福建仙云洞石笋记录的Heinrich 1事件突变特征

Characteristics of the Heinrich 1 abrupt climate event inferred from a speleothem record from Xianyun Cave, Fujian Province

Heinrich 1事件(H1事件)是末次冰期一系列冰漂碎屑事件中最为突出的寒冷事件.本文基于闽西连城县仙云洞石笋(编号:XYⅣ-3)8个^(230)Th年龄和256个氧同位素结果,获得了16.8~15.2 ka BP期间平均分辨率达7 a的东亚夏季风演变序列.该石笋氧同位素记录最为显著的特征是在16.3~16.1 ka BP时段出现两个阶段的偏正过程,振幅达1.8‰,指示了一次显著的弱季风事件突变过程,对应于北大西洋显著的H1事件.与具有年层时标的南京葫芦洞和神农架青天洞石笋记录对比发现,尽管在定年误差范围内3个洞穴的石笋记录都捕捉到了H1事件的这次突变过程,但我国东南仙云洞记录的H1突变过程的时间(~260 a)明显比长江中下游的青天洞(18 a)和葫芦洞(19 a)长得多,表现出显著的差异响应.分析表明,在北半球突变事件诱发后,热带西太平洋的海气过程有可能是造成这种差异响应的原因.

The climate system of the Earth experienced a series of abrupt changes such as Heinrich events and Dansgaard-Oeschger cycles during the last glaciation. Among them, the Heinrich 1 event（H1 event） is the most prominent cold event. It was recorded in Greenland ice cores, deep sea sediments, loess, stalagmites and lacustrine sediments and was proposed to be a global climate event. During H1 event（~18–15 ka）, the ice raft extended southward and large amount of cold and fresh water was integrated into the North Atlantic Ocean. It led to the reduction of the Atlantic Meridional Overturning Circulation（AMOC）, cooling of the northern hemisphere, and the weakening of the East Asian summer monsoon（EASM） as the results of the southward shift of the Inter-tropical Convergence Zone（ITCZ）. Pausata et al.（2011） used a numerical climate model with an embedded oxygen-isotope model to assess what caused the shifts in the oxygen-isotope signature of precipitation during a climate perturbation designed to mimic the Heinrich event. The simulations reproduced the enrichment of δ18 over Northern India and East Asia evident in speleothem records during Heinrich events. They then concluded that changes in the δ18 of Chinese stalagmites associated with Heinrich events reflect changes in the intensity of Indian rather than East Asian monsoon precipitations. The simulation result also shows no significant change in δ18O in precipitation over southeastern China, where its climate is influenced by EASM only. However, there is a lack of well-dated stalagmite record covering the H1 event from the southeastern China to test this hypothesis. Here we present a high resolution stalagmite isotope record（XYⅣ-3） from Xianyun Cave in western Fujian Province to reveal the detailed precipitation history of the EASM from 16.8 to 15.2 ka BP, based on eight high-precision 230 Th ages and 256 oxygen isotope data, yielding an average resolution of 7 a. This high resolution stalagmite δ18O record constrained detailed changes of the EASM during H1 event. It is characterized by a two-step positive shift process beginning at ~16.2 ka BP with the amplitude of 1.8‰, which indicates a significant weakened monsoon period. This result does not support the model simulation which was proposed by Pausata et al.（2011） that the δ18 of Chinese stalagmites reflected changes of the Indian Summer Monsoon rainfall. Indeed our record suggests that changes in δ18 of Chinese stalagmites reflect changes of EASM, especially during Henrich events. Comparing with stalagmite records from Hulu Cave and Qingtian Cave, stalagmites records from all 3 caves captured the H1 abrupt climate changes within dating errors. However, changes in Xianyun Cave are more prolonged（~260 a） than the abrupt（18–19 a） changes in Qingtian and Hulu caves. We conclude that the prolonged process of the Xianyun Cave is influenced, to some extent, by ocean-atmosphere coupling processes over the tropical western Pacific Ocean. The hydroclimate of the tropical west Pacific may cause the different responses. Due to the influence of sea-surface temperature changes in the tropical western Pacific warm pool and ENSO, the response of δ18 in precipitation in Fujian Province evident by our stalagmites to North Atlantic Ocean change would last longer than other EASM regions.