闽西仙云洞石笋记录的Heinrich2事件年龄及亚旋回特征

Timing and Structure of the Heinrich 2 Abrupt Event Inferred from a Speleothem Record from Xianyun Cave,Western Fujian Province

选取福建西部仙云洞石笋(XYⅢ-28),通过利用U系定年和氧碳同位素测试方法分别对9个U/Th年龄和301个氧同位素样品进行分析,从而建立了26.33～22.98 kyr B.P.期间平均分辨率为11 yr的东亚夏季风演化序列。该石笋记录最显著的特征是在24.09～23.39 kyr B.P.时段内δ^(18)O偏正,振幅为0.7‰,对应于北大西洋显著的Heinrich2(H2)突变事件。根据主要变化过程中间点的位置可以标定H2事件的起止时间分别为24.09±0.06 kyr B.P.、23.39±0.07 kyr B.P.。XYⅢ-28石笋记录的H2事件内部呈现出明显的百年至十年际尺度的"两谷一峰"结构的亚旋回特征。这种百年至十年际尺度的亚旋回事件在H1事件、新仙女木事件以及小冰期等内部同样存在,表明这些千年尺度极端变冷事件内部气候也是不稳定的,可能受控于相同的驱动因素。仙云洞石笋记录与高低纬古气候记录对比发现,南北半球呈现反相位关系,即"see-saw"模式。

The Last Glacial Maximum （LGM）, which was the last and largest glacial period in the history of the Earth＇s climate, has long been a hot issue in paleoclimate research. The Heinrich2 abrupt event （H2 event） was a typical ice raft debris event formed in the LGM, which occurred about 25-24 kyr B.P. During the H2 event, ice raft debris extended southward, and large amounts of cold and fresh water were integrated into the North Atlantic. This led to the reduction of the Atlantic Meridional Overturning Circulation （AMOC）, cooling of the Northern Hemisphere, and weakening of the East Asian Summer Monsoon （EASM） as a result of the southward shift of the Inter-tropical Convergence Zone （ITCZ）. Although the H2 event is recorded in global records, there are still some discrepancies about the beginning and ending times of the H2 event due to the accuracy of dating and sample resolution. The NGRIP δ 18 O record with layer-counting time scales is not significant at the H2 event. The age of the H2 event can be calibrated in stalagmite records of high-precision 230 Th ages in the Asian monsoon region, but the time and structure still need to be further investigated.Xianyun Cave （116°59′E, 25°33′N, 970 m a.s.l） is located in Laiyuan Town, Liancheng County, Fujian Province, Southeast China. The cave is approximately 970 m in altitude and overlain by 30-50 m of Permian limestone bedrock. Annual average temperature in the region is 19.3 ℃, and the mean precipitation is 1 500-1 700 mm. The cave temperature is about 17.5 ℃. The relative humidity inside is close to 100%. The precipitation of δ 18 O in Xianyun Cave area is strongly influenced by the EASM： when the summer monsoon is strong, the δ 18 O value of precipitation is negatively biased, and vice versa.This paper presents a high-resolution stalagmite isotope record （XYIII-28） from Xianyun Cave in western Fujian Province to reveal the detailed precipitation history of the EASM from 26.33 to 22.98 kyr B.P., based on nine high-precision 230 Th ages and 301 oxygen isotope data points and yielding an average resolution of 11 years. The high-resolution stalagmite δ 18 O record is characterized by a positive shift process during the interval 24.09-23.39 kyr B.P. with an amplitude of 0.7‰, which demonstrates this remarkable H2 event from the North Atlantic. According to the midpoint of the main transition process, the beginning and ending times of the H2 event can be dated as 24.09±0.06 kyr B.P. and 23.39±0.07 kyr B.P., respectively. The duration of the H2 event was 0.69±0.06 kyr. The H2 event recorded by XYIII-28 shows a very obvious centenary-decade scale “double-positive” sub-cycle feature structure. This feature, which was recorded in the XYIII-28, also existed in the H1 event, the Younger Dryas event, and the Little Ice Age, illustrating that the internal structure of these extreme cooling events is unstable and that their driving mechanisms may be similar. Compared with paleoclimate records for high and low latitudes, XYIII-28 shows an anti-phase relationship between the Northern and Southern Hemispheres, namely the “see-saw” model.