用古土壤有机质碳同位素探讨青藏高原东南缘的隆升幅度

TENTATIVE ESTIMATE OF THE SOUTHEAST MARGIN UPLIFT OF QINGHAI-XIZANG PLATEAU USING ORGANIC CARBON ISOTOPE COMPOSITION OF PALEOSOLS

本文首先对玉龙山东麓不同海拔高度现代表土样品有机碳同位素组成进行研究 ,探讨土壤碳同位素组成与海拔高度的关系 ,然后对 0 .5～ 0 .7MaB .P .以来 3期古土壤样品的有机碳同位素组成进行了测定。结果表明 ,现代表土的有机碳同位素组成与海拔高度之间存在良好的相关性 ,应是植被随海拔高度变化的反映 ;不同年龄的古土壤虽然目前均出露于地表 ,但其深部碳同位素组成存在显著差异 ,表明古土壤有机质中仍可能包含了过去植被的信息。根据上述结果对青藏高原 0 .5～ 0 .7MaB .P .以来的隆升幅度进行尝试性估算 ,得到的隆升幅度约为 80 0m。

The Earth′s environment since the mid Miocene has experienced a series of drastic changes,such as the formation of the polar ice sheets,the consequent reorganization of the atmosphere ocean circulation systems and the deterioration of environments in a number of regions. The most remarkable environmental events in Asia were the inception and evolution of the monsoon systems and the aridification in central Asia,which led to the formation of the basic pattern of the modern environments in China,characterized by humid conditions in the southern part and arid conditions in the northwestern territory. The causes of these environmental changes remain unclear. It has been suggested by a number of studies that the tectonic events in East Asia,especially the uplift of Qinghai Xizang Plateau,may have played a vital role in forcing the global environmental changes.\;Although numerous progresses have been made in the past years in the reconstruction of the uplift history of Qinghai Xizang Plateau,controversies still exist in interpreting the timing,amplitudes and spatial variability of the uplift phases. It is therefore highly necessary to broadly collect evidence for reconstructing an accurate uplift history of Qinghai Xizang Plateau,which is crucial for understanding the relationship between the tectonic events and the major environmental changes.\;In this paper,based on twenty seven surface soil samples,we show firstly that the carbon isotope compositions of modern soil samples are closely related to elevation, and are therefore indicative to vegetation changes. Then,carbon isotope compositions of the paleosols formed on three glacial deposits of the last 0.5～0.7Ma were analyzed. The results indicate that the isotope compositions of the lower profiles of the paleosols formed at different ages have distinct value ranges,suggesting that they are still indicative to the past vegetations. Based on these data,we subsequently show that the southeast margin of Qinghai Xizang Plateau has experienced drastic uplift since 0.5～0.7Ma B.P. and the elevation increase is estimated to about 800m. This tentative estimate is consistent with our earlier one based on the properties of the paleosols.