以我国西南地区邛海钻孔沉积物为研究对象,利用高分辨率粒度数据,结合正构烷烃指标特征,探讨了研究区30cal.kaB.P.以来的植被演化和气候环境变化过程。研究表明,邛海沉积有机质主要来自于外源的高等陆生植物和内源的水生生物(低等菌藻类生物和挺水、沉水和漂浮植物)。沉积物粒度组成及正构烷烃代用指标变化特征表明,邛海地区过去30cal. ka B.P.以来的气候环境变化过程大致分为4个阶段:1)29.1~ 23.0 cal. ka B.P.,沉积物中值粒径(Md)均值为22.4 mum ,正构烷烃平均碳链长度(ACL值)和陆生植物正构烷烃相对输入量指标(P_(wax)值)呈高值,指示相对暖湿的气候条件,湖区以草本植物为主;2)23.0~19.0cal.ka B.P.,沉积物中值粒径明显减小,均值为15.4 mum,同时ACL值、P_(wax)值为低值,指示温度较低,气候干燥,这一时期植被类型以木本植物为主;3) 19.0~11. Ocal. ka B.P.,中值粒径减小,均值为1.4 mum, ACL值总体较低,P_(wax)值由高转低,研究区气候从由暖湿向冷干转变,木本植物扩张;4)自11.0 cal. ka B.P.以来沉积颗粒表现出细一粗一细的变化特点,中值粒径均值为11.5 mum, ACL值、P_(wax)值由低值先升高后降低,揭示这一时期邛海气候冷干一暖湿一冷干的变化过程,湖区经历了草本植物先扩张后收缩的过程。特别的是,邛海沉积物记录的23.0~ 19.0 cal. ka B.P.、 12.8~ 11.7 cal. ka B.P.和9.0~5.0 cal. ka B.P.这3次典型气候时期,在时间和气候特征上与末次冰盛期(LGM)、新仙女木事件(YD)和全新世气候适宜期(HCO)相符,可能是对西南季风强度变化及地质历史时期典型气候事件的响应。
英文摘要:
Qionghai Lake is located in the transition zone of southeast Tibetan Plateau and southwest of Sichuan Province,SW China. In order to reconstruct the paleoclimate and paleoenvironment of Qionghai Lake area, a continuous 23.9-m long sediment core(27°50'42.92"N, 102°20'9.25"E) was obtained in 2013 in the northeast of Qionghai Lake. The lithology is mainly lacustrine brown-yellow and grey silt. The chronology was established based on the AMS 14C data of twelve plant remains, extending back to about 29.1 cal. ka B.P. A total of 72 samples were taken at a 30-cm interval for n-alkanes analysis and 186 samples were collected at a 10-cm interval for grain-size analysis. The bimodal distribution of n-alkanes from Qionghai Lake sediments indicate that the organic matters were derived from both exogenous higher land plants and endogenous aquatic plants (including aquatic algae,and emergent, submerged and floating macrophytes). Based on the geographical location of Qionghai Lake and its characteristics, the paleoclimate significances of grain-size of Qionghai Lake sediments were discussed. In general,variations of grain-size data of Qionghai Lake sediments could effectively indicate paleo-precipitation history in the study area. When the southwest summer monsoon was strong, the coarse particles were easy to deposit in the study area due to the heavy precipitation and strong hydrodynamics, indicating a humid climate condition. Whereas, the increase of fine fraction of Qionghai Lake sediments may correspond to a relatively dry climate. Besides, Paq(aquatic plant n-alkane proxy) and P_(wax) (land plant n-alkane proxy) were also applied to infer the climate conditions. Generally, high Paq and low P_(wax) values reflect a low input of terrestrial plants, and thus a weak surface runoff under a relatively dry climate. According to the grain size characteristics and n-alkane-based proxies, the paleoclimate conditions in Qionghai Lake area can be divided into four stages somewhat arbitrarily : Stage I(29.1 ~23.0 cal. ka B.P.),the median particle diameter(Md) of Qionghai Lake sediments were 22.4 mum,reaching the highest value in the entire profile,and the values of ACL (average chain length of n-alkanes) and P_(wax) were high, indicating a relatively warm and moist climate. And the vegetation types were maily grasses. Stage II(23.0~19.0 cal. ka B.P.),the Md value decreased to 15.4 mum, with low values of ACL and P_(wax), suggesting a cold and dry climatic condition. And the vegetation was dominated by woody plants during this stage. Stage III(19.0~ 11.0 cal. ka B.P.),the Md value of sediments decreased to 10.4 mum. Additionally the values of ACL were low and P_(wax) decreased, indicating climate changed from warm-moist to cold-dry condition. And the woody plants expanded during this period. Stage IV(11.0 cal. ka B.P. to present),the median grain size were 11.5 mum. However, the sediment particles exhibited fine-coarse-fine periodical fluctuations, and values of ACL and P_(wax) increased and then decreased,reflecting the cold and dry climate changed to warm and humid condition, and then shifted to cold and dry climate again. Meanwhile,the grasses in Qionghai Lake area expanded in the Early to Middle Holocene and then shrinked in the Late Holocene. It is worthy to note that the specific climate conditions in Qionghai Lake area in the interval of 23.0 ~19.0 cal. ka B.P.,12.8 ~11.7 cal. ka B.P. and 9.0 ~5.0 cal. ka B.P.,were coincident with the Last Glacial Maximum (LGM),Younger Dryas(YD)event, and Holocene Climatic Optimum(HCO),respectively, probably in response to the varying intensities of southwest summer monsoon and typical climatic events.