2017, Vol.37
② 国土资源部岩溶生态环境-重庆南川野外基地, 重庆 408435)
气候的稳定性和突变是人们关注的焦点，剧烈的气候变化事件会对地球生态系统和社会文明演化产生巨大的影响。末次冰期最显著特征是气候变化呈现出周期性的千年尺度突变事件[1]，即Dansgaard-Oeschger(D/O)循环[2]和Heinrich (H)事件[3]。上述事件在变化幅度和时间上具有巨大、快速的特征[4, 5]，并且在冰芯[2]、海洋[6]、黄土[7]和石笋[8~10]记录中等都留下了清晰的印记。现有研究比较关注与全新世相似的，温暖、湿润的MIS 3[11~14]和MIS 5[15~18]阶段，而对较寒冷的MIS 4阶段研究较少。Wang等[9]指出相对于50~10ka B.P.而言，在110~50ka B.P.期间千年尺度强夏季风事件持续时间更长但频率却低一些。Wolff等[19]指出约在60~70ka B.P.阶段D/O循环表现的极其微弱。MIS 4为典型的冰期时段，持续约15ka[1]，人们过去普遍认为冰期气候极为不稳定，但现有的记录显示MIS 4时段内仅出现3次较大的千年尺度气候波动事件：D/O18、D/O19暖事件和H6冷事件[19]。由于区域差异、分辨率、测年误差等多种因素影响，这些气候突变事件即使在同一替代指标上也存在很大的差异。例如D/O18事件和H6事件在南京葫芦洞[9]和三宝洞记录[10]中振幅并不显著，对其是否具有全球性尚需要高分辨率相关记录进一步验证。H6事件无论是在格陵兰冰芯记录还是东亚季风区石笋记录中都缺乏明确的时间限制，与H1~H5事件有明显的区别[3, 4]。因此，本文利用重庆羊子洞定年数据和氧同位素数据建立高分辨率MIS 4季风记录，明确千、百年尺度的快速突变事件发生的时间和过程，通过与同时期低纬过程和高纬过程的比较，为探讨千百年尺度快速气候变化的机制，理解过去和预测未来气候变化提供依据。
2 研究区域以及样品与方法本文研究的石笋样品Y-02采自羊子洞，羊子洞(29°47′N，107°47′E)为三峡库区重庆丰都雪玉洞群的高层洞穴(图 1)。羊子洞所属的雪玉洞群为典型的季风气候区，气候类型为中亚热带湿润季风气候，该地年均温约16~18℃，降水量约1400mm[20]。Y-02石笋整体呈圆柱体，直径约6.5cm，石笋表面呈乳白色，沿生长轴切开、抛光，内部由纯净方解石组成，方解石结晶致密(图 2a)。沿生长轴测得石笋总长78cm，本文研究距顶部0~25.5cm之间的部分，集中讨论MIS 4阶段的气候记录。
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图 1 羊子洞地理位置以及文中提到的亚洲季风区其他洞穴 羊子洞(29°47′N，107°47′E)；新崖洞(30°45′N，109°28′E)；三宝洞(31°40′N，110°26′E)；天鹅洞(31°26′N，110°26′E)；葫芦洞(32°30′N，119°10′E)；小白龙洞(24°12′N，103°21′E)；董哥洞(25°17′N，108°50′E)黑色箭头表示东亚冬季风、夏季风和印度夏季风的方向 Fig. 1 Location of Yangzi Cave site and other cave sites of the East Asian monsoon area discussed in this paper:Yangzi Cave(29°47′N, 107°47′E); Xinya Cave(30°45′N, 109°28′E); Sanbao Cave(31°40′N, 110°26′E); Tian'e Cave(31°26′N, 110°26′E); Hulu Cave(32°30′N, 119°10′E); Xiaobailong Cave(24°12′N, 103°21′E); Dongge Cave(25°17′N, 108°50′E). Black arrows denote the directions of the East Asian winter monsoon, East Asian summer monsoon and the Indian summer monsoon |
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图 2 石笋Y-02照片(a)和年代模型(b) Fig. 2 Section (a) and age-depth model (b) of stalagmite Y-02 |
石笋δ18O和δ13C在西南大学地球化学与同位素实验室分析完成，分析仪器为Delta-V-Plus型质谱联动碳酸盐自动进样装置(Kiel Ⅳ)测试。在进行稳定同位素测试时，每7个样品加测一个标准样品NBS19，分析误差(± 1σ)δ18O值＜± 0.1 ‰，δ13C＜± 0.06 ‰，结果相对于VPDB(Vienna Pee Dee Belemnite)标准。Y-02共测试503个样品，平均时间分辨率约为25.8年。
沿着石笋剖面上的生长轴方向用直径为5mm的牙钻钻取12个年龄样品(表 1)，每个年龄样品重量约150~200mg之间。Y-02石笋的年龄样品中Y-02-4、Y-02-5、Y-02-6和Y-02-9按Shen等[21]方法在台湾大学High-precision Mass Spectrometry and Environment Change(HISPEC)实验室测试，其余8个年龄样品在美国明尼苏达大学地质与地球物理系同位素实验室测试，石笋年龄样品测试仪器为MC-ICP-MS Nepture，年龄误差≤1 % (2σ)。
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表 1 羊子洞石笋Y-02 ICP-MS测年结果* Table 1 Uranium and thorium isotopic compositions and 230 Th ages for Yangzi Cave speleothem Y-02 by ICP-MS |
表 1给出石笋Y-02的12个230 Th年代数据，跨越年龄段约60~73ka B.P.。从表 1可以看出，所有年龄数据均按石笋沉积的先后顺序排列，说明数据可信。Y-02石笋的12个年代数据中除4.6cm处误差较大外，其余多在200~300年内，误差较小。通过对230 Th年代数据进行线性内插建立了Y-02的年代模型(图 2b)。
3.2 石笋氧同位素Y-02的δ18O记录除了在约63ka B.P.、64ka B.P.和70ka B.P.附近有较大的波动外，整体趋势较为平缓。δ18O值的波动范围为-9.2 ‰ ~-6.6 ‰，δ18O平均值为-7.71 ‰。73.0~64.2ka B.P.时段内，石笋δ18O呈现出逐渐偏负的趋势，但波动不大，平均值为-7.59 ‰；64.2~63.1ka B.P.时段内，δ18O急剧偏正，由最小值-8.98 ‰转变为最大值-6.75 ‰，二者相差2.23 ‰，平均值为-8.03 ‰；63.1~60.0ka B.P.时段内，δ18O开始偏负并且表现的也较为和缓，平均值为-7.87 ‰ (图 3)。
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图 3 60~73ka B.P.石笋Y-02的δ18O记录 Fig. 3 The variation of Y-02 δ18O record during 60~73ka B.P. |
石笋氧、碳同位素是否达到平衡分馏是作为气候变化的代用指标的基本条件[22]。图 4所示：同一纹层石笋δ18O值变幅不超过0.45 ‰，说明羊子洞Y-02石笋同一生长纹层中碳酸盐的δ18O值基本一致，向外无富集；Y-02石笋单个生长纹层中δ18O和δ13C之间没有显著的相关关系(图 4)。上述结果说明羊子洞石笋δ18O达到平衡分馏。
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图 4 羊子洞石笋Y-02的Hendy检验[22]结果(方块曲线表示距顶部4.5cm，三角曲线距顶部17.4cm) (a)距离顶部不同深度各层位δ18O值的变化＜0.45 ‰；(b)同一层位δ18O和δ13C的关系 Fig. 4 Results of the Hendy Test on two growth layers of stalagmite Y-02, and the squares and their curve denote 4.5cm from the top, and the triangles and their curve are 17.4cm from the top. (a)One-sigma δ18O variability within growth layers varies by < 0.45 ‰ at different depths from the top; (b)Plots of δ18O versus δ13C for coeval subsamples |
此外，Dorale和Liu[23]认为重复性检验对于验证同位素是否达到平衡分馏更加的严谨。因此，我们选取了纬度相近的葫芦洞[9]和三宝洞[10]生长于同一时期的石笋进行对比，结果显示三者具有较好的重复性(图 5)。多数研究显示东亚季风区石笋δ18O主要反映东亚季风信息[8~10, 24~34]。Wang等[8, 9]指出石笋氧同位素记录反映了与夏季风强弱变化相联系的冬夏季降水的比率；Yuan等[24]认为石笋δ18O记录主要反映了水汽由源区向内陆移动的过程中水汽冷凝比率的多寡，进而反映了夏季风强度的变化；Cheng等[10, 34]认为中国南方地区石笋氧同位素的变化主要反映大气降水的氧同位素组成。本文中Y-02的δ18O曲线与新合成的三宝洞-葫芦洞石笋δ18O曲线变化趋势一致[34]，石笋δ18O记录的千年尺度气候波动事件，在定年误差范围内能较好的吻合。羊子洞、葫芦洞、三宝洞三地石笋δ18O记录保持一致，可以证实在千年尺度上，亚洲季风区的降雨在区域上具有一致性[15, 25]。因此，本文中Y-02石笋δ18O所代表的古气候意义与之前的研究得出的结论[8~10, 24, 34]一致，即：洞穴石笋δ18O值偏负时指示强季风，反之指示弱季风。
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图 5 羊子洞与葫芦洞[9]和三宝洞[10]重复性检验 Fig. 5 Replication Test for Yangzi Cave with Hulu Cave[9] and Sanbao Cave[10] |
已有的冰芯记录[1, 35]和石笋记录[36, 37]研究对D/O事件作出定义并给出判定标准：D/O事件发生时的气候状况明显的偏离之前气候的基本水平并且以急剧上升部分的第一个数据点作为起点；同时D/O事件以急剧下降部分的最后一个数据点作为每一次快速的气候转变结束阶段。
羊子洞石笋Y-02的δ18O记录显示：千年尺度至百年尺度的气候波动是MIS 4期间季风变化的一个显著特征。Y-02石笋δ18O曲线在MIS 4期间记录了3次强的千年尺度夏季风增强事件：D/O18、D/O19a和D/O19。D/O18事件开始于约64.6ka B.P.，结束于63.6ka，持续时间近1.0ka，其间石笋δ18O值在短短400a时间内由-7.63 ‰偏负到-8.98 ‰，偏负了1.35 ‰，随后逐渐偏正至-6.75 ‰。Y-02的δ18O记录显示D/O18事件开始时，东亚夏季风强度急剧增强，然后逐渐减弱，增强过程用了近400a，而减弱过程用了600a，表明整个过程是不对称的(图 6b和图 7b)。这在东亚季风区其他石笋记录中也得到了验证，天鹅洞和新崖洞也是如此(图 6d和6f)。
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图 6 羊子洞石笋Y-02的δ18O记录与其他古气候记录对比 (a)NGIRP δ18 O记录[38]，(b)羊子洞，(c)小白龙洞[39]，(d)天鹅洞[40]，(e)阿拉伯海L *(50点滑动距平)[41]，(f)新崖洞[42]；其中(b)、(c)、(d)和(f)下部误差棒表示230 Th数据和误差(±σ)，D/O18至19事件标注在相对应图形上部 Fig. 6 Comparison of the stalagmite δ18O record from Yangzi Cave with other paleoclimatic records. (a)The δ18O record of NGRIP from the Greenland ice core[38], (b)Yangzi Cave, (c)Xiaobailong Cave[39], (d)Tian'e Cave[40], (e)L * from Arabian Sea(50-point running mean)[41] and (f) Xinya Cave[42]. The horizontal bars are below (b), (c), (d), and (f) indicate the 230 Th dates and errors(±σ). The D/O events 18 to 19 are marked on (b), (c), (d), and (f) |
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图 7 石笋Y-02记录的季风急剧减弱阶段与其他记录的详细对比 (a)NGIRP δ18O记录[38]；(b)羊子洞；(c)小白龙洞[39]；(d)南极冰芯Dome C δD记录[43]黄色小长方形之间的部分表示季风急剧减弱阶段在各个记录中所处的位置，红色箭头表示各记录的变化趋势 Fig. 7 Detailed comparison between Y-02 and other records for the monsoon sharply weakened. (a)NGRIP δ18O record[38]; (b)Yangzi Cave; (c)Xiaobailong Cave[39]; (d)Antarctic δD record from ice core EDC[43]. The yellow vertical bars mark the monsoon sharply weakened in each of the records, and the red lines with arrows denote the general trends around D/O18 |
格陵兰冰芯记录的D/O18暖事件[38]开始于64.17ka B.P.，中心位置在约64.01ka B.P.附近，结束于63.81ka B.P.，持续了约360a(图 6a)。整个过程表现为脉冲式的变化(图 7a)，迅速增温，维持时间很短，然后迅速降温[19]。与格陵兰冰芯记录相比，Y-02石笋δ18O记录的D/O18事件持续时间要更长，但是D/O18事件时间的中心位置与格陵兰冰芯一致。
Y-02的δ18O记录D/O19事件的中心位置在70ka B.P.附近，石笋δ18O值在百年之内偏负0.6 ‰，显示东亚夏季风急剧增强，但紧接着又急剧减弱，东亚夏季风的增强和减弱过程都是在百年之内完成，持续时间也不超过400a。格陵兰冰芯记录的D/O19事件开始的时间为72.28ka B.P.，在持续2ka后结束[19]，这与Y-02石笋δ18O记录的D/O19事件起止时间有很大的差异(图 6a和6b)。
D/O19事件结束以后，格陵兰冰芯记录显示在中心约69.4ka B.P.附近，又出现了持续200~300a的北半球高纬度地区温度升高事件，即D/O19a。Y-02的δ18O值在中心约69.4ka B.P.附近也形成一个小峰值，整个过程持续仅仅百年，说明东亚夏季风强度又一次短暂的增强，可能是D/O19a事件。Y-02记录的D/O19a事件的中心位置在69.4ka B.P.附近，二者在起止时间和转换形态上十分的相似。Y-02的δ18O记录D/O18、D/O19a和D/O19事件在定年误差范围内，与东亚季风区其他洞穴石笋记录相符合(图 6b、6c、6d和6f)。
Y-02的δ18O记录D/O18、D/O19a和D/O19事件在定年误差范围内与格陵兰冰芯记录的D/O事件基本一致；尤其是Y-02记录的D/O18和D/O19a事件与冰芯记录相比：二者均表现出快速开始和迅速结束的转换形态，而且二者的起止时间也大体相同(图 6a和6b)，进一步证实中国季风变化与北半球高纬度地区温度变化遥相关[8]。
但是，我们发现：格陵兰冰芯记录与中国石笋记录也还存在较大的区别，冰芯记录的D/O19事件无论在持续时间和变化幅度上都是一次重要的事件，但是D/O19事件在中国季风区石笋记录中并不是太显著，在小白龙洞[39]和天鹅洞[40]的石笋记录中均没有完全显示出来，甚至出现石笋生长中断，即使在葫芦洞[9]和三宝洞[10]石笋记录中，其变化幅度也非常有限(图 5)。而来自同一地区的重庆羊口洞石笋JFYK7记录显示：D/O19事件开始于71.4±0.2ka B.P.，东亚夏季风在D/O19事件时快速增强之后，仍经历了约1500年的缓慢增强过程，但结束过程不清晰，与冰芯记录的D/O19事件形态不同[44]。
同时以石笋Y-02为代表的东亚季风区石笋记录显示(图 7)：D/O18事件是一次较强的千年尺度季风增强事件，整个过程持续近千年，而冰芯记录的D/O18事件仅持续了360a。格陵兰冰芯记录显示D/O19事件明显强于D/O18事件，而越来越多来自中国季风区石笋记录[39, 40, 42, 44]表明：D/O18事件时东亚夏季风经历了一次显著的增强过程，甚至有超越D/O19事件时东亚夏季风强度的倾向。
而来自印度季风区的石笋以及海洋记录所表现出的千年尺度夏季风波动事件与中国季风区相一致[39, 41, 45]。印度北部的Bittoo洞穴石笋记录的D/O18事件转换形态与Y-02记录相一致，但D/ O19事件时段内石笋停止生长[45]。这一现象与小白龙洞石笋记录相一致[39]，小白龙洞石笋记录代表典型的印度夏季风变化，同样未能完整的记录下D/O19事件。来自阿拉伯海钻孔SO130-289KL记录同样显示D/O18事件在持续时间和强度上不弱于D/O19事件[41](图 6e)。
印度夏季风作为亚洲夏季风系统的一部分，其演化和发展对亚洲夏季风变化产生十分重要的影响。印度夏季风和东亚夏季风的演化直接响应北半球高纬度地区太阳辐射变化，二者共同受到北半球高纬度地区气候变化的影响[8, 41, 45~48]。来自印度洋的水汽不仅控制印度夏季风的降雨，而且水汽运移到中国季风区，对中国夏季风产生重要影响[49~54]。所以，我们认为MIS 4时段内，东亚夏季风的演化不仅受到北半球高纬度地区气候变化的影响，同时也受其他影响因素控制。
3.4 H6事件Y-02的δ18O记录显示D/O18事件以后，δ18O值出现一个较大幅度的加重波动，暗示一个短期大幅度季风减弱波动事件，3个测年数据为这次事件提供了较为精确的年代限制。石笋深度7.3cm处对应测得的年龄为64.02±0.51ka B.P.；深度4.6cm处对应年龄为63.47±0.75ka B.P.；深度3.5cm处对应年龄为62.62±0.55ka B.P. (表 1)。我们以63.74ka B.P.作为此次事件的开始，以62.68ka B.P.作为事件的结束，共持续约1.0ka，此次事件开始时δ18O值为-8.33 ‰，事件结束时δ18O值为-8.22 ‰。在63.74~62.68ka B.P.内，Y-02的δ18O值在中心约63ka B.P.附近达到最高点，为-6.75 ‰。Y-02的δ18O值在这次大幅度波动事件中偏差达到1.58 ‰，其波动十分剧烈(图 7b)。Y-02石笋记录这次东亚夏季风突然且短暂的一次减弱过程，也得到了东亚季风区其他记录支持。小白龙洞[39](图 7c)和董哥洞[24]记录都显示东亚季风在D/O18事件后经历了一次快速的季风减弱事件。
Y-02石笋δ18O记录的这次弱季风事件与格陵兰冰芯记录中的H6事件发生的时间范围相符合[19]。格陵兰冰芯δ18O曲线显示H6事件为持续时间较长的平缓低谷[19]，反映的北半球高纬度地区温度的降低并不是十分的剧烈，这可能与此时处于MIS 4冰期、全球温度原本就很低有关，所以此时段高纬度地区的降温现象在冰芯记录中表现的不明显[4, 19, 38, 55](图 7a)。冰芯δ18O记录显示H6事件在大约63~62ka B.P.时段是温度最低[4]，与Y-02石笋δ18O曲线显示的季风最弱时段相对应。基于格陵兰冰芯记录的年层计数年代数据(GICC05)，H6持续时间为3.1ka(63.2~60.1ka B.P.)[1]。在年龄误差范围内，Y-02石笋记录的H6事件开始时间虽早于冰芯记录，它的δ18O值在63.74~62.68ka B.P.内急剧偏正，在中心约63ka B.P.附近达到最大值，Y-02石笋δ18O所反映的东亚夏季风减弱事件的中心位置在63ka B.P.附近，刚好对应北半球高纬度地区出现最低温度的时间(图 7a和7b)。Oster等[14]认为H6事件开始的时间在60.9±0.5ka B.P.，H6事件在持续约1.1ka后，在约59.8±0.6ka B.P.，石笋的δ13C突然正偏2.1 ‰，伴随着生长速率降低，H6事件结束；Li等[42]对重庆新崖洞石笋XY-2研究时发现，在约60ka B.P.附近，XY-2的δ18O值出现较大幅度的正偏移且达到峰值，代表了当时夏季风的急剧减弱，H6事件应该发生在60ka B.P.附近；同时，Rohling等[56]研究也指出：H6事件最有可能发生的时间段为：60.8~61.6ka B.P.。Y-02石笋δ18O记录捕捉到的D/O18事件以后的这次东亚夏季风突然且短暂的一次减弱过程，可能就是H6事件或者是H6事件季风最弱的时段。
现有研究表明：H事件发生期间，北半球高纬度地区气候变化与亚洲季风区域内气候状况，二者之间存在着紧密的联系[57]。Wang等[8]研究指出：末次冰期，东亚季风的突变与北半球高纬度地区出现的D/O循环和H事件存在关联；东亚夏季风减弱阶段与北大西洋地区寒冷阶段相对应，反之亦然。随后越来越多来自中国中部和南部地区的石笋记录证实了这一观点[9, 10, 24, 58~60]。Deplazes等[41]通过对来自阿拉伯海钻孔SO130-289KL分析所得到的地球化学和沉积学数据的研究，并结合其他重要的古气候记录指出：在H1~H6事件，北热带地区呈现出变干的趋势，环北大西洋地区温度变化的信号传输到热带地区，并影响到热带地区的水汽变化，对受季风影响区域的局部地区的环境变化产生重要的影响。
而现代观测和模拟研究均表明：淡水的注入/北大西洋地区海冰覆盖范围的扩大可能会导致北大西洋深层水和北大西洋经向翻转环流(AMOC)发生显著的变化，这会引发北半球高纬度地区温度降低和热带辐合带(ITCZ)向南移动[46, 61~63]。此外，北大西洋地区海冰的变化也会放大和传递这种信号，热带辐合带向南移动会导致东亚夏季风的减弱[64]。在东亚季风区，北半球高纬度地区温度的降低可能会导致东亚冬季风的增强和青藏高原积雪覆盖面积的增加[65, 66]，这也可能会使东亚夏季风减弱。
同时，Y-02石笋δ18O记录显示在约63.65~62.87ka B.P.期间东亚夏季风突然而短暂的减弱过程可能也受到南半球温度变化影响。Cai等[67]指出，D/O12阶段XBL-1石笋记录与南极冰芯Byrd记录的温度变化模式相似，并在千年尺度上遥相关；Zhou等[68]在对中国中部季风区边缘SJ1石笋记录进行研究时也发现，SJ1石笋与中国季风区MSL石笋和XBL-1石笋记录的H4事件的开始、结束时间是一致的，并且与南极EDML冰芯记录的开始增温和暖事件结束时间同步，他指出该现象可能受南半球温度变化的影响；Jiang等[15]对三星洞石笋记录研究时指出：在113.5~86.6ka B.P.期间，三星洞石笋δ18O记录与南极冰芯呈现反相位关系，他指出亚洲夏季风与南半球温度二者之间存在联系。
如图 7所示，Y-02石笋δ18O记录的D/O18事件开始于约64.6ka B.P.，在经历约400a后季风达到最强盛，随后开始减弱。而南极地区在此时段内经历着迅速降温和逐渐升温的相反变化过程。亚洲地区季风系统主要受南印度洋和亚洲大陆间的热力差异驱动[9, 69]，南半球高纬度地区的气温变化可能通过跨越澳大利亚东部地区的大气环流由海洋进入东亚大陆，对东亚夏季风产生影响[70]。而当南半球变暖而北半球变冷时，南北半球之间的温度梯度差会增加，反之亦然，这会导致热带辐合带的平均位置朝向较为温暖的半球偏移[46, 63, 71]。
如图 7b所示，Y-02石笋δ18O记录显示在约63.65~62.87ka B.P.期间东亚夏季风突然而短暂的减弱对应于北半球高纬度地区温度的降低，而南极此时正处在温度逐步上升的阶段，这可能会进一步引发热带辐合带的平均位置向南移，使得东亚夏季风强度进一步减弱。因而，Y-02石笋δ18 O记录捕捉到的D/ O18事件以后的这次东亚夏季风突然且短暂的一次减弱过程事件不仅受到北半球高纬度地区气候变化的影响，与此同时，南极地区温度的变化也可能会对它产生影响。
4 结论利用来自中国西南地区重庆市丰都县羊子洞高分辨率石笋Y-02的δ18O记录重建了MIS 4阶段东亚夏季风千年尺度变化，通过与季风区其他石笋记录以及高纬冰芯记录分析对比，初步得出以下结论：
(1) Y-02石笋δ18O曲线显示MIS 4阶段，东亚夏季风强度表现为明显的千年尺度变化。尤其是Y-02的δ18O记录D/O18、D/O19a、D/O19事件的转换形态：均表现为脉冲式的快速开始和迅速结束的转换模式；与此同时，除了千年尺度事件发生时，Y-02的δ18O曲线出现剧烈波动，其余时段十分平缓。上述变化特征与MIS 4时段内格陵兰冰芯记录保持高度一致。这证实：MIS 4阶段，东亚夏季风强度演化快速响应北半球高纬度地区温度的变化。
(2) Y-02石笋δ18O记录显示，D/O18事件是一次较强烈的千年尺度季风增强事件，持续时间近千年，而D/O19事件在持续时间和强度上不如D/ O18事件，这种形态特征不同于格陵兰冰芯记录。这表明，虽然东亚季风受到高纬气候的影响，但是对D/O事件的响应可能存在不同模式。
(3) Y-02石笋δ18O记录显示：紧接着D/O18事件以后，在中心约63ka B.P.附近δ18O值急剧偏正，反映东亚夏季风强度急剧减弱，在持续约1ka年后，又迅速的恢复，这个突变可能对应H6事件或者是H6事件季风最弱的时段，这一弱季风事件也得到了来自亚洲季风区其他洞穴石笋记录的支持。我们认为：北半球高纬度地区气候变化是这次季风减弱事件的主导因素，而南极地区温度的变化也会对它产生一定的影响。
致谢 感谢张月明、黄帆、李辰丝、吕春艳和李国军等研究生参与野外采样和实验分析工作；十分感谢审稿专家和编辑部老师提出的宝贵评审意见和修改建议，帮助提高和完善了本文。
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② Field Scientific Observation & Research Base of Karst Eco-environments at Nanchuan in Chongqing, Ministry of Land and Resources of China, Chongqing 408435)
Abstract
Yangzi Cave (29°47'N, 107°47'E), belonging to the upper caves of Xueyu Cave group, is a longitudinal cave system developing along the monoclinic strata, and it is situated on the left bank of downstream of Dragon River, about 18 kilometers from new downtown of Fengdu, Chongqing, Southwest China. Yangzi Cave is more than 500m in length, composed of lower and wide passage and two halls, with some residual underground and flood muddy sediments therein. This region is dominated by the typical subtropical humid monsoon climate. The current mean annual temperature is 16~18℃ and the average annual rainfall is ca.1400mm. Stalagmite Y-02, collected in Yangzi Cave, is 78cm in length. In this paper, we present the data from the top 25.5cm. The shape of Y-02 is cylindrical and the color of surface is milky. The stalagmite was cut along the central axis. It is composed of pure calcite, and the structure of calcite crystals appears mighty dense. 12 subsamples, 150~200mg for each, were drilled parallel to the growth axis of Y-02.4 subsamples (Y-02-4, Y-02-5, Y-02-6 and Y-02-9)were dated with U-series method at the Laboratory of Taiwan University, and the rest of subsamples were dated at the Minnesota Isotope Laboratory, and errors are given with two standard deviation (±2σ). Stable oxygen and carbon isotope of stalagmite Y-02 was analyzed at Geochemistry and Isotope Laboratory of Southwest University. Analysis was performed using a Delta-V-Plus Mass Spectrometer, combined with a Kiel Ⅳ Carbonate Device. Every seven samples were intercalated with one standard, NBS 19. Isotopic results are given with respect to Vienna Pee Dee Belemnite (VPDB)standard with one-sigma external error < ±0.1 ‰ for δ18O and < ±0.06 ‰ for δ13C. The results suggested that the age of Y-02 between the top and the depth of 25.5cm grew from 60~73ka B.P., and all age data are ranked in the order of stalagmite deposition, which illustrates the data are valid and trusted. There is no obvious sedimentary discontinuity in this period, which we can use linear interpolation to establish its time scale. In total 503 stable oxygen and carbon isotope samples of stalagmite Y-02 were analyzed. The average resolution of the δ18O series is about 25.8a. The δ18O record of Y-02 displays a significant millennial-scale change that correlates well in timing and duration with Dansgaard/Oeschger (D/O)events identified in high-latitude regions of the Northern Hemisphere. We reconstructed the evolution of the East Asian summer monsoon during the MIS 4 glacial period. Millennial-scale events activity appears particularly weak in the meantime, which is also the most obvious character of the δ18O record in Y-02. The Y-02 record also displays that the intensity of the Asian summer monsoon immediately weakened after the D/O event 18, then rapidly recovered in about 1ka later. After comparing the stalagmite Y-02 δ18O record with other records from the Asian monsoon area, we find that this is not an isolated phenomenon. We consider this event may be the part of Heinrich event 6 or the weakest monsoon episode of it. We believe that climate change in the northern hemisphere high latitudes is the dominant factor, and that changes in temperature in the Antarctic region will also have some effect on it.