Yang Da-yuan

Features, Impacts and Causes of the High Temperature and Large Precipitation Event in the Tibetan Plateau and Its Adjacent Area During 40-30kaBP

Based on records of the Guliya Ice Core (35.6°N,80.5°E),pollen and high lake levels in the Tibetan Plateau and its adjacent area, there appeared a particular warm and wet stage with the mean temperature 2-4℃ higher and large precipitation much more than that of the present. The reconstructed precipitations with Kutzbachs water and energy balance model of closed lake basin are estimated up to 640mm, 560mm and 260mm in the Qinghai Lake, Chabyer Caka/Laguo Co, and Akesaiyi/Tianshuihai Lake, which are 1.7, 3, 5 times of that in the present respectively. The large precipitation, extended to the north slope of Qilian Mountains at the north, and to Yunnan Province at the south east of Qinghai Xizang plateau, had played an important role in the geomorphological evolution of some rivers originated from the Tibetan Plateau. The upstream river pattern of the Yellow River, same as present, had come into being for the Zoige Palaeolake overflowing to meet the headward erosion of the Yellow River in the borderland of Gansu and Sichuan Provinces. The formation of the deep channels with their bottom lower than present sea level in the Three Gorge area, in the middle reach of the Yangtze River, might be related to high speed flood resulted from the upstream large precipitation in the Tibetan Plateau. While flood currents were narrowish and hindered in the gorge channel, the high energy turbulent flow carried large quantitative gravels to cut the river bottom and formed the deep channels (formed before 40kaBP). Some 14 C dates of the buried woods between 40-30kaBP, in the gravel deposits at the channel bottom indicated the upper mechanism process. Some separated closed lake catchments in the Tibetan Plateau, such as, Serling Co, Nam Co and Pangkog Co, were merged into a great closed catchment by water overflow. This high lake level event is mainly caused by the particularly strong Indian monsoon for enhanced cross equator aircurrent from south hemiphere over Indian Ocean activated by high solar radiation of precession cycle in low latitude. On the other hand, we infer, in the period between the event H4 (35.5kaBP) and H3(28-27kaBP), the southward shifted westerlies, from the warm North Atlantic Ocean for northern Eurasia ice sheet gave passive impact on high lake levels for arousing much precipitation.

Chemical element transfer of weathering granite regolith in the Three Gorges Dam region of Yangtze River

Clearing up sediment and regolith on the foundation of the dam in the Three Gorges of the Yangtze River in 1999, riverbed were exposed. On the basis of weathering granite regolith sampled from different portions of the valley landforms, by analysing total chemical contents with X rays fluorescent slice and calculating proper value of chemical element transferring ratio and intensity, the transferring law of chemical elements in different portions of the landforms were concluded: 1) In various landforms of the river valley, the process of desilication is not distinct; 2) in weathering granite regolith of riverbed, easy soluble CaO and MgO are relatively enriched whereas A12O3 tends to decrease. The enriching rate of Fe2O3 is the greatest in various landforms of the river valley; 3) in weathering granite regolith of flood-plain, K2O and MgO contents are relatively enriched; 4) the weathering granite regolith of valley slope is a typical north subtropical weathering regolith, and its chemical weathering degree is in the transition phase from early to middle period; and 5) there is an opposite layer where K2O is relatively leaching and Na2O relatively enriching in 6.5 m depth of all weathering granite regolith.

Environmental changes in the eastern China during the Late Pleistocene

The environmental changes during the Late Pleistocene were more obvious in the eastern China than in other areas at the same latitude, which either between northern and southern, or between land and sea of the eastern China were mostly non-synchronous. The transitional period prior to the last glaciation came about in the northern part of the eastern China about 115,000 yr. ago. The desert environments of the inland of the north China were developed both in the glacial maximum and in the warm interglacial period, but the loess accumulation mostly took place during the glacial period. The sand dunes and the periglacial solifluctions in the lower Changjiang (Yangtze) River region were formed during the last glacial period. The event of lowest surface temperature occurred at 98,000 yr. B.P. and 59,000 yr. B.P. in the northern part of the South China Sea but not during the full-glacial stage. The phenomena mentioned above were the result of the following reasons that the paleo-environmental changes in the eastern China were controlled by the common factors leading to the changes of global environment, on the other hand, the different changes were related to the particular of East Asia as well as a time lag required by the adjusting process of varied changes themselves.The environmental changes during the Late Pleistocene were more obvious in the eastern China than in other areas at the same latitude, which either between northern and southern, or between land and sea of the eastern China were mostly non-synchronous. The transitional period prior to the last glaciation came about in the northern part of the eastern China about 115,000 yr. ago. The desert environments of the inland of the north China were developed both in the glacial maximum and in the warm interglacial period, but the loess accumulation mostly took place during the glacial period. The sand dunes and the periglacial solifluctions in the lower Changjiang (Yangtze) River region were formed during the last glacial period. The event of lowest surface temperature occurred at 98,000 yr. B.P. and 59,000 yr. B.P. in the northern part of the South China Sea but not during the full-glacial stage. The phenomena mentioned above were the result of the following reasons that the paleo-environmental changes in the eastern China were controlled by the common factors leading to the changes of global environment, on the other hand, the different changes were related to the particular of East Asia as well as a time lag required by the adjusting process of varied changes themselves.

Geomorphologic characteristics and origin of the valley bottom troughs at the site of Three Gorges Dam

This paper describes valley bottom troughs of the Changjiang River and infers the geomorphologic development of troughs. Based on the morphology of the troughs, the following conclusions are drawn. (1) The deep troughs on the Three Gorges valley bottom are formed by river downcutting along the structural zones on the background of regional tectonic uplift at about 40–30 ka BP. (2) When river downcutting occurred in the river bed of Changjiang, the jets current (particularly eddy current) with a large number of pebbles ground and eroded the valley bottom, resulting in trough formation and deepening. Meanwhile, water currents with gravels and pebbles eroded the bank and the left wall of No.76 trough as well as the right wall of No.77 trough by striking, scouring, horizontal and vertical grinding. (3) The depth of the trough is mainly determined by the intensity of the water current and the consistency of bedrock against erosion, and is not controlled by the altitude of the sea level as the base level of erosion.

Geographic environment change and flood catastrophe in huaihe river basin during last 2000 years

During the last 2000 years, flood and waterlogging catastrophes took place quite frequently in the Huaihe River Basin. In the authors’ opinion, these natural calamities have a very close relation to the evolution of Hongze Lake. Formed initially within a man-made dyke that was built in the Han Dynasty about 2000 years ago, Hongze Lake brought out headward accumulation developing in the middle reaches of the Huaihe River, with its continuous aggravation on lake-bottom and consequent water-level rise. It was estimated that, on an average, there were 3400 × 104 t sediment per kilometre per year deposited on the river bed from Lutaizi to Bengbu. Therefore, the rising of water-level and the drainage difficulty in the middle reaches of the Huaihe River aggravated local flood and waterlogging catastrophe here.