第四纪研究  2020, Vol.40 Issue (4): 918-925   PDF    
末次冰盛期东亚夏季风变化的重庆石笋记录
张伟宏1, 张振球2,3, 陈剑舜1, 周汪洋1, 邵庆丰3, 李凤全1, 王天阳1     
(1 浙江师范大学地理与环境科学学院, 浙江 金华 321004;
2 南京师范大学生命科学学院, 江苏 南京 210023;
3 南京师范大学地理科学学院, 江苏 南京 210023)
摘要:基于重庆小山岩洞XSY1石笋的5个230Th年龄和582个氧同位素数据,重建了23134~19345 a B.P.期间分辨率约6.5 a的东亚夏季风变化历史。此石笋δ18O记录的长期趋势逐渐负偏,在其上叠加了一系列千年-百年尺度季风振荡,呈现为4谷3峰的结构形态。重庆XSY1和南京MSD石笋δ18O记录在数百年尺度的反相关系表明冰盛期时东亚大陆东部水汽并非来自印度洋。与北高纬记录对比显示东亚季风的增强与减弱基本对应格陵兰温度的升高与降低时期,暗示在冰量最大时期东亚季风与北高纬气候仍密切联系。此外,XSY1的δ18O与冰芯10Be通量亦存在明显的对应关系,表明太阳活动对东亚季风的调控作用受冰量边界条件影响较小。功率谱分析结果显示出1011 a、722 a、460 a、337 a和163 a的周期,与太阳活动周期相近,进一步确认太阳活动对东亚季风的驱动作用。石笋XSY1与澳大利亚石笋MC-S2 δ18O记录在千年尺度呈反相位变化,表明冰盛期时东亚季风与全球其他气候系统的动力联系。
关键词末次冰盛期    石笋    氧同位素    东亚夏季风    太阳活动    
中图分类号     P597+.2;P532                     文献标识码    A

0 引言

末次冰盛期(Last Glacial Maximum,简称LGM)是末次冰期全球冰量缓慢积累的鼎盛阶段[1]。该时期之后,快速冰消过程发生[2]。研究LGM的气候变化对预测未来冰期气候变化过程和理解当今冰量较少暖期的气候变化有重要意义。末次冰期发生了一系列千年尺度事件,如Dansgaard-Oeschger(DO)和Heinrich (H)事件,这些事件集中发生在冰量中等的MIS3阶段[3]。其中,在冰量最大的LGM(H1和H2事件期间)仅发生有DO2一次明显的千年尺度事件[4];在LGM之后的冰消期发生有H1、BØlling-AllerØd(BA)和Younger Dryas(YD)显著的千年尺度事件。这些冰期和冰消期的千年尺度事件在东亚石笋记录中均有体现,当北高纬冷事件如H事件和YD事件发生时,东亚夏季风减弱;反之,当北高纬发生DO增暖事件时,东亚夏季风增强[5~8]。这些千年尺度事件在南极冰芯记录于中亦有所记录,与北高纬温度呈see-saw模式变化[9~10];但在冰量最大的LGM,这种see-saw机制不复存在[10~11],意味着该时段全球气候系统变化及关联模式与以往阶段不同。在气候极端的LGM,尽管已知千年尺度事件发生频率较低[4],但百年尺度的气候振荡状况目前仍不明确。高分辨率石笋记录显示太阳活动对全新世百年尺度的气候振荡具有重要驱动作用[12~13]。Bond等[14]指出太阳活动对全球气候变化影响通过温盐环流放大。在季风动力学上,太阳活动可通过影响海陆热力差异来直接影响季风[15~16]。因此,在LGM时期南北两极see-saw解耦以及北大西洋温盐环流停止或在全球气候变化中的作用发生了变化[17~18]。那么,在冰盖最大的LGM,东亚夏季风的百年尺度变化是否直接受太阳活动影响目前还不清楚,这一问题对理解不同气候边界下季风变化的主要驱动力有重要意义。

本文基于我国西南高分辨率石笋氧同位素记录重建了东亚夏季风在LGM时期的详细变化历史,发现东亚夏季风在LGM时期存在一系列百年-千年尺度的振荡。这些振荡与格陵兰冰芯和太阳活动记录都有较好的对应关系。此外,这些振荡与南京葫芦洞记录[19]在百年尺度上呈反相位变化。

1 研究区域、材料与方法

石笋XSY1采自重庆市石柱土家族自治县小山岩洞(29°42′N,108°22′E),洞穴位于四川盆地东部(图 1),该地年均降水量在1110mm以上(数据来源于http://data.cma.cn/),降水主要集中于夏半年;最高气温出现在7月,最低气温出现在1月,为典型的亚热带季风气候。小山岩洞口海拔为456m,洞深约700m,洞内温度约17.4℃,相对湿度为100 %,洞穴封闭性好。洞穴上覆植被以常绿阔叶林为主,植被繁茂。石笋XSY1长约823mm,形状细直。沿生长轴切开并抛光,发现0~600mm层段呈乳白色,600~823mm层段呈暗褐色,590~600mm处可能为沉积间断。

图 1 重庆小山岩洞和葫芦洞[21]地理位置示意图 Fig. 1 Locations of Xiaoshanyan and Hulu caves

在石笋抛光面上,使用直径为0.9mm的牙钻沿生长轴钻取5个粉末年龄样,样品层宽1~2mm。铀系年龄测定在南京师范大学同位素实验室完成,测试仪器为MC-ICP-MS Neptune型电离质谱仪,化学实验方法参照Shao等[20],年龄误差为±2σ。使用直径为0.5mm的牙钻沿石笋生长轴方向以1mm间隔共钻取582个同位素样品粉末,同位素数据通过碳酸盐自动进样装置与FinniganMAT-253型质谱仪联机完成测试,分析误差(±1σ)小于0.06 %,VPDB标准,在南京师范大学同位素实验室完成。

2 结果 2.1 年龄时标的建立

表 1为XSY1石笋230Th年龄测试结果。基于5个230Th年龄及石笋抛光面判断,石笋距顶590~600mm处存在沉积间断,故本文研究石笋距顶0~582mm层段。整体上,XSY1石笋定年精度较高,定年误差不超过70a。本文利用MOD-AGE模型[22]建立年龄模式。该模型通过年龄深度、实测年龄、采样宽度及年龄误差进行Monte-Carlo模拟。建立的年龄时标见图 2,结果显示石笋生长区间为23134~19345aB.P.,平均生长速率为0.154mm/a。

表 1 小山岩洞XSY1石笋230Th测年结果* Table 1 U-series dating results of stalagmite XSY1 from Xiaoshanyan Cave

图 2 石笋XSY1年龄模式图 中间黑线为年龄模式,两侧灰色线为95 %置信度范围,黑圆点和误差棒表示实测年龄和2σ误差 Fig. 2 Age model for stalagmite XSY1. The black line denotes modeled age frames. The gray lines denote 95 % confidence levels. The black dots and bars indicate 230Th aged and 2σ errors, respectively
2.2 石笋δ18O记录

石笋XSY1δ18O记录显示出明显的气候振荡特征。该石笋δ18O记录的平均分辨率约为6.5a,变化范围为-7.51 ‰ ~-6.11 ‰,平均值为-6.77 ‰。此δ18O记录呈现出逐渐负偏的长期趋势,表明在此阶段东亚季风逐渐增强,在此长期趋势上叠加了一系列千年-百年尺度的季风振荡事件。δ18O记录多项式拟合后呈现出4谷3峰的结构形态,大致表现为4次千年-百年尺度振荡旋回(图 3)。其中,3次明显的季风减弱事件分别发生在22.0kaB.P.、20.7kaB.P.和19.7kaB.P.。功率谱分析显示出1011a、722a、460a、337a和163a的周期(图 4),除460a周期外,这些周期与太阳活动的975a、706a、353a和160a周期相近[23],呈现了LGM时太阳活动在东亚季风中的印记。

图 3 石笋XSY1δ18O记录 黑色实线为石笋XSY1δ18O多项式拟合结果 Fig. 3 Stalagmite XSY1 δ18O record. The black solid line, superimposed on the XSY1 δ18O record, suggests a polynomial result

图 4 石笋XSY1δ18O功率谱分析结果 虚线为90 %置信度水平 Fig. 4 Spectral analysis of the XSY1 δ18O record. The dotted line is at the 90 % confidence level
3 讨论 3.1 石笋XSY1 δ18O的百年尺度振荡及气候意义

石笋δ18O继承了大气降水δ18O的信息,记录了大气水循环的变化过程。石笋作为一种新兴的气候载体,在全球气候变化研究中取得重要成果[24~28]。在石笋δ18O解释方面,取得较为一致的认识为整个东亚季风区石笋δ18O在轨道-千年尺度上主要反映了季风强度的变化,而东亚季风区边缘位置的石笋δ18O在百年尺度上主要受季风降水量的影响[24, 28]。然而,有学者对石笋氧同位素提出新的解释,认为从轨道到百年尺度中国石笋δ18O记录主要反映了源自印度洋和太平洋不同水汽源区同位素混合比例的综合信号[29~33]。当近源的太平洋水汽增多时,石笋δ18O值增大;而当远源的印度洋水汽增多时,石笋δ18O值减小[33]。模拟结果显示,当H1冷事件发生时,印度洋海温降低导致海洋蒸发减弱,印度季风区水汽δ18O值增大,当此水汽传输至风向下游的东亚地区时,促使中国石笋δ18O值增大。据此认为在H1时中国石笋记录了西南季风的减弱过程,并非东亚季风本身减弱[30]。在数理统计上,从西南至东北方向,全新世石笋δ18O值逐渐递减,表明δ18O在水汽通道上的瑞利分馏过程[34]。将重庆石笋XSY1与南京葫芦洞石笋MSD的δ18O记录[19]对比,可检验LGM时段的季风水汽循环过程。为便于对比,把不同分辨率的XYS1和MSD δ18O记录[19]分别进行15点和5点移动平均,比较后发现此两支石笋记录在千年-百年尺度上存在较明显的反相位变化(图 5)。当XSY1δ18O负向漂移时,MSD δ18O值增大;当XSY1δ18O正向漂移时,MSD δ18O值减小。这种反相关系表明在LGM时东亚大陆东部水汽并非完全源自西南季风带来的印度洋水汽。倘若来自印度洋水汽,XSY1与MSD δ18O记录则依据瑞利分馏法则同相变化。Pausata等[30]通过模拟发现,在H1时期中国石笋δ18O主要继承了印度季风降水的δ18O信号。此反相变化表明XSY1与MSD δ18O信号并不存在传承关系,据此我们推测XSY1δ18O记录可能主要反映了印度洋水汽至重庆地区的综合分馏水平,此水汽并未在很大程度上影响东部沿海地区;而MSD δ18O记录可能主要反映了太平洋水汽至南京地区的综合分馏状况。得出此结论的原因主要有二:1)Cai等[35]的模拟结果显示在LGM时期,印度洋输送到中国的水汽大幅度减少,中国南海和太平洋输送到中国东部的水汽增多;2)在LGM,石笋XSY1要比MSD的δ18O平均值负偏约0.5 ‰。XSY1与MSD δ18O记录的反相关系似乎表明印度洋与太平洋水汽输入量呈现此消彼长的状态,呈现出明显的反相位变化。近千年来,中国季风区大气降水δ18O与ENSO模态变化联系紧密(称为ENSO-环流效应理论)[31, 36],如小冰期时(石笋δ18O正偏)赤道太平洋地区为偏El Niño模态,而中世纪暖期(石笋δ18O负偏)则偏La Niña模态。图 3显示XSY1石笋LGM时百年尺度季风振荡的幅度范围约为0.3 ‰ ~0.8 ‰,与全新世振荡幅度相当[12],表明在冰盖较稳定的LGM和全新世,冰盖活动在地球气候变化中的正反馈作用减弱,太平洋海气活动占主导控制作用。受ENSO-环流效应理论[31, 36]的启示,这种LGM时期重庆与南京δ18O记录的反相变化可能与太平洋的类ENSO海气活动变化[37~39]相关。不过有区别的是,重庆与南京石笋δ18O记录若用经典的ENSO-环流效应科学理论来解释的话,似乎与ENSO-环流效应理论的本意并不一致,这是因为依据该理论,重庆与南京石笋δ18O记录应该具有同向的振荡过程。尽管已有研究表明南京葫芦洞与西南地区的其他洞穴在LGM存在显著差异[40],但在LGM这种数百年尺度的反相关系仅供学者做一参考研究,因为这种反相关系还需更多高精度定年和高分辨率石笋记录的佐证和支持。

图 5 石笋XSY1和MSD δ18O记录[19]对比 黑线为小山岩洞XSY1δ18O记录15点移动平均结果,灰线为葫芦洞MSD δ18O记录5点移动平均结果;黑色误差棒为石笋XSY1年龄误差(2σ),灰色误差棒为石笋MSD年龄误差(2σ) Fig. 5 Comparison of δ18O records of stalagmites XSY1 and MSD[19]. The black line indicates a 15-point running average of the stalagmite XSY1 δ18O record. The gray line indicates a 5-point running average of the stalagmite MSD δ18O record. Black and gray bars indicate errors of 230Th ages(2σ)for XSY1 and MSD, respectively
3.2 东亚季风气候的遥相关及在全球气候中的作用

从冰期到冰消期时期,东亚夏季风与北高纬气候在千年尺度上存在很好的对应关系[21, 41~42]。虽然在LGM全球气候相对稳定,但东亚夏季风与北大西洋气候在百年尺度上仍然密切相关。图 6显示东亚夏季风增强与减弱的时段基本上对应于格陵兰温度[43]的升高和降低时期。欧亚大陆冰盖及上覆冷气团的进退影响到夏季风在大陆的延伸程度[44]。过去研究显示亚洲夏季风受北大西洋气候和太阳辐射能量影响显著,在早全新世北高纬温度变化主控了亚洲夏季风的变化,直到晚全新世太阳辐射才占主导作用[45~46]。在LGM太阳活动能量变化与东亚夏季风的变化有良好的对应关系(图 6)。10Be通量变化指示了太阳活动的变化过程,其增大和减小值指示了太阳活动减弱和增强的时期[47]10Be通量增大和减小时期对应于XSY1δ18O值的高值和低值时段,暗示了太阳活动对东亚夏季风的控制作用(图 6)。如在22.0kaB.P.和20.7kaB.P.,10Be通量显著增大,对应XSY1δ18O明显正偏时期,表明太阳活动减弱导致东亚夏季风衰退。Bond等[14]指出北大西洋温盐环流放大太阳活动信号,进而影响全球气候。在LGM时期,北大西洋温盐环流变弱甚至停止[11, 17]10Be通量与XSY1δ18O记录的对应关系表明甚至在冰量最大时期,太阳活动对亚洲夏季风的控制作用仍然存在,太阳活动可直接调控季风的动力变化。

图 6 石笋XSY1δ18O记录(灰色)与格陵兰GISP2冰芯10Be通量[47](橙色)及δ18O记录[43](紫色)的对比 黑线为5点平滑后的石笋XSY1δ18O记录,两个浅蓝色柱表示了两次东亚夏季风衰退期 Fig. 6 Comparison between the stalagmite XSY1 δ18O record(gray)and GISP2 ice core 10Be flux[47](orange)as well as δ18O time series[43](purple). The black curve indicates a 5-point running average of the stalagmite XSY1 δ18O record. Two powder blue bars depict two weakened monsoon periods

H1事件的发生标志着LGM的结束和冰消期的开始[48]。冰消期的H1、BA和YD事件引起全球气候发生快速重组,南极大陆周边海冰退缩,南大洋深海CO2释放,引起全球变暖[49]。Denniston等[50]指出北高纬冷事件发生时南北半球西风带和季风环流南移。南半球石笋记录显示在H1、BA和YD事件发生时,南北半球季风呈反相位变化[51]。在气候变化较稳定的LGM时期,XSY1与澳大利亚石笋MC-S2的δ18O记录[52]在千年尺度上也存在反相位变化(图 7),这可能与热带辐合带(Intertropical Convergence Zone,简称ITCZ)的南北移动密切相关[53~54]。此外,这种变化表明除了在全球气候快速重组的冰消期,LGM时东亚季风变化与全球其他气候系统仍存在密切联系,是全球气候变化的重要组成部分。

图 7 石笋XSY1δ18O记录与石笋MC-S2δ18O记录[52]对比 黑色曲线为10点平滑之后的石笋XSY1和MC-S2δ18O记录 Fig. 7 Comparison of δ18O records for stalagmite XSY1 and MC-S2[52]. Black curves indicate 10-point running averages for both the stalagmite δ18O records
4 结论

利用高精度的230Th年龄和高分辨率的氧同位素数据,重建了LGM时期(23134~19345aB.P.)平均分辨率约6.5a的重庆小山岩洞石笋XSY1δ18O序列,获得LGM时期东亚夏季风的详细演化历史,主要得出以下结论:

(1) 石笋δ18O在-7.51 ‰ ~-6.11 ‰范围内变化,平均值为-6.77 ‰。该记录整体上表现为逐渐负偏趋势,多项式拟合后大致表现出4次千年-百年尺度振荡。在22.0kaB.P.、20.7kaB.P.和19.7kaB.P.时分别发生3次显著的季风减弱事件。

(2) 小山岩洞石笋XSY1和葫芦洞石笋MSD δ18O记录在数百年尺度上呈反相位变化。此反相变化可能表明XSY1δ18O主要反映了印度洋水汽至重庆地区的综合分馏水平,而MSD δ18O则主要反映了太平洋水汽至南京地区的综合分馏水平。此反相变化可能与太平洋类ENSO海气活动相关。XSY1δ18O在数百年尺度上的振荡幅度为0.3 ‰ ~0.8 ‰,与全新世振幅相当,暗示在冰量较稳定的LGM和全新世,冰盖活动对气候变化的正反馈作用变弱,东亚夏季风受太平洋海气活动影响显著。

(3) 东亚夏季风与北高纬气候密切相关,东亚夏季风的增强与减弱对应于格陵兰温度的升高和降低时期。此外,XSY1δ18O和GISP2冰芯10Be通量存在显著的峰谷对应特征,暗示LGM时太阳活动对东亚夏季风的控制作用。石笋XSY1δ18O谱分析结果显示出显著的1011a、722a、460a、337a和163a周期,除460a周期外,其他周期均与太阳活动周期相近,进一步证实了LGM时太阳活动对东亚夏季风的驱动作用。

致谢: 感谢两位审稿专家和编辑部杨美芳老师对本文提出的建设性修改建议,使此文得以完善。

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Variability of the East Asian summer monsoon during the Last Glacier Maximum recorded by a stalagmite oxygen isotope record in Chongqing, Southwestern China
Zhang Weihong1, Zhang Zhenqiu2,3, Chen Jianshun1, Zhou Wangyang1, Shao Qingfeng3, Li Fengquan1, Wang Tianyang1     
(1 College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, Zhejiang;
2 College of Life Sciences, Nanjing Normal University, Nanjing 210023, Jiangsu;
3 College of Geography, Nanjing Normal University, Nanjing 210023, Jiangsu)

Abstract

A highly resolved and precisely dated record characterizes in detail the East Asian Summer Monsoon (EASM) during the Last Glacial Maximum (LGM). Here we report a stalagmite (no. XSY1) δ18O record from Xiaoshanyan Cave (29°42'N, 108°22'E; 456 m above sea level). This cave is located in Shizhu Tujia Autonomous County, Chongqing, Southwestern China. The area is climatically dominated by the EASM, featured by warm and wet summer, as well as cold and dry winter. The annual precipitation is more than 1110 mm in the region, most of which falls during the summer seasons. Vegetation above Xiaoshanyan Cave is dense and is mostly composed of deciduous plants. The temperature and relative humidity are 17.4℃ and 100% inside the cave, respectively. The stalagmite XSY1 is 832 mm in length. When halved along its growth axis and polished, the sample becomes ivory in the upper section of 0~600 mm and fuscous in the lower part of 600~823 mm.A total of 5 230Th dates were measured on the stalagmite XYS1. The 230Th dating were conducted by Thermo-Finnigan Neptune multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at Nanjing Normal University. The 230Th dating results indicate that a growth hiatus took place at the depth of 590~600 mm. In this study, the upper continuous section is focused on. We drilled 582 subsamples for stable isotope analyses along the central growth axis using a dental drill with a diameter of 0.5 mm. The δ18O measurements were performed by Finnigan-MAT-253 mass spectrometer at Nanjing Normal University. Precision of δ18O values is 0.06 ‰, at 1-sigma level.We reconstruct an evolutionary history of the EASM during the period of 23134~19345 a B.P. based on the 5230Th ages and 582 δ18O data. The XSY1 δ18O values vary from -7.51 ‰ to -6.11 ‰ and the averaging value is -6.77 ‰. The δ18O record displays a long-term decreasing trend overall, which indicates a gradually intensifying EASM during the LGM. This long-term trend is interrupted by a series of multi-centennial to millennial δ18O oscillations, marked by 4 valleys and 3 peaks in shape. We find that the XSY1 δ18O variation is anti-phased with the stalagmite MSD δ18O record from Hulu Cave, eastern China on the multi-centennial scale. This relationship indicates that moistures at least in coastal East Asia were not totally from the Indian Ocean during the LGM. If the moisture had been from the remote Indian Ocean, both the stalagmite δ18O records would fluctuate in the same direction. The anti-phased behavior may be caused by ocean-atmosphere interactions in the Pacific Ocean, such as ENSO-like circulations. This is because the ENSO-like circulations regulate relative intensity of the Indian and East Asian monsoon. In addition, the XSY1 δ18O record correlates well with the GISP2 ice core δ18O and 10Be records on the centennial scale. The good correlations point to tight links of the EASM to the North Atlantic climate and solar activity during the LGM. The solar influence is further supported by spectral analyses of the XSY1 δ18O record. The analysis exhibits significant cycles at 1011 a, 722 a, 460 a, 337 a and 163 a at the 90% confidence level, close to solar cycles. Moreover, additional comparisons reveals that the EASM fluctuated reversely with the hydroclimate change in the Southern Australia on the millennial scale. This phenomenon indicates a dynamic connection of the EASM with the global climate system during the LGM.
Key words: Last Glacial Maximum    stalagmite    oxygen isotope    East Asian summer monsoon    solar activity