中晚全新世东亚季风演化历史及其气候事件研究

【作者】 李伟；

【导师】 陈仕涛；

【作者基本信息】 南京师范大学 ， 自然地理学， 2015， 硕士

【摘要】 当前极端异常气候事件频繁发生,并由此带来了环境恶化、物种灭绝、水资源短缺等一系列全球问题,不仅给社会经济发展带来巨大损失,而且给人类生存带来严重威胁。研究这类事件发生的频率,探索气候突变的原因是目前迫切需要解决的问题。中晚全新世百年尺度季风事件的内部结构及其驱动机制存在争议,年际尺度异常干旱和降水事件的发生频率、幅度尚不清楚。本文基于湖北神农架龙腑宫洞一支石笋的14个高精度230Th年龄和1410组氧碳同位素重建了6.60～0.45kaBP时段高分辨率东亚季风演化历史。石笋6180约在5.3kaBP、4.2kaBP、2.8kaBP、1.9kaBP、0.9kaBP及小冰期显著偏正,幅度达0.7‰～1.1‰,指示若干百年尺度的弱季风事件；其中,永兴洞石笋记录的"2.8ka”事件与和尚洞石笋记录的"8.2ka"事件内部结构和转型特征十分相似：弱季风事件以夏季风缓慢减弱开始,迅速增强结束,事件内部有两个次一级的振荡旋回(两峰三谷结构)；另外,龙腑宫和董哥洞记录"4.2ka"事件内部结构与这两个弱季风事件也基本相似。这些季风突变与北大西洋Bond事件几乎同步发生、内部结构非常相似,且基本都发生在太阳活动极小期,说明全新世百年尺度的弱季风事件可能受太阳活动和北高纬气候共同驱动。石笋δ13C在重建古环境、古气候研究方面显示出巨大潜力,需要我们进一步加大对碳同位素的沉积过程和机理研究,然而,石笋δ13C记录在沉积过程中受到多种局地因素的控制且在大区域范围内缺乏重现性,导致其在古气候重建方面困难重重。据此,我们选取龙腑宫洞、董哥洞内3根高分辨率(2-4年)石笋调查方解石δ6C记录的时空变化模式及其与亚洲季风变化的关系。观察发现,中晚全新世石笋δ13C与δ18O信号虽在低频变化上不一致,即碳同位素不存在显著轨道-千年尺度的长期趋势,而是围绕一个稳定的平均值上下振荡,但δ13C十年-百年的高频变化与δ180记录却非常一致,并贯穿整个记录,相关系数R2都达到0.65以上,说明613C在百年-十年尺度上和δ180一样也能很好的反映当地的气候、环境变化。中晚全新世石笋δ13C和太阳活动代用指标的交叉谱分析结果表明两者具有显著的～200年共同周期,暗示δ13C百年尺度的变化受控于太阳活动。这些观察表明区域性的水文循环通过影响土壤湿度平衡控制着百年-十年尺度的δ13C变化；也就是说石笋613C通过气候变化控制土壤过程能很好的示踪百年-十年尺度气候变化。其次,为了研究更短时间尺度的异常气候事件,我们刮样获得980AD～1504AD时段2370组氧碳素数据,分辨率可达季节,记录显示中世纪时期大部分属于暖干气候,仅在980～1055AD.1340～1480AD时期相对湿润。值得注意的是,石笋δ18O、δ13C在～1390AD快速正偏,可能指示小冰期的开始。通过进一步分析,在980～1504AD时段我们鉴定出大约12次持续1～4年的极端干旱事件和13次极端降水事件,石笋δ18O偏离30年气候平均状态达0.5‰～0.7‰(相当于4.2ka气候突变事件的1/2～2/3),这些异常气候事件给人类的生产、生活带来了巨大威胁。将这些年际尺度的气候振荡与太平洋十年际振荡(PDO)、厄尔尼诺(ENSO)变幅年代际振荡对比发现他们在变化趋势上基本相似,并拥有60-70年的共同周期,说明PDO/ENSO有可能通过亚洲夏季风强度影响中国季风区降水变化。更多还原

【Abstract】 The Longfugong Cave (31°38’N, 110°46’E,564m above sea level) is located at Mt.Shennongjia, Hubei Province, China. The mean annual precipitation at the study site is between 1500mm and 2000mm, most of which falls in May to October. The relative humidity and temperature in the cave is about 100% and 180C, respectively.We present a stalagmite (No.LFG10) collected from this cave, which is 2040-mm-long, with a diameter of 150 to 200 mm. And few lithological changes can be observed from the sample along the growth axis. Age model for the stable isotopic sequences (δ18O and δ13C) is established by linear interpolation between individual dated points. Fourteen 230Th dates with typical analytical errors (2σ) ranging from 3 to 72 yrs, were conducted at the Isotope Laboratory of Institute of Global Environmental Change, Xi’an Jiaotong University, China. The dating results are in stratigraphic order. A total of 1410 stable isotope samples drilled along the growth axis with 0.5-mm-diameter carbide dental drill. The analysis was performed by Finnigan-MAT 253 mass spectrometer fitted with a Kiel Carbonate Device, at the Isotope Laboratory of College of Geography Science, Nanjing Normal University. It provides a detailed record of East Asian monsoon variability between 6.60 ka B.P. and 0.45 ka B.P. When compared with high-resolution and precisely-dated stalagmite DA, DAS, HS4 and SB43 generated from East Asian monsoon site, the general well correlation of the record with other cave records in the contemporary growth period, suggesting that variations of the stalagmite LFG10 δ18O is reliable and can reflect broadly regional changes in variations of East Asian monsoon circulation.Although the record generally follows boreal summer insolation, it is punctuated by several weak monsoon events (5.3 ka BP,4.2 ka BP,2.8 ka BP,1.9 ka BP,0.9 ka BP and LIA). These century-scale abrupt monsoon events probably relate to distinct decrease in the solar activities, and link to the North Atlantic Ocean ice rafting events. In addition, their inner characteristics were agreed, especially our high resolution δ18O record (YX57) reveals a striking weak summer monsoon event (2.8 ka event) with δ18O progressively increase at onset and sudden decrease at the end. It displays a double peak and three plunging structure, similar to that of 8.2 ka event recorded by stalagmite H4 from Heshang Cave in Hubei. The internal structure of the 4.2 ka event recorded from stalagmite LFG10 is as well similar to that of DA. Interestingly, the period of ancient culture and civilization decline in our country well accords with the weak monsoon event, indicating climate change may play an important role in the development of human society.Speleothem δ13C used in paleoclimate reconstruction is limited by multiple controls associated with site-specific processes and lack of reproducibility between regional records. The δ13C fluctuations in our record remains stable, around its mean value, and is characterized by persistence of centennial cycles like other two stalagmite δ13C record (DA and DAS); while precessional-to millennial-scale changes are observable in the δ18O signal. Despite of the disagreement in low-frequency pattern, but when detrended, centennial-scale variations of the residual δ18O (δ18Od) signals are well mirrored in the δ13Cd. This tight linkage indicates that centennial-scale changes of both isotopic signals may have the same forcing mechanism, and/or the climatic and environmental changes influence them in the same direction. Moreover, the magnitude of δ13C changes is about twice as large as that of the δ18O signal. These observations suggest that the soil humidity balance associated with regional hydrological cycles is important to control centennial 813C changes, and the ecosystem may amplify the climate signal. Thus, it is likely that the speleothem δ3C can be a reliable indicator to track centennial-scale climate changes through the climatically-controlled soil processes.Additional 2370 isotopic data covering 980 AD-1504 AD, were collected by striking off on the polished surface of LFG10 between 0-225mm, with a resolution approaching seasons. It shows that the Medieval Period is warm and arid. The signal of isotope rapidly increases, implying the beginning of the LIA. This high resolution record depicts twelve extreme arid events and thirteen extreme wet events which last for one to four years. The amplitudes, on thirty years climate average state, reach 0.5‰～0.7%o when these events occur, which are a half to two thirds that of the 4.2 ka event. The δ18O oscillations on decadal to interannual time scale are positive correlation to PDO and inverse correlation to ENSO variance. They all have 60-70 years cycle in common, demonstrated that PDO and ENSO may affect the precipitation of monsoon region in China associated with the Asian summer monsoon intensity.更多还原