第四纪研究  2020, Vol.40 Issue (4): 898-908   PDF    
CIS13气候事件特征的山西高分辨率石笋记录
黄伟1, 董进国2     
(1 宜春学院, 江西 宜春 336000;
2 南通大学地理科学学院, 江苏 南通 226007)
摘要:文章基于山西黎城白云洞高分辨率石笋(BY2)δ18O记录,重建了53.82~47.41 ka B.P.东亚夏季风演化历史。BY2δ18O值在52.2 ka B.P.和49.5 ka B.P.前后显著偏负,指示两次夏季风显著增强阶段,分别对应于格陵兰冰芯记录的DO14和DO13事件。山西白云洞石笋清晰记录了CIS13事件"双峰"结构,与NGRIP冰芯记录相对应,反映了东亚季风边缘区对北大西洋海-气耦合振荡,以及ITCZ南北移动引起的雨带范围变化的敏感响应。但BY2 δ18O记录的CIS13事件转换过程较NGRIP缓慢,其中开始过程呈现"三相位"变化特征:即亚洲夏季风提前出现缓慢增强趋势、在达到水热阈值条件后表现出与格陵兰温度的同步快速爆发、最后仍持续增强直至峰值;这与热带大西洋和印度洋海盆岩芯反照率记录相似,可能与南大洋越赤道气流以及ITCZ持续北移有关。另一方面,千年尺度事件和百年-年代际振荡幅度分别高达5 ‰和2 ‰,明显较东亚季风区其他石笋δ18O记录大,尤其是在百年-年代际尺度上呈现出"高频、大幅"的变化特征;表明其敏感响应于不同时间尺度气候波动。功率谱结果表明,其具有明显的50 a、60 a和206 a变化周期,可能受到太阳活动和海洋水文变化的双重影响。
关键词石笋δ18O    白云洞    东亚夏季风    DO/CIS事件    “双峰”结构    
中图分类号     P597+.2;P532                     文献标识码    A

0 引言

一系列快速转换、剧烈波动的千年尺度气候事件是末次冰期气候波动的主要特点,其中以格陵兰冰芯记录的温度波动最为典型,数十年内变幅高达8~15℃[1]。南极冰芯[2]、中低纬大气环流及水文记录[3~5]均呈现出类似的气候事件,表明其具有全球性特征,尽管各自的变化模式有所不同。在此基础上,研究千年尺度气候事件内部细节特征对进一步理解冰期气候驱动机制具有重要的参考价值。然而,由于受到噪音、信号敏感性等差异影响,格陵兰不同冰芯记录之间存在局部细节性差异。例如,位于冰原中部GRIP和GISP2冰芯δ18O记录的Dasgaard-Oeschger(DO)13事件表现出快速上升的“单峰”模式[6~7],而后期钻取的NGRIP冰芯记录则具有明显的“双峰”结构特征[8];正如Johnsen等[9]所言,NGRIP信号可能在响应气候波动过程中更为敏感。值得注意的是,其他地区高分辨率地质记录则对DO13内部细节特征鲜有报道。虽然后续中国中部石笋记录隐约显示其两次内部振荡[10],但由于相对振幅较小,对该事件的刻画仍然不清晰。因此,DO13事件的“双峰”结构是局域信号还是具有区域气候意义,有待更多高分辨率地质记录的验证。

地质记录和模拟研究表明,太阳活动对全新世百年-年代际尺度气候波动产生了重要影响[11~15],尽管其直接作用或间接放大效应的具体驱动机制存在争议[16~20],但太阳黑子、耀斑、总太阳辐射量等太阳活动的周期性波动在现代观测研究中得到了证实。然而,由于年代误差、样品分辨率低等原因,在冰期时段,更多的研究关注于千年突变事件,对于短尺度气候波动的报道较少。Prasad等[21]通过对死海Lisan古湖盆MIS2阶段纹泥记录分析显示,其在百年-年代际尺度具有与太阳活动相似的变化周期;Lal等[22]通过对极地冰芯(Summit)32ka以来14C记录分析表明,其显著的百年尺度波动存在太阳活动的印记。因此,考虑到中低纬气候与格陵兰温度变化之间的紧密联系,亚洲季风在百年-年代际尺度的行为特征是否也受到太阳活动的调控需要更多高精度、高分辨率地质样品的支持。

我国北方地质记录能够敏感地响应夏季风降水范围变化,为深入探讨夏季风演化及其与高纬之间的气候机制提供了良好载体。位于黄土高原的山西莲花洞石笋记录在轨道-千年尺度上与已发表的其他区域洞穴记录相一致,并在捕捉诸如YD、“8.2ka”等重大气候事件方面表现出更大的波动幅度,具有更好的信号敏感性[23]。本文利用山西白云洞石笋(BY2)δ18O序列,重建了53.82~47.41kaB.P.东亚夏季风演化历史。在分析亚洲季风区与北高纬地区DO14~DO13事件特征基础上,尝试探讨亚洲夏季风千年-百年尺度事件的动力学机制。

1 材料与方法

山西省地处大陆东岸中纬度内陆,是典型的黄土覆盖区,其地势东北高西南低,有利于亚洲夏季风的入侵。同时,该区域又处于亚洲夏季风影响的边缘区,对冬、夏季季风环流的进退变化非常敏感。

白云洞(36°40′N,113°24′E)位于山西省东南部黎城县北30km处(图 1)。该洞洞口较大,洞长约500m,过道碳酸盐景观丰富,主要有石花、羊奶石和石坝等。洞外植被覆盖较好,主要由灌木和乔木组成。洞内实测温度为11℃,湿度为80 %。受东亚夏季风的影响,区域年平均降雨量约为530mm,集中分布在5~10月,约占全年降雨量的87 %。

图 1 本文研究区(三角形)和亚洲季风区部分地质记录(圆形)的地理位置分布图 图中虚线表示现代季风/非季风区分界线 Fig. 1 Map showing the location of this study(triangle)and part of the geological records (circle) of the Asian monsoon region. The dashed line through graph represents the boundary of the modern monsoon/non-monsoon zone

石笋BY2采自距洞口200多米处一石坝上,高约137mm。其顶、底直径均一(图 2)。沿石笋生长轴对半切开并抛光,显示其岩性呈不透明乳白色至黄褐色,由方解石组成,生长层清晰可见,并在距离中心沉积位置一定距离内呈近似直线排布。

图 2 石笋BY2剖面图 黑色实心点表示测年点位置及对应的年代结果 Fig. 2 Profile of stalagmite BY2. The solid black dots represent the positions of dated sampling and their corresponding chronological results

用直径为0.9mm牙钻沿石笋剖面垂直生长轴钻取230Th年代样粉末,每个样品重量约为50mg;样品采集和测试工作前后分别于台湾大学高精度质谱仪与环境变迁实验室和南京师范大学石笋同位素实验室完成。年代样品的化学处理与Edwards等[24]纯化方法类似。测试分析仪器为多接收器电感耦合等离子体质谱仪(MC-ICP-MS,Neptune型),具体技术方法和流程现已比较成熟[25~27];测量统计误差均为±2σ,年代测试结果见表 1

表 1 BY2石笋230Th测年结果* Table 1 230Th dating results for stalagmite BY2 from Baiyun Cave

用直径为0.5mm的金属牙钻沿抛光面生长轴以0.5~1.0mm为间隔采集大约50~100μg的氧同位素测试样品,共计268对,样点平均分辨率为24a。样品测试采用第三代碳酸盐自动进样装置(KIEL CARBONATE DEVICE Ⅲ)与Finnigan质谱仪(MAT-253型)联机测试,每隔10个样品插入一个标准样品(NBS-19)进行仪器监测。最后将测定的同位素丰度比值通过公式计算成δ18O,VPDB标准,分析误差优于0.06 ‰,由南京师范大学石笋同位素实验室测试完成。

2 结果 2.1 U/Th年代

U/Th测年结果显示(见表 1),该支石笋样品沉积于53.82~47.41kaB.P.。BY2铀含量较高,在500~1500ng/g之间;而钍含量相对较低,在1.5~30.0ng/g之间,这样使得实测年龄误差较小(2 ‰ ~3 ‰);正如图 3所示,本文采用线性内插法建立年龄模式。由于深度24mm和34mm的测年结果在误差范围内一致,直接采用两者之间的内插结果可能导致沉积速率信号失真。考虑到在DO13事件(对应深度34~11mm)期间,沉积速率较快,且在整个事件的平均态上相对稳定;因此,此时段的年龄模式采用深度34mm和11mm的内插结果。在连续生长时段内,石笋生长速率变化总体稳定,在16~47μm/a之间变化。

图 3 BY2石笋δ18O变化序列年龄-深度图 Fig. 3 Age-depth curve with the stalagmite δ18O series of BY2
2.2 石笋δ18O序列

基于上述时标,本文建立了53.82~47.41kaB.P.石笋δ18O变化序列,其平均分辨率为24a(图 3)。δ18O值总体呈偏正趋势,在-8.5 ‰ ~-1.0 ‰之间波动。整个过程可分为3个阶段:1)53.82~51.01kaB.P.,石笋δ18O值整体偏负,期间呈现频繁的百年尺度快速波动变化,振幅达1 ‰ ~2 ‰。2)50.98~49.91kaB.P.,δ18O值总体偏正,振幅较小,围绕平均值(-4.9 ‰)上下波动。3)δ18O值在49.82~49.51kaB.P.阶段持续偏负,振幅达到3 ‰ (-4.5 ‰ ~-7.5 ‰);随后,在49.47~49.29kaB.P.期间出现一次持续200年左右的快速偏正过程,振幅达到2 ‰。最后,δ18O值逐渐偏正到-1 ‰,直至停止生长。

由于地理位置的特殊性,东亚季风区同时受到印度洋、西太平洋、中国南海水汽的混合影响[28],导致降水δ18O在发生源、信号传输与改造、信号表达等方面的影响因素较为复杂[29],由此引发了基于降水δ18O继承关系的中国南方石笋δ18O释义争议。从目前的地质证据来看,尽管当地降雨量可能不是中国南方石笋δ18O变化的主要因素[30~31],但后者变化在大尺度空间范围内的一致性代表了不同时期大气环流的空间规模;正如Cheng等[32~34]所指出:在长时间尺度上,石笋δ18O实际上指示了一种平均态夏季风强度的变化或是水汽源至沉降点综合水汽输送的结果。

现代季风研究认为,东亚夏季风降水带由南向北推移具有年循环特征,其北移界线能够在逻辑上反映夏季风强度变化[35];从长时间尺度而言,中国北方雨季平均状态可作为夏季风强度指标。另一方面,现代降水同位素观测及模拟研究表明,尽管亚洲季风区各地夏季降水峰值在时间上呈现差异性,但其δ18O在空间上的变化具有一致性[34, 36],夏季风爆发前后的降水δ18O值存在显著差异[37]。这说明:1)“雨量效应”的不充分性,降水δ18O更多地反映了海陆热力差异、大尺度大气环流变化;2)空间上的夏季降水(雨季)占据了全年总降水量的大部分,即决定了全年整体的干湿程度。换言之,降水δ18O变化不仅是对大尺度大气动力学机制的响应,还间接反映了年际或更长时间尺度的夏季降水强度变化,尤其在季风边缘区表现得更为显著。

我们过去的研究工作[23, 38]已表明,末次冰期以来山西石笋δ18O记录在轨道-千年尺度上与中国南方洞穴记录具有良好的重现性,并与当地黄土、湖泊等不同地质记录[4, 39](图 1)恢复重建的区域降水量变化序列也具有较好的一致性。这表明北方石笋δ18O能够较好地指示东亚夏季风强度变化,这一理解也得到模拟研究和现代洞穴沉积记录结果的支持[40~41]。当东亚夏季风强度增强时,华北地区夏季降水量增大,石笋δ18O总体偏负;反之夏季风强度减弱,石笋δ18O明显偏正。

3 讨论 3.1 千年尺度气候突变事件的区域响应

石笋BY2沉积始于Chinese Interstadial(CIS)14早期(图 3),并止于H5冷事件强盛期。BY2石笋δ18O主要变化特征及趋势(图 4c)与中国中部的狮子洞[10](图 4d)、东部的葫芦洞[5, 42](图 4e)、西南地区的羊口洞[43~44]和小白龙洞[45](图 4f4g),以及印度季风区的Moomi洞[46](图 4h)石笋记录基本一致,表明不同区域洞穴环境共同受到大尺度亚洲季风环流的控制。对于本文记录的CIS13和CIS14事件,首次清晰发现于格陵兰冰芯记录[7],并在后期众多地质记录中均有明确报道。其中,CIS14持续了长达数千年时间,在阿拉伯海岩芯反照率和不同洞穴石笋δ18O变化中也有清晰记录(图 4),表明北高纬温度变化与低纬夏季风增强受到共同驱动因素的影响[5],即此时“北半球增温,南半球降温”模式导致ITCZ北移[47],为亚洲季风区带来大量降水。同时,低纬热带海洋热量通过海-气耦合路径输送至北高纬地区[48],导致其快速增温。反之,当北大西洋径向翻转环流异常减弱时,北半球西风带增强使亚洲夏季风强度减弱(或冬季风增强)[49],进而导致该区域出现可能的干冷事件(如H5事件);这一事件也可能导致石笋BY2停止生长。

图 4 BY2记录与其他地质记录对比 (a)NGRIP冰芯δ18O记录[8];(b)阿拉伯北部海盆岩芯反照率记录[3];(c)BY2石笋δ18O记录(本文);(d)四川狮子洞(SI3)δ18O记录[10];(e)南京葫芦洞δ18O记录[5, 42];(f)重庆羊口洞(JFYK7)δ18O记录[43~44];(g)云南小白龙洞(XBL-1)δ18O记录[45];(h)也门Moomi洞(M1-2)δ18O记录[46]图中阴影部分代表DO13/CIS13事件 Fig. 4 Regional comparisons of BY2 record(c, this study)to other geological records from NGRIP ice core δ18O(a)[8], the reflectance record from Arabian Sea basin (b)[3], SI3 δ18O from Shizi Cave (d)[10], stalagmite δ18O from Hulu Cave (e)[5, 42], JFYK7 δ18O from Yangkou Cave (f)[43~44], XBL-1 δ18O from Xiaobailong Cave (g)[45], and M1-2 δ18O from Moomi Cave, Yemen (h)[46]. The shaded bar through the curves marks DO13/CIS13 event

另一方面,由于地理空间及影响因素的区域差异性,低纬季风与格陵兰温度的DO变化模式存在“解耦”现象[43, 45, 50~52]。在CIS14事件中,格陵兰温度在快速达到峰值后呈锯齿状逐渐下降[8](图 4a),而阿拉伯海盆沉积物反照率则指示了热带季风强度逐步增强状态[3](图 4b)。以羊口洞(图 4f)、小白龙洞(图 4g)和Moomi洞(图 4h)为代表的印度季风型记录与其相似,尤其在51.5~50.5kaB.P.时段石笋δ18O总体偏负,似乎并未受到或较小受到北大西洋低温环境影响。同样在CIS13变化过程中,阿拉伯海盆岩芯反照率和BY2石笋记录代表的低纬水文变化表现出明显的“三相模式”(图 5b5c),即在格陵兰温度仍处于低值时,亚洲夏季风出现缓慢增强趋势,可能是南大洋热量积累的结果[50],以及热带海洋环流对低纬季风的激发和维持作用[53]。在达到水热阈值条件后表现出与格陵兰温度的同步爆发,最后仍继续增强直至峰值(-7.6 ‰)。在CIS13/H5转换阶段,NGRIP的持续时间仅为40a(图 5a),而BY2石笋记录的夏季风变化则持续了900a(图 5c)。这进一步证实了亚洲季风不仅受到北半球气候的“拉伸”作用,还受到南大洋的“助推”影响[54]。因此,在强调北高纬对低纬季风环流影响的同时,热带海洋以及南大洋越赤道气流对亚洲季风的影响不可忽视,尤其是受印度季风控制区域。

图 5 DO13/CIS13事件内部特征及变化模式对比 (a)NGRIP冰芯δ18O记录[8];(b)阿拉伯北部海盆岩芯反照率记录[3];(c)BY2石笋δ18O记录(本文);(d)四川狮子洞(SI3)δ18O记录[10];(e)土耳其北部Sofular洞(So1)石笋δ18O记录[55]图中阴影部分表示各地质载体记录DO13/CIS13事件可能的“双峰”结构,箭头表示不同变化阶段 Fig. 5 Comparison of the internal characteristics and patterns for DO13/CIS13 event among records from NGRIP ice core δ18O(a)[8], the reflectance record from Arabian Sea basin (b)[3], BY2 record(c, this study), SI3 δ18O from Shizi Cave (d)[10], and So1δ18O from Sofular Cave, Turkey (e)[55]. Gray bars represent "double peaks" within DO13/CIS13 event likely printed in above records. Arrows indicate the different stages of process change between low and high latitudes
3.2 CIS13气候事件的双峰结构特征

尽管不同空间地质记录在响应区域或全球气候变化趋势,以及识别重要千年突变事件方面表现出整体一致性,但在气候事件内部细节特征及模式方面表现出一定的差异性。从NGRIP冰芯记录可以看出[8],其δ18O值记录的DO13事件表现出明显的双峰结构(图 4a5a)。在低纬地区,高分辨率的卡里亚科(Cariaco)海盆以及阿拉伯海岩芯反照率均显示该时段也存在两次明显的波动[3](图 5b),尽管其“谷值”相对幅度较NGRIP变化小(1/2);这种一致性表明热带水文变化能够敏感、快速地响应北大西洋气候。然而,在亚洲季风区,已发表的众多高分辨率石笋δ18O记录却未能提供明确的CIS13双峰结构的证据(图 4)。例如,四川狮子洞隐约地记录了CIS13的双峰振荡过程[10](图 5d),其总体振幅达到1.6 ‰ ~1.8 ‰。同样来自中部地区的永兴洞石笋δ18O记录也呈现出该事件的“双峰”特征轮廓[56~57];然而,限于分辨率低、相对振幅小等不足,其CIS13事件整体不清晰,无法详细刻画内部细节特征。土耳其北部的Sofular洞两支石笋δ18O记录均呈现出CIS13事件类似的“双峰”,而且在发生时态上与其δ13C记录一致[55];但由于该事件早期缺少完整的碳酸盐沉积过程,其“双峰”结构特征仍然不够清晰(图 5e)。

BY2石笋δ18O则首次清晰记录了亚洲夏季风在CIS13事件期间两次显著偏负波动,与NGRIP记录[8]一一对应;其振幅达到1.8 ‰ ~3.5 ‰ (图 5c),明显较中部地区狮子洞[10]和永兴洞[56~57]记录大,反映了该洞穴记录对夏季风强度及北高纬气候变化敏感响应的空间优势。但BY2石笋δ18O记录的CIS13事件在时间和变化模式方面与NGRIP存在差异;前者记录的DO13始于49.72kaB.P.,而后者突变爆发时间则为49.25kaB.P.(图 5)。考虑到NGRIP在该时段的数年误差(±0.03ka)、累计数年误差(>2ka)以及石笋两个230Th测年点总误差(±0.3ka),其似乎能够解释两者在开始阶段的时间差问题。另一方面,BY2记录的CIS13开始时间与葫芦洞[42]和羊口洞[44]在测年误差范围内基本一致(图 4e4f),尤其是与葫芦洞最新样品(HL161)记录的时间差异更为微小[58],但与狮子洞偏差较大(0.3ka)[10]。因此,正如前文所述,不同空间地质记录的时间差异是由各自测年误差造成的,还是受控因素不同有待进一步验证。

3.3 太阳活动和海洋水文波动对百年尺度季风的调控作用

BY2记录的另一个特点在于千年-年代际尺度上的振幅均较大,其在千年尺度上的振幅达到2.5 ‰ ~3.5 ‰,在CIS13/H5事件转换的变化幅度更是高达5 ‰,是其他石笋δ18O记录的两倍(图 4)。类似信号特征在狮子洞记录中也有体现[10](图 4d5d),贵州竹蹓坪洞石笋δ18O记录的末次冰消期至早中全新世阶段同样刻画了“高频、大幅”的气候波动特征[59]。这反映了本研究洞穴滴水对大气降水信号的削弱作用较小,亦或碳酸盐沉积环境对外部大气环流变化响应敏感,进一步支持其能够清晰识别CIS13内部“双峰”特征的信号真实性。在百年-年代际尺度上,BY2石笋δ18O“高频、大幅”(0.5 ‰ ~2.0 ‰)波动反映了该区域季风环流的不稳定性,同时间接印证了东亚夏季风北界的不稳定性。

本文石笋δ18O和Δ14C[60]以及冰芯10 Be[61]去趋势后的残差信号对比结果显示(图 6),三者在重要变化特征方面具有相似性。例如,在48.5kaB.P.、50kaB.P.、51kaB.P.以及53kaB.P.前后记录了显著的弱季风事件,对应于较高的大气Δ14C和 10Be浓度;在47.6kaB.P.、51.3kaB.P.和53.5kaB.P.前后夏季风增强阶段,则对应于大气Δ14C和10 Be浓度低值,暗示两者之间可能存在气候上的联系。然而,正如以往研究认为,太阳能量变化较小(<1 %),不足以直接驱动气候突变[14, 16, 62]。这可能还需其他作用机制进一步放大并传输到气候系统中。

图 6 BY2石笋δ18O与大气Δ14C和 10Be记录对比 (a)大气Δ14C去趋势后的残差信号[60];(b)BY2石笋Δδ18O(本文);(c)格陵兰冰芯Δ10Be (ss09sea时标)[61];(d)石笋Δδ18O(灰色)与冰芯Δ10Be信号(黑色)其中,(b)和(c)中的黑色粗线分别表示7点和3点滑动平均结果 Fig. 6 Mechanism relation of centennial-decadal timescale oscillations. (a)Residual signal of atmospheric Δ14C after detrended[60]; (b)Stalagmite Δδ18O processed from the same as (a)(this study); (c)NGRIP Δ10Be (ss09sea timescale)[61]; (d)Comparison between(b, gray)and(c, black). The bold back lines in (b) and (c) stand for 7-point average and 3-point average, respectively

BY2石笋δ18O时间序列的功率谱分析结果显示,其表现出明显的50~60a气候周期(图 7a),其中50a可能是大气Δ14C振荡周期之一[16],而60a周期则被认为是亚洲季风短尺度波动的重要周期[63],其在全新世印度夏季风波动历史中可能充当了重要角色,即与北大西洋年代际振荡(AMO)调控有关[62]。为了进一步突出季风百年-年代际尺度变化,本文对原数据进行去趋势处理,其功率谱结果显示(图 7b),除了典型的60a周期外,还突出了206a的太阳活动周期(De Vries周期),表明季风短尺度高频波动有可能受到太阳活动的驱动。另外,还有49a周期信号,虽然置信度不高(90 %),但综合图 7a的频谱信号结果,40~50a周期可能指示了北大西洋温盐环流振荡历史[64]。因此,不仅限于全新世时段,太阳活动和海洋水文循环在冰期时段仍然对季风的内部波动起着重要的调控作用。太阳活动可能通过改变大洋表层温度和盐度,影响深层水形成的强度和规模[16, 18],最终以“大洋传输”为纽带对半球或全球气候产生“放大效应”。其中,北大西洋水文异常信号可通过海-气耦合路径对低纬季风气候施加影响[5, 11]

图 7 Redfit[65]功率谱分析结果 (a)53.82~47.41kaB.P.时段BY2石笋δ18O序列分析结果;(b)Δδ18O功率谱分析结果图中实线和虚线分别表示95 %和90 %置信度水平 Fig. 7 Spectral results by Redfit tool[65]. (a)Significant periods detected from δ18O time series of stalagmite BY2 (53.82~47.41kaB.P.); (b)Possible cycles detected from Δδ18O signature of stalagmite BY2. The solid and dashed lines show 95 % and 90 % confidence level, respectively
4 结论

(1) 利用山西BY2高分辨率石笋δ18O时间序列,重建了53.82~47.41kaB.P.期间平均分辨率为24a的东亚夏季风演化历史。δ18O值在52.2kaB.P.和49.5kaB.P.前后时段显著偏负,振幅达到3 ‰,指示了东亚夏季风显著的增强过程;其对应于格陵兰冰芯记录的DO14和DO13事件,即中国石笋记录的CIS14和CIS13事件,进一步支持千年尺度北大西洋气候与亚洲季风的遥相关关系。在百年尺度上,本文首次记录了CIS13清晰的“双峰”结构,与NGRIP冰芯记录相对应,反映了东亚季风边缘区对北高纬气候以及ITCZ南北移动引起的雨带范围的敏感响应。

(2) 在突变事件的变化模式上,BY2记录的CIS13事件开始和结束过程较NGRIP缓慢,呈现“三相位”模式:在北高纬气候仍处于冰阶状态时,低纬夏季风出现缓慢增强趋势;在达到水热阈值条件后表现出与格陵兰温度的同步爆发,最后再次与格陵兰温度变化形成“解耦”。这种响应过程具有典型的低纬热带水文变化特征,可能与南大洋越赤道气流以及ITCZ持续北移有关。因此,亚洲夏季风强度变化特征和模式受到南北纬气候的共同驱动作用。

(3) 在研究时段内,BY2δ18O记录的振荡幅度明显较东亚季风区其他石笋记录大,尤其是在百年-年代际尺度上呈现出“高频、大幅”的变化特征,振幅高达2 ‰,敏感响应于不同时间尺度气候波动。功率谱结果综合表明,其具有明显的50a、60a和206a变化周期,可能受到太阳活动和海洋水文变化的双重影响,亦或后者放大了前者的气候效应。

致谢: 感谢台湾大学地质系沈川洲博士,南京师范大学地理科学学院邵庆丰博士在测年方面给予的帮助;感谢审稿专家和编辑部杨美芳老师提出的建设性修改意见。

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Characteristics of CIS13 event recorded by a high resolution stalagmite δ18O in Shanxi Province, China
Huang Wei1, Dong Jinguo2     
(1 Department of Geographical Science, Yichun University, Yichun 336000, Jiangxi;
2 School of Geographical Science, Nantong University, Nantong 226007, Jiangsu)

Abstract

Rapid millennial-scale climate oscillations during the last glaciation, associated with Dansgaard-Oeschger (DO) events, are the most interesting focus. The detailed features for some of them, however, are still open discussion. Here we reconstructed the evolution history of the East Asian summer monsoon (EASM) during the time interval of 53.82~47.41 ka B.P. (including DO14 and DO13 events), based on absolute-dated and high-resolution (24 a average) δ18O record of a stalagmite from North China. In this study, we mainly focus on the detailed features of DO13 event and associated mechanisms.Baiyun Cave (36°40'N, 113°24'E) is located 30 km from northern Licheng County, Shanxi Province. The entrance of Baiyun Cave is large, about 500 m long, and the passageway is rich in carbonate landscape, mainly including stone flower, claw stone and stone dam. Vegetation overlying the cave are mainly composed of shrubs and trees. The temperature and humidity in-situ is 11℃ and 80%, respectively. Controlled by the EASM, the annually-average rainfall in this region is approximately 530 mm, with rainy season from May to October, which accounts for 87% of the total precipitation. Stalagmite BY2 was collected from a rock dam, 200 meters away from the entrance of cave. It is 137 mm in length, and the top and bottom diameters are uniform. The cut and polished profile shows that the lithofacies is opaque milky white to yellowish-brown, with clearly visible growth layers, which is arranged in an approximate straight line within a certain distance from the central deposition position.Six subsamples were collected along the deposition layer with 0.9-mm-diameter carbide dental drill for U/Th dating, and each powder amount is about 50 mg. The measurements were performed by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Fisher Thermo-Neptune. For stable isotopic measurements, 268 subsamples were taken with 0.5-mm-diameter carbide dental drill along the growth axis with powder masses varying between 50 μg and 100 μg. Sampling interval varies from 0.5 mm to 1.0 mm, corresponding to a temporal resolution of mainly ranging from 10 a to 30 a. The analysis was conducted by Finnigan-MAT 253 mass spectrometer fitted with a Kiel Carbonate Device.The tendency of BY2 δ18O is gradually increasing, varying from -8.5 ‰ to -1.0 ‰. The δ18O values are significantly negative surrounding 52.2 ka B.P. and 49.5 ka B.P., with the amplitude of 3 ‰, indicating two significant intensification periods of summer monsoon, and most likely corresponding to DO14 and DO13 events recorded by Greenland ice core, respectively. The stalagmite in Shanxi clearly records the Chinese interstadial (CIS) 13 event with "twin peaks" structure, which mirrors the NGRIP ice core record. And it therefore reflects the sensitive response of the East Asian monsoon (EAM) margin to the North Atlantic Ocean-atmosphere coupling oscillation and the rain belt range variation caused by the north-south movement of the intertropical convergence zone (ITCZ). However, the CIS13 event transition processes recorded by BY2 are slower than that of NGRIP. Specifically, the starting process of summer monsoon presents three stages, which are similar to the reflectance records from the tropical Atlantic and Indian Ocean basins, and may be related to the cross-equatorial airflow in the southern ocean and the continuous northward movement of the ITCZ.On the other hand, the range of millennial-scale events and century-decadal oscillations was up to 5 ‰ and 2 ‰, respectively, which was significantly larger than that recorded by other stalagmites within EAM region. Especially for it remarkably displays "high frequency and large amplitude" characteristics on the century-decadal timescales. It thus demonstrates that the BY2 record-based EASM is sensitive to climate fluctuations on different timescales. The results of power spectrum show that it has obvious change periods of 50 a, 60 a and 206 a, which may be influenced by both solar activity and ocean hydrology. Among, the origin signature of solar output is possibly amplified by the oceanic temperature and salinity oscillators, associated with formation of the north Atlantic deep water.
Key words: stalagmite δ18O    Baiyun Cave    East Asian summer monsoon    DO/CIS event    "twin peaks" structure