季风与西风对青藏高原全新世气候变化的影响:同位素证据

青藏高原气候变化在冰期-间冰期、千年、十年际和季节尺度上受亚洲季风和西风环流的交互影响,表现出显著的区域性特征。然而全新世以来青藏高原气候变化的机制还不甚清楚,主要原因之一是缺少指示意义明确的古气候代用指标。课题组近年来利用叶蜡氢同位素(δDwax)重建了高原东北部的青海湖、中北部的令戈错、中部的达则错、西部的班公错和阿翁错不同时间尺度的大气降水同位素记录,本文对上述工作进行总结,并结合青藏高原全新世以来已发表的其他地点的同位素和古水文记录,揭示全新世以来季风与西风对青藏高原不同区域气候变化的影响。结果表明:1)早全新世青海湖、令戈错、班公错和阿翁错4个湖泊均主要受夏季风影响,夏季风可以影响到青藏高原的大部分地区,此时夏季风在青藏高原的最北界限可能位于青海湖以北、克鲁克湖以南。2)中全新世青海湖、班公错和阿翁错受夏季风影响逐渐减弱;然而令戈错在7.0~4.5 ka水汽主要来源于西风环流。3)晚全新世青海湖和班公错受季风的影响进一步减弱;西风在3.5~1.7 ka和2.0~1.0 ka分别影响到高原中部的令戈错和达则错;晚全新世阿翁错受冰川融水补给影响降水同位素异常偏负。4)本研究表明在中晚全新世季风较弱的时期,西风能够深入到青藏高原内部地区,给高原内部地区带来冷湿的水汽。

Influence of monsoon and westerlies on Holocene climate change in the Tibetan Plateau: Isotopic evidence

In recent years, we have reconstructed five independently dated compound specific hydrogen isotope records of sedimentary leaf waxes at different time scales from lakes on the Tibetan Plateau, Qinghai Lake(37°01'N, 100°12'E), Lingge Co(33°51'N, 88°35'E), Dagze Co(31°50'N, 87°30'E), Bangong Co(33°42'N, 78°41'E) and Aweng Co(32°42'N, 81°38'E). This study summarized the above records, and combined these records with existing isotopic and hydrological data to investigate variations in the influence of the summer monsoon and the westerly jet on the moisture budget of the Tibetan Plateau during the Holocene. The results show that:(1)δD values of precipitation at the above lakes were relatively negative, suggesting the influence of summer monsoon, indicating that the summer monsoon can affect most parts of the Tibetan Plateau. During the early Holocene the boundary of the summer monsoon located between Lake Qinghai and Lake Hurleg.(2) During the Middle Holocene δD values of precipitation at Qinghai Lake, Bangong Co and Aweng Co were gradually positive, indicating the weakening of the summer monsoon. However, during 7.0~4.5 ka, periods of more positive δD values of precipitation correspond to wetter intervals inferred from lake level high stands at Linggo Co in the northern Tibetan Plateau and likely reflect variations in moisture associated with the westerly jet.(3) During the Late Holocene, the positive δD values of precipitation at Qinghai Lake and Bangong Co suggesting the continuing weakening of the summer monsoon. The depleted δD values of precipitation at Aweng Co during the Late Holocene was due to the influence of the melt water as a result of the increased temperature. The positive δD values of precipitation at Lingge Co correspond to high lake levels indicating the influence of Westerlies during 3.5~1.7 ka. Precipitation isotope ratios tended to be lower between 2.0~1.0 ka, implying a cool and humid climate and reflecting the influence of Westerlies when the monsoon weakened.(4) Collectively, the hydrogen isotope records at these lakes document millennial and centennial scale variations in the strength of the summer monsoon systems and concurrent changes in the westerly jet. During the middle to Late Holocene, when the monsoon weakened, the Westerlies could have been able to penetrate into the region, supplying cold and wet precipitation.