Li Yu, Wang Nai'ang, Zhang Chengqi
College of Earth and Environmental Sciences, Center for Hydrologic Cycle and Water Resources in Arid Region, Lanzhou University, Lanzhou, China.
PLoS One. 2014 Mar 5;9(3):e90241. doi: 10.1371/journal.pone.0090241. eCollection 2014.
The mid-latitudes of East Asia are characterized by the interaction between the Asian summer monsoon and the westerly winds. Understanding long-term climate change in the marginal regions of the Asian monsoon is critical for understanding the millennial-scale interactions between the Asian monsoon and the westerly winds. Abrupt climate events are always associated with changes in large-scale circulation patterns; therefore, investigations into abrupt climate changes provide clues for responses of circulation patterns to extreme climate events. In this paper, we examined the time scale and mid-Holocene climatic background of an abrupt dry mid-Holocene event in the Shiyang River drainage basin in the northwest margin of the Asian monsoon. Mid-Holocene lacustrine records were collected from the middle reaches and the terminal lake of the basin. Using radiocarbon and OSL ages, a centennial-scale drought event, which is characterized by a sand layer in lacustrine sediments both from the middle and lower reaches of the basin, was absolutely dated between 8.0-7.0 cal kyr BP. Grain size data suggest an abrupt decline in lake level and a dry environment in the middle reaches of the basin during the dry interval. Previous studies have shown mid-Holocene drought events in other places of monsoon marginal zones; however, their chronologies are not strong enough to study the mechanism. According to the absolutely dated records, we proposed a new hypothesis that the mid-Holocene dry interval can be related to the weakening Asian summer monsoon and the relatively arid environment in arid Central Asia. Furthermore, abrupt dry climatic events are directly linked to the basin-wide effective moisture change in semi-arid and arid regions. Effective moisture is affected by basin-wide precipitation, evapotranspiration, lake surface evaporation and other geographical settings. As a result, the time scales of the dry interval could vary according to locations due to different geographical features.
东亚中纬度地区的特点是亚洲夏季风与西风带相互作用。了解亚洲季风边缘地区的长期气候变化对于理解亚洲季风与西风带之间千年尺度的相互作用至关重要。突变气候事件总是与大尺度环流模式的变化相关联；因此，对突变气候变化的研究为环流模式对极端气候事件的响应提供了线索。在本文中，我们研究了亚洲季风西北边缘石羊河流域全新世中期一次突然变干事件的时间尺度和全新世中期气候背景。从该流域中游和终端湖收集了全新世湖相记录。利用放射性碳和光释光测年法，确定了一个百年尺度的干旱事件，其特征是流域中下游湖相沉积物中有一层砂层，绝对年代为公元前8.0 - 7.0 cal kyr。粒度数据表明，干旱期流域中游湖面水位突然下降，环境干燥。先前的研究已经表明季风边缘区其他地方存在全新世干旱事件；然而，它们的年代学还不够精确，无法研究其机制。根据绝对测年记录，我们提出了一个新的假说，即全新世干旱期可能与亚洲夏季风减弱以及中亚干旱地区相对干旱的环境有关。此外，突变干旱气候事件与半干旱和干旱地区全流域有效湿度变化直接相关。有效湿度受全流域降水、蒸散、湖面蒸发和其他地理环境的影响。因此，由于地理特征不同，干旱期的时间尺度可能因地点而异。
