Cosmogenic nuclide measurements in southernmost South America and implications for landscape change

We measured in situ ¹⁰Be, ²⁶Al and³⁶Cl on glacial deposits as old as 1.1 Myr in the southernmost part of Patagonia and on northern Tierra del Fuego to understand boulder and moraine and, by inference, landscape changes. Nuclide concentrations indicate that surface boulders have been exposed for far less time than the ages of moraines they sit upon. The moraine ages are themselves constrained by previously obtained ⁴⁰Ar/³⁹Ar ages on interbedded lava flows or U-series and amino acid measurements on related (non-glacial) marine deposits. We suggest that a combination of boulder erosion and their exhumation from the moraine matrix could cause the erratics to have a large age variance and often short exposure histories, despite the fact that some moraine landforms are demonstrably 1 Myr old. We hypothesize that fast or episodic rates of landscape change occurred during glacial times or near the sea during interglacials. Comparison with boulder erosion rates and exhumation histories derived for the middle latitudes of semi-arid Patagonia imply different geomorphic processes operating in southernmost South America. We infer a faster rate of landscape degradation towards the higher latitudes where conditions have been colder and wetter.     

关于南国冰臼群成因的商榷之二

南国冰臼群分布区大量冰川堆积、冰川条痕石及变形砾石的发现，再次有力地证明是“冰臼”不是“壶穴”的正确结论，由冰川漂砾经融冻作用形成的“石海”，与化学风化作用产生的花岗岩“石蛋”地貌特征，有天渊之别，不应混为一谈；边滩是由河流冲积物组成，不能与基岩冰床相提并论；圆滚水钻与急流旋涡及风蚀作用，形成的动力学性质、方向和方式完全不同，不该阴错阳差；节理、构造只影响冰臼的形态和发育程度，不是控制形成的动力学性质、方向和方式完全不同，不该阴错阳差；节理、构造只影响冰臼的形态和发育程度，不是控制冰臼的发育部位，不致于是非不辨，南国地区古冰川遗迹的大量发现，证明距今约二三百万年的第四纪早期确实曾发生过大规模的古冰川运动，为争论了半个多世纪的中国东部中低山区有无第四纪冰川问题，从此画上一个圆满的句号。     

质疑粤西北的冰川遗迹

从地貌、沉积物、第四纪地层关系、古气候、古环境等方面论证了广东封开-怀集-带所谓的冰川谷、冰蚀三角面、悬谷、角峰、刃脊、冰半、鼻山尾、冰碛砾石、蛇形丘以及近百万年来发生过3次冰期，是既不符合地学基础理论也不符合野外实际，所以是不确实的，那里没有第四纪冰川遗迹。     

Non-classical types of loess

The purpose of this contribution is to describe the sequence of physical and chemical processes resulting in the sediment-type named loess, a fine-grained sediment deposit of universal occurrence. Owing to historical causes, loess has been (and still is) implicitly linked to glacial/periglacial environments among most naturalists. However it is known today that most eolian dust is deflated from tropical deserts. Hence, that sequence of processes is more comprehensive than the former narrow cold scenario. Six examples of different “non-classical” cases (from South America and Europe) that fit well to the loess definition are developed: 1) volcanic loess in Ecuador: pyroclastic eruptions/valley wind/mountain praire/silica structuring; 2) tropical loess in northeastern Argentina, Brazil and Uruguay: deflation of river and fan splays/savanna/iron sesquioxide structuring; 3) gypsum loess in northern Spain: destruction of anhydrite/gypsiferous layers in a dry climate/valley wind/Saharian shrub peridesert/gypsum structuring; 4) trade-wind deposits in Venezuela and Brazil: deflation in tidal flats/trade wind into the continent/savanna/iron hydroxide structuring; 5) anticyclonic gray loess in Argentina: continental anticyclone on plains/anti-clockwise winds and whirls/steppe/carbonate structuring. All these non-classical types conform to the accepted loess definitions and they also share the most important field characteristics of loess such as grain size, friability, vertical or sub-vertical slopes in outcrops, subfusion and others. Other cases can probably be recognized when systematically scrutinized.     

OSL dating of glacial deposits during the Last Glacial in the Terskey-Alatoo Range, Kyrgyz Republic

Optically stimulated luminescence (OSL) dating was applied to glacial and loess deposits in the north flank of the Terskey-Alatoo Range, Kyrgyz Republic, to elucidate the glacier chronology of the central Asian mountains during the Last Glacial. Moraines in five parts of study area were classified into four stages (Terskey Stages I–IV) based on their geographical position and elevation, and their moraine rock weathering. According to this classification, the oldest moraines (Terskey Stage I) were at 2100–2250 m a.s.l. and the second-oldest moraines (Terskey Stage II) were at 2400–2700 m a.s.l. Quartz samples from moraines of these two stages were used for OSL dating. The OSL ages of the quartz samples indicate that glacier expansion in the Terskey Stage II occurred between 21 and 29 ka BP.     

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.     

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.     

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。     

Pattern of abrupt climatic fluctuation in the East Asian Monsoon during the Last Glacial: Evidence from Chinese loess records

Records of two loess sections located in mid-eastern and western margins of the East Asian Monsoon area captured 20 Dansgaard-Oescher events and six Heinrich events. All these suggested that the climate in the East Asian Monsoon area fluctuated rapidly on millennial to century timescales during the whole Last Glacial. We found that these loess-based events of rapid climate fluctuations were generally synchronous with those of GRIP records, but that there were differences between the Shagou loess section in the west and the Wangguan loess section in the east: the former was more sensitive to climate change than the latter. Compared with earlier studies on loess records covering the Last Glacial from neighboring areas, we discovered that the magnitude of Dansgaard-Oeschger cycles decreased gradually from west to east and we suggest that it resulted from the combined effect of the Westerlies and the East Asian Monsoon.