The Last 15 kyr Paleoclimate Record of Yanhaizi Lake, Inner Mongolia

• Main Authors: Hsin-chi Lan, 藍信企
• Other Authors: Chen-tung Arthur Chen
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/67065960195206129157

碩士 === 國立中山大學 === 海洋地質及化學研究所 === 87 === This thesis is focused on the paleoclimatic reconstruction of Yanhaizi, a saline lake located near the northern limit of the East Asian Summer Monsoon. With paleoclimatic records collected nearby and from East Asia, I have attempted to understand the mechanisms controlling the climate in this region. Chronological data show that the sediments in the core Ya01 has deposited since 14.1 kyr B.P. Information from stable isotopes, the first appearance of evaporite, sedimentary texture and structure of salt minerals, and processes of mirabilite crystallization, suggests that the stable isotopes and soluble elements in the core may have been altered by early diagenesis. Coarse sediments are deposited during the shrinkage phase of the lake, or during the period when sand dune reactivated. These sediments have low organic carbon contents and high maturity indices (i.e. ratios of feldspar to the sum of feldspar and quartz), indicating that they were deposited in an arid environment. On the contrary, fine sediments were deposited during periods of high lake stand due to the peripheral migration of the sand-flat sedimentary environment and the better preservation of the wind-carried dusts in the water body. Their high organic contents and low maturity indices imply that they were deposited in a humid environment. Three humid phases, 11.7-9.1, 6.4-5.8 and 3.8-2.6 kyr B.P., are recovered from the core Ya01, with the first the wettest, followed by the third phase. The above dry and wet phases are consistent with those recovered from the same arid-semiarid transition zone, but are contrary to the humid Holocene Optimum revealed in east China and the alpine Retreat Lake in Taiwan. The discrepancy may be due to the relative insensitivity to humidity changes of these two areas, because they were under the total influence of the summer monsoon. Besides, the high temperature Holocene Optimum as revealed in the Okinawa Trough and the northern South China Sea is correlated to the dry phases in Yanhaizi. However, the 4-2 kyr B.P. coldest period in the Holocene corresponds to the third wet phase in Yanhaizi. We argue that enhanced evaporation over higher monsoon precipitation is the key factor in determining the effective humidity in the region near the northern bourdary of summer monsoon