2019, Vol.39
2 河北师范大学泥河湾考古研究院, 河北 石家庄 050024;
3 中国科学院地质与地球物理研究所, 中国科学院新生代地质与环境重点实验室, 北京 100029;
4 中国科学院生物 演化与环境卓越创新中心, 北京 100044;
5 中国科学院大学地球与行星科学学院, 北京 100049;
6 中国地震局地质研究所, 北京 100029)
北大西洋冰漂碎屑事件(即Heinrich事件)，是发生于末次冰期的周期性极端寒冷事件，是晚近地质时期最重要的极端气候事件[1~4]。Heinrich事件最早由德国海洋地质学家Hartmut Heinrich[5]于1988年在北大西洋东部Dreizack海沉积物中发现。1992年，Broecker等[6]和Bond等[7]分别对北大西洋DSDP609岩芯沉积物进行分析，结果证实末次冰期北大西洋地区广泛分布着6层冰漂碎屑沉积(依次记为H1、H2、H3、H4、H5和H6)[3]。随后，在北半球大部分地质[3, 8~9]、生物[10~12]和化学记录[13~15]中都找到了Heinrich事件的印迹：如北大西洋西北部Labrador海[8]及北欧诸海沉积的岩屑含量记录[3]，中高纬北大西洋沉积的底栖有孔虫氧同位素记录[13]和有机碳、氮同位素记录[15]，亚北极太平洋沉积的生物蛋白石与陆源碎屑比值记录[10]，东亚地区黄土堆积粒度[16~18]、古风化强度[19]、蜗牛化石[11]和洞穴石笋氧同位素记录[14, 20]等。
上述研究表明，Heinrich事件是北半球普遍存在的气候突变事件，但6次Heinrich事件的强度和持续时间均存在明显差别[3, 14, 21]，其中Heinrich 1事件(发生于17000~15000 cal.a B. P.前后)的强度和影响最为显著和突出[22~23]，因此得到古气候学界的广泛关注。已有研究显示，Heinrich 1事件发生期间，欧洲[24~25]、北美[26]、热带大西洋[27]和非洲[22, 28]等地区气候环境发生了显著变化；同时，区域陆地生态系统也发生了巨大转型，主要表现为：环北大西洋和北美地区温度骤降[24~27]、非洲地区显著干旱化[22, 28]；低纬地区森林收缩、草地扩张[29]，中高纬地区植物群落衰退、耐寒属种增多[26, 30~31]；不同区域对Heinrich 1事件的响应形式和变化幅度均存在显著差异。
近年来，东亚地区Heinrich 1事件发生时期的气候环境变化也获得了较多关注。Heinrich 1事件期间，黄土堆积石英颗粒中值粒径变粗、粗粒度组分增加、冬季风显著增强[16~17]；洞穴石笋氧同位素偏正、夏季风减弱[14, 32~34]；同时，季风区湖泊沉积(湖光岩玛珥湖[35~36]、天才湖[37]、腾冲青海湖[38]、洱海[39]、星云湖[40]、四海龙湾[41]、哈农玛珥湖[42]等)、西北干旱区湖泊沉积(青海湖[43]和巴里坤湖[44]等)和冰缘地貌[45]等也均对Heinrich 1事件响应显著。然而，包括石笋在内的所有高质量古环境记录多来自东亚中低纬地区；东亚中高纬地区Heinrich 1事件气候记录仅见四海龙湾[41]、小龙湾[46]、贝加尔湖[47]和月亮湖[48]等初步研究，仍缺乏可靠的高分辨率古气候记录。Heinrich 1事件在东亚中高纬地区的表现形式如何？其具有怎样的结构特征？区域陆地生态系统如何响应Heinrich 1事件的气候突变？是目前学界亟需解决的关键科学问题。
呼伦湖位于东亚夏季风边缘区最北端(图 1a)，是区域气候变化的良好载体[49~50]。本文基于AMS 14C定年技术和孢粉分析，重建了呼伦湖21500~13000 cal.a B. P.区域植被高分辨率变化历史，以期揭示Heinrich 1事件在东亚中高纬地区的表现形式，探讨区域植被对Heinrich 1事件的响应特征，为应对未来全球变暖和气候突变提供科学依据。
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图 1
呼伦湖地理位置图
(a)东亚地区卫星影像及呼伦湖地理位置，虚线表示现代夏季风北界；(b)呼伦湖区卫星影像 卫星影像来源：谷歌地球，http://www.earth.google.com Fig. 1 Location of the Hulun Lake. (a)Map of East Asia showing the current northern limits of the East Asian summer monsoon; (b)Satellite image of the Hulun Lake region). Satellite image is from http://www.earth.google.com |
呼伦湖(48°30′40″~49°20′40″N，117°00′10″~117°41′40″E；湖面海拔543 m)位于内蒙古自治区满洲里市正南30 km，地处蒙古高原东部，是形成于晚上新世时期的内陆构造湖[51]。湖水依靠地表径流、湖面降水和地下水补给，主要入湖河流有源自湖盆以西肯特山东麓的克鲁伦河和湖盆以东大兴安岭中段的乌尔逊河(图 1b)。
研究区属中温带大陆性气候，受东亚季风和大陆性气团交替影响，夏季温凉短促、冬季严寒漫长[51]。湖区年均温- 0.7 ℃至1.1 ℃，夏季均温18.3 ℃至20.1 ℃，冬季均温- 21.3 ℃至- 19.3 ℃；年降水量248 mm至292 mm，70%以上集中在6月至8月[52]。
湖盆区域地带性植被属中温型干草原亚带，主要植物群落为大针茅(Stipa grandis)、克氏针茅(Stipa krylovii)和羊草(Leymus chinensis)等为主的禾草草原，代表性物种以禾本科(Gramineae)、菊科(Asteraceae)和蔷薇科(Rosaceae)等为主。隐域性植被包括沙生、盐生和草甸植被：其中沙生植被分布于湖区东部和嵯岗沙地，主要包括沙蓬(Agriophyllum squarrosum)、差巴嘎蒿(Artermisia halodendron)和冰草(Agropyron cristatum)等；盐生植被生长在盐湿低地，主要包括芨芨草(Achnatherum splendens)、碱蓬(Suaeda glauca)、碱茅(Puccinellia distans)和碱蒿(Artemisia anethifolia)等；草甸植被分布于沟谷洼地和河道两岸，主要包括无芒雀麦(Bromus inermis)和鹅绒委陵菜(Potentilla anserina)等[53]。湖盆以东大兴安岭海拔1000 m以上发育山地针叶林，以兴安落叶松(Larix gmelinii)为主，混生有白桦(Betula platyphylla)和山杨(Populus davidiana)等树种；海拔800~1000 m为森林草原过渡带，以中生杂类草草甸为主，白桦-山杨林呈岛状或片状分布，林下灌丛茂密[53]。湖盆以西肯特山海拔2000 m以上为亚高山草甸，主要由莎草科(Cyperaceae)、菊科和蓼科(Polygonaceae)植物组成；海拔1600~2000 m发育山地针叶林，以西伯利亚落叶松(Larix sibirica)为主，白桦为主要伴生种，西伯利亚云杉(Picea obovata)和樟子松(Pinus sylvestris)等常有出现；海拔800~1600 m为森林草原过渡带，杨树和榆树为主要乔木树种，草本植物由禾本科、蒿属(Artemisia)和藜科(Chenopodiaceae)等组成[54]。
2 材料与方法 2.1 岩芯钻探2008年1月，课题组采用日式TOHO钻探设备(D1-B型)在呼伦湖开展钻探取芯工作，钻孔位于呼伦湖沉积中心(水深约5 m)，对应坐标49°06′52.4″N，117°31′56.0″E(图 2a)。该次钻探获得一套完整、连续岩芯9.21 m，命名为HL08；现场对其进行对剖、拍照，详细描述并记录其颜色、岩性等沉积特征；对剖岩芯带回室内后以1 cm间隔切分为样品。
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图 2 呼伦湖HL08岩芯钻孔位置(a)、岩性及年代-深度模型(b)(5.75 m以上) 卫星影像来源：https://www.ArcGIS.com Fig. 2 Location (a), lithology and age-depth model(b) of core HL08 from Hulun Lake (upper 5.75 m). Satellite image of the Hulun Lake region is from https://www.ArcGIS.com |
本研究采用AMS 14C方法对沉积岩芯进行年代测试，测年材料选取富含有机质的全岩样品。样品测试采用HVEE AMS-Ⅱ系统在日本Paleo Labo公司完成，样品前处理和年代测试参考Nakamura等[55]制定的实验方法。年代-深度模型采用贝叶斯沉积速率模型建立[56]，模型构建前对所有14C测年数据进行碳库校正和日历年龄校正。
2.3 孢粉分析孢粉提取采用常规的HCl-NaOH-HF处理法[57]，每个样品取干重1 g。为计算孢粉浓度，化学处理前每个样品中加入现代石松孢子27637±563粒。孢粉鉴定统计在Olympus BX41光学生物显微镜下进行，每个样品鉴定统计陆生孢粉600粒以上。花粉形态鉴定主要参考《中国植物花粉形态》[58]和《日本植物的花粉形态》[59]。
3 研究结果 3.1 HL08岩芯岩性与年龄呼伦湖HL08岩芯为连续的湖相沉积，未发现沉积间断，其岩性变化可分为三段(图 2b)：孔深921~591 cm，由青灰色粘土和粘土质粉砂组成；孔深590~376 cm，沉积物颗粒变粗，主要为青灰色粉砂，深540~531 cm和488~402 cm夹杂有砾石；孔深375~0 cm，主要为青灰色-灰色粘土和粉砂。
本研究以25~50 cm间隔对HL08孔5.75 m以上沉积岩芯进行AMS 14C年代测试，共获得16个年代数据(表 1)。岩芯顶部1 cm 14C年龄为419年，表明呼伦湖沉积物14C定年受碳库效应影响，其碳库年龄约为419年。假定整个岩芯沉积过程中受到的碳库影响基本恒定，本文利用该表层碳库年龄对原始14C测年数据进行了碳库校正；而后，利用IntCal 13数据集[60]和OXCal4.3程序[61]对碳库校正后的年代进行日历年龄校正，获得对应层位2σ误差范围的校正年龄(表 1)。贝叶斯沉积速率模型运行结果显示，呼伦湖HL08孔5.75 m以上岩芯沉积速率变化函数呈伽玛分布且相邻层位之间依赖性适中，表明不同层位岩芯的沉积速率变化小，年龄-深度模型可靠。据此，本文计算了HL08孔5.75 m以上每厘米岩芯样品的年龄值(95%置信区间内)，取加权平均值作为每个样品的年龄(图 2b)；结果显示，HL08孔5.75 m以上岩芯涵盖过去约35000年。
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表 1 呼伦湖HL08沉积岩芯5.75 m以上AMS 14C年龄 Table 1 AMS 14C radiocarbon dates of samples from the upper 5.75 m of core HL08 |
本文重点关注呼伦湖区Heinrich 1事件发生前后的植被变化历史，故按1 cm间距对HL08岩芯415~275 cm段(21500~13000 cal.a B. P.)140个样品进行孢粉分析，共计鉴定出74个科属植物孢粉类型：乔木类型主要包括松属(Pinus)、云杉属(Picea)、桦属(Betula)等；草本类型中蒿属、藜科和禾本科占绝对优势，其次为莎草科、紫菀属(Aster)和蒲公英属(Taraxacum)等；灌木类型、蕨类孢子和水生植物花粉含量低。根据呼伦湖孢粉组合变化可将其划分为3个主要阶段(图 3)。
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图 3 呼伦湖HL08岩芯21500~13000 cal.a B. P.主要孢粉类型组合图 Fig. 3 Pollen assemblage diagram of major types for core HL08 from Hulun Lake during 21500~13000 cal.a B. P. |
阶段1 (415~385 cm，21500~19000 cal.a B. P.)：本阶段花粉组合以草本类型为主，莎草科(17.0%~42.0%，平均30.2%)占绝对优势，其次为禾本科(9.0%~19.2%，平均13%)、藜科(8.0%~29.4%，平均15%)和蒿属(4.7%~17.3%，平均10.8%)；木本类型中云杉属和松属平均含量分别为7.8%(3.8%~11.5%)和7.3%(4.8%~10.0%)，阔叶属种含量极低。花粉总浓度低，草本花粉浓度约0.6×104粒/g，木本花粉浓度约0.14×104粒/g。
阶段2 (385~315 cm，19000~14900 cal.a B. P.)：本阶段花粉组合仍以草本类型为主，与阶段1相比，蒿属和藜科花粉平均含量显著增加，分别增加17%和15%，莎草科(3.0%~36.7%，平均14.2%)、禾本科(3.2%~17.0%，平均8.4%)和其他亚高山草甸类型含量明显减少；乔木类型中，松属(0.5%~9.1%，平均2.8%)和云杉属(0.3%~13.4%，平均3.5%)含量显著下降。花粉总浓度逐渐上升，草本花粉浓度约2.28×104粒/g，木本花粉浓度约0.27×104粒/g。根据主要孢粉类型变化特征，可将本阶段孢粉组合进一步划分为4个子阶段：
阶段2-1 (385~360 cm，19000~17500 cal.a B. P.)乔木植物类型中云杉属(3.2%~13.4%，平均8%)、松属(2.5%~9.1%，平均5.6%)占有一定比例，桦属含量相对较低(1.1%~9.0%，平均3.6%)。草本植物类型中蒿属平均含量占17.4%(6.0%~28.4%)，藜科平均含量占24.9%(15.1%~30.8%)，莎草科、禾本科、紫菀属和蒲公英属花粉含量相对降低。木本和草本植物花粉浓度分别为0.12×104粒/g和0.6×104粒/g。
阶段2-2 (360~345 cm，17500~16500 cal.a B. P.)乔木植物类型中云杉属(0~2.2%，平均1.2%)和松属(0.5%~4.3%，平均1.8%)比例下降，桦属含量相对升高(6.7%~9.4%，平均8.0%)。草本植物类型中蒿属(34.0%~49.6%，平均40.0%)和藜科(27.7%~35.5%，平均30.0%)含量明显增加，莎草科、禾本科、紫菀属和蒲公英属花粉含量相对减少。木本和草本植物花粉浓度逐渐增加，平均为0.6×104粒/g和5.2×104粒/g。
阶段2-3 (345~325 cm，16500~15400 cal.a B. P.)孢粉组合以藜科、莎草科显著增加和蒿属、桦属明显降低为特征。藜科花粉占绝对优势(9.4%~47.4%，平均37.2%)，莎草科(3.7%~21.9%，平均14.5%)含量相对较高，蒿属(15.2%~46.6%，平均25.9%)含量显著降低。花粉总浓度低，草本花粉浓度约2×104粒/g，木本花粉浓度约0.19×104粒/g。
阶段2-4 (325~315 cm，15400~14900 cal.a B. P.)本阶段孢粉组合变化主要表现为蒿属含量快速增加(34.3%~45.5%，平均37.3%)，藜科(20.1%~38.1%，平均28.4%)和莎草科(6.3%~15.9%，平均11%)含量逐渐下降，其他属种类型含量变化不明显。草本花粉浓度(2.2×104粒/g)和木本花粉浓度(0.19×104粒/g)无明显变化。
阶段3 (315~275 cm，14900~13000 cal.a B. P.)：桦属和蒿属花粉在短期内快速增加，分别上升10%和5%；藜科花粉进一步降低(8.0%~25.9%，平均17.1%)，莎草科(3.8%~10.4%，平均6.6%)和紫菀属(0~1.9%，平均0.6%)含量也略有下降；木本和草本植物花粉浓度明显上升，分别达1.2×104粒/g和5.2×104粒/g。
4 讨论 4.1 呼伦湖区21500~13000 cal.a B. P.植被演替过程呼伦湖属有河流注入的大型湖泊，主要入湖河流有源自湖盆以西肯特山东麓的克鲁伦河和湖盆以东大兴安岭中段的乌尔逊河；因此，呼伦湖花粉沉积主要依靠空气、河流和地表片流搬运[62]。结合呼伦湖区现代植被分布特征[53~54]：湖盆以东大兴安岭生长落叶松属和桦属为主的针阔混交林；湖盆以西肯特山生长莎草科和禾本科为主的亚高山草甸和松属、云杉属、桦属等为主的针阔混交林；湖盆区域发育以蒿属为主的典型草原和藜科为主的荒漠草原。本文认为，呼伦湖沉积物花粉组合可以较好地反映呼伦湖流域范围内植被组成，即呼伦湖沉积物中出现的木本和亚高山草甸花粉类型主要源自周围山地，指示山地植被组成；出现的蒿属和藜科花粉来源于湖盆区域，反映湖盆植被变化[62]。本文研究显示，21500~13000 cal.a B. P.时期呼伦湖区主要属种类型发生了显著变化，据此，可将该时期的植被演替过程划分为3个主要阶段(图 3)，它们分别是21500~19000 cal.a B. P.对应末次冰盛期(Last Glacial Maximum，简称LGM)、19000~14900 cal.a B. P.对应HS1冰阶(Heinrich Stadial 1，简称HS1)以及14900~13100 cal.a B. P.对应博令/阿勒罗德暖期(Bølling-Allerød Warming，简称B/A暖期)。
4.1.1 LGM(21500~19000 cal.a B. P.)研究区周围山地以莎草科和禾本科为主的亚高山草甸占绝对优势，同时生长有一定比例云杉属和松属为主的针叶林植被，表明该阶段周围山地耐寒针叶林和亚高山草甸发育，森林植被盖度相对较低。湖盆区域生长以蒿属为主的典型草原，但区植被盖度极低、生态环境相对恶劣。
4.1.2 HS1冰阶(19000~14900 cal.a B. P.)19000~16500 cal.a B. P.，研究区周围山地松属和云杉属为主的针叶林植被逐渐被桦属为主的落叶阔叶林替代，莎草科和禾本科为主的亚高山草甸植被面积逐渐下降；湖盆区域蒿属为主的典型草原和藜科为主的荒漠草原均呈扩张趋势，区域植被盖度显著增加。16500~15400 cal.a B. P.，研究区周围山地森林植被锐减，以莎草科和禾本科为主的亚高山草甸比例相对增加，表明周围山地森林植被稀疏、亚高山草甸发育；湖盆区域典型草原比例为整个阶段最低，以藜科为主的荒漠草原占优势，表明该阶段湖盆地区生态环境恶劣、荒漠草原发育，植被盖度极低。15400~14900 cal.a B. P.，研究区周围山地落叶阔叶林略有增加，以莎草科和禾本科为主的亚高山草甸相对减少，周围山地森林植被仍较稀疏、植被盖度低；湖盆区域典型草原显著增加，以藜科为主的荒漠草原逐渐减少，表明该阶段湖盆地区生态环境相对好转，但区域植被盖度仍较低。
4.1.3 B/A暖期(14900~13000 cal.a B. P.)研究区周围山地桦属为主的先锋树种迅速扩张，针叶林比例相对减少；同时，亚高山草甸比例显著降低，表明周围山地落叶阔叶林面积扩张、森林盖度增加。湖盆区域生态环境明显好转，原有的荒漠草原逐渐被蒿属为主的典型草原替代，中生和喜暖湿的草本植物属种增多、植被盖度明显上升。
4.2 呼伦湖区孢粉记录的Heinrich 1事件及其表现特征前文已述，全球不同区域对Heinrich 1事件的响应形式存在显著差异[22, 24~27]。Stager等[22]通过多指标记录综合分析认为，Heinrich 1事件在亚-非季风区整体表现为极端干旱化；来自东亚中低纬地区的石笋记录[63]和湖泊沉积记录[64]均显示，Heinrich 1事件期间东亚季风和印度季风相对减弱、区域降水减少、气候干旱化。而格陵兰[65]、欧洲[24~25]和西伯利亚[66]气候记录却显示，Heinrich 1事件以降温为主要特征。东亚中高纬地区是衔接北半球极地与东亚低纬地区的关键区域，对认识和理解Heinrich 1事件的过程机制至关重要。
为辨识呼伦湖孢粉序列记录的Heinrich 1事件，揭示呼伦湖区植被组成对Heinrich 1事件的响应过程，本文选取孢粉记录中主要的9种花粉类型(平均含量> 1%)进行主成分分析(Principal Component Analysis，简称PCA)，结果显示：PCA第一得分轴和第二得分轴分别解释了72.8%和19.9%的孢粉属种百分比变化累积方差值(图 4)；莎草科、禾本科、云杉属和松属等类型分布于第一得分轴正值方向，蒿属和桦属分布于负值方向，表明PCA第一得分轴指示区域温度变化；藜科位于第二得分轴正值方向，桦属位于负值方向，表明PCA第二得分轴指示区域降水/相对湿度变化。16500~15400 cal.a B. P.，PCA第一轴和第二轴得分均显著偏正；同时，其发生时限也与格陵兰冰芯δ18O值[65]和中国南方石笋δ18O值[33, 63]记录的Heinrich 1事件相近(图 5)。据此，本研究认为该事件应与发生于北大西洋地区[3]的Heinrich 1事件相对应。
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图 4 呼伦湖HL08岩芯21500~13000 cal.a B. P.主要孢粉类型PCA排序图 Fig. 4 PCA ordination of major pollen types for core HL08 from Hulun Lake during 21500~13000 cal.a B. P. |
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图 5 呼伦湖区21500~13000 cal.a B. P.气候-植被变化过程及区域对比图 中国南方石笋δ18O记录有衙门洞[63]、青天洞[33]；孢粉记录为月亮湖[48]；δ18O记录为格陵兰NGRIP冰芯(GICC05)[65] Fig. 5 Climate-vegetation succession in Hulun Lake region during 21500~13000 cal.a B. P. and their comparison with other regional indicators. The δ18O records of stalagmite are from Yamen Cave[63] and Qingtian Cave[33] in South China; The pollen record is from Moon Lake[48]; The δ18O record of NGRIP ice core is from Greenland(GICC05)[65] |
Heinrich 1事件发生期间，呼伦湖湖盆区域荒漠草原扩张、区域植被盖度显著降低，指示极端干旱气候；来自呼伦贝尔沙地的粒度记录[66]也证实，在16000 cal.a B. P.前后，研究区风沙活动强烈，与本文孢粉记录相印证，表明区域干旱化是Heinrich 1事件在东亚中高纬地区的表现特征之一。同时，呼伦湖区周围山地亚高山草甸相对发育，指示山地林线下降、区域气温显著降低；然而，来自贝加尔湖[47, 67]、兴凯湖[68]、月亮湖[48]和四海龙湾[41]孢粉记录以及小龙湾硅藻记录[46]均显示，Heinrich 1事件期间东亚中高纬地区气候以降温为主要特征，干旱特性并不显著，因此，我们认为这可能与上述湖泊多属小型湖泊或山地湖泊有关，其受湖盆、地形和海拔等因素影响较大，对区域干旱响应不敏感有关。此外，呼伦湖孢粉记录显示，Heinrich 1事件期间呼伦湖区植被变化具有明显的阶段性，以16000 cal.a B. P.为界可将呼伦湖区Heinrich 1事件划分为两个时期：16500~16000 cal.a B. P.，蒿属为主的典型草原快速减少、莎草科和禾本科为主的亚高山草甸相对扩张，区域植被显著恶化；16000~15400 cal.a B. P.，湖盆地区典型草原比例降至最低，藜科为主的荒漠草原占主导，区域植被进一步退化。
4.3 不同植被类型对Heinrich 1事件的差异响应已有高分辨率气候记录显示(格陵兰冰芯[65]和东亚石笋记录[33~34, 63]等)，Heinrich 1事件以急剧转冷为标志：格陵兰冰芯δ18O值显著偏负，北半球高纬温度下降明显[65]；Zhang等[33]得到的结果为东亚地区石笋δ18O值在20年内偏正2 ‰，指示夏季风短期内显著减弱。呼伦湖孢粉记录显示，区域植被对Heinrich 1事件的响应也十分敏感，但不同植被类型的响应速率和幅度存在明显差别(图 3)：Heinrich 1事件伊始(16500 cal.a B. P.前后)，呼伦湖区植被组成中以莎草科和禾本科为主的亚高山草甸和蒿属为主的典型草原对其响应最为快速、敏感，100年内分别增加15%和减少20%；同时，木本和草本花粉浓度显著减少，指示区域植被盖度明显降低；而以桦属为主的落叶阔叶林和藜科为主的荒漠草原响应较为缓慢，且变幅相对较小(< 5%)(图 3)。Heinrich 1事件结束期(15400 cal.a B. P.前后)，呼伦湖区植被变化也表现为蒿属为主的典型草原快速增加、亚高山草甸显著减少，100年内分别增加12%和减少10%；而藜科为主的荒漠草原和桦属为主的落叶阔叶林变化仍较缓慢。上述结果表明呼伦湖区莎草科和禾本科为主亚高山草甸和蒿属为主的典型草原对Heinrich 1事件的响应较藜科为主的荒漠草原和落叶阔叶林更为快速、敏感。本文认为，导致上述差异的原因可能与不同植物属种的生活型不同有关，如蒿属和莎草科植物多为一年生草本，其生长周期短，对区域气候变化响应更为敏感。因此，在评估未来全球气候变化对陆地生态系统影响评价时，还应考虑不同植被类型的响应差异。
5 结论基于HL08孔415~275 cm段高分辨率孢粉序列，本文重建了呼伦湖区21500~13000 cal.a B. P.植被变化的详细历史，在此基础上揭示了Heinrich 1事件在东亚中高纬地区的表现特征，并探讨了呼伦湖区植被组成对Heinrich 1事件的响应差异，获得以下主要结论：1)21500~ 13000 cal.a B. P.，呼伦湖区周围山地植被经历了由针叶林和亚高山草甸向落叶阔叶林的转变，湖盆区域典型草原逐步发育，区域植被盖度逐渐增加；2)呼伦湖区Heinrich 1事件发生于16500~15400 cal.a B. P.，以剧烈降温和显著干旱化为主要特征；事件发生期间湖区周围山地亚高山草甸发育、森林植被稀疏；湖盆区域荒漠草原显著扩张、植被盖度降低；3)Heinrich 1事件期间呼伦湖区植被变化具有明显的阶段性：16500~16000 cal.a B. P.，蒿属为主的典型草原快速减少、莎草科和禾本科为主的亚高山草甸相对扩张，区域植被显著恶化；16000~15400 cal.a B. P.，蒿属为主的典型草原进一步减少、藜科为主的荒漠草原占主导，区域植被进一步退化；4)呼伦湖区莎草科和禾本科为主亚高山草甸和蒿属为主的典型草原对Heinrich 1事件的响应较藜科为主的荒漠草原和落叶阔叶林更为快速、敏感；因此，在评估未来全球气候变化对陆地生态系统影响评价时，应考虑不同植被类型的响应差异。
致谢: 中国地质大学(北京)刘宝林教授研究团队协助完成呼伦湖HL08岩芯钻探，日本Paleo Labo公司Hideki Yamagata博士帮助完成AMS 14C样品年代测试，匿名审稿专家和杨美芳老师提出了宝贵修改意见，在此一并表示感谢。
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,
Xiao Jule3,4,5,
Wen Ruilin3,4,5,
Fan Jiawei3,6,
Huang Yun3,5,
Li Manyue2,
Xu Qinghai1,2
2 Institute of Nihewan Archaeology, Hebei Normal University, Shijiazhuang 050024, Heibei;
3 Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029;
4 CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044;
5 College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049;
6 Institute of Geology, China Earthquake Administration, Beijing 100029)
Abstract
The Heinrich event 1 was the most prominent cooling events in the North Atlantic region during the last deglaciation, and had a profound influence on global atmospheric circulation and terrestrial ecosystems; however, its behavior and consequence in eastern Asia remain unclear. In order to reveal the character and impact of the Heinrich event 1 in the middle-high latitude of East Asia. Hulun Lake was selected as a research target, which is on the northeastern edge of the current monsoon margin. In this paper, we presented a detailed record of the development of regional vegetation during 21500~13000 cal.a B. P. based a total of 16 AMS 14C samples for the upper 5.75 m of core HL08 (49°06'52.4"N, 117°31'56.0"E) and 140 pollen samples at the depth of 4.15~2.75 m of core HL08 from Hulun Lake, and discussed the response process of regional vegetation around the Hulun Lake to the Heinrich event 1. The results show that:(1) During the period of 21500~13000 cal.a B. P., the vegetation changed from subalpine meadow and coniferous forest to deciduous forest in the surrounding mountains, and typical steppe occupied the area around the lake basin gradually, implying an overall increase in the density of the vegetation cover. (2) Based on principal component analysis of major pollen taxa, we suggested that the cold and dry event that occurred in the Hulun Lake region at 16500~15400 cal.a B. P. would be the regional manifestation of the Heinrich event 1 occurred in the North Atlantic region, and the climate was extreme cold and catastrophic drought in the middle-high latitude of East Asia. (3) During the Heinrich event 1, Cyperaceae and Gramineae denominated subalpine meadow and scattered sparse forest were present in the surrounding mountains, and Chenopodiaceae denominated desert steppe developed around the lake basin, suggesting that the regional ecological environment deteriorated and the vegetation cover was reduced around the Hulun Lake. (4) The change of regional vegetation around the Hulun Lake region can be divided into two stages:16500~16000 cal.a B. P., Artemisia dominated typical steppe decreased significantly and Cyperaceae and Gramineae denominated subalpine meadow expanded, the vegetation cover also reduced rapidly; 16000~15400 cal.a B. P., Artemisia dominated typical steppe further reduced and Chenopodiaceae denominated desert steppe occupied the lake basin, the vegetation cover further reduced and the regional ecological environment deteriorated obviously. (5) The responses of different vegetation type to the Heinrich event 1 were significantly different around the Hulun Lake region:the response of subalpine meadow and Artemisia dominated typical steppe to the onset and end of the Heinrich event 1 was more sensitive than Chenopodiaceae denominated desert steppe and deciduous broad-leaved forest, which would be related to the difference of life form for different species; Therefore, the differential response of vegetation to climate change needs to be addressed in future global change research.