雅鲁藏布江中游杰德秀古湖的发现及其意义

在雅鲁藏布江中游山南宽谷段发现了一套湖相沉积地层,形成时代为晚更新世晚期,为冰川阻江形成的堰塞湖沉积物。野外调查发现该套湖相地层在桑日县、乃东县、扎囊县等地均有出露,综合研究认为其为一个东起桑日县扎巴村、西至贡嘎机场附近的大型古堰塞湖泊,面积达700多km2。通过区域调查,该堰塞湖形成的湖相沉积地层在贡嘎县杰德秀镇出露最厚,厚10余m,主要由粉砂质黏土层、细砂层组成,水平纹层等湖相沉积特征明显,本研究称之为杰德秀古湖。杰德秀剖面顶部14C日历校正年龄为15 680～15 105 aBP,属于末次盛冰期。对比林芝古湖、格嘎古湖和松宗古湖等雅鲁藏布江下游堰塞湖的沉积物特征和形成时间,认为杰德秀古湖发育于末次盛冰期。沉积相特征和湖水库容量模拟分析表明,该堰塞湖发育时期,雅鲁藏布江中游山南宽谷段并未完全封闭,为一个湖水面积巨大的过水湖或吞吐湖,类似于现代过水水库。桑日县扎巴村雅鲁藏布江河谷两岸分布有冰碛物,其与雅鲁藏布江中游山南宽谷段的湖相沉积地层的时空关系指示杰德秀古湖与冰川阻塞河道有关,杰德秀古湖是由于冰川阻江形成的冰川堰塞湖。研究结果也进一步说明,雅鲁藏布江现代水系形成以前,其中游不存在面积巨大的众多古湖泊,雅鲁藏布江也不是溯源侵蚀疏干多个古湖泊而形成的,在古湖泊形成之前,雅鲁藏布江已经贯通。雅鲁藏布江流域内的古湖泊是由于气候变化、新构造运动或地震活动等原因造成河道堵塞形成的堰塞湖。     

Climatic Instability Study of Lanzhou since Last Glacial Maximum

INTRODUCTIONA great number of records from ice cores andocean sediments ( Bond et al., 1993; Dansgaard etal., 1993; Grootes, et al., 1993 ) reveal that theearth has experienced an integrated cool warm cli matic cycle since the LGM, in the course of which aseries of rapid and high amplitude oscillations oc curred (Chondrogianni et al., 2004). These impor tant oscillations, known as Heinrich events, Youn ger Dryas event and last deglaciation, initia…     

Climatic changes in Eurasia and Africa at the last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modelling

In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and modern CO₂ concentrations reveals that the effect of CO₂ on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in Europe show warmer winter temperatures under LGM CO₂ concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high altitude regions of Africa are wetter for LGM CO₂ concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency.     

The geomorphological impact of Loch Lomond (Younger Dryas) Stadial plateau icefields in the central Lake District,northwest England

Detailed geomorphological mapping has revealed evidence for the development of plateau icefields in the central fells of the English Lake District during the Loch Lomond (Younger Dryas) Stadial (ca. 12.9–11.5 ka). The largest plateau icefield system, which covered an area of approximately 55 km² (including outlet glaciers), was centred on High Raise. To the west, smaller plateau icefields developed on Grey Knotts/Brandreth and Dale Head, covering areas of 7 km² and 3 km² respectively. The geomorphological impact of these plateau icefields appears to have been minimal on the summits, where the survival of blockfields and other frost‐weathered debris (mostly peat‐covered) implies the existence of at least patches of protective, cold‐based ice. Ice‐moulded bedrock at some plateau edges, however, documents a transition to wet‐based, erosive conditions. Prominent moraine systems were produced by outlet glaciers, which descended into the surrounding valleys where their margins became sediment traps for supraglacial debris and inwash. In some valleys, ice‐marginal moraines record successive positions of outlet glaciers, which actively backwasted towards their plateau source. This interpretation differs from that of previous workers, who assumed an alpine style of glaciation, with reconstructed glaciers emanating from corries and valley heads. It is likely that plateau icefields were more common at this time in upland Britain than hitherto has been appreciated. Copyright © 2001 John Wiley & Sons, Ltd.     

AN AGCM +SSiB MODEL SIMULATION ON CHANGES IN PALAEOMONSOON CLIMATE AT 21 KA BP IN CHINA*

The numerical simulation experiment of climate at Last Glacial Maximum (LGM.21 ka BP) in China is made by using an atmospheric general circulation model (AGCM) coupled with land surface processes (AGCM+SSiB) and earth orbital parameters and boundary forcing conditions at21 ka.The modeled climate features are compared with reconstructed conditions at 21 ka from paleo-lake data and pollen data.The results show that the simulated climate conditions at 21 ka in China are fairly comparable with paleo-climatological data.The climate features at 21 ka in China from the experiment are characterized by a drier in the east and a wetter in the west and in the Tibetan Plateau as well.According to the analysis of distribution of pressure and precipitation,as well as the intensity of atmospheric circulation at 21 ka,monsoon circulation in eastern Asia was significantly weak comparing with the present.In the Tibetan Plateau,the intensity of summer monsoon circulation was strengthened,and winter monsoon was a little stronger than the present.The simulation with given forcing boundary conditions,especially the different vegetation coverage,can reproduce the climate condition at the LGM in China,and therefore provides dynamical mechanisms on the climate changes at 21 ka.     

末次盛冰期气候模拟及青藏高原冰盖的可能影响

在国际古气候模拟比较设计设定的边界条件下（其中包括：海洋表面温度，陆地冰盖，CO2浓度，地球轨道参数等变化）。本文汇报了利用中国科学院大气物理研究所的9层全球大气环流格点模式对末次盛冰期气候进行数值模拟的研究成果。结果表明：末次盛冰期全球年平均温度在低5．3℃，降雨量下降9％，其中大陆年平均降水量仅为目前的71％。通过与其他模式模拟结果和重建古气候资料的对比表明，该模式捕捉到了末次盛冰期干，冷的冰期气候特征。在此基础上，我们根据刘东生等的科研成果，初步探讨了青藏高原在末次盛冰期倘若存在一定范围的冰盖，那么该冰盖会对东亚区域古气候模拟结果产生怎样的冲击。     

西藏纳木错末次盛冰期以来的古植被、古气候和湖面变化

西藏纳木错湖相沉积的U系、14C年龄和孢粉分析结果表明,纳木错沿岸的拔湖约1.5～8.3m和8.3～15.6m的T1和T2分别形成于末次盛冰期以来约(11.81±0.10)～(4.22±0.09)kaB.P.期间和(28.2±2.8)kaB.P.左右。该套湖相层的孢粉组合、地层和湖岸堤的分布表明,在末次盛冰期期间,纳木错湖面主要波动于拔湖12～20m之间,但湖面最低可达拔湖约8m。区域植被主要为以蒿和莎草科为主、含松和桦的草原。在约11.8～4.2kaB.P.期间,湖面波动于拔湖2～9m之间,区域气候整体较为暖湿。其中全新世大暖期出现在约8.4～4.2kaB.P.期间,气候温暖湿润,区域出现针叶林或针阔叶混交林,气温可能比现今高约5℃,降水量可能比现今多100～200mm,湖面扩张并升高,最高可达拔湖约10m。     

冰川模型及其在古冰川模拟研究中的应用

冰川模型已广泛应用于预测未来的冰川变化。随着冰川地貌制图、测年和古气候研究的不断发展,冰川模型也逐渐用于模拟古冰川变化、估算古冰川发育时期的气候信息和探讨古冰川演化的气候驱动因素。本文综述用于古冰川模拟的两类模型：地貌-冰面剖面形态模型和物质平衡-冰川动力耦合模型,介绍不同冰川模型的原理、用于古冰川模拟的流程和利用地貌体进行模型参数校验的方法。在此基础上,以青藏高原及其周边山地为例,总结利用冰川模型恢复古冰川的范围、体积、平衡线等参数,估算不同冰川发育时期的温度和降水,以及评估测年数据及其恢复的古冰川期次和规模的案例研究。最后指出了利用冰川模型进行古冰川模拟研究存在的问题和未来发展趋势,为进一步加强和改进冰川模型在古冰川模拟研究中的应用,恢复古冰川的规模、演化过程及其气候驱动机制奠定基础。     

青藏高原末次盛冰期和全新世大暖期多年冻土边界变化的探讨

位于中低纬的青藏高原多年冻土是第四纪高原隆升和冰期气候叠加的产物,与高纬多年冻土相比,具有厚度薄和不稳定的特点,对全球变化反应敏感.因此,评价冰期-间冰期多年冻土扩张-收缩过程和其范围重建,是研究高原环境变化的重要工作.本文依据青藏高原及周边地区温度数据和《中国冰川冻土沙漠图》,对青藏高原现代大片多年冻土、岛状多年冻土和高山多年冻土分布进行恢复.依据来自冰川、冰缘和湖泊等证据,采用末次盛冰期气温较现代低7℃,全新世大暖期气温较现代高4℃,进行末次盛冰期和全新世大暖期多年冻土分布重建.重建结果表明:末次盛冰期多年冻土扩张明显,面积约为现代冻土面积的195％;末次盛冰期大片多年冻土几乎覆盖整个高原,岛状多年冻土向东扩张明显,向西范围逐渐收缩变窄,高山多年冻土在喜马拉雅山、祁连山和横断山脉等地区扩张明显.全新世大暖期多年冻土明显收缩,面积是现代多年冻土的73％;大片多年冻土收缩幅度较小,岛状多年冻土在高原东南部收缩明显,高山多年冻土在喜马拉雅山脉、祁连山脉、横断山脉等高海拔山地发育.