2019, Vol.39
2 中国科学院青藏高原地球科学卓越创新中心, 北京 100101;
3 中国科学院大学, 北京 100049;
4 云南大学地球系统科学研究中心, 云南 昆明 650091)
千年尺度全新世气候突变事件的发现是20世纪90年代古全球变化研究的一项重大突破[1]。基于格陵兰冰芯地球化学元素[2]和北大西洋深海沉积物冰筏碎屑指标[3]重建的古气候记录，改变了传统上基于冰芯氧同位素[4]所得出的全新世气候系统总体比较稳定的认识。这些全新世气候事件变化速率快、波动幅度大、持续时间长，对自然环境和人类社会产生了重大影响，因此受到国际学术界和公众的广泛关注[1, 5~7]，成为近二十多年来全球变化研究领域重大学术前沿和研究热点[1, 8~12]。
在这种研究背景下，一些全新世气候事件被识别并得到了系统地研究，如早全新世气候震荡[13]、8.2 ka事件[14~15]、5.5 ka事件[16~18]、4.2 ka事件[19~20]等。另一些气候事件也逐渐被识别出并引起了学术界的关注，其中包括7.5~7.0 cal.ka B.P.(cal.ka B.P.指距今千年)气候事件。全球多个地区测年准确、分辨率较高的古气候档案明显记录了这次气候事件[21~27]，其中也包括中国地区[28~31]。进一步研究表明这次气候事件可能发生在7.5~7.0 cal.ka B.P.[24, 32]。基于该气候事件低谷发生的时间，不同研究者将其称之为7.0 ka事件[33]、7.1 ka事件或IRD-5b事件[34]、7.3 ka事件[32]以及7.4 ka事件[35]等。
进一步的研究还发现，该气候事件可能对全球多个地区的考古学文化转变产生了明显影响。如在欧洲地区，该气候事件被认为是触发当地中石器文化向新石器文化转变的关键因素[36~37]。在中国多个地区，包括汉中[38]、甘青[39]、内蒙古农牧交错地带[40]、东北西辽河流域[41]以及中国整个北方地区[42]，新石器早中期考古学文化的转变可能与该气候事件有关。因此7.5~7.0 cal.ka B.P.气候事件对于认识气候系统行为以及揭示全新世早中期考古学文化转变动因具有重要的科学价值；另外，与8.2 ka事件发生在大暖期开始，4.2 ka事件发生在大暖期结束的情形不同，7.5~7.0 cal.ka B.P.气候事件发生在全新世大暖期期间[43]，因此能为全球变暖背景下未来气候变化及其影响提供更为合适的历史气候相似型，具有明显的现实意义。
然而，目前为止，在中国，对这次气候事件的区域环境表现特征及其驱动机制还缺乏系统的了解。除了需要继续开展古气候重建，利用测年准确、分辨率高的古气候记录识别和刻画该气候事件外，还有必要对已发表的古气候记录进行总结。这种综述通过系统地对比和分析不同古气候记录，利用其相互印证和互相补充关系，整合不同记录的优点，减少单站点古气候重建的诸多不确定性，从而获得对区域气候事件更为全面、清晰和客观的认识[6, 44~46]。中国是世界上拥有高分辨率古气候档案种类最多的地区之一，过去二十多年来，不同学者利用这些古气候档案在全新世高分辨率古气候重建方面取得了大量进展，为识别和理解7.5~7.0 cal.ka B.P.气候事件提供了丰富的证据。本文，我们首先收集中国地区26条古气候记录，系统总结7.5~7.0 cal.ka B.P.事件在中国地区的环境表现；并与全球其他地区的古气候记录进行对比，阐明该气候事件是否具有全球尺度特征；在此基础上将古气候记录与气候驱动因素记录对比，分析其产生的动力机制；最后，基于7.5~7.0 cal.ka B.P.事件的研究现状，提出未来研究热点和重要方向。
1 中国7.5~7.0 cal.ka B.P.事件记录为了获得对7.5~7.0 cal.ka B.P.事件的认识，我们基于以下5条标准收集了中国地区24条高分辨率的古气候记录(图 1)：1)时间分辨率小于100年；2)测年误差小于7.5~7.0 cal.ka B.P.事件的持续时间；3)古气候记录比较连续；4)代用指标气候指示意义比较明确；5)能够记录到7.5~7.0 cal.ka B.P.气候事件。除此之外，还选取了2条低分辨率，或沉积不太连续但明确记录到该气候事件的记录，总计26条古气候记录。这些记录覆盖了中国大部分地区，其具体位置如图 1所示，每条记录的地理位置、测年个数、时间分辨率、代用指标和测年误差等相关信息列在表 1中。
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图 1 古气候记录点的地理位置 (1)红原泥炭；(2)青藏高原中部Ahung Co；(3)青海湖；(4)古里雅冰芯；(5)敦德冰芯；(6)岱海；(7)调角海子；(8)安固里淖；(9)山西莲花洞；(10)渭南剖面；(11)东海；(12)黄海；(13)暖和洞；(14)王家崴洞；(15)哈尼泥炭；(16)二龙湾玛珥湖；(17)大九湖泥炭；(18)青天洞；(19)和尚洞；(20)竹溜平洞；(21)董哥洞；(22)西盟龙潭；(23)云南西湖；(24)戴云山泥炭；(25)雷州半岛珊瑚礁；(26)台湾Retreat湖 Fig. 1 The locations of paleoclimate records. (1)Hongyuan Peat; (2)Ahung Co in central Tibetan Plateau; (3)Qinghai Lake; (4)Guliya ice core; (5)Dunde ice core; (6)Daihai Lake; (7)Diaojiaohaizi Lake; (8)Anguli-Nuur Lake; (9)Lianhua Cave; (10)Weinan profile; (11)East China Sea; (12)Yellow Sea; (13)Nuanhe Cave; (14)Wangjiawai Cave; (15)Hani Peat; (16)Erlongwan Maar Lake; (17)Dajiuhu Peat; (18)Qingtian Cave; (19)Heshang Cave; (20)Zhuliuping Cave; (21)Dongge Cave; (22)Ximenglongtan Lake; (23)Xihu Lake in Yunnan; (24)Daiyun Mountains peat; (25)Leizhou Peninsula; (26)Retreat Lake in Taiwan |
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表 1 中国记录到7.5~7.0 cal.ka B.P.气候事件的地点 Table 1 List of paleoclimate records of 7.5~7.0 cal.ka B.P. event from China |
在青藏高原东部，测年准确、时间分辨率为30年的红原泥炭单一植物残体苔草纤维素的δ13C记录了全新世以来的印度夏季风变化，δ13C值在7.5~7.3 cal.ka B.P.期间偏正(图 2a)，指示印度夏季风减弱，降水减少[47]；在青藏高原中部，Ahung Co湖相沉积物碳酸盐含量、有机质含量和C/N等多项指标的综合研究表明7.5~7.0 cal.ka B.P.期间发生了全新世期间最为显著的一次气候异常事件，表现为季风降雨减少，湖泊水位下降(图 2b)[48]；在青藏高原北部，有57个年代控制点、分辨率较高的青海湖湖泊沉积物碳酸盐和TOC含量等多种气候替代性指标记录了32 ka以来的亚洲夏季风变化，结果表明在7.4~7.0 cal.ka B.P.期间，夏季风显著减弱，降水减少(图 2c)[29]。除了上述季风强度记录外，青藏高原地区还存在一些该气候事件的温度记录，如西昆仑山古里雅冰芯δ18O值从7.5 cal.ka B.P.左右开始明显偏负，指示温度下降(图 2d)[49]。祁连山敦德冰盖δ18O值在7.3 cal.ka B.P.处于低谷，也明确显示了这次降温事件(图 2e)[50]。
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图 2 中国地区7.5~7.0 cal.ka B.P.气候事件记录 (a)红原泥炭单一植物残体苔草纤维素δ13C[47]；(b)Ahung Co地球化学指标第一主成分值[48]；(c)青海湖夏季风指数[29]；(d)古里雅冰芯δ18O[49]；(e)敦德冰芯δ18O[50]；(f)岱海7月温度重建[51]；(g)青天洞石笋δ18O[31]；(h)大九湖温度重建[59]；(i)董哥洞石笋δ18O[28]；(j)云南西湖碳酸盐含量[30]；(k)黄海海表温度重建[65]；(l)和尚洞石笋等温剩磁(IRM)[60] (a~e)位于青藏高原区，(f，k)位于北方地区，(g~j，l)位于南方地区 Fig. 2 Paleoclimate records from China for the 7.5~7.0 cal.ka B.P. climatic event. (a)Hongyuan Peat δ13C of C.mulieensis remains cellulose[47]; (b)Ahung Co first principal component of geochemistry[48]; (c)The summer monsoon indice of Qinghai Lake[29]; (d)Guliya ice core δ18O[49]; (e)Dunde ice core δ18O[50]; (f)Reconstructed July temperature of Daihai Lake[51]; (g)Qingtian Cave δ18O[31]; (h)Reconstructed temperature of Dajiuhu Peat[59]; (i)Dongge Caveδ18O[28]; (j)The carbonate content of Lake Xihu in Yunnan Province[30]; (k)SSTs record of the Yellow Sea[65]; (l)Isothermal Remanent Magnetization(IRM)of Heshang Cave[60]. (a~e)are located in the Tibetan Plateau, (f, k)are located in Northern China, and (g~j, l)are located in Southern China |
在北方地区，东南夏季风边缘地带的多个湖泊沉积物记录一致性地揭示出7.5~7.0 cal.ka B.P.期间的气候异常。利用岱海湖相沉积物孢粉定量重建的全新世温度变化显示，7.6~7.4 cal.ka B.P.期间7月平均气温比现今低2 ℃(图 2f)[51]；安固里淖近50年时间分辨率的沉积物孢粉记录显示，7.4~7.0 cal.ka B.P.期间植被组成发生明显变化，桦木属(Betula)和其他落叶阔叶乔木花粉百分比减少，松属(Pinus)、云杉属(Picea)和冷杉属(Abies)花粉百分比增加，表明气候比前一阶段更为寒冷干燥；其他代用指标如环境磁学参数、粒度、TOC和C/N等也显示降水减少，气候出现明显异常[52]。上述利用湖泊沉积物重建得出的7.5~7.0 cal.ka B.P.期间温度下降的结果也得到了非连续性古气候记录的支持，如位于农牧交错带的调角海子湖剖面在全新世发育了3期埋藏古冰楔，其中一次形成于7.5~7.4 cal.ka B.P.，估计当时年均温可能比现今要低2~3 ℃[40]。
在黄土高原地区，对关中盆地渭南剖面多种气候替代性指标的综合研究，发现7.3~6.8 cal.ka B.P.期间暖湿的蜗牛种类、植硅体类型和阔叶木本植物明显减少，表明气候寒冷干燥[38]；同样位于黄土高原地区的山西莲花洞具有4~47年时间分辨率的石笋δ18O值从7.5 cal.ka B.P.开始升高，并于7.4 cal.ka B.P.左右达到峰值，指示夏季风强度明显减弱[53]。
在东北地区，辽宁本溪的暖和洞[55]和王家崴洞[56]两个高分辨率的石笋δ18O记录均显示7.3~7.0 cal.ka B.P.期间发生了一次百年尺度的夏季风减弱事件。这一结果得到了同一地区泥炭记录的支持，比如吉林哈尼泥炭近20年时间分辨率的混合植物残体苔草纤维素的δ13C记录显示，7.4~7.2 cal.ka B.P.期间，降水明显减少，可能与夏季风减弱有关[57]；附近的二龙湾玛珥湖沉积物高分辨率的δ13C和干密度值在7.4 cal.ka B.P.左右出现异常显著的峰值，同时总有机碳和总氮含量降低，反映气候向干冷化方向转变[58]。
在南方长江中游地区，湖北神农架青天洞2~6年时间分辨率的石笋δ18O记录显示，7.4~7.2 cal.ka B.P.期间发生了一次弱夏季风事件，且呈现出突变的特征(图 2g)[31]；附近的大九湖泥炭微生物脂类代用指标显示7.5~7.0 cal.ka B.P.期间温度明显下降(图 2h)[59]。
在华南地区，福建戴云山泥炭沉积物孢粉记录显示，7.3~7.0 cal.ka B.P.期间木本花粉百分比明显下降，炭屑浓度急剧增加，可能与夏季风减弱引起的降水减少有关[63]。南海北部雷州半岛的角孔珊瑚礁剖面的Sr/Ca记录显示，7.5~7.0 cal.ka B.P.期间冬季温度比现代低约2~3 ℃，反映当时冬季风强劲；进一步的研究发现，该珊瑚礁剖面还发育了至少9层间断面，对间断面附近珊瑚骨骼Sr/Ca比值和骨骼密度等指标分析表明，每20~50年发生一次的异常强劲的冬季风增强事件，引起冬季大幅度(>7 ℃)降温，导致珊瑚死亡并形成间断面。7.5~7.0 cal.ka B.P.降温事件及其期间发生的气候异常被称为“雷州事件[64]。
在西南地区，贵州董哥洞测年准确、5年时间分辨率的石笋氧同位素记录表明过去9000年共发生了8次弱夏季风事件，分别对应于北大西洋冰筏事件[22]，其中一次发生在7.5~7.1 cal.ka B.P.(图 2i)[28]。这次夏季风减弱事件在贵州竹溜平洞石笋δ18O记录中也有所体现[61]；云南西湖泥炭在7.2 cal.ka B.P.、7.4 cal.ka B.P.和7.5 cal.ka B.P.都有高精度的AMS14 C年代控制点，对沉积物磁化率、干密度、碳酸盐含量(图 2j)等多项指标的综合研究表明，7.5~7.0 cal.ka B.P.期间印度夏季风减弱，导致降水明显减少[30]；云南西盟龙潭沉积物粒度和地球化学元素指标也显示，7.6~7.0 cal.ka B.P.期间印度夏季风减弱、降雨明显减少[62]。
来自中国黄海和东海的海表温度记录同样揭示了这次显著的降温事件[54, 65]。利用黄海北部沉积物的长链烯酮指标定量重建的海表温度结果表明(图 2k)，全新世期间发生了一系列百年尺度的降温事件，其中一次发生在7.2~7.0 cal.ka B.P.，当时温度下降了2~3 ℃[65]；同样地，利用东海东南部沉积物的长链烯酮指标重建的海表温度记录显示，7.3 cal.ka B.P.左右出现了一次明显的降温事件，可能与这一时期黑潮减弱、冬季风增强有关[54]。最近，对冲绳海槽北部沉积物粒度、锶同位素等多种替代性指标的综合研究表明7.2~7.0 cal.ka B.P.黑潮显著减弱[67]；台湾岛北部Retreat湖沉积物TOC、C/N、干密度等多种代用性指标一致显示，7.5~7.0 cal.ka B.P.期间发生了一次明显的夏季风减弱事件[66]。
研究还表明，在长江中游地区，7.5~7.0 cal.ka B.P.期间极端降水事件可能增加，主要标志是出现了异常古洪水事件。湖北清江和尚洞石笋矿物磁性参数—等温剩磁(IRM)记录表明全新世期间共发生了10次古洪水事件，其中一次发生在7.5~7.2 cal.ka B.P.(图 2l)[60]；汉江上游地区的野外地质调查显示7.5~7.2 cal.ka B.P.期间洪水频发[68]；王绍武和黄建斌[69]综合分析了考古证据和古气候资料，发现7.5~7.0 cal.ka B.P.为一洪水期。
总体上，覆盖中国大部分地区的测年准确、分辨率较高的石笋、冰芯、湖泊、泥炭和海洋等沉积物记录均显示7.5~7.0 cal.ka B.P.期间发生了一次明显的气候异常事件。整体上看，气候特征表现为温度下降、夏季风减弱，但季风减弱在不同地区引起的降水状况可能不同。北方和青藏高原地区表现为降水减少，而南方地区的降水模式比较复杂。在华南地区，福建戴云山泥炭沉积物表明这一时期季风减弱、降水减少，而长江中游地区和尚洞石笋磁性矿物参数显示古洪水发生频率增加，可能与降水增加有关。长江中下游地区在降温期间由于夏季风减弱引起降水增加的现象已经被多位学者的研究所证实。如，吴文祥和刘东生[70]发现4.2~4.0 cal.ka B.P.事件期间，降温引起的夏季风减弱事件导致中国东南季风区出现南涝北旱的环境格局；Tan等[71]也发现4.2 ka事件和小冰期期间，中国北方降水减少，而中部和南方地区降水增加；最近，Zhang等[72]利用石笋中的微量元素重建了长江中游末次冰消期以来的降水变化，并综合了其他古气候记录，发现了夏季风降水的三极模态变化，即东亚夏季风减弱时，长江中游地区降水增多，华南和华北地区降水减少，这种夏季风强度变化与降水的空间变化关系得到了古气候模拟结果的支持。然而，由于古气候记录存在较大的不确定性，同时有关7.5~7.0 cal.ka B.P.事件的降水记录在我国淮河和长江流域还比较少，难以准确刻画其降水特征。未来需要更多高分辨率的古气候记录理清这次气候事件降水的空间分布特征。
2 与全球其他地区7.5~7.0 cal.ka B.P.事件记录对比全球其他地区大量古气候记录同样揭示了7.5~7.0 cal.ka B.P.事件。测年准确、分辨率高的极地冰芯记录不仅能反映局地气候状况，而且能反映大尺度气候条件，因此提供了可靠的古气候证据[73]。格陵兰冰芯(GISP2)中的δ18O显示7.5~7.0 cal.ka B.P.期间，温度明显下降(图 3a)[21]；最近利用冰芯中的氩同位素和氮同位素重建的温度变化记录也揭示了这一时期的显著降温现象(图 3b)[74]；同时，格陵兰冰芯钙离子浓度升高[75]、甲烷浓度下降(图 3c)[76]，分别反映了西风强度增加以及赤道湿地面积萎缩。南极Taylor Dome冰芯氧同位素(图 3d)[77]和Siple Dome冰芯钙离子浓度[78]，分别指示7.5~7.0 cal.ka B.P.期间温度下降和西风环流加强，这与格陵兰冰芯记录具有较好的一致性，暗示该气候事件的全球性分布特征。
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图 3 全球其他地区7.5~7.0 cal.ka B.P.气候事件记录 (a)GISP2冰芯δ18O[21]；(b)GISP2温度重建[74]；(c)GRIP甲烷浓度[76]；(d)Taylor Dome δ18O[77]；(e)北大西洋染色赤铁矿颗粒记录[22]；(f)挪威北部Tsuolbmajavri湖7月气温重建[80]；(g)中欧高水位测年个数[85]；(h)中欧低水位测年个数[85]；(i)地中海西部木本花粉百分比[35]；(j)地中海西部Padul泥炭木本花粉百分比含量[89]；(k)乞力马扎罗冰芯粉尘浓度[91]；(l)北非Sidi Ali介形虫δ18O[27]；(m)阿曼北部Hoti洞石笋δ18O[92]；(n)印度Mawmluh洞δ18O[94]；(o)阿拉伯海浮游有孔虫G.bulloides的百分比[23]；(p)北美Oregon洞δ18O[25]；(q)巴西Botuver洞Sr/Ca[26] Fig. 3 Paleoclimate records from the other parts of the world for the 7.5~7.0 cal.ka B.P. climatic event. (a)GISP2δ18O[21]; (b)Reconstructed temperature from argon and nitrogen isotopes[74]; (c)GRIP CH4[76]; (d)Taylor Domeδ18O[77]; (e)Ice-rafted hematite stained grains record in the North Atlantic[22]; (f)Reconstructed July temperature for Tsuolbmajavri Lake[80]; (g)Distribution of the dates of higher lake-level events in central Europe[85]; (h)Distribution of the dates of lower lake-level events in central Europe[85]; (i)Western Mediterranean forest pollen record[35]; (j)Mediterranean forest percentages for Padul wetland[89]; (k)Kilimanjaro ice core dust record[91]; (l)Sidi Ali ostracod derivedδ18O record[27]; (m)Oman Hoti Cave δ18O[92]; (n)Indian Mawmluh cave δ18O[94]; (o)Arabian Sea G.bulloides percent[23]; (p)North American Oregon Cave δ18O[25]; (q)Brazil Botuver Cave Sr/Ca[26] |
北大西洋深海沉积物显示，7.5~7.2 cal.ka B.P.期间冰筏碎屑含量达到峰值，指示显著降温(图 3e)[22]。对法罗群岛沉积物的综合研究表明，这一时期极地冷海水扩张[79]；利用挪威北部Tsuolbmajavri湖沉积物硅藻定量重建的全新世温度变化显示，7.2 cal.ka B.P.左右7月平均气温明显下降(图 3f)[80]；斯堪的纳维亚半岛[81~82]和阿尔卑斯山地区[83]在7.5 cal.ka B.P.左右则发生了明显的冰川前进；对阿尔卑斯山及其周边地区孢粉、植物大化石和沉积学的综合研究表明，7.5~7.0 cal.ka B.P.期间温度下降、蒸发减弱、林线下降[84]；对中欧地区26个湖泊180个年代数据的综合集成分析显示，高水位测年数据集中在7.5~7.2 cal.ka B.P.(图 3g)，而这一时期低水位测年数据较少(图 3h)，表明降水增加[85]。
地中海沿岸地区的多种古气候档案显示，7.5~7.0 cal.ka B.P.期间发生了明显的气候异常。在地中海北部，对南亚里亚德海沉积物多种替代性指标的综合研究表明，7.5~7.3 cal.ka B.P.期间海水盐度出现明显下降，可能与这一时期降雨增加引起的入海河流径流量增加有关[86]；利用沉积物孢粉定量重建的降水序列也显示这一时期降水增加[33]。在地中海中部，对意大利南部Lago Preola湖沉积物的综合研究表明，7.5~7.0 cal.ka B.P.期间，湖泊水位明显下降[24]；意大利西西里岛测年准确、分辨率较高的石笋δ18O和δ13C记录显示，7.5 cal.ka B.P.左右气候变冷变干[87]。在地中海西部，两个深海沉积岩心都记录了这一时期的气候异常：对MD95-2043沉积物孢粉的研究表明，7.4 cal.ka B.P.左右木本花粉浓度明显降低，指示降水减少(图 3i)[35]；对MD99-2343沉积物有孔虫δ18O、粒度和地球化学元素的综合研究表明，7.2 cal.ka B.P.左右降水减少，温度降低[88]。邻近地区的Padul泥炭沉积物记录显示，7.5 cal.ka B.P.左右木本花粉浓度降低(图 3j)[89]。在地中海东部，黎巴嫩Jeita洞7年时间分辨率的石笋δ18O显示，7.4~7.1 cal.ka B.P.期间降水明显减少[90]。Magny等[32]综合分析了地中海地区的多个古气候记录，发现7.5~7.0 cal.ka B.P.该区40°N以北气候偏湿润，以南地区气候偏干旱。
位于非洲赤道附近乞力马扎罗山6个冰芯的粉尘浓度在7.3 cal.ka B.P.左右出现异常峰值(图 3k)，被解释为与显著的干旱事件有关[91]；非洲北部Sidi Ali湖测年较为准确、分辨率较高的粒度和地球化学元素指标显示7.5 cal.ka B.P.左右气候向干旱化方向发展(图 3l)[27]。
在印度季风区，高分辨率的气候记录主要来自石笋和海洋沉积物气候记录档案。阿曼北部Hoti洞4年分辨率(图 3 m)[92]、南部Qunf洞4~5年分辨率[93]以及印度东北部Mawmluh洞5年分辨率的石笋δ18O记录(图 3n)[94]均显示7.5~7.1 cal.ka B.P.期间，印度夏季风强度明显减弱，降水减少；阿拉伯海西北部海洋沉积物有孔虫记录(图 3o)[23]也显示了类似的季风强度减弱现象。
在北美东北部，对加拿大纽芬兰岛泥炭沉积物植物大化石和腐殖质度的研究表明这一时期，温度明显下降[95]；在北美西部，美国俄勒冈州西南部测年较准确，分辨率较高的石笋δ18O记录显示全新世期间发生了8次百年尺度的冷气候事件，其中一次发生在7.4~7.0 cal.ka B.P.(图 3p)[25]；在北美西南部，美国新墨西哥州Pink Panther洞测年准确，17年时间分辨率的石笋δ18O记录显示，7.2 cal.ka B.P.左右降水明显增加[96]；墨西哥南下加利福尼亚州西海岸沉积物的Mg/Ca比值显示7.2~7.0 cal.ka B.P.期间温度明显下降[97]；佛罗里达海峡沉积物有孔虫δ18O记录显示，7.6~7.4 cal.ka B.P.海水盐度增加，指示气候变干导致蒸发增加[98]。
南半球的古气候档案也明显记录了7.5~7.0 cal.ka B.P.气候事件。除了前述南极Taylor Dome冰芯中的氧同位素值[77]和Siple Dome中的钙离子浓度[78]两个记录外，南美季风区两个高分辨率的石笋记录也揭示了7.5~7.0 cal.ka B.P.期间的气候异常[26, 99]。巴西东南部的石笋记录显示，7.2 cal.ka B.P.左右，氧同位值偏负0.5 ‰，Sr/Ca比值减小，表明夏季风明显增强，降雨增加(图 3q)[26]；巴西中东部Lapa Grande洞石笋δ18O值在7.5~7.3 cal.ka B.P.偏负，指示夏季风增强，降水增加[99]。对南美西海岸加拉帕戈斯群岛El Junco湖生物标志物的研究表明，7.6~7.4 cal.ka B.P.期间El Niño事件频繁发生[100]；对智利南部蓬塔阿雷纳斯Lago Hambre湖的多种气候替代性指标的综合研究表明，7.2 cal.ka B.P.左右气候明显变干[101]；在半球尺度上，Marcott等[102]对北半球温度变化曲线的集成记录也显示，7.5 cal.ka B.P.以来气温逐渐下降。
综上所述，来自全球多个地区的古气候记录表明，7.5~7.0 cal.ka B.P.期间出现了明显的气候异常。在北大西洋沿岸、北中欧和北美地区，这一时期主要表现为温度下降，部分地区因为蒸发减弱，湿度增加[22, 85~86, 95]；相反在赤道非洲、地中海沿岸区和印度季风区，主要表现为降水减少[24, 27, 92~94]。南美季风区的两个石笋记录表现与北半球石笋记录相反的季风强度变化模式[26, 99]。
3 7.5~7.0 cal.ka B.P.事件产生的可能机制对比气候代用指标序列和气候强迫序列表明，4种因素可能是引起7.5~7.0 cal.ka B.P.期间气候异常的驱动力。
第一种影响因素是太阳活动。大量研究表明，亚洲夏季风强度变化与太阳活动密切相关[23, 28, 94]。GISP2冰芯的10 Be记录[103](图 4a)和14 C记录(图 4b)[104]显示，7.5~7.0 cal.ka B.P.期间的太阳活动明显减弱。然而太阳活动导致的太阳辐射变化量相当小，难以直接引起明显的气候变化。因此太阳活动可能只是这次气候事件的触发机制，必然存在放大机制才能引起明显的气候变化，对此学术界提出了不同的观点。Gray等[107]认为太阳活动减弱引起的紫外线辐射变化能改变平流层臭氧和温度，向下传递到对流层，影响Hadley环流和Walker环流，进而导致亚洲夏季风的变化；Meehl等[108]认为太阳活动减弱时，热带海洋地区温度降低、蒸发减弱、水汽输送和潜热释放减少，将直接影响亚洲夏季风强度；Kodera[109]则发现太阳活动减弱时，近赤道地区的对流活动增强，而远赤道地区的对流活动减弱，从而导致印度夏季风减弱；Bond等[22]发现全新世北大西洋冰漂碎屑事件与太阳活动之间存在密切关系；而太阳活动信号可能通过温盐环流放大，再通过大气传输作用影响亚洲夏季风变化[110]。东亚地区8.2 ka和2.8 ka弱季风事件分别与北大西洋冰漂碎屑Bond 5和Bond 2事件在内部结构上表现一致且同步发生，说明太阳活动通过经向环流的变化影响半球甚至全球性的百年尺度变化[28]。尽管我们目前尚不清楚太阳活动具体的放大机制及其放大过程，但地球气候系统海气耦合过程可能是这种百年尺度变率放大和传输的重要动力学机制。
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图 4 气候强迫记录 (a)GISP2 10 Be浓度[103]；(b)GISP214 C浓度[104]；(c)北纬60°夏季太阳辐射量[105]；(d)GISP2 SO42-浓度[106] Fig. 4 The climate forcing series. (a)GISP2 10 Be[103]; (b)GISP2 14 C[104]; (c)60°N summer solar insolation[105]; (d)GISP2 SO2-4[106] |
第二种因素是融冰淡水入海。地质证据表明7.6 cal.ka B.P.左右劳伦泰德冰盖(Laurentide Ice Sheet)发生快速融化[111]，导致大量冰川湖泊形成，触发了16次高强度的冰融水倾泻事件，引起大量融冰淡水入海[112]。Yu等[113]通过对瑞典波罗的海东南部小流域盆地沉积物的研究发现，当地海平面在7.6 cal.ka B.P.前后快速上升了4.5 m；Herrle等[114]发现爱琴海的盐度在7.5 cal.ka B.P.左右突然下降了1.3 ‰ psu，可能是由海平面上升引起盐度较低的黑海海水溢出，流向爱琴海所致；Bird等[115]利用50个生长在潮间带的红树林泥炭测年数据和浅海沉积物，重建了新加坡9.5~6.5 cal.ka B.P.期间的海平面变化，发现7.5~7.0 cal.ka B.P.海平面上升了4 m；Liu等[116]研究了黄海北部的水下斜坡沉积物，发现7.0 cal.ka B.P.左右海平面快速上升了2~3 m；Wang等[117]利用一系列基底盐沼泥炭和潮上带沉积物的AMS14 C测年数据，重建了长江三角洲南部全新世早中期高精度海平面变化曲线，结果显示7.6~7.4 cal.ka B.P.海平面快速上升了2 m；Zong等[118]利用长江下游跨湖桥遗址区孢粉、海藻、真菌孢子等多项指标，重建了该区9.0~7.5 cal.ka B.P.以来的环境变化，结果显示7.5 cal.ka B.P.左右淡水环境转为海水环境，表明出现了一次海侵。上述不同地区的海平面在这一时期都出现明显上升，一致性支持了融冰淡水入海这一观点。大量融冰淡水入海必然会破坏海表的温度和盐度，导致北大西洋经向翻转环流变慢[112, 119]。古气候记录和模拟结果都表明这一时期北大西洋经向翻转环流减慢[120~121]。北大西洋经向翻转环流变慢导致高北纬地区温度降低，作为补充，热量将通过哈德利环流圈向北输送，导致热带辐合带向南移[122~123]，引起亚洲夏季风强度减弱[124]。在亚洲季风区，高北纬地区温度降低也可能通过增强东亚冬季风和增加青藏高原的冰雪覆盖，进而通过耦合响应机制减弱夏季风[110]。劳伦泰德冰盖融化引发温盐流减弱触发的8.2 ka事件在中国多个地区的古气候记录中都有所体现[28~30]，表明温盐流减弱影响着中国地区的气候变化。
第三种因素是火山活动。格陵兰冰芯SO42-记录显示，7.5~7.0 cal.ka B.P.期间至少发生了两次大规模的火山喷发事件(图 4d)[106]。模拟结果显示火山喷发产生的硫酸盐气溶胶滞留在平流层中，通过反射太阳光照导致地球表面温度降低[125]。较近的研究还表明火山活动导致的温度变化，能改变大气环流，进而对降水产生显著影响。火山喷发引起的陆地降温比海洋更剧烈，在一定程度上减弱了东亚夏季风强度[126]；另外火山喷发活动产生的气溶胶引发的南北半球温度差能导致热带辐合带(ITCZ)南移，进而对亚洲夏季风降水产生影响[127]。Zdanowicz等[128]认为发生在7627±150 cal.a B.P.左右的火山喷发导致北半球中纬度温度下降0.6~0.7 ℃，低温持续了1~3年。单次火山活动的突发性和短暂性似乎与长时间持续性的气候异常相矛盾。但最近模拟结果表明一次大规模的火山喷发能引起海洋低温持续大约16年，在高北纬地区温度下降导致海冰扩张，通过这种反馈机制低温甚至能持续上百年[74]。
第四种影响因素是北半球夏季太阳辐射量变化。气候模拟和古环境地质证据表明亚洲夏季风强度与地球轨道参数所决定的北半球夏季太阳辐射量有关，当太阳辐射量增大时，夏季风强度增强；反之，夏季风强度减弱[28, 129~130]。北半球夏季太阳辐射量自10 cal.ka B.P.以来总体上表现为逐渐减少(图 4c)[105]，将会引起夏季风强度减弱[130]；另外，研究还发现，太阳辐射量的减少速率在8.0~7.0 cal.ka B.P.期间达到最大，这种快速变化能改变纬度之间的热梯度，引起气候极不稳定[131]，并进而影响夏季风强度变化。不过，目前还没有证据表明北半球夏季太阳辐射量缓慢的变化会导致类似7.5~7.0 cal.ka B.P.气候事件。
因此，上述4种因素可能在7.5~7.0 cal.ka B.P.气候事件产生的过程中都发挥了作用，太阳活动和融冰淡水入海可能是主要的触发因素，然而目前还难以对上述4种因素的相对贡献做出准确评估。
4 结论与展望从以上综述可以看出7.5~7.0 cal.ka B.P.气候事件在中国多个地区都有明显的反映，整体上表现为温度下降，夏季风强度减弱，但不同地区降水模式可能不同，在北方地区主要表现为干旱，在长江中下游地区可能表现为降水增加。与全球其他地区的古气候记录对比显示7.5~7.0 cal.ka B.P.气候事件可能是一个全球尺度的气候事件。古气候代用指标和气候驱动因素之间较好的时间对应关系表明这次事件主要与北半球夏季太阳辐射量减少、太阳活动减弱、大规模火山喷发和劳伦泰德冰盖融化导致的赤道辐合带南移和北大西洋经向翻转环流减慢共同作用引发的夏季风强度减弱有关。
本文只是选取了部分古气候记录，对7.5~7.0 cal.ka B.P.气候事件在中国的环境表现做了一个初步的总结，更像是一种启发性的工作，还存在许多不完善之处，今后需要开展更深层次的研究。首先是不同气候档案以及不同地区同一气候档案对7.5~7.0 cal.ka B.P.事件记录的起止时间、变化幅度以及变化速率都有所不同。古里雅冰芯表明降温开始于7.5 cal.ka B.P.左右；湖北神农架青天洞氧同位素记录表明事件开始于7.4 cal.ka B.P.，呈突变特征，事件持续了200年；黄海海表温度记录表明事件发生于7.2~7.0 cal.ka B.P.。这些差异是由于气候代用指标的指示意义、记录分辨率以及测年误差的不同导致的？还是气候事件的响应存在着区域差异？需要进一步厘清。其次，7.5~7.0 cal.ka B.P.事件降温幅度的定量重建是认识气候系统行为的关键，但目前古气候研究更多是定性和半定量重建，未来需要更多测年准确、高分辨率的古气候记录定量—半定量刻画该气候事件的变化幅度、起止时间、以及区域差异。最后关于事件的驱动机制，古气候记录与各气候驱动因素在发生时间上的良好对应关系，表明它们之间可能存在一定的成因联系，未来需要结合定量的古气候重建与数值模拟，阐明7.5~7.0 cal.ka B.P.的触发和反馈因子，评估各影响因素的相对贡献。
致谢: 感谢审稿专家和编辑部杨美芳老师宝贵的修改意见!
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2 CAS Center for Excellence in Tibetan Plateau Earth Science, Beijing 100101;
3 University of Chinese Academy of Sciences, Beijing 100049;
4 Research Center for Earth System Science, Yunnan University, Kunming 650091, Yunnan)
Abstract
Identification of Holocene climate events and their regional environmental expressions, exploration of their temporal and spatial characteristics, and investigation of the dynamic mechanisms behind them are important for understanding climate system, forecasting future climate changes and interpreting the cultural transformations. With the advances in Holocene paleoclimate reconstructions, one climate event at 7.5~7.0 cal. ka B.P. is increasingly recognized but lack a systematic review. In this paper, we review paleoclimate evidences from 26 sites covering most of China to detect 7.5~7.0 cal.ka B.P. climate event, including 24 well-dated and highly-resolved paleoclimate time series together with 2 lower-quality paleoclimate records.The widely distributed proxy records clearly show decreased temperature during 7.5~7.0 cal.ka B.P. over most areas of China. However precipitation indicators show pronounced spatial differences in different parts of China, with drier conditions in the Tibetan Plateau and in Northern China, and probably wetter conditions in the middle Yangtze River region. This spatial pattern of precipitation variation could be attributed to the behavior of a reduced summer monsoon intensity associated with decreased temperature during 7.5~7.0 cal.ka B.P. climate event. There are some discrepancies in terms of onset timing, duration and magnitude of this climate event from site to site. It is difficult to clarify whether these discrepancies are caused by the dating errors, the sensitivity of climate proxies, or their different regional responses. Comparison with various proxy records from other parts of the world indicate that the 7.5~7.0 cal.ka B.P. is of global scale. Such climatic anomaly is characterized by cooling and wetter conditions in north-central Europe and North America and South America monsoon domain, widespread drying around the tropical Africa, the Indian monsoon domain and the Mediterranean.The close correspondence in timing between solar irradiance minima, strong volcanic eruptions, meltwater flux into the North Atlantic, orbitally-induced decrease in solar insolation, and 7.5~7.0 cal.ka B.P. climate event suggests possible causal links. Of all the potential climate forcing mechanisms, solar activity and meltwater flux seem to play more important role in the southward migration of the Intertropical Convergence Zone and the slowing of North Atlantic Thermohaline Overrunning, triggering weakening of the Asian summer monsoon. In the future, more quantitative reconstructions and precisely-dated and highly-resolved climate records are needed to describe the magnitude, duration, and regional differences of 7.5~7.0 cal.ka B.P. climate event, and paleoclimate simulations should be incorporated to elucidate the relative contributions of these four forcing factors