H2事件是发生于末次冰盛期的一次显著的气候突变事件,对理解千年气候事件和内部机制具有重要意义。然而,目前具有高精度和精确定年的H2事件气候记录依然很少,这在很大程度上限制了对H2事件起止时间、内部细节和触发机制的进一步研究。本研究基于印度东北部乞拉朋齐洞(Cherrapunji Cave)具有年纹层的石笋CHE-2 (总长940 mm)的21个U-Th年代、2701条纹层的计数和259个delta~(18)O数据,重建了25. 50~24. 76 ka B.P.和24.38~ 22.42ka B.P.(99.5~437.0 mm样品段)期间石笋delta~(18)O的高分辨率(24.38~23.08 ka B.P.时段平均分辨率8年,其他时段平均分辨率14年)、精确时间序列,刻画了印度东北部地区H2事件的结束过程和精细结构。研究表明,乞拉朋齐洞石笋记录的H2事件结束时段为24.2800.028~23.4360.028 ka B.P.,共持续8443年,振幅约为1.9,事件发生时间在误差范围内与东亚季风区石笋记录和格陵兰冰芯记录同步,在此期间,delta~(18)O记录呈现了两次负偏过程(24. 28~24. 17 ka B.P.和23. 90~23. 44 ka B.P.)和一段相对稳定过程(24. 17~23.90 ka B.P.),叠加了多个百年-十年际尺度的气候振荡。本研究得到的高分辨率delta~(18)O精确时间序列对于改善气候模型和检验气候事件假说有一定意义。从亚洲季风区与高纬地区同时段H2事件记录的对比来看,其发生机制可能主要包括以下几个方面:1)淡水注入北大西洋导致AMOC减弱,北高纬温度降低,推动ITCZ南移;北高纬温度变化的信号通过西风带和蒙古冷高压传递到亚洲季风区,从而影响亚洲夏季风;2) H2事件结束过程可能受到南极缓慢变暖的影响;3)低纬和青藏高原的变化也可能对H2事件产生复杂的影响。这些依然有待更多的高分辨率记录和气候模拟结果的进一步验证。
英文摘要:
The Heinrich 2( H2) Event is among one of the abrupt cold events, which is first recognized from the North Atlantic Ocean as an ice-rafted debris event during the Last Glacial Period. The detailed structure (e. g., timing, duration) under constraint chronology of this millennial scale climatic event can provide a better understanding of the underlying forcing mechanisms. However, up to now, there are only a few high-resolution and precisely-dated paleoclimate records across the H2 Event, which largely impedes our ability to understand its timing, structure and the potential forcing mechanisms. Here, we present a cave speleothem oxygen isotope ( delta~(18)O ) record ( stalagmite CHE-2,total length is 940 mm) from the Cherrapunji Cave (25°11'59"N, 92°27'11"E; 1100 m a.s.l.), Northeast India. This record has high resolution ( 259 oxygen isotope data, an average resolution of 8 years between 24. 38 ka B. P. ( before present, present = 1950 A. D. ) to 23. 08 ka B.P. and 14 years for other period ), and reconstruction of the Indian monsoon during H2 Event is constrained not only by 21 high-precision U-Th ages ( average age uncertainty is 60 years),additionally via annual fluorescent lamination counting ( 2701 annual bands, further supported by the U-Th ages) using the Laser Scanning Confocal Microscope. The unprecedented delta~(18)O resolution and chronology of our record allow us to characterize in detail the Indian monsoon history from 25. 50 ka B.P. to 24. 76 ka B.P. and 24. 38 ka B.P. to 22. 42 ka B.P.(sample section between 99. 5 mm and 437.0 mm). Our new delta~(18)O record suggests that the total duration of H2 termination spans for 844 3 years ( error is determined by the annual fluorescent lamination counting). The termination of the event occurred from 24. 280 0. 028 ka B.P. to 23. 4360. 028 ka B.P. inferred by a delta~(18)O gradual shift of ca. 1. 9%o. This process incorporates two negative excursions, from 24. 28 ka B.P. to 24. 17 ka B.P. and 23.90 ka B.P. to 23.44 ka B.P. and a relatively stable period from 24. 17 kaB.P. to 23. 90 ka B.P., superimposed on the termination trend are a number of centennial-decadal scale oscillations. The H2 Event in our record coincides within uncertainties with stalagmite records in the East Asian monsoon domain and ice core records from Greenland. Our delta~(18)O record during the H2 Event,compared with existing records from high and low latitudes,suggests : ( 1 ) A large amount of fresh water released into the North Atlantic Ocean weakened the Atlantic Meridional Overturning Circulation, thus reduced temperature in north high latitude and shifted the Intertropical Convergence Zone southward; the signal was propagated to the Asian monsoon domain through the Westerlies; (2) The long and gradual termination of the H2 Event may be causally linked to the gradual warming in Antarctica; ( 3 ) Changes in low-latitude and the Tibetan Plateau may have a critical response/feedback on the event, thus further model simulations and more high-resolution H2 records are critical to test these hypothesis. In addition, our precise delta~(18)O time series may provide new important constraints on the climate model simulation and the causal link of the H2 Event between high-low latitude climate changes.