2万年来南亚季风演化历史

文章综合中国西南部位于不同纬度、不同海拔，代表不同地貌、气候和植被单元的星云湖、腾冲青海湖、泸沽湖和伍须海这4个湖泊2万年以来的孢粉、硅藻记录，揭示了中国西南部2万年来的气候变化与南亚季风演化历史。结果表明，在中国西南地区，冰盛期后首次开始升温的时间发生在19.0~18.0 ka（1 ka=1000 cal.a B.P.）之间。自冰消期首次开始升温以后，中国西南地区气候具有区域差异，即在中国西南部偏西偏南、受较单一南亚季风影响的区域，冰后期都存在H1冷事件、B/A暖期和YD冷事件，并于11.5 ka左右进入全新世，开始是缓慢增温增湿，随后在10.0 ka左右进入明显变暖湿的时期；全新世适宜期都出现在中全新世，然后进入晚全新世降温期，并逐步开始受到人类活动的影响。在中国西南部偏东、偏北的区域，还兼受东亚季风影响，导致气候效应更加复杂。中国西南地区2万年来的气候变化揭示南亚季风从19.0 ka左右开始逐渐增强，在季风逐渐增强的过程中，出现两次明显的季风减弱期，分别对应H1和YD时期；在11.5 ka左右，南亚季风进一步增强，并在中全新世达到最强，随后又逐渐减弱。对其机制探讨认为，冰盛期后首次升温及此后逐渐增温的趋势主要受夏季太阳辐射的控制，同时，温室气体浓度的变化也起了重要的调制作用。而增温过程中存在的H1和YD突然变冷事件与大西洋温盐环流减弱有关；南亚季风区的全新世适宜期发生在中全新世可能是受海平面上升、海表温度、冰量范围、9月太阳辐射量等多种因素的共同影响。

Evolution of the South Asian monsoon during the last 20ka recorded in lacustrine sediments from Southwestern China

Xingyun Lake is located in central Yunnan Province, Southwestern China with 1722 m a.s.l. Qinghai Lake is situated in southwestern Yunnan Province with 1885 m a.s.l. Lugu Lake, the third largest deepwater lake in China, is located on the boundary between Yunnan and Sichuan provinces with 2690 m a.s.l. Wuxu Lake is located in southwestern Sichuan Province, the eastern branch of the Hengduan Mountains on the southeastern margin of the Qinghai-Tibetan Plateau and its altitude is 3760 ma.s.l. These four lakes are located in different latitude and altitude regions in Southwestern China, and they represent different topographic, climatic and vegetation units. Four continuous sedimentary cores were recovered using a UWITEC piston corer from Xingyun, Qinghai, Lugu, and Wuxu lakes, respectively. The geographical location and water depth of these four cores are 24°21'33.4″N, 102°47'11.1″E, 8.5 m in depth; 25°7'56.8″N, 98°34'19.2″E, 6.3 m in depth; 27°43'08.4″N, 100°46'33.9″E, 69.3 m in depth; and 29°9'11.48″N, 101°24'21.6″E, 30 m in depth, respectively. 127 pollen samples from Xingyun Lake, 377 pollen samples and 600 diatom samples from Lugu Lake, 278 pollen samples from Qinghai Lake, and 276 pollen samples from Wuxu Lake were analyzed, respectively. These pollen and diatom records spanning ca. 20 ka(1 ka=1000 cal. a B.P.) reveal histories of climate change and the South Asian monsoon since 20 ka in Southwestern China. The results show that the initial Late Glacial warming in Southwestern China was at 19.0~18.0 ka. After the initial Late Glacial warming, the climate was different in different regions of Southwestern China. Namely, the Heinrich Event 1, the Bølling-Allerød warm period and the Younger Dryas event during the last deglaciation are ubiquitous in the west and south regions of Southwestern China, affected solely by the South Asian monsoon. The start of the Holocene is recorded at 11.5 ka. Since then, the temperature and humidity began to slowly increase. From ca. 10.0 ka, the climate warmed and humidified faster, and culminated at the mid-Holocene. Then, the climate deteriorated, accompanied by evidence for human impact. However, the east and north regions of Southwestern China are simultaneously affected by the East Asian monsoon, resulting in a more complicate climate effect. These climate changes since 20 ka in Southwestern China reveal that the South Asian monsoon began to strengthen gradually at ca. 19.0 ka. In the course of the gradual strengthening of the monsoon, there are two obvious monsoon weakening periods, corresponding H1 and YD events, respectively. The South Asian monsoon strengthened further at 11.5 ka and reached to the maximum at the mid-Holocene, and then the monsoon gradually weakened. Based on this study, we consider that the initial late glacial warming and the tendency of gradual increase of temperature are controlled by summer solar insolation. At the same time, the variations in greenhouse gas concentrations also modulate the overall structure. Whereas the Heinrich Event 1 and the Younger Dryas event may be related to the slowdown of the ocean's thermohaline circulation. Holocene climatic optimum occurred at the mid-Holocene in the South Asian monsoon regions, which was likely caused by the interplays of multiple large-scale boundary conditions, including direct and indirect insolation controls, sea-level change, sea-surface temperature, and remnant ice sheets.