明显。2000 m以上高海拔地区:最高温度和最低温度的变化趋势在20世纪90年代以前变化较小,而在近十年增幅十分明显;日较差季节变化大:夏季减小,冬季增加。20世纪90年代以前,最高、最低温度随海拔高度变化不大,而近20年随海拔高度升高,最高、最低温度的变化趋势几乎都是先减小后增加。高海拔地区比低海拔地区对全球变化反应更明显。     

Response of a climate model to tidal mixing parameterization under present day and last glacial maximum conditions

Experiments with a climate model were conducted under present day and last glacial maximum conditions in order to examine the model’s response to a vertical mixing scheme based on internal tide energy dissipation. The increase in internal tide energy flux caused by a 120 m reduction in sea level had the expected effect on diffusivity values, which were higher under lower sea level conditions. The impact of this vertical diffusivity change on the Atlantic meridional overturning is not straightforward and no clear relationship between diffusivity and overturning is found. There exists a weak positive correlation between overturning and changes to the power consumed by vertical mixing. Most of the climatic response generated by sea level change was not related to alterations in the internal tide energy flux but rather to the direct change in sea level itself.     

Late Quaternary glaciation and equilibrium-line altitudes of the Mayan Ice Cap, Guatemala, Central America

The Sierra los Cuchumatanes (3837 m), Guatemala, supported a plateau ice cap and valley glaciers around Montaña San Juan (3784 m) that totaled ∼ 43 km² in area during the last local glacial maximum. Former ice limits are defined by sharp-crested lateral and terminal moraines that extend to elevations of ∼ 3450 m along the ice cap margin, and to ca. 3000-3300 m for the valley glaciers. Equilibrium-line altitudes (ELAs) estimated using the area-altitude balance ratio method for the maximum late Quaternary glaciation reached as low as 3470 m for the valley glaciers and 3670 m for the Mayan Ice Cap. Relative to the modern altitude of the 0°C isotherm of ∼ 4840 m, we determined ELA depressions of 1110-1436 m. If interpreted in terms of a depression of the freezing level during maximal glaciation along the modern lapse rate of − 5.3°C km^− 1, this ΔELA indicates tropical highland cooling of ∼ 5.9 to 7.6 ± 1.2°C. Our data support greater glacial highland cooling than at sea level, implying a high tropical sensitivity to global climate changes. The large magnitude of ELA depression in Guatemala may have been partially forced by enhanced wetness associated with southward excursions of the boreal winter polar air mass.     

Holocene paleoenvironmental changes inferred from diatom assemblages in sediments of Kusawa Lake, Yukon Territory, Canada

The southwest Yukon Territory, Canada, is an important region for recovering sensitive records of Holocene paleoclimatic change. More information is needed, however, to constrain the timing of the major Holocene climatic transitions, and to understand associated impacts on different ecosystems. For example, paleolimnological studies have focused on small lakes and ponds, but the history of large lakes has received little study. We analyzed diatom assemblages, species richness, valve concentrations, and biogenic silica, in the sediments of Kusawa Lake (60°16.5'N; 136°10.9'W; 671 m a.s.l.) to reconstruct the responses of this large (surface area = 142 km²), deep (Z_max = 135 m) freshwater ecosystem to Holocene climatic transitions. Diatoms colonized the lake soon after ice retreat, around 11,000 cal yr BP; assemblages throughout the record were dominated by planktonic types. Diatom concentrations and biogenic silica were high during the Holocene Thermal Maximum between 10,700 and 7300 cal yr BP, then began to decrease in response to cooling associated with orbitally driven reductions in insolation. Diatom assemblages shifted towards taxa with lower surface water temperature optima after 8300 cal yr BP, perhaps in response to abrupt and progressive cooling. Our study confirms that diatom assemblages in large lakes are sensitive to regional-scale paleoclimatic changes.     

Glacial paleoceanography off the mouth of the Mekong River, southern South China Sea, during the last 500 ka

We have analyzed core MD01-2392, ∼ 360 km east of the Mekong River mouth in the South China Sea (SCS). Over the past 500 ka, planktonic foraminiferal oxygen-isotopic values are consistently lighter than northern SCS and open-ocean records by up to 0.5‰, indicating the influence of run-off from the Mekong River during both glacial and interglacial periods. Carbonate content is higher during interglacials; sedimentation rates were higher during glacials. Increased sedimentation rates since 30 ka imply increased run-off during the last glacial maximum and Holocene Period. Contrary to general experience, in which it is classed as a warm species for temperature estimates, the thermocline-dwelling species Pulleniatina obliquiloculata increased its numbers during glacial periods. This implies an estuarine circulation and even brackish-water caps during glacial periods, reinforcing the sense of strong run-off. In an overall decline of warm water, the thermocline shoaled stepwise, with rapid rises across the glacial terminations. We infer that the southern SCS was opened to an influx of Indian Ocean waters through southern passages at those times of rising sea levels.     

驳施雅风“冰臼”“负球状风化”成因论

施雅风院士最近撰文指出笔者等近年来在中国东部发现的若干"冰臼"并非冰川遗迹,而是花岗岩经"负球状风化"而成。然而,他所谓的"负球状风化说"与当地的气候环境条件和客观存在的事实完全相悖。冰臼是地表负地形之一,现今是"堆积区"而不是"侵蚀区",以现代常见的侵蚀作用产生冰臼的所谓"风说"、"壶说"、"风雨说"、"差异风化说"、"负球状风化"等,都是不可能的。这些假说是对过去业己形成冰臼群进行不同成因推断,是对冰臼群成因的最大误解。冰臼只能由大量冰川融水沿冰川裂隙自上向下,以滴水穿石的方式形成滚流水钻,强烈冲击下伏基岩产生。从河流流水动力学特点上看,急流旋涡主要产生水平力为主,而冰臼的形成主要是垂直力才有可能产生,因此河流流水也是无法产生冰臼的。