大兴安岭阿尔山天池沉积物中正构烷烃记录揭示的全新世古气候变化

PALEOCLIMATIC CHANGES RECORDED BY N-ALKANES FROM THE LACUSTRINE SEDIMENTS OF ARXAN CRATER LAKE,IN THE CENTRAL GREAT KHINGAN MOUNTAINS,NORTHEAST CHINA DURING THE HOLOCENE

我国东北地区地处东亚季风北缘,在高、低纬过程的共同影响下,区域气候和环境变化较为复杂。文章选取大兴安岭中段的阿尔山天池长2.5 m的全新世以来的湖泊沉积岩芯(ACL17C)作为研究对象,对源于植物叶蜡的正构烷烃及相关代用指标进行分析,结果表明阿尔山天池自10.0 cal.ka B.P.以来,正构烷烃浓度所代表的湖泊及流域的生产力逐渐增强,并在5.0~3.0 cal.ka B.P.期间达到峰值,结合其他指标推测该阶段湖泊水位较高,有机物输入稳定,沉积速率较慢,气候温暖湿润,可能是全新世气候适宜期;2.3~1.5 cal.ka B.P.期间,正构烷烃浓度下降,沉积物颗粒变粗,指示湖泊水位突降,藻类含量上升,植被状况较差,可能发生了区域性的气候变干事件;1.5 cal.ka B.P.之后,相关指标显示植被状况转好,湖泊水位上升。通过对比大、小兴安岭及附近区域的植被及气候演化结果,表明该区域全新世气候适宜期可能发生在中、晚全新世,与其他地区记录的早、中全新世适宜期存在显著差异。

Arxan Crater Lake(ACL17C:47.32°N,120.41°E;1280 m a.s.l.)is one of the closed volcanic crater lakes at the Arxan-Chaihe volcanic field in the middle of Great Khingan Mountains,NE China,which is located at the northern margin of the East Asian monsoon.A 2.5 m sequence lake sediments of ACL17C was sampled at 3~6 cm intervals from the bottom to the top.The chronological sequence was determined by AMS 14C dating,and the age-depth model was established by Locally Weighted Spline method in the CLAM model which covered the whole Holocene.Here,we present n-alkane analysis on ACL17C to research the lake evolution process and climate change during the Holocene.The distribution of n-alkanes showed the following characteristics:a homologous series of n-alkanes was detected with carbon numbers ranging from nC17 to nC35,most samples show a single peak and the main peak is nC27;The short chain(<nC21)n-alkanes do not have obvious carbon number characteristics.The middle(nC23~nC25)and long(>nC27)chain n-alkanes have an odd over even carbon number predominance.These characteristics show that the organic matter in the lake came from both terrestrial and aquatic plant.Based on the result of n-alkane and above proxy indicators,the changes of lake environment and the evolution history of regional climate in Arxan Tianchi Lake during the Holocene as following:during the Early Holocene,the initial productivity of the lake and area were low,and the productivity of the lake gradually increased with the gradual warming and humidity of the climate and the rise of the lake water level.During the period of 5.0~3.0 cal.ka B.P.,the input of algae decreased,the input of terrestrial plants increased,the concentration of n-alkane reached the maximum,the productivity of the lake and area was more,the lake water level was high and stable,and the vegetation reached the most prosperous period,which was the most optimum period for the climate.Since 3.0 cal.ka B.P.,the water level decreased,the input of algae and herbs increased significantly.The climate was relatively unstable.In addition,during the period of 7.5~6.0 cal.ka B.P.,the water level rose suddenly,and a large number of terrestrial plants were imported.During 2.3~1.5 cal.ka B.P.,the water level dropped significantly and algae blooms proliferated,presumably as a result of a dry event of the region climate.Combined with the vegetation change and climate evolution of the in Arxan Tianchi Lake and nearby areas,the results show that the Middle-Late Holocene was the optimum period during the Holocene in Great Khingan Mountains,rather than Early-Middle Holocene.