冰期之青藏高原新研究

冰碛测年技术近些年有很大进展。古里雅冰芯底部3 6Cl测年和昆仑山垭口老冰碛ESR测年已表明 ,青藏高原在昆仑黄河运动之后的 70万年前就进入了当时的冰冻圈 ,与全球性的冰期旋回相耦合 ,发生了最早的冰川作用。最近的冰碛ESR测年表明 ,高原东北边缘的祁连山地区至少在 40多万年前的氧同位素 12阶段发生了冰川作用 ,天山高望峰冰碛测年显示了同样的结果。最近的研究进一步表明 ,高原第四纪冰川仍然是以横亘在高原上的高大山系为依托发育的 ,各地各次冰川作用的清晰界限和有限范围不允许作连续大冰盖的推断。稻城冰帽南面断陷谷地中深风化红色冰碛物粘土的硅铝率和硅铝铁率达到了红壤的水平 ,其北面理塘盆地灰岩风化壳也显示很深的风化 ,此二者海拔在40 0 0m上下 ,有相当老的年龄 ,说明它们从未受到大冰盖的袭劫。研究进一步表明 ,现代和冰期雪线由高原边缘向腹地升高 ,羌塘地区高出边缘 15 0 0m左右 ,生动表现了“亚洲干极”的作用。广泛分布的湖群说明羌塘地区是一个大江大河尚未伸入的内流地区 ,意味着青藏高原是个年青的高原 ,而不是什么大冰盖的证据。

A NEW STUDY ON QINGHAI-TIBET PLATEAU IN ICE AGES

Many technologies have been used to date moraines in recentyears. The 36Cl dating of bottom of ice core of Guliya ice cap, western Kunlun and the ESR dating of moraine at Kunlun pass indicate that the Qinghai-Tibet Plateau had grown up into the Cryosphere at 0.7 Ma B.P.The recent work on moraine ESR dating shows that glaciation occurred at least at 0.46 Ma B.P. ( stage 12 of oxygen isotope of deep sea ) in the Qilian Mountains. The same situation was also shown in the Tianshan Mountains. Recent studies indicate again that Quaternary glaciations occurred just around the high mountains over the Plateau. The clear bounds and limited scales of multiple glaciations are incompatible with imagining a successive ice sheet on the Plateau. The very deeply weathered moraine with 2.42 of w(SiO2)/w(Al2O3) and 2.35 of w(SiO2)/w(R2O3) at the southern margin (3 850m) of Daocheng ice cap and the red crust in Litang Basin (4 100m) show that they are of considerable age and never suffered mopping-up operations of an ice sheet. The present and ice age snowlines of Qinghai-Tibetan Plateau have been distributed with an ascendant trend (the difference is about 1 500m) from the surrounding margin to the hinterland Qiangtang, describing vividly its function of “The Dry Pole of Asia” suggested by some scientists early. The lakes mean that Qiangtang Plateau is an interior drainage area where the big rivers have not yet stretched in, showing an younger Qinghai-Tibetan Plateau rather than an ice covered Plateau!