赣北石笋记录的新仙女木事件

Timing and structure of the Yougner Dryas event recorded by a stalagmite from northern Jiangxi, China

新仙女木事件(Younger Dryas,简称YD)是末次冰消期升温过程中发生的一次北半球急剧降温事件,目前全球范围多个地区的地质记录中均有发现该事件的印记.然而,受限于定年误差或采样分辨率等因素,关于YD事件的精确起止时间、精细内部结构和触发机制仍存在争议.本研究利用江西北部神农宫溶洞内一支文石石笋SN29(总长度426 mm,其中距离顶部220~388 mm用于本研究),通过高精度 230Th定年和氧、碳稳定同位素测试分析,建立了13614~10811 a B. P.期间平均分辨率12年的石笋氧同位素(δ18O)变化序列.该记录最为显著的特征是δ18O值在12851~11575 a B. P.时段偏正1. 7‰,指示了一次显著的弱季风突变事件,响应于北大西洋YD事件. SN29为高铀含量文石石笋且生长速率较快,精确界定了响应于YD事件的东亚弱季风事件的起止时间和精细内部结构.结果表明: YD期间, SN29记录的东亚弱夏季风事件的开始时间为 12851±16 a B. P.,经历了约249年;结束过程开始于11575±19 a B. P.,经历了约146年;整个YD弱季风事件共持续了 1276±35年.东亚季风区各石笋δ18O记录的YD弱季风事件在误差范围内基本一致,且与格陵兰冰芯δ18O记录和低纬度其他记录的发生时间一致,支持YD时期东亚夏季风减弱事件可能是对大量淡水进入北大西洋导致径向翻转流减弱、北半球变冷、ITCZ南移的快速响应的假说.

The Younger Dryas (YD) event is the major and most extensively studied climatic event during the transition from the last glacial period into the present Holocene interglacial, so far it has been reported in many geological archives/records in the world.However, due to dating uncertainties and/or temporal-resolution, the precise timing and structure of the YD event and its trigger mechanism remain in debate. Shennonggong Cave (28°42'39"N, 117°15'4.2"E;383 m a.s.l.) is located in Wannian Town, Shangrao City, Jiangxi Province, Southeastern China.The cave developed in the Carboniferous limestone of the Chuanshan and Huanglong groups, which are mainly composed of limestone and interbedded dolostone.The thickness of the cave roof ranges from about 20 m to about 80 m, with average of ca.50 m.The length of the cave is 7200 m.The temperature inside the cave varied from 16.5℃ to 21.5℃ and the relative humidity inside was close to 100% during the period of 2011~2013.This region is strongly influenced by the East Asian summer monsoon (EASM).The annual average temperature is 18.5℃ and the mean precipitation is 1875 mm. A stalagmite (SN29) was collected 200 m behind the cave entrance with 426 mm in height.This study focuses on the 220~388 mm of distance from the top.A total of 169 subsamples for stable isotopic measurements were drilled along the central axis of SN29 at an interval of 1 mm.We provide a new precisely dated high-resolution (12 yrs) stalagmite δ18O record (SN29) from Shennonggong Cave, which characterized in detail the timing and structure of the weakened EASM during the YD event.The study section of the stalagmite SN29 spans time range from 13614 a B.P.to 10811 a B.P., and the δ18O record is reconstructed by 169 δ18O data and 20 high-precision 230 Th dates.The sample SN29 is composed of aragonite with high growth rate, which allows us to reconstruct a high-resolution record with an unparalleled age constraints with typical errors less than 20 yrs.The YD event in our record manifests a heavier δ18O excursion from 12851 a B.P.to 11575 a B.P.with an amplitude of 1.7 ‰, indicating a significantly weakened summer monsoon period.Our results show that the onset of the EASM YD began at 12851±16 a B.P.and lasted for ca.249 yrs, The end of the YD began at 11575±19 a B.P.and lasted for ca.146 yrs.The duration of the event is thus 1276±35 yrs.The timing of the YD event in the SN29 δ18O record is within errors consistent with previously reported stalagmite δ18O records in the EASM region.A close comparison with other records from high and low latitudes supports a current notion that the weakened EASM during the YD event might be caused by the southward shift of the Intertropical Convergence Zone resulted from the reduction of the Atlantic Meridional Overturning Circulation and cooling of the Northern Hemisphere, which is possibly triggered by fresh water injection into the North Atlantic Ocean.