, 林晓, 王君波Paleoclimate changes reflected by lake terrace in Tangra Yumco of Central Tibet since middle-late Holocene
LIUYuqing, LINXiao, WANGJunbo通讯作者:
收稿日期:2017-01-6
修回日期:2017-03-12
网络出版日期:2017-06-30
版权声明:2017《地理研究》编辑部《地理研究》编辑部
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1 引言
季风和西风是影响中国气候变化的两个重要系统,季风进退和西风强弱有着此消彼长的关系。大量研究表明,全新世东亚季风由北向南逐渐退缩[1],改变了区域的水热配置。西风—季风过渡区自东北向西南横贯中国[2],在位于青藏高原东北缘及其以东区域有着较为明确的过渡带,而位于高原腹地的广大地区,对气候变化响应过程复杂,虽有冰芯、树轮和湖泊沉积等多种载体记录西风和季风的相互作用,但其影响范围尚未达成共识。其中高原湖泊分布区域广泛,成为研究西风—季风相互作用的理想材料[3,4]。季风—西风过渡带由东亚季风—西风和西南季风—西风两个子系统组成。目前青藏高原的湖泊沉积记录表明,全新世以来高原不同区域对两个子系统有不同的响应。其中,对位于东南部的拉龙错[5]、仁错[6]、海登湖[6]、沉错[7]等研究表明受西南季风影响较明显。如拉龙错孢粉揭示10 ka BP~4.4 ka BP受季风影响且比现代要强[5];沉错多指标综合分析指示全新世时期温暖湿润是由西南季风带来[7]。对位于青藏高原西部的龙木错[8]、班公错[9]等研究表明受西风影响较显著。如龙木错孢粉A/C值揭示在5.7 ka BP~4 ka BP是干旱环境,与东南部环境变化趋势相反[8]。而高原中部和东北缘的青海湖[10]、更尕海[11]、苦海[12]、寇察湖[13]、兹格塘错[14]、塔若错[15]等记录表明,这一区域的全新世气候不但具有季风的变化特征,同时也受西风的影响,体现出过渡带的特征。如青海湖湖芯钻孔记录晚更新世以来气候变化为西风和东亚季风两个系统的相互作用所致[10];塔若错中全新世湿润期是由于西风环流衰弱西南季风加强,晚全新世干旱则是西风加强的结果[15]。上述研究表明,青藏高原东北部受东亚季风—西风影响较为明显,高原东南部和中部受西南季风—西风影响较为明显。然而,无论是东亚季风还是西南季风,其与西风的相互作用范围和程度在高原上并不明确,也不能单一地认为该地仅受某一种风系的影响。当惹雍错位于西藏中部羌塘湖盆区南缘,处于西风与东亚季风、西南季风共同作用的过渡带[2],研究其气候变化对于认识西风—季风的作用范围有重要意义。
目前,对当惹雍错的研究主要围绕湖岸阶地年代学[16-19]和多指标重建古环境序列[20]进行。其中,年代学研究主要通过直接地貌证据与测年结果推断水位变化,如东北部阶地的3个10Be暴露年龄显示4.3 ka湖泊有所退缩[21];环湖9个光释光年代学结果推断2.1 ka时发生了水位下降[18];放射性碳测年与介形虫研究揭示了该区2.6开始湖泊退缩为浅滩等[21]。在多指标重建古环境中,已建立了当惹雍错17.6 ka BP的环境序列,该序列与YD事件、Bφiling暖期等全球性气候事件有较好的一致性,表明其结果具有较好的区域可比性[22]。上述研究中,通过测年揭示的水位变化是单个时间点的,而相对缺乏连续性记录;而在17.6 ka古环境序列中,由于时间尺度较长而使中晚全新世较短时间尺度的分辨率有所降低,能知其全貌而不知其细节。因此,目前当惹雍错还比较缺乏中晚全新世气候变化的信息。
本文以当惹雍错T1阶地剖面为研究对象,利用光释光测年方法获取年代标尺,通过粒度、TOC、TN、C/N等气候代用指标的分析,对该地区中晚全新世以来的古气候变化进行重建,并进一步讨论其变化过程受西风—季风影响的程度。
2 研究方法与数据来源
2.1 研究区概况
当惹雍错位于西藏自治区那曲地区尼玛县南部,冈底斯山中段北麓断陷盆地内（图1）。盆地外围高山环绕,南部有达果藏布,西部有农寨藏布为主要补给水源。湖泊南北延向呈长靴状,面积约为996.9 km2,南北最大长度为72 km,东西最大宽度为19 km。湖面海拔4528 m,北部湖区实测最大水深为230 m,南部湖区实测最大水深为110 m[16,23]。
显示原图|下载原图ZIP|生成PPT图1当惹雍错湖盆示意图
-->Fig. 1Sketch map of the Tangra Yumco Basin
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T1阶地剖面位于惹雍错西北端,距离河口有一定距离（图1）。剖面高252 cm,高出现在湖面约3 m,河流沿阶坡倾向切割剥露出阶地沉积物。该剖面自下而上分为5层,各层岩性描述如下：
（1）252~132 cm,灰白色、灰黄色粉砂与粉砂质泥互层;岩性均一,水平层理发育,块状构造,下伏砾石层,未见底,与上层呈平行不整合接触。
（2）132~114 cm,灰黄色粉砂、粉砂质泥层与灰白色含泥粉砂层互层,下部岩性不均一,上部较硬,水平层理发育,块状层理构造。
（3）114~60 cm,灰白色中细砂层,平行层理,呈块状构造,偶见植物碎屑以及碎裂螺壳。
（4）60~40 cm,上部为灰黄色中粗砂,下部为干燥灰白色粗砂层,水平层理,块状构造,呈粒序变化,局部含水生植物碎屑,偶见细砾。
（5）40~0 cm,黄褐色砂砾层与灰黑色中细砂层,上部现锈迹,下部粉砂含量增高,水平层理,块状构造。
该剖面128 cm以上部分按2 cm间隔采样,以下按1 cm间隔采样,共采集样品184个。样品自然干燥后进行了粒度测试,仪器为Beckman Coulter L S230全自动;总氮（TN）采样凯氏定氮法测定,两项均在中国地质大学（武汉）生物地质与环境地质国家重点实验室完成。总有机碳（TOC）测定在中科院青藏高原研究所环境变化与地表过程重点实验室完成,仪器为岛津TOC-VCPH有机碳测试仪。上述指标分别测试样品180个。
2.2 年代确定
在剖面15 cm、100 cm和251 cm处采集3个光释光样品（编号为分别为DRY-10、DRY-08和DRY-09）,送往南京大学地理与海洋科学学院光释光实验室测试。利用钾长石红外后释光法（pIRIR50,150）,该方法是在红外光束激发钾长石获得光释光信号的基础上,通过增加温度得到比标准红外释光信号更稳定的释光信号[24],以获得等效剂量,通过公式A=De/D（De为等效剂量,D为环境计量率）,计算得到钾长石年龄（表1）。Tab. 1
表1
表1当惹雍错T1剖面沉积物光释光测年结果
Tab. 1OSL dating results of the T1 terrace in the northwestern part of Tangra Yumco
| 样品号 | 深度(cm) | U(ppm) | Th(ppm) | K(%) | 等效剂量(Gy) | 含水量(%) | 环境计量率(Gy/ka) | 样品年龄(ka) |
|---|---|---|---|---|---|---|---|---|
| DRY-10 | 15 | 17.1±0.38 | 13.8±0.37 | 1.45±0.05 | 11.06±0.10 | 10 | 6.45±0.42 | 1.71±0.11 |
| DRY-08 | 100 | 10.1±0.24 | 13.6±0.37 | 1.79±0.06 | 15.71±1.39 | 10 | 5.29±0.34 | 2.97±0.33 |
| DRY-09 | 251 | 13.0±0.31 | 18.4±0.48 | 2.15±0.06 | 38.94±5.24 | 10 | 6.44±0.43 | 6.05±0.91 |
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计算结果显示该剖面15 cm处年龄为1.71±0.11 ka,100 cm处年龄为2.97±0.33 ka,251 cm处年龄为6.05±0.91 ka（表1）,可见不同深度处年龄具有很好的线性关系,因此利用拟合公式y=53.375x-68.904（R2=0.994）来建立年代—深度序列（图2）。
显示原图|下载原图ZIP|生成PPT图2T1剖面年龄—深度模型
-->Fig. 2Age-depth model of T1 terrace
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3 结果分析
3.1 粒度
粒度参数是分析沉积物动力条件和沉积环境的良好指标[25,26]。通过沉积物多组分粒度特征概率分布曲线来判断沉积相类别,再运用沉积相所对应的粒度环境意义来重建古气候[27]。T1剖面粒度由6个组分叠加构成,中值粒径范围分别为① <1 μm,② 1~10 μm,③ 10~70 μm,④ 70~150 μm,⑤ 150~700 μm,⑥ >700 μm,粗粒组分含量越高,指示沉积物越靠近湖滨;细粒组分含量越高,指示沉积物越靠近湖心（图3）。可以判断T1剖面峰型是典型的湖相沉积,剖面自下而上呈现出从湖心—湖滨过渡到湖滨的演变过程特征,表明了湖泊水位的变化过程。
显示原图|下载原图ZIP|生成PPT图3当惹雍错T1沉积物粒度分布
-->Fig. 3Grain size distribution of sediments in Tangra Yumco T1 terrace
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对于这种封闭湖区,粒度大小与湖区降水量、蒸发作用密切相关,具有干湿变化的指示意义[28],即粗粒指示气候相对干旱,反之相对湿润。另外,通过偏态可判别粒度分布的对称性,通过峰态表明沉积物的粒径分布集中程度,作为粒度判定的辅助指标。T1剖面粒度分析结果表明,沉积物平均粒径分布在3~190 μm区间,根据粒度频率曲线和粒度参数特征,并基于以上对粒度古环境意义的分析,将整个剖面自下而上可划分出两个主要的岩性段（图4）。
显示原图|下载原图ZIP|生成PPT图4当惹雍错中晚全新世各项气候代用指标变化规律
-->Fig. 4Variations of climate indexes in Tangra Yumco since middle Holocene
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下段（252~115 cm,约6 ka BP~3.4 ka BP）,平均粒径范围3.7~19.7 μm,根据粉马登—温德华粒径分级标准,属于粉砂和黏土级沉积物,整体趋势平稳,平均粒径下半段（252~196 cm,约6 ka BP~4.9 ka BP）比上半段（195~115 cm,约4.9~3.4 ka BP）偏细（图4a）。对应频率分布曲线（图3）,前者位于靠近湖心的湖心—湖滨过渡带,因此水动力条件更小,水位更高,后者水位稍低。偏态负偏,在180 cm附近达到该阶段最低值（图4b）。峰态在240 cm以下较为平稳,有从常峰态向尖峰态过渡发展的趋势（图4c）。上段（114~0 cm,约3.4 ka BP~1.4 ka BP）,平均粒径范围26.3~190 μm,属于砂与粉砂组分,波动幅度明显,平均粒径自上而下呈变细趋势,在62 cm（2.4 ka BP）达到峰值190 μm,分别在32 cm（1.8 ka）和104 cm（3.2 ka）附近到达谷值,分别为26.3 μm和54.8 μm（图4a）。对应概率分布曲线,该段位于湖滨地带,水位比上一阶段大大降低,表现出湖泊退缩的过程。偏态较下段明显负偏,与粒度变粗相对应（图4b）。峰态变化趋势与粒度变化趋势基本一致,表明粒度组分的集中程度与粒度大小呈正相关（图4c）。
3.2 TOC与C/N
沉积物中TOC和TN含量,是判断湖区初级生产力以及沉积有机质保存状况的基本指标,用以揭示气候波动状况[29,30],即TOC含量高指示温暖湿润的气候条件,反之亦然。C/N比值可以用来判断有机质的来源,水生浮游藻类蛋白质含量高,C/N比值在4~10之间,陆生植物纤维素含量高,C/N比值大于20[31]。TOC、TN的含量大小还受运移路径、沉积过程以及保存能力等因素影响[29],T1剖面在114 cm（约3.4 ka BP）出现一个峰值,结合粒度频率分布曲线可知,当时水动力条件处在转折期,沉积物向湖滨相发生转变,因此TOC含量的增加可能更多的是由于沉积环境的改变而导致。114 cm是一个转折点,前后两端的TOC含量反映了气候的相对波动幅度,而在114 cm处则反映了沉积环境的变化。T1剖面252~115 cm（约6 ka BP~3.4 ka BP）TOC含量介于0.7%~1.8%之间,平均值为1.2%。趋势整体稳定,无较大波动。114~0 cm（约2.4 ka BP~1.4 ka BP）TOC含量介于0.1%~1.9%之间,平均值为0.7%,由下自上呈减少趋势。其中,80~62 cm（约2.8 ka BP~2.4 ka BP）的TOC含量从1.7%快速下降至0.1%,指示的是一次快速降温气候事件。62 cm以上（约2.4 ka BP）TOC含量波动上升至峰值1.2%后波动下降,但总体处于整个剖面的低水平（图4d）。
整个剖面TN含量在0.02%~0.22%之间,变化趋势与TOC的基本一致,但总体含量较TOC低（图4f）。C/N比值介于3.5~17.1之间,绝大部分位于10以下,表明湖泊有机质以浮游藻类贡献为主。其中114 cm以上C/N比值大小与变化幅度呈明显变大趋势,指示的是有机质来源出现了从浮游藻类向陆生植物过渡的变化（图4f）。
3.3 当惹雍错中晚全新世古气候变化
根据上述测试结果,可以将该区中晚全新世的古气候演化分为两个阶段（图4）。阶段 I（约6 ka BP~3.4 ka BP）：平均粒径低值反映了这一时期水量丰沛,气候湿润。湖泊处于较高水位。该时期气候整体处于稳定阶段,变化趋势不明显。该阶段可分成两个亚阶段：约6 ka BP~5 ka BP（图4中Ia）,粒度组分为黏土,气候最为湿润。TOC在5.6 ka BP~5.8 ka BP出现低值,指示了一次程度较低的冷事件。5.8 ka之后其含量上升,波动幅度增大,气候在波动中保持总体温暖。C/N比值指示湖泊有机质主要由浮游藻类贡献,陆地有机质输入少。约5 ka BP~3.4 ka BP（图4中Ib）,平均粒径有所增大,气候有向干旱方向发展的趋势,但整体仍然湿润。水位有所降低但总体还是偏高。TOC波动幅度增大但总体含量较高,总体温暖。C/N比值基本在7~8之间波动,表明湖泊有机质输入稳定。
阶段II（约3.4 ka BP~1.4 ka BP）：平均粒径显著增大,变化幅度增加,指示了气候波动增大,向干旱方向发展。水位出现了较大程度降低,导致沉积环境发生变化。3.4 ka处TOC的增加与沉积环境的变化有关,但总体TOC变化趋势与平均粒径反相关,可认为在气候变干的同时也朝着寒冷的方向变化。该阶段又可分作三个气候亚阶段：约3.4 ka BP~2.8 ka BP（图4中IIa）,水位开始降低,气候开始变干。温度有下降趋势但仍保持总体温暖,TOC含量较高,波动幅度明显。约2.8 ka BP~2.1 ka BP（图4中IIb）,气候经历了一次幅度较大的冷暖变化,其中在2.4 ka时湿度与温度降低至6千年来最低水平,气候寒冷干燥,是一次长达400年的气候冷干事件。2.4 ka BP时粒度达到最大,可以认为此时气候最干旱。此后气候回暖,但较前个阶段依然较寒冷。约2.1 ka BP（图4中IIc）,气候回暖后继续波动变冷。C/N比值在阶段II相对于前两阶段处在高值,波动幅度较大,指示陆生植物输入增加,环境变化较大。
当惹雍错中晚全新世温度与湿度配置模式表现为前期暖湿、后期冷干两大特征,既与全球性气候序列基本一致[32,33],又与当惹雍错现有研究成果有较好的可比性,如通过不同测年方法揭示的当惹雍错湖区在中晚全新世发生了不同程度的水位下降事件[20-22],与本文气候序列揭示的3.4 ka时发生较大的沉积环境变化具有一致性。当惹雍错气候变化整体趋势也与现有建立的17.5 ka古环境序列较为吻合[18],这为后面作区域对比提供了前提条件。
4 讨论
分别选取青藏高原上典型的季风型湖泊、过渡带湖泊和西风控制湖泊,这些湖泊在各自的区域具有代表性的环境意义。比较当惹雍错中晚全新世气候序列与它们的异同点,可以发现该区湿度受季风影响较为明显,而温度变化则与季风—西风的关系不大。在湿度方面,6 ka BP~3.4 ka BP当惹雍错平均粒径指示了气候湿润（图5d）,与高原季风区的气候记录有较好的一致性。这一时期,拉龙错（图5a）孢粉PCA含量总体较高,反映了湿润的环境[34]。若尔盖（图5b）泥炭孢粉组合A/C呈现高值且有减少趋势,反映气候湿润,东亚季风盛行[35],季风有减弱发展的趋势,与T1剖面5 ka BP粒度增加对应（图5d）。当惹雍错湿度变化与西风控制区哈拉湖[36]（图5g）古水位和托勒库勒湖[37]（图5h）A/C比显示的水位下降明显不同。3.4 ka BP后沉积环境发生较大变化,气候朝干冷方向发展,并在2.4 ka出现一次较大干旱事件（图5d）,这与当惹雍错湖岸阶地水位变化研究相互印证。如利用孢粉和介形重建当惹雍错古水位变化研究揭示2.6 ka水位下降,浅水位一直持续到1.4 ka BP[17,21]。通过长石冷发光和石英OSL测年发现水位下降始于2.1 ka,直至0.9 ka回升[17]。该时期季风区湿润指数与当惹雍错平均粒径增加同步指示了湿润程度下降[38]。相反的,西风控制区为主的乌伦古湖[39]（图5i）和新疆湿润指数[40]（图5j）在该时期上升,与当惹雍错地区的湿润状况有较大差异。
显示原图|下载原图ZIP|生成PPT图5中晚全新世以来T1剖面气候序列区域气候记录对比
-->Fig. 5The comparison between climate sequence of T1 terrace and region climate records since the middle-late Holocene
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在温度方面,当惹雍错中晚全新世温度变化与季风和西风区的温度变化差异并不明显。如前期当惹雍错TOC含量（图5e）处在稳定高值,变化幅度较少,后期TOC含量减少,揭示了时期气候先温暖后寒冷的变化过程。这与高原东北缘冬给错纳[41]TOC（图5c）指示湖泊有机质该前期输入比后期丰富较为一致。位于高原腹地的错鄂[42]TOC含量（图5f）显示,3.4 ka后温度趋于寒冷趋势。而西风控制区也显示了相似的温度变化趋势,如博斯腾湖[43]C/N比值（图5k）和赛里木湖[44]PCA axis2（图5l）变化趋势与当惹雍错TOC含量（图5e）较为一致,由此推断西风区的相对温度变化在中晚全新世与青藏高原没有太大的差异性。这可能是由于30°N夏季太阳辐射在中晚全新世的辐射逐渐减弱,其引起的气温变化是全球性而非区域性。
综上所述,当惹雍错中晚全新世湿度变化与季风区有较好的一致性,与西风区差异较大。温度变化在与季风和西风区对比上并没有明显差异。而从水热配置情况来看,当惹雍错气候序列基本遵循暖湿—冷干的组合模式,这与季风模式是相匹配的。但这种模式是受印度季风或是东亚季风支配,或者兼而有之,仍需要进一步深入的研究。
5 结论
通过对当惹雍错湖泊阶地沉积物的测年和粒度、TOC、TN、C/N分析,重建了该地区中晚全新世的古气候,并初步对西风—季风过渡带在研究区的影响过程进行了区域对比,得出以下认识：（1）当惹雍错地区中晚全新世古气候变化具有明显的两阶段：6 ka BP~3.4 ka BP,气候温暖湿润,水量充沛,湖泊水位高;3.4 ka BP~1.4 ka BP,环境发生较大变化,朝干旱方向发展,水位波动下降,总体从暖湿向冷干转变,其中2.4 ka BP记录了一次较为明显的干冷事件。
（2）通过区域对比发现,当惹雍错地区中晚全新世湿度变化受季风的影响较为显著,而温度变化则与季风—西风在区域上的表现不明显。这种暖湿—冷干的水热配置模式,是受印度季风或是东亚季风支配,或者兼而有之,仍需要进一步深入地研究。
The authors have declared that no competing interests exist.
参考文献 原文顺序
文献年度倒序
文中引用次数倒序
被引期刊影响因子
| [1] | . The spatial and temporal distribution of summer monsoon precipitation (or effective moisture) during the Holocene has been reconstructed on the basis of geological data, including lake levels, pollen profiles, and loess/paleosol records. In addition, the summer (July) precipitation increment, effective precipitation, and monsoon strength index have been obtained from numerical modeling experiments. Both geological data and numerical modeling indicate that the Holocene optimum, as defined by peak East Asian summer monsoon precipitation, was asynchronous in central and eastern China, reaching a maximum at different times in different regions, e.g., ca. 10,000–800002yr ago in northeastern China, 10,000–700002yr ago in north-central and northern east-central China, ca. 7000–500002yr ago in the middle and lower reaches of the Yangtze River, and ca. 300002yr ago in southern China. In southwestern China the maximum appeared ca. 11,00002yr ago, but probably was related to the maximum landward extension of the Indian summer monsoon. The regional shift in the maximum precipitation belt from northwest to southeast over the past 10,00002yr is interpreted as a response to changing seasonality related to orbital forcing of the climate. The southeastward shift of the East Asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the Holocene and the East Asian monsoon index declined, while the early maximum in southwestern China matches the maximum of the Indian monsoon index. |
| [2] | . We synthesize palaeoclimate records from the mid-latitude arid Asian region dominated today by the Westerlies (“arid central Asia” (ACA)) to evaluate spatial and temporal patterns of moisture changes during the Holocene. Sediment records from 11 lakes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five-class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200-year time slices. The proxies used in these records include pollen and diatom assemblages, sediment lithology, lake levels, and geochemistry (mainly isotope) data. The results of our synthesis show that ACA as a whole experienced synchronous and coherent moisture changes during the Holocene, namely a dry early Holocene, a wetter (less dry) early to mid-Holocene, and a moderately wet late Holocene. During the early Holocene most of the lakes experienced very low water levels and even dried out before ca 802ka (102ka=100002cal02a02BP). Hence the effective-moisture history in ACA is out-of-phase with that in monsoonal Asia as documented by numerous palaeoclimate records. In monsoonal Asia, a strong summer monsoon and humid climate characterized the early Holocene, and a weakened summer monsoon and drier climate prevailed during the late Holocene, which were mainly controlled by changes in low-latitude summer insolation. In contrast, we propose that the pattern of Holocene effective-moisture evolution in the westerly dominated ACA was mainly determined by North Atlantic sea-surface temperatures (SSTs) and high-latitude air temperatures that affect the availability, amount and transport of water vapor. Also, topography of the Tibetan Plateau and adjacent Asian highlands could have contributed to the intensification of dry climate in ACA during the early Holocene, as a result of strengthening the subsidence of dry air masses, associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation. Summer insolation might have played a key role in directly controlling moisture conditions in ACA but only after the northern hemisphere ice-sheets had disappeared in the mid- and late Holocene. |
| [3] | . 中国古湖泊数据库收录的42个湖泊,提供的湖泊水量每千年变化的空间信息,可以用来系统分析中国区域末次冰盛期以来大气环流变化的状况.研究结果表明:我国西部从末次冰盛期以来直至全新世中期均为较湿润的气候状况,推测冰期内的湿润条件主要与西风带的降水以及低温低蒸发密切相关,而全新世主要为夏季风降水增加所致;全新世晚期气候趋干明显.我国东部的大部分区域在冰盛期和晚冰期较为干旱;只是在全新世有效降水状况才有大幅度的改善,全新世中期夏季风降水的效应仍然相当显著,控制的范围可达整个中国西部,同时位于现代季风气候区的中国东部,有效降水的峰值区的变化似乎存在从北往南的穿时性,南方有效降水峰值出现在晚全新世.而西南季风区湿润状况的明显改善发生在晚冰期,比东南季风区发生的早,显然这与两个季风系统的相互消长有一定的关系.我国东北区的湿润状况改善的也较早,显示了独特的季风气候机制. . 中国古湖泊数据库收录的42个湖泊,提供的湖泊水量每千年变化的空间信息,可以用来系统分析中国区域末次冰盛期以来大气环流变化的状况.研究结果表明:我国西部从末次冰盛期以来直至全新世中期均为较湿润的气候状况,推测冰期内的湿润条件主要与西风带的降水以及低温低蒸发密切相关,而全新世主要为夏季风降水增加所致;全新世晚期气候趋干明显.我国东部的大部分区域在冰盛期和晚冰期较为干旱;只是在全新世有效降水状况才有大幅度的改善,全新世中期夏季风降水的效应仍然相当显著,控制的范围可达整个中国西部,同时位于现代季风气候区的中国东部,有效降水的峰值区的变化似乎存在从北往南的穿时性,南方有效降水峰值出现在晚全新世.而西南季风区湿润状况的明显改善发生在晚冰期,比东南季风区发生的早,显然这与两个季风系统的相互消长有一定的关系.我国东北区的湿润状况改善的也较早,显示了独特的季风气候机制. |
| [4] | . . |
| [5] | . <h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">We present a late glacial pollen record (17,700 to 8500 cal yr BP) from a Lake Naleng sediment core. Lake Naleng is located on the southeastern Tibetan Plateau (31.10°N 99.75°E, 4200 m) along the upper tree-line. Variations in the summer monsoon are evident from shifts in vegetation that correspond to late glacial climate trends from other monsoon-sensitive regions. Alpine steppe was recorded between 17,700 and 14,800 cal yr BP, indicating low effective moisture at the study site. Expansion of alpine meadows followed by advances in the position of tree-line around Lake Naleng suggest that climate became warmer and wetter between ∼ 14,800 and 12,500 cal yr BP, probably representing an enhancement of the Asian monsoon. Climatic cooling and reduced effective moisture are inferred from multivariate analysis and the upward retreat of tree-line between ∼ 12,500 and 11,700 cal yr BP. The timing and nature of these shifts to warm, wet and then cold, dry climatic conditions suggest that they correspond to the Bølling/Allerød and Younger Dryas intervals. <em>Abies</em>-<em>Betula</em> forests, representing warm and moist conditions, spread during the early Holocene.</p> |
| [6] | . 从西藏东南部的海登湖和仁错两个封闭湖泊的钻孔岩心中得到良好的AMS14C测年及高分辨率的孢粉记录.和邻近区域的资料对比揭示了研究区末次冰盛期晚期以来的气候记录:大约20000～14000 cal.aBP期间,气候寒冷干燥,植被以耐旱植物为主,1和7月的气温均低于现在,年降水量只有现在的40%,湖面较低;14000～12500 cal.aBP期间,气候仍较寒冷干燥,为草原植被,季风萎缩,低湖面;12500～9400 cal.aBP期间,气温和降水量都逐步上升,开始出现木本植物,如桦木和松树;9400～2400 cal.aBP期间,气候温暖湿润,季风的增强导致湖面的上升,植被以针阔混交林为主,有桦、栎、松、云杉和冷杉等.2400 cal.aBP之后,气候变冷变干,孢粉谱中落叶树的比例下降,草本植物花粉上升. . 从西藏东南部的海登湖和仁错两个封闭湖泊的钻孔岩心中得到良好的AMS14C测年及高分辨率的孢粉记录.和邻近区域的资料对比揭示了研究区末次冰盛期晚期以来的气候记录:大约20000～14000 cal.aBP期间,气候寒冷干燥,植被以耐旱植物为主,1和7月的气温均低于现在,年降水量只有现在的40%,湖面较低;14000～12500 cal.aBP期间,气候仍较寒冷干燥,为草原植被,季风萎缩,低湖面;12500～9400 cal.aBP期间,气温和降水量都逐步上升,开始出现木本植物,如桦木和松树;9400～2400 cal.aBP期间,气候温暖湿润,季风的增强导致湖面的上升,植被以针阔混交林为主,有桦、栎、松、云杉和冷杉等.2400 cal.aBP之后,气候变冷变干,孢粉谱中落叶树的比例下降,草本植物花粉上升. |
| [7] | . <a name="Abs1"></a>Climatic and environmental changes since the last glacial period are important to our understanding of global environmental change. There are few records from Southern Tibet, one of the most climatically sensitive areas on earth. Here we present a study of the lake sediments (TC1 core) from Lake Chen Co, Southern Tibet. Two sediment cores were drilled using a hydraulic borer in Terrace 1 of Lake Chen Co. AMS <sup>14</sup>C dating of the sediments showed that the sequence spanned >30,000 years. Analyses of present lake hydrology indicated that glacier melt water is very important to maintaining the lake level. Sediment variables such as grain size, TOC, TN, C/N, Fe/Mn, CaCO<sub>3</sub>, and pollen were analyzed. Warm and moderately humid conditions dominated during the interval 30,000–26,500 cal year BP. From 26,500 to 20,000 cal year BP, chemical variables and pollen assemblages indicate a cold/dry environment, and pollen amounts and assemblages suggest a decline in vegetation. From 20,000 to 18,000 cal year BP, the environment shifted from cold/dry to warm/humid and vegetation rebounded. The environment transitioned to cold/humid during 16,500–10,500 cal year BP, with a cold/dry event around 14,500 cal year BP. After 10,500 cal year BP, the environment in this region tended to be warm/dry, but exhibited three stages. From 10,500 to 9,000 cal year BP, there was a short warm/humid period, but a shift to cold/dry conditions occurred around 9,000 cal year BP. Thereafter, from 9,000 to 6,000 cal year BP, there was a change from cold/dry to warm/humid conditions, with the warmest period around 6,000 cal year BP. After 6,000 cal year BP, the environment cooled rapidly, but then displayed a warming trend. Chemical variables indicate that a relatively warm/dry event occurred around 5,500–5,000 cal year BP, which is supported by time-lagged pollen assemblages around 4,800 cal year BP. Our lake sediment sequence exhibits environmental changes since 30,000 cal year BP, and most features agree with records from the Greenland GISP2 ice core and with other sequences from the Tibetan Plateau. This indicates that environmental changes inferred from Lake Chen Co, Southern Tibet were globally significant. |
| [8] | |
| [9] | . A 12.4 m core collected from the eastern part of Lake Bangong provided a continuous Holocene record. This core was analyzed for diatoms, ostracods, charophytes, content in total organic carbon and identification of organic particles contained in the sediments. The record is remarkable for the abundance and diversity of its microorganisms. An oligo- to mesosaline marsh occupied the core site before 9.5 ka B.P. Bangong was most likely closed. A sudden filling of the lake basin by dilute, nutrient-poor water occurs around 9.4 ka B.P. Maximum lake extent seems to have occurred around 9.4-8.7 ka B.P. Generally freshwater, relatively deep conditions were maintained up to 6.3 ka B.P., although interrupted by short-term regressive events between 5.6 and 7.5 ka B.P. A trend towards palustral conditions begins after 6.3 ka B.P. and the interval 3.9-3.2 ka B.P., rich in detrital organic material, coincides with an extremely low lake level. A last lacustrine episode of minor amplitude is recorded from about 3 to 2 ka B.P., before the closure of the system leading again to slightly saline conditions. The resemblances and synchronism between environmental changes recorded at Bangong and in Lake Sumxi, situated 200 km northwards and 800 m higher in elevation, indicate that regional climatic changes have been the major driving factor on ecological and hydrobiological changes in lakes of western Tibet. |
| [10] | . Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes. |
| [11] | . Abstract<br/><p class="a-plus-plus">Genggahai Lake, a small, shallow water body on the northeastern Qinghai–Tibetan Plateau, is presently occupied by dense aquatic vegetation. The distribution of macrophytes is strongly associated with water depth. Macrofossils from a 7.82-m sediment core indicate that the dominant taxa in past aquatic plant communities were almost the same as those that dominate the lake today. In combination with sediment geochemical variables TOC, TN, and δ<sup class="a-plus-plus">13</sup>C<sub class="a-plus-plus">bulk-org</sub>, macrofossil assemblages (aquatic plant remains, stem encrustations, and mollusc shells) were employed to reconstruct lake-level fluctuations over the past 16?ka. The lake formed or refilled at 15.3?cal?ka and sustained a low level until 11.4?cal?ka. From 11.4 to 6.3?cal?ka, a remarkably high level was punctuated by a period of relatively low level between 9.2 and 7.4?cal?ka. Stepwise drops in lake level occurred from 6.3 to 5.5?cal?ka. Water level was characterized by more frequent fluctuations between 5.5 and 4.1?cal?ka. Low level persisted from 4.1 to 2.1?cal?ka and from 1.6 to 0.3?cal?ka. Short-lived episodes of high water level were centered around 1.8?cal?ka and since 0.3?cal?ka. The lake-level history of Genggahai Lake appears to be largely consistent with the Asian monsoon records from nearby and distant locations. Our preliminary results suggest that monsoon variability on centennial to millennial timescales was superimposed on the long-term pattern of the Asian monsoon, modulated by orbitally induced summer insolation, and that the Asian monsoon may play an important role in water balance in marginal regions.</p><br/> |
| [12] | . A sediment core from a closed basin lake (Lake Kuhai) from the semi-arid northeastern Tibetan Plateau was analysed for its pollen record to infer Lateglacial and post glacial vegetation and climatic change. At Lake Kuhai five major vegetation and climate shifts could be identified: (1) a change from cold and dry to relatively warmer and more moist conditions at 14.8 cal ka BP: (2) a shift to conditions of higher effective moisture and a stepwise warmer climate at 13.6 cal ka BP; (3) a further shift with increased moisture but colder conditions at 7.0 cal ka BP; (4) a return to a significantly colder and drier phase at 6.3 cal ka BP; (5) and a change back to relatively moist conditions at 2.2 cal ka BP. To investigate the response of lake ecosystems to climatic changes, statistical comparisons were made between the lake Kuhai pollen record and a formerly published ostracod and sedimentary record from the same sediment core. Furthermore, the pollen and lacustrine proxies from lake Kuhai were compared to a previously published pollen and lacustrine record from the nearby Lake Koucha. Statistical comparisons were done using non-metric multidimensional scaling and Procrustes rotation. Differences between lacustrine and pollen responses within one site could be identified, suggesting that lacustrine proxies are partly influenced by in-lake or local catchment processes, whereas the terrestrial (pollen) proxy shows a regional climate signal. Furthermore, we found regional differences in proxy response between lake Kuhai and Lake Koucha. Both pollen records reacted in similar ways to major environmental changes, with minor differences in the timing and magnitude of these changes. The lacustrine records were very similar in their timing and magnitude of response to environmental changes; however, the nature of change was at times very distinct. To place the current study in the context of Holocene moisture evolution across the Tibetan Plateau, we applied a five-scale moisture index and average link clustering to all available continuous palaeo-climate records from the Tibetan Plateau to possibly find general patterns of moisture evolution on the Plateau. However, no common regional pattern of moisture evolution during the Holocene could be detected. We assign this to complex responses of different proxies to environmental and atmospheric changes in an already very heterogeneous mountain landscape where minor differences in elevation can cause strong variation in microenvironments. (C) 2010 Elsevier Ltd. All rights reserved. |
| [13] | . Abstract Predominantly laminated lake sediments from a saline closed-basin lake on the northeastern Tibetan Plateau were investigated using a multi-proxy approach ( 14 C-accelerator mass spectrometry dating, smear-slide analysis, loss on ignition, grain size, X-ray diffraction, elemental concentration, ostracod assemblages, stable isotopes of ostracod shells) to trace the regional environmental and climatic history in the Lateglacial and Holocene. Before about 1565cal.65ka BP, small saline water bodies probably filled the basin under unstable cold and harsh environmental conditions. Soon after about 14.965cal.65ka BP, a relatively deep saline lake was established, probably as a result of runoff from melting snow, ice and frozen ground in the lake's catchment. Large changes in flux of aeolian material to the lake were recorded during this initial period of formation of Lake Kuhai. Highest lake levels, a low sediment accumulation rate and less saline conditions were maintained between about 12.8 and 7.165cal.65ka BP when the aeolian influx diminished significantly. After about 7.165cal.65ka BP, the aeolian influx remained at a moderate level apart from a strongly increased dust delivery to the lake between about 6.1 and 5.465cal.65ka BP and a minor short-lived period of slightly enhanced aeolian influx at about 2.765cal.65ka BP. The strongly enhanced dust input to the lake between 6.1 and 5.465cal.65ka BP represented the largest influx of aeolian material to Lake Kuhai during the entire Holocene. However, evidence for climatic deterioration during this period is not seen at most other palaeoclimate sites on the Tibetan Plateau, but instead a significant increase in aridity has been recorded at numerous sites in the northern foreland of the Tibetan Plateau and on the Chinese Loess Plateau. The large dust input to Lake Kuhai between 6.1 and 5.465cal.65ka BP probably did not result from a severe climate deterioration on the Tibetan Plateau itself, but from the pronounced aridity in its northern and eastern foreland. In contrast, the increase in dust influx about 2.765cal.65ka BP seems to correspond to a brief warming spell recorded at other sites on the Tibetan Plateau too. A slight increase in lake level and decrease in salinity after about 0.665cal.65ka BP suggests a slightly higher effective moisture during the final lake stage, accompanied by a somewhat larger dust influx. This apparent contradiction possibly results from enhanced human activities on the northeastern Tibetan Plateau during the last 600 years. Copyright 08 2009 John Wiley & Sons, Ltd. |
| [14] | . 通过青藏高原中部兹格塘错湖泊沉积物总碳（TC）、总有机碳（TOC）、总氮（TN）、总硫 （TS）、氢指数（HI）、氧指数（OI）和有机质的碳同位素（δ^13Corg）等多项指标的综合分析,在判断沉积物中有机质来源的基础上,根据各指标 的变化特征阐明了各自的气候指示意义,建立了兹格塘错全新世以来的古气候演化序列.10100 cal a BP兹格塘错地区进入全新世,全新世早中期为暖湿气候特征,在8600～8400和7400～7000 cal a BP发生两次强烈冷事件;中晚全新世以来气候变冷变干.这一气候演化过程与其邻近的错鄂的研究结果相近,代表了青藏高原中部全新世的气候演化特点.青藏高 原中部全新世气候变化主要受太阳辐射控制. . 通过青藏高原中部兹格塘错湖泊沉积物总碳（TC）、总有机碳（TOC）、总氮（TN）、总硫 （TS）、氢指数（HI）、氧指数（OI）和有机质的碳同位素（δ^13Corg）等多项指标的综合分析,在判断沉积物中有机质来源的基础上,根据各指标 的变化特征阐明了各自的气候指示意义,建立了兹格塘错全新世以来的古气候演化序列.10100 cal a BP兹格塘错地区进入全新世,全新世早中期为暖湿气候特征,在8600～8400和7400～7000 cal a BP发生两次强烈冷事件;中晚全新世以来气候变冷变干.这一气候演化过程与其邻近的错鄂的研究结果相近,代表了青藏高原中部全新世的气候演化特点.青藏高 原中部全新世气候变化主要受太阳辐射控制. |
| [15] | . 利用活塞采样器在位于青藏高原西南部塔若错60 m水深处获得3.1 m沉积岩芯.通过AMS14C测年、花粉、粒度和无机碳分析,恢复了青藏高原西南部全新世以来的植被和气候变化.结果显示,全新世初期(10200~8900 cal a BP),植被由高寒草原向高寒草甸转换,气候由冷干向暖湿发展;全新世早期(8900~7400 cal a BP),植被由高寒草甸到高寒草原的转换,气候相对干旱,但偏暖;全新世中期(7400~3300 cal a BP),植被从高寒草原向高寒草甸或草甸化草原转换,气候以冷湿为主;全新世晚期(3300 cal a BP至今),植被逐渐被高寒草原取代,气候变得寒冷干旱.塔若错全新世早期和中晚期的特征气候事件表明,该地区明显受到西风区环境变化的影响,而在全新世中期更多受印度季风环流控制.尤其是中晚全新世塔若错的湖泊沉积环境记录对于进一步阐述西风/季风在该地区的影响程度具有非常重要的意义. . 利用活塞采样器在位于青藏高原西南部塔若错60 m水深处获得3.1 m沉积岩芯.通过AMS14C测年、花粉、粒度和无机碳分析,恢复了青藏高原西南部全新世以来的植被和气候变化.结果显示,全新世初期(10200~8900 cal a BP),植被由高寒草原向高寒草甸转换,气候由冷干向暖湿发展;全新世早期(8900~7400 cal a BP),植被由高寒草甸到高寒草原的转换,气候相对干旱,但偏暖;全新世中期(7400~3300 cal a BP),植被从高寒草原向高寒草甸或草甸化草原转换,气候以冷湿为主;全新世晚期(3300 cal a BP至今),植被逐渐被高寒草原取代,气候变得寒冷干旱.塔若错全新世早期和中晚期的特征气候事件表明,该地区明显受到西风区环境变化的影响,而在全新世中期更多受印度季风环流控制.尤其是中晚全新世塔若错的湖泊沉积环境记录对于进一步阐述西风/季风在该地区的影响程度具有非常重要的意义. |
| [16] | |
| [17] | . Radiocarbon dating of bulk organic matter is the most commonly used method for establishing chronologies of lake sediments for palaeoclimate reconstructions on the Tibetan Plateau. However, this method is likely to be problematic because the dated material often suffers from old carbon contamination. Recently, advances in luminescence-based chronological techniques have provided new options for dating lacustrine sediments. In the current study, we tested for the first time the applicability of a new post-IR IRSL (pIRIR) measurement protocol for dating fine-grained polymineral material from a deep-lake sediment core from the central part of Tangra Yumco, on the southern Tibetan Plateau. Our results show that: (i) radioactive disequilibria in the uranium decay chain were observed in the studied lake sediments, and thus taken into account for dose rate calculation by using a dynamic modelling approach; (ii) the suitability and robustness of the pIRIR protocol measured at 150 C (pIRIR150) for our samples are confirmed by a set of luminescence characteristic tests as well as the agreement with an independent age control; (iii) turbidite deposition partly caused an insufficient resetting of luminescence signals and thus apparent overestimation in luminescence dating; (iv) compared with the luminescence-based age-depth model, the 14C ages of bulk organic matter from the studied core generally yielded an age difference of 2 ka, which is attributed to hardwater reservoir effects in Tangra Yumco. This study highlights the need for multi-dating approaches of lake sedimentary archives on the Tibetan Plateau. |
| [18] | |
| [19] | . Palaeolimnological studies together with geomorphological investigations of exposed lacustrine sections on the Tibetan Plateau provided valuable palaeoclimate records. Radiocarbon dating is the most commonly used method for establishing chronologies of lake sediments. However, 14C dating of such sediments could be problematic due to the lack of organic matter or a reservoir effect, which commonly appears in radiocarbon ages of lacustrine sediments from the Tibetan Plateau. OSL dating is an alternative for dating the lake sediments and also provides the opportunity to independently test radiocarbon chronologies. The current study tries to compare OSL and 14C dating results in order to evaluate the reservoir effect of 14C dating, and then based on quartz OSL dating and stratigraphic analysis, to construct the chronostratigraphy of a lacustrine sedimentary sequence (TYC section), an offshore profile from Tangra Yumco lake on the southern Tibetan Plateau. Results suggest that: (1) it is possible to obtain robust OSL age estimates for these lake sediments and the OSL ages of the three samples range from ca. 7.6ka to ca. 2.3ka; (2) The discrepancy between the OSL and 14C ages is ca. 4 5ka, which possibly results from the age overestimate of 14C dating due to a reservoir effect in the studied lake; (3) the chronostratigraphy of TYC section and sedimentological environmental analysis show a large lake with a lake level distinctively above the present during ca. 7.6 2.7ka indicating a wet mid-Holocene in the study area. |
| [20] | . Many lakes on the Tibetan Plateau are surrounded by palaeo-shorelines that document former lake-level highstands and hence past changes in climate. Establishing accurate chronologies for these shorelines is crucial to correlate lake-level fluctuations with palaeo-environmental histories that are commonly inferred from lake sedimentary records. Here we apply surface exposure dating to constrain the age of two lake-level highstands at one of the largest Tibetan lakes – Tangra Yumco – which has well developed palaeo-shorelines located up to 65185m above the current lake. For sampling we focussed on wave-cut bedrock terraces that vary considerably in width, because they were carved into rocks of different erodibility. Samples from the highest strath terrace (65180–185m above the lake) at three different sites yield tightly clustered 10 Be ages between 7.39±0.19 ka and 7.87±0.27 ka (internal errors), which constrain the end of an early Holocene lake-level highstand at 7.6±0.6 ka (external error). The consistency of the 10 Be ages indicates that bedrock erosion by wave attack reduced the inherited 10 Be component to negligible amounts, because otherwise samples would show considerable age scatter. Two bedrock samples from a lower terrace (65140–145m above the lake) and one amalgamated clast sample from a beach ridge on the same terrace yield 10 Be ages between 4.04±0.14 ka and 4.50±0.15 ka. These ages document that the lake level dropped by 6540m between 657.6 ka and 654.3 ka. Granite boulders from the two investigated terraces yield apparent 10 Be ages that are 656 ka and 6527 ka older than the ages of the respective terraces, indicating that this sample type is not reliable in this setting. Overall, our results demonstrate that exposure dating of bedrock terraces provides a valuable tool to reconstruct lake-level histories on the Tibetan Plateau. |
| [21] | . The closed Tangra Yumco Basin underwent the strongest Quaternary lake-level changes so far recorded on the Tibetan Plateau. It was hitherto unknown what effect this had on local Holocene vegetation development. A 3.6-m sediment core from a recessional lake terrace at 4,700 m a.s.l., 160 m above the present lake level of Tangra Yumco, was studied to reconstruct Holocene flooding phases (sedimentology and ostracod analyses), vegetation dynamics and human influence (palynology, charcoal and coprophilous fungi analyses). Peat at the base of the profile proves lake level was below 4,700 m a.s.l. during the Pleistocene/Holocene transition. A deep-lake phase started after 11 cal ka BP, but the ostracod record indicates the level was not higher than similar to 4,720 m a.s.l. (180 m above present) and decreased gradually after the early Holocene maximum. Additional sediment ages from the basin suggest recession of Tangra Yumco from the coring site after 2.6 cal ka BP, with a shallow local lake persisting at the site until similar to 1 cal ka BP. The final peat formation indicates drier conditions thereafter. Persistence of Artemisia steppe during the Holocene lake high-stand resembles palynological records from west Tibet that indicate early Holocene aridity, in spite of high lake levels that may have resulted from meltwater input. Yet pollen assemblages indicate humidity closer to that of present potential forest areas near Lhasa, with 500-600 mm annual precipitation. Thus, the early mid-Holocene humidity was sufficient to sustain at least juniper forest, but Artemisia dominance persisted as a consequence of a combination of environmental disturbances such as (1) strong early Holocene climate fluctuations, (2) inundation of habitats suitable for forest, (3) extensive water surfaces that served as barriers to terrestrial diaspore transport from refuge areas, (4) strong erosion that denuded the non-flooded upper slopes and (5) increasing human influence since the late glacial. |
| [22] | . |
| [23] | . . |
| [24] | . Northeastern China is located in the East Asian monsoon region; it is sensitive to both high and low latitude global climate systems. Loess deposits in the region have considerable potential as sensitive archives of past climate changes. However, research into loess deposition and climate change in this region is restricted by the lack of independent age control. In this study, coarse-grained quartz SAR OSL and K-feldspar post-IR infrared (IR) stimulated luminescence (post-IR IRSL; pIRIR 290 ) methods have been used to date the Sanbahuo loess site in northeastern China. The quartz OSL characteristics are satisfactory. The measured pIRIR 290 D e 's do not vary significantly with IR stimulation temperatures between 50°C and 260°C; a first IR stimulation temperature of 200°C was adopted. Dose recovery tests were performed by adding different laboratory doses to both laboratory bleached (300h SOL2) samples and natural samples; the results are satisfactory up to 65800Gy. Resulting quartz OSL and feldspar pIRIR 290 ages are in good agreement at least back to 6544 ka; beyond this feldspar pIRIR 290 ages are older. The feldspar ages are consistent with the expected age of the S1 palaeosol (MIS 5). There appears to have been a period of fast loess deposition at 6562 ka, perhaps indicative of winter monsoon intensification with a very cold and dry climate that lead to a serious desertification of dunefields in northeastern China. |
| [25] | . |
| [26] | . <p>对内蒙古等地68个湖库共139个表层沉积物样品的粒度组分分布及其影响因子进行研究,探讨湖泊沉积物粒度分布特征及其成因机制和粒度组分的环境指示意义。结果表明：湖泊表层沉积物粒度可分为C1~C6共6个组分（0.4~1.9 μm、2.0~12.0 μm、17.0~58.0 μm、70.0~150.0 μm、170.0~500.0 μm和>600.0 μm）,近湖心样品粒度组分以C1、C2、C3为主,过渡带样品以C3和C4为主,近湖滨样品以C5主导。受入湖河流影响的样点处水动力条件决定了粒度组分的空间分布,波浪作用造成的二次悬移再沉积可能造成了C3组分向湖心迁移并导致近岸C5组分富集。C1及C2组分中径流输入不容忽视,C3组分中风力悬移搬运贡献较大。C4、C5和C6含量可指示样点距河口的相对位置,C3可反映样点距岸相对位置。</p> . <p>对内蒙古等地68个湖库共139个表层沉积物样品的粒度组分分布及其影响因子进行研究,探讨湖泊沉积物粒度分布特征及其成因机制和粒度组分的环境指示意义。结果表明：湖泊表层沉积物粒度可分为C1~C6共6个组分（0.4~1.9 μm、2.0~12.0 μm、17.0~58.0 μm、70.0~150.0 μm、170.0~500.0 μm和>600.0 μm）,近湖心样品粒度组分以C1、C2、C3为主,过渡带样品以C3和C4为主,近湖滨样品以C5主导。受入湖河流影响的样点处水动力条件决定了粒度组分的空间分布,波浪作用造成的二次悬移再沉积可能造成了C3组分向湖心迁移并导致近岸C5组分富集。C1及C2组分中径流输入不容忽视,C3组分中风力悬移搬运贡献较大。C4、C5和C6含量可指示样点距河口的相对位置,C3可反映样点距岸相对位置。</p> |
| [27] | . 以210Pb和AMS14C年代为框架，基于青藏高原南部羊卓雍错（简称羊湖）短钻岩芯的粒度记录，通过对沉积物粒度敏感粒级、粒度参数及频率特征曲线分析，重建了青藏高原南部过去近2000年来的有效湿度变化。结果显示：约100～820A．D．，沉积物粒度较细，湖泊水位和有效湿度较高，气候温凉偏湿，对应黑暗时代冷期；820～1200A．D．，粒度变粗，湖泊水位下降，流域气候干旱或有效湿度较低，对应中世纪暖期；1200—1910A．D．，对应小冰期，粒度变细，湖泊水位上升，有效湿度增加，气候湿润，其中在约1200～1320A．D．、1400～1550A．D．和1780～1900A．D．出现3次显著的湿润期，可能对应小冰期的3次较寒冷亚阶段；20世纪以来，湖泊急剧收缩，流域趋于干旱。与太阳活动、北半球温度和亚洲季风指标的对比分析表明：羊湖粒度反映的过去近2000年来有效湿度的变化与青藏高原及其他地区的记录有较好的一致性，但程度和起止时间有所不同；青藏高原南部地区过去2000年来的气候模式为暖干一冷湿，可能主要受太阳辐射强度所控制。该记录可补充青藏高原南部地区过去2000年来湿度变化记录的缺乏，对深人理解季风区高海拔地区过去2000年来气候变化过程及原因有重要的意义。 . 以210Pb和AMS14C年代为框架，基于青藏高原南部羊卓雍错（简称羊湖）短钻岩芯的粒度记录，通过对沉积物粒度敏感粒级、粒度参数及频率特征曲线分析，重建了青藏高原南部过去近2000年来的有效湿度变化。结果显示：约100～820A．D．，沉积物粒度较细，湖泊水位和有效湿度较高，气候温凉偏湿，对应黑暗时代冷期；820～1200A．D．，粒度变粗，湖泊水位下降，流域气候干旱或有效湿度较低，对应中世纪暖期；1200—1910A．D．，对应小冰期，粒度变细，湖泊水位上升，有效湿度增加，气候湿润，其中在约1200～1320A．D．、1400～1550A．D．和1780～1900A．D．出现3次显著的湿润期，可能对应小冰期的3次较寒冷亚阶段；20世纪以来，湖泊急剧收缩，流域趋于干旱。与太阳活动、北半球温度和亚洲季风指标的对比分析表明：羊湖粒度反映的过去近2000年来有效湿度的变化与青藏高原及其他地区的记录有较好的一致性，但程度和起止时间有所不同；青藏高原南部地区过去2000年来的气候模式为暖干一冷湿，可能主要受太阳辐射强度所控制。该记录可补充青藏高原南部地区过去2000年来湿度变化记录的缺乏，对深人理解季风区高海拔地区过去2000年来气候变化过程及原因有重要的意义。 |
| [28] | . 青藏高原西北部甜水海盆地地处内陆干旱区和喀喇昆仑山的东侧雨影区,气候环境极端干旱.但多次考察结果揭示晚更新世时期存在着大湖和高湖面的地质证据.为了探讨该地区过去气候与环境的演变历史,在甜水海盆地(海拔4840m)进行钻探采得沉积岩芯56.32m(TS95),通过对沉积岩芯其中一段的粒度、生物、同位素及地球化学多指标综合分析,探讨了MIS 3时期甜水海地区气候环境演化过程.沉积岩芯显示沉积物岩性变化频繁,记录着湖面多次波动、沉积环境多变的演化过程,分别在59～56kaB.P.,49～47kaB.P.,45～41kaB.P.,35.5～34.OkaB.P.和28～25kaB.P.期间存在高湖面和湖水稀释.在其多环境指标的记录中,Heinrich事件和D-O旋回均有反映,揭示了全球变化信号在研究区的响应.同时,不同高湖面阶段内部环境波动也较明显,差别较大,其产生原因也各有不同,归结为3种情况:1)由于气候变暖导致冰雪融水补给增加而产生的高湖面;2)由于冷湿气候导致的高湖面;和3)由于暖湿气候导致的高湖面.本研究揭示出了MIS 3时期气候频繁波动的特征和气候的不稳定性. . 青藏高原西北部甜水海盆地地处内陆干旱区和喀喇昆仑山的东侧雨影区,气候环境极端干旱.但多次考察结果揭示晚更新世时期存在着大湖和高湖面的地质证据.为了探讨该地区过去气候与环境的演变历史,在甜水海盆地(海拔4840m)进行钻探采得沉积岩芯56.32m(TS95),通过对沉积岩芯其中一段的粒度、生物、同位素及地球化学多指标综合分析,探讨了MIS 3时期甜水海地区气候环境演化过程.沉积岩芯显示沉积物岩性变化频繁,记录着湖面多次波动、沉积环境多变的演化过程,分别在59～56kaB.P.,49～47kaB.P.,45～41kaB.P.,35.5～34.OkaB.P.和28～25kaB.P.期间存在高湖面和湖水稀释.在其多环境指标的记录中,Heinrich事件和D-O旋回均有反映,揭示了全球变化信号在研究区的响应.同时,不同高湖面阶段内部环境波动也较明显,差别较大,其产生原因也各有不同,归结为3种情况:1)由于气候变暖导致冰雪融水补给增加而产生的高湖面;2)由于冷湿气候导致的高湖面;和3)由于暖湿气候导致的高湖面.本研究揭示出了MIS 3时期气候频繁波动的特征和气候的不稳定性. |
| [29] | . The organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions, evaluate histories of climate change, and assess impacts of humans on local ecosystems. The elemental, isotopic, and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area, and they serve as proxies of organic matter delivery and accumulation. Sedimentary records from the North American Great Lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions. The records of these lakes date from retreat of the Laurentide ice sheet around 12 ka, include the mid-Holocene Hypsithermal, and show consequences of recent human changes. Low C org /N total ratios indicate that most of the sediment organic matter in the Great Lakes is from algal production, yet changes in biomarker molecule compositions also show that varying amounts of land-plant organic matter have been delivered to the lakes. Elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13 C values in the organic matter of sediments that were deposited in the 1960s and 1970s. Increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the Great Lakes. Accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the Great Lakes. Emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising, yet some potentially important measurements remain underutilized. |
| [30] | . <p>通过对滇池流域宝象河水库沉积物TOC、TN、C/N、δ<sup>13</sup>C及粒度等指标的测定,分析了沉积物中有机碳的主要来源及其变化趋势。结果表明:水体中不同位置沉积物有机碳来源并不相同,距三岔河入库河口相对较近的沉积柱芯A中C/N比值介于2.93~11.73,δ<sup>13</sup>C值介于 -23.66‰~-21.78‰,有机碳主要来源于水生生物,陆源输入贡献较小,有机碳含量受粒径大小影响显著;距三岔河入库河口相对较远的沉积柱芯B中,其TOC和TN含量较高,分别介于14.62~24.93 g・kg<sup>-1</sup>和2.11~3.3 g・kg<sup>-1</sup>,δ<sup>13</sup>C值变化范围为-28.36‰~-26.36‰,粒度以黏土和细粉砂为主,有机碳主要来源于水库周边陆源输入,受人类活动影响强烈。</p> . <p>通过对滇池流域宝象河水库沉积物TOC、TN、C/N、δ<sup>13</sup>C及粒度等指标的测定,分析了沉积物中有机碳的主要来源及其变化趋势。结果表明:水体中不同位置沉积物有机碳来源并不相同,距三岔河入库河口相对较近的沉积柱芯A中C/N比值介于2.93~11.73,δ<sup>13</sup>C值介于 -23.66‰~-21.78‰,有机碳主要来源于水生生物,陆源输入贡献较小,有机碳含量受粒径大小影响显著;距三岔河入库河口相对较远的沉积柱芯B中,其TOC和TN含量较高,分别介于14.62~24.93 g・kg<sup>-1</sup>和2.11~3.3 g・kg<sup>-1</sup>,δ<sup>13</sup>C值变化范围为-28.36‰~-26.36‰,粒度以黏土和细粉砂为主,有机碳主要来源于水库周边陆源输入,受人类活动影响强烈。</p> |
| [31] | . |
| [32] | . Glaciochemical time series developed from Summit, Greenland, indicate that the chemical composition of the atmosphere was dynamic during the Holocene epoch. Concentrations of sea salt and terrestrial dusts increased in Summit snow during the periods 0 to 600, 2400 to 3100, 5000 to 6100, 7800 to 8800, and more than 11,300 years ago. The most recent increase, and also the most abrupt, coincides with the Little Ice Age. These changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods, and that temperatures in the mid to high northern latitudes were potentially the coldest since the Younger Dryas event. |
| [33] | . We present the first area-average time series reconstructions of warmest month, coldest month and mean annual surface air temperatures across Europe during the last 12,000 years. These series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four-dimensional gridding procedure. This approach combines three-dimensional spatial gridding with a fourth dimension represented by time, allowing data from irregular time series to be ocussed onto a regular time step. We provide six regional reconstructed temperature time series as well as summary time series for the whole of Europe. The results suggest major spatial and seasonal differences in Holocene temperature trends within a remarkably balanced regional and annual energy budget. The traditional mid-Holocene thermal maximum is observed only over Northern Europe and principally during the summer. This warming was balanced by a mid-Holocene cooling over Southern Europe, whilst Central Europe occupied an intermediary position. Changes in annual mean temperatures for Europe as a whole suggest an almost linear increase in thermal budget up to 7800 BP, followed by stable conditions for the remainder of the Holocene. This early Holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west, which have continued to rise at a progressively decreasing rate up to the present day. |
| [34] | The upper 8.45 m of a sediment core from Lake Naleng were analysed for its pollen and charcoal content to detect vegetation, climate, and human activity changes on the southeastern Tibetan Plateau (TP) since the Pleistocene–Holocene transition (11.7 cal kyr BP). The application of pollen indicator taxa, pollen sums, biomisation, and multivariate analysis helped to reconstruct environmental conditions during the Holocene. Furthermore, 64 surface samples from the southeastern TP and its foreland were analysed to investigate the relationship between modern pollen assemblages and vegetation. The modern vegetation was recorded in the surface pollen assemblages by pollen indicator types, although sediments also contain pollen spectra extra-regional pollen transported from lower vegetation belts, which is seen as a consequence of steep elevation gradients at the southeastern margin of the TP. The relation between different vegetation belts and temperature from the modern sample sites was used to infer quantitative temperature trends for the Holocene record. Spreading of forests at the beginning of the Holocene marks the transition to warmer conditions. From 10.7 to 65 4.4 cal kyr BP open Abies– Betula forests were found at Lake Naleng probably reflecting a stronger-than-present summer monsoon. Temperature range reconstructions suggest a temperature that was about 2–3 °C warmer and a treeline position that was 400–600 m higher than today. A severe climate deterioration with temperatures about 1–2 °C colder than before is documented by a retreating forest between 8.1 and 7.2 cal kyr BP. Forests retreated while subalpine Rhododendron shrubs spread around Lake Naleng indicating colder conditions with temperatures 1–2 °C below early and mid Holocene level after 4.4 cal kyr BP. The forest decline at 4.4 cal kyr BP is considered to reflect climatic conditions as low concentrations of charcoal particles found in this period exclude human-induced fires as reason for the forest retreat. However, grazing indicators imply human influence on the environment since 3.4 cal kyr BP around Lake Naleng. We therefore consider the rapid treeline shift seen at 3.4 cal kyr BP as partly generated by human activity and partly by climatic conditions. |
| [35] | . The climates on the eastern Tibetan Plateau are strongly influenced by direct insolation heating as well as monsoon-derived precipitation change. However, the moisture and temperature influences on regional vegetation and climate have not been well documented in paleoclimate studies. Here we present a well-dated and high-resolution loss-on-ignition, peat property and fossil pollen record over the last 10,000 years from a sedge-dominated fen peatland in the central Zoige Basin on the eastern Tibetan Plateau and discuss its ecological and climatic interpretations. Lithology results indicate that organic matter content is high at 60-80% between 10 and 3 ka (1 ka = 1000 cal yr BP) and shows large-magnitude fluctuations in the last 3000 years. Ash-free bulk density, as a proxy of peat decomposition and peatland surface moisture conditions, oscillates around a mean value of 0.1 g/cm(3), with low values at 6.5-4.7 ka, reflecting a wet interval, and an increasing trend from 4.7 to 2 ka, suggesting a drying trend. The time-averaged mean carbon accumulation rates are 30.6 gC/m(2)/yr for the last 10,000 years, higher than that from many northern peatlands. Tree pollen (mainly from Picea), mostly reflecting temperature change in this alpine meadow-forest ecotonal region, has variable values (from 3 to 34%) during the early Holocene, reaches the peak value during the mid-Holocene at 6.5 ka, and then decreases until 2 ka. The combined peat property and pollen data indicate that a warm and wet climate prevailed in the mid-Holocene (6.5-4.7 ka), representing a monsoon maximum or "optimum climate" for the region. The timing is consistent with recent paleo-monsoon records from southern China and with the idea that the interplays of summer insolation and other extratropical large-scale boundary conditions, including sea-surface temperature and sea-level change, control regional climate. The cooling and drying trend since the mid-Holocene likely reflects the decrease in insolation heating and weakening of summer monsoons. Regional synthesis of five pollen records along a south north transect indicates that this climate pattern can be recognized all across the eastern Tibetan Plateau. The peatland and vegetation changes in the late Holocene suggest complex and dramatic responses of these lowland and upland ecosystems to changes in temperature and moisture conditions and human activities. (C) 2011 Elsevier Ltd. All rights reserved. |
| [36] | . 61Multiple sediment records show diverse sediment patterns and ostracod assemblages.61Holocene water depth fluctuations could be individually estimated for each site.61Four phases of lake filling were traced for the past 24 kyr BP.61Unstable summer monsoon impact is recorded for the early Holocene.61Since the mid-Holocene Westerly-derived moisture supply became an important source. |
| [37] | . . |
| [38] | . There has been much recent debate about Holocene climate variation in the monsoon region of China, especially the temporal pattern of variations in precipitation, the time-transgressive nature of the Holocene precipitation maximum, and the extent to which variations in regions influenced by the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) have been synchronous. We summarize and compare carbonate oxygen-isotope records (delta O-18(carb)) from ten lakes within the present-day ISM region. We discuss their paleoclimate significance considering the present-day moisture source, isotopic composition of precipitation and the hydrological setting. The delta O-18(carb) records are controlled mainly by the isotopic composition of lake water, which in turn is a function of regional Precipitation/Evaporation (P/E) balance and the proportion of precipitation that is monsoon-derived. We normalized the delta O-18(carb) data and used these records to generate an integrated moisture index. This index, along with oxygen-isotope records from speleothems and carbon-isotope records (delta C-13(org)) from peats within the monsoon region, suggests that Holocene climate was broadly synchronous across the monsoon region and, within the limits of accuracy of the existing age models, provides no strong evidence for previously-proposed anti-phasing of the ISM and the EASM. Stable-isotope records from lake sediments and peat bogs have excellent potential for providing high-quality paleoclimate data for monsoon Asia, and complement high-resolution speleothem sequences, which are only found in certain localities. (C) 2011 Elsevier Ltd. All rights reserved. |
| [39] | . <h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Sedimentological, geochemical and palynological data from Wulungu Lake in northern Xinjiang, China, are used to reconstruct environmental and climate changes since 9550 cal yr BP. High abundance of <em>Sparganium</em> and Poaceae, low Md (median diameter) and δ<sup>13</sup>C<sub>organic</sub> values indicate aridity between 9550 and 6730 cal yr BP. High Md and δ<sup>13</sup>C<sub>organic</sub> values, and the prevalence of desert-steppe and steppe vegetation between 4200 and 560 cal yr BP, indicate that effective moisture increased after 6730 cal yr BP, peaking at 4200 and 560 cal yr BP. Low Md values, a negative excursion of δ<sup>13</sup>C<sub>org</sub>, and the transition from steppe to desert vegetation since 560 cal yr BP reflect a decrease in effective moisture during the latest Holocene. Late Holocene human activities were indicated by sharp increase in the abundance of <em>Pediastrum</em> then. Variations in carbonate contents indicate that temperature was generally high between 9550 and 7740 cal yr BP, low between 7740 and 6730 cal yr BP, intermediate between 6730 and 560 cal yr BP, and low during the last 560 yr. Regional comparison indicates that the Asian monsoon did not extend to Wulungu Lake and westerlies were the main factor in determining the moisture availability during the Holocene.</p> |
| [40] | . |
| [41] | 338( |
| [42] | |
| [43] | |
| [44] | . 通过对西风区新疆赛里木湖沉积岩芯的精确定年和孢粉、炭屑分析,重建了该地区全新世以来的植被和气候环境演化历史.结果表明,~9.6cal.kaBP前的早全新世,研究区以荒漠植被为主,A/C比值和第一主成分样品得分所指示的有效湿度明显偏低,第二主成分样品得分所指示的气温回升,气候干旱.9.6~5.5cal.kaBP的中全新世湖区为典型的荒漠草原/草原植被,区域有效湿度明显增加,气温较高,为温暖时期.其中6.5~5.5cal.kaBP期间,研究区植被由荒漠草原/草原迅速转变为荒漠,有效湿度明显降低,同时温度升至最高,可能是一次千年尺度的高温干旱事件.5.5cal.kaBP以来的中-晚全新世,研究区呈现出典型的草原/草甸景观,有效湿度相对较高,温度大幅降低,气候温凉湿润,相对较为适宜.赛里木湖孢粉、炭屑记录的温度和湿度变化与区域其他湖泊记录对比的一致性表明,太阳辐射是造成区域温度变化的主要驱动因素,同时温度变化影响了区域的有效湿度变化.该地区早全新世气候干旱,中-晚全新世气候相对湿润的气候环境演化特征与季风影响区有着明显的差异.早全新世干旱是中纬度西风减弱和上风向蒸发减少的环流背景下局地温度高蒸发强烈的结果. . 通过对西风区新疆赛里木湖沉积岩芯的精确定年和孢粉、炭屑分析,重建了该地区全新世以来的植被和气候环境演化历史.结果表明,~9.6cal.kaBP前的早全新世,研究区以荒漠植被为主,A/C比值和第一主成分样品得分所指示的有效湿度明显偏低,第二主成分样品得分所指示的气温回升,气候干旱.9.6~5.5cal.kaBP的中全新世湖区为典型的荒漠草原/草原植被,区域有效湿度明显增加,气温较高,为温暖时期.其中6.5~5.5cal.kaBP期间,研究区植被由荒漠草原/草原迅速转变为荒漠,有效湿度明显降低,同时温度升至最高,可能是一次千年尺度的高温干旱事件.5.5cal.kaBP以来的中-晚全新世,研究区呈现出典型的草原/草甸景观,有效湿度相对较高,温度大幅降低,气候温凉湿润,相对较为适宜.赛里木湖孢粉、炭屑记录的温度和湿度变化与区域其他湖泊记录对比的一致性表明,太阳辐射是造成区域温度变化的主要驱动因素,同时温度变化影响了区域的有效湿度变化.该地区早全新世气候干旱,中-晚全新世气候相对湿润的气候环境演化特征与季风影响区有着明显的差异.早全新世干旱是中纬度西风减弱和上风向蒸发减少的环流背景下局地温度高蒸发强烈的结果. |
