西藏纳木错过去200年来的环境变化*

文章通过纳木错浅钻沉积硅藻研究,结合青藏高原湖泊现代硅藻-电导率转换函数,对过去200年来的湖水盐度(电导率)变化进行了定量重建。纳木错在小冰期冷期为淡水环境;小冰期结束后,湖水盐度开始增加;至20世纪60年代中期以来,盐度增加幅度更加明显。过去100年来湖水盐度的增加与钻孔粒度变化所揭示的入湖径流量的增多,反映了增温背景下湖泊水文的响应特点。温度的上升,一方面引起了流域冰雪融水补给量的增加,但另一方面,湖泊水量平衡明显偏负,说明小冰期结束以来,尤其是最近40年,冰融水的增加并不足以弥补湖泊水量的负平衡。由此提出蒸发量在湖泊水量平衡中起重要作用,温度是影响湖泊水文变化的关键因子。区域湖泊综合对比结果进一步表明,不同湖泊盐度和水文变化趋势一致,反映了封闭湖泊对区域气候变化的共同响应特点。

ENVIRONMENTAL CHANGES OF NAMU CO, XIZANG DURING THE PAST 200 YEARS

Namu Co is a large oligo-saline lake in Xizang Plateau. It lies at an altitude of ca. 4718m a.s.l. and covers an area of ca.1961.5km 2 . The runoff into the lake is mainly supplied by the melt-water of glacier originated from Nianqingtanggula Mountain to the south of the lake. In order to examine the relationship between lake level fluctuation and strong retreat of modern glacial under the warming background and to understand how lake water balance responded to climate changes in the past decades,paleolimnological techniques were used to reconstruct the past environment changes based on 210 Pb,grain size,total organic carbon and fossil diatom from a shallow sediment core obtained from the south part of the lake,as well as the regional diatom transfer function built in Qinghai-Xizang Plateau lakes. The quantitative reconstruction of lake salinity shows that Namu Co was freshwater in the last cold period of the Little Ice Age (LIA),and water salinity started to increase at the end of the nineteenth century. A large-scaled increase of water salinity occurred after the middle of 1960s. However,the increasing percentage of coarse particle size after 1870A.D. indicates an obvious increase of runoff after LIA. The increase of both salinity and water discharge into the lake since 1870A.D. reflects the hydrologic response to the warming climate in Namu Co. The rise of the temperature drove more melt-water of the glacier,resulting in the increase of runoff inferred from the grain size. On the other hand,the negative water balance inferred from diatom and water salinity after LIA,especially in recent 40 years,suggests that the increase of runoff as well as summer rainfall caused by the enhanced Indian summer monsoon failed to compete the lose of lake water. We propose that the evaporation driven by increased temperature plays an important role in water balance and influences the hydrologic process of Namu Co. The consistency between the lake salinity and hydrological changes by regional comparison reveals that the common response of these inland lakes to regional climate changes further supports our conclusion.