湖北神农架年纹层石笋记录的YD与8.2ka事件转型模式研究

Transitional Patterns of YD and 8.2 ka Event Recorded by Annually-laminated Stalagmites from Qingtian Cave,Mt. Shennongjia

基于青天洞两支年纹层石笋QT16和QT40高分辨率δ18O序列,重建了Younger Dryas（YD）和8.2 ka期间季风突变细节过程。结果显示,亚洲季风强度在YD和8.2 ka事件内部并不稳定。在年层时标控制下,季风强度在YD早期最弱,随后缓慢上升,其间叠加三次百年尺度次级振荡;在8.2 ka期间,季风强度整体较弱,内部出现一次持续40 a左右的强季风事件,曲线形态类似＂W＂结构。运用＂方差法＂和＂RAMPFIT＂法分析显示,在两事件开始,季风衰减均表现出缓慢特征（转型时间占据事件整体历时的40%~50%）,但在结束期间季风迅速增强,相似变化模式得到了同区其他石笋记录的支持。与格陵兰冰芯记录对比发现,尽管高、低纬气候在两事件结束时段变化特征基本类似,但在两事件开始,亚洲季风衰减过程却显著长于北高纬气温变化。与低纬记录对比显示,热带水文变化在两事件开始阶段均表现出与亚洲季风基本一致的演化趋势,表明亚洲季风的突变过程可能具有鲜明的低纬特色。这些细节差异若得到更多高分辨率地质记录验证,将有利于进一步认识亚洲季风对突变事件的响应方式及其动力学机制。

High-resolution δ18O records from two annually-laminated stalagmites（ QT16 and QT40） from Qingtian Cave,Mt. Shengnongjia reveal a detailed Asian monsoon（ AM） variability throughout the Younger Dryas（ YD） and the8.2 ka event. During the mid-YD,the AM variability exhibits three centennial oscillations,while the 8.2 ka event is characterized by two 20-year weak AM events separated by one 40-year AM strengthening event. The general pattern of AM variability in two time windows at this site agree well with other cave records from the AM area and the Greenland records. To quantitatively evaluate the structure of the YD and 8.2 ka event,two mathematical methods are applied. In these analyses,both climate events show an asymmetrical structure,i. e.,a gradual transition into the event and an abrupt ending. These transitional patterns are further supported by previous stalagmite records in the AM area. However,the AM changes during these events are significantly different from the temperature changes over the Greenland.To some degree,the gradual AM decline at the onset is similarly reflected by low-latitude terrestrial or oceanic records regarding their onsets. This implicates that a forcing factor associated with hydrological circulations in the tropical ocean might modulate the AM evolution during these events.