2017, Vol.37
地理学国家级教学示范中心(陕西师范大学), 西安 710119)
深海氧同位素曲线第三阶段(MIS 3，即56~25ka B.P.)是末次冰期中气候相对温湿的特殊时段[1~4]，其反映的气温明显低于末次间冰期和全新世时期[5]。而古里雅冰芯的研究认为青藏高原在MIS 3晚期(40~30ka B.P.)的气候异常温暖湿润，可达到间冰期的温湿程度，被称为“特强夏季风事件”或“高温大降水事件”[6~8]。已有证据表明这一时期青藏高原及其邻区的湖泊呈高湖面特征[9~11]，在中国沿海地区也出现海侵特征[12~16]。不同研究成果显示末次冰期中的这次“高温大降水事件”在中国大陆地区的表现与其他地区有所不同，这种差异引起诸多学者对我国MIS 3时期气候变化特征的广泛关注和争议。
黄土-古土壤序列被认为是记录古环境演变的最好陆地载体之一[17]。黄土高原的众多剖面记录到末次冰期中存在较为温凉湿润的气候阶段[17~23]，与其干冷气候形成显著对照，并认为气候温湿程度低于全新世及末次间冰期[20~25]，其发生时间可对应于MIS 3时期[24~28]。这一较温湿的气候变化事件在川西高原、新疆、长江中下游等地的黄土剖面中也有记录[29~31]。尽管许多黄土剖面均记录到MIS 3时期气候较温湿特征，但不同剖面记录的这一时期气候变化特征仍存在较大分歧，主要有：1) 不同地区黄土记录的MIS 3阶段气候变化幅度存在差异[21, 24, 25]；2) 不同黄土剖面这一时期气候变化规律不同，即冷暖变化阶段划分方案不同[21, 24, 26, 27]；3) MIS 3晚期和早期的温湿程度不一致[21, 24, 26, 27]。显然，我国MIS 3时期气候变化特征如何？MIS 3晚期的“高温大降水事件”是否存在？这些问题仍需要更多研究做进一步的验证。
汉江位于秦岭南麓，地处温带季风和亚热带季风气候的过渡地带，对气候变化的响应十分敏感。近些年对汉江黄土的空间分布、地层、年代序列、风化成壤特征等相关问题进行了深入研究，并取得了一系列新的认识[32~41]，例如，在汉江一级阶地上分布有55ka B.P.以来的连续黄土堆积，其无疑记录了55ka B.P.以来的气候环境演变信息[32~36]。这些黄土是否存在MIS 3时期气候变化的记录？记录的MIS 3期间气候变化特征如何？与其他区域研究结果是否相同？是区域气候特征的表现还是对全球气候变化的响应？这些问题尚无人研究。基于此，本文以汉江上游郧西段庹家湾(TJW)黄土-古土壤剖面为研究对象，对其磁化率、粒度、色度、OSL年代等进行了综合研究，试图揭示该区域MIS 3时期的气候变化特征，并通过不同黄土剖面记录对比，进而探讨不同区域MIS 3时期气候记录的差异性。
2 区域概况汉江发源于秦岭南麓汉中宁强县，干流流经陕西、湖北两省，于武汉市汇入长江。流域大体位于30°8′~34°11′N，106°12′~114°14′E，其中丹江口水库以上为上游地区(图 1)，河道长925km，流域面积9.52×104km2。区内属于北亚热带季风气候，干湿季节分明，夏季高温多雨，冬季低温少雨，年均气温约12~16℃，年均降水量约700~1100mm。地带性植被为亚热带常绿阔叶林，土壤主要为淋溶土(发生学上认定为黄褐土)[41]。位于中国南北气候交界、温带季风和亚热带季风的地带，该区域对我国季风气候和全球变化的响应十分敏感。
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图 1 汉江上游流域及研究剖面图 Fig. 1 Map showing the location of the study profile in the upper Hanjiang River valley |
汉江流域北以秦岭为界、东北以伏牛山毗邻、西南以大巴山相邻，整体地势西北高东南低，地形以峡谷与盆地相间为主[32]。区内分布有很多大面积平坦盆地，如：汉中盆地、安康盆地、旬阳盆地、郧县盆地、商丹盆地等。盆地地段的河流阶地发育良好，一般可见到Ⅰ~Ⅳ级河流阶地，分别高出汉江平水位10~15m、30~40m、60~70m和90~100m[42]。野外调研发现该区分布有大量黄土沉积物，这些黄土不仅分布在河谷、盆地等低洼地区，还分布在地势较高的山坡和山顶面上，具有风成黄土的披覆沉积特征。其中Ⅰ级阶地面宽阔平坦，为黄土的堆积与保存提供了有利场所，常覆盖5~20m厚的黄土，且黄土地层完整未间断；而Ⅱ~Ⅳ级阶地地形多为丘陵，冲沟侵蚀破坏严重，地层常常缺失不全[43]。
3 研究材料和方法 3.1 研究材料本文选取湖北省郧西县观音镇庹家湾村TJW剖面(32°05′47″N，110°22′51″E)为具体研究材料，其位于汉江左岸Ⅰ级河流阶地，是当地采砂金过程中形成的完整断面。该剖面海拔高度约168m，黄土堆积厚度为8m左右，地层完整且界线清晰。结合前人的研究和划分[32, 36]以及野外特征和室内理化指标分析，剖面自上向下依次划分为：表土层(TS)、近代黄土(L0)、古土壤(S0)、过渡黄土(Lt)、马兰黄土(L1)和河流相沉积(T1-al1)。该剖面地层特征与汉江流域其他地点黄土剖面完全相同，如郧县段前房村(QFC)和黄坪村(HPC)剖面[32, 33]等，见图 2。无疑，TJW剖面完整地记录了55ka B.P.以来区域气候环境的演变信息。
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图 2 汉江上游TJW剖面与QFC、HPC剖面[32, 33]的地层年代对比图 Fig. 2 Stratigraphy and chronology of TJW profile and its correlation with QFC and HPC profiles in the upper Hanjiang River valley |
野外观察清楚显示，马兰黄土L1的顶界在178cm，黄土L1堆积厚度约6m，其中600~818cm深度夹有透镜状细砂-砂层。这说明，河流阶地刚刚形成时风尘堆积不稳定，之后(600cm以上)便开始连续堆积。野外观察和室内实验数据显示，228~260cm和294~370cm深度呈暗红棕、粘土-粉砂质地、比较紧实坚硬特征，明显不同于上下黄土层，应属于经历了一定程度成壤改造的弱古土壤层(L1-2-S1和L1-2-S2)。具体地层描述见表 1。
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表 1 汉江上游TJW剖面地层划分及特征描述 Table 1 Stratigraphic description of TJW profile in the upper Hanjiang River valley |
自剖面顶部向下进行高密度连续采样至马兰黄土L1底部，其中0~570cm间隔2cm、570~818cm间隔4cm取全样，共采集347个样品。同时采集了11个光释光(OSL)测年样品。OSL样品采集时剥除表面已风化部分，采取后用锡箔纸和黑色塑料袋密封，避免样品曝光和水分散失。所有测量均在陕西师范大学环境变迁实验室完成。颜色特征依据中国标准比色卡(Munsell)[44]进行描述。磁化率选用英国MS-2B型磁化率仪测量；粒度采用美国LS13320型激光粒度仪测量；色度(L*、a*和b*)特征采用美国非接触分光光度仪VS450测量；OSL年代是选取90~125μm的提纯石英颗粒进行测量，等效剂量(De)采用单片再生剂量法(SAR)在丹麦生产的RISØ-TL/OSL-20型自动释光断代仪上测定。该仪器的人工β辐射源为90 Sr/90 Y源，活度为1.48G Bq，蓝光激发光源波长为470±30nm，红外激发光源波长为880±80nm，光电倍增管为EMI9523QB15，滤光片为Hoya U-340型。
4 光释光(OSL)年代可靠性及结果准确可靠的年代框架是研究古气候变化的前提和关键[45, 46]。近十几年来，OSL测年技术已经成功应用于各种第四纪沉积物的测年研究[35, 47~50]，其中，风成黄土被认为是良好的OSL测年材料[51~53]。课题组多年来采用相对成熟稳定的SAR方法对汉江上游低阶地多处黄土剖面进行了OSL测年研究[35, 54~58]。其中，本文在TJW剖面关键层位共采取了11个OSL测年样品，有关OSL测年的详细步骤和过程另撰文发表1)样品测量过程中通过预热坪区检验(预热温度范围180~300℃，间隔20s，预热10s)、剂量恢复试验等一系列实验条件测试，发现预热温度220~280℃之间出现一个明显的温度坪区，剂量恢复系数介于0.9~1.1之间，同时结合循环比和回收率等试验结果，最终选取预热温度260℃，cut heat 220℃作为样品等效剂量(De)值的测量条件。为了获得准确可靠的De值，对所测样品的De值进行统计分析，采用信号对比方法[59]，选择样品自然释光信号离散度(RSDN-OSL)和第一次再生剂量校正后的释光信号(RSDR-OSL)最接近的De值参与年代计算。经过挑选后的De值分布相对集中，基本呈正态分布，反映这些样品最后一次被埋藏前经历了较好地晒退，以上这些说明本文OSL测年结果是合理可靠的。
1)张文桐，庞奖励，黄春长等. 汉江上游郧县庹家湾剖面光释光测年研究及意义. 中国沙漠(待发表)
具体的测年数据见表 2。结合图 2地层关系发现，这些OSL年代结果与剖面地层相一致，即OSL年代随着剖面深度从上到下逐渐增大，与黄土地层沉积顺序相对应，从另一个面验证了本文所测OSL年代结果是可信的。TJW剖面OSL测年结果显示马兰黄土L1顶部190~195cm和200~205cm深度的测年结果分别为13.57±1 33ka和15.26±1 05ka；其底部以及下伏河流相沉积物顶部805~810cm、820~825cm和830~835cm深度测年结果分别为53.49±3.97ka、54.65±2.90ka和55.11±5.20ka。这些结果可与诸多汉江已研究剖面建立的年代[54, 55]直接对比(图 2)，再次证明了本文OSL年龄数据的准确可靠性。根据黄土地层关系和OSL测年结果，可将研究点马兰黄土L1的底界年龄定为55.0ka B.P.，这也暗示汉江一级阶地快速抬升时间约为55.0ka B.P.，其后风成黄土在该区域开始连续堆积；马兰黄土的顶界年龄定为11.5ka B.P.。
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表 2 汉江上游TJW剖面马兰黄土OSL年龄数据 Table 2 OSL age data of Malan loess in the TJW profile in the upper Hanjiang River valley |
野外调研中发现TJW剖面228~260cm和294~370cm深度有两层弱古土壤(L1-2-S1和L1-2-S2)，其成壤强度变化同时也得到理化性质参数(磁化率、粒度、色度)的印证。
5.1.1 磁化率证据磁化率作为表征沉积物磁性特征的指标，能够反映黄土风化成壤强度的变化，间接地指示古季风气候环境的演变[23, 60, 61]。TJW剖面的磁化率值介于35.5×10-8~207.8×10-8 m3/kg，其中古土壤S0含量(平均166.1×10-8 m3/kg)较高，说明其经历了强烈的成壤改造作用，形成大量次生铁磁性矿物，指示温暖湿润的气候条件，与中全新世大暖期[62]相吻合。黄土L1的含量(平均104.0×10-8 m3/kg)较低，指示微弱的成壤改造作用和寒冷干旱的气候特征。值得注意的是，在马兰黄土L1中有两个深度分别为228~260cm和294~370cm的磁化率明显偏高(图 3和表 3)，其均值分别为136.9×10-8 m3/kg和161.3×10-8 m3/kg，显著高于黄土L1层，说明它们经历了一定程度的成壤改造，应属于弱古土壤(L1-2-S1和L1-2-S2)。但与古土壤S0相比，其磁化率值偏低，暗示它们受到的成壤改造强度弱于古土壤S0。
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图 3 汉江上游TJW剖面磁化率、粒度、色度变化曲线图 Fig. 3 OSL age data of Malan loess in the TJW profile in the upper Hanjiang River valley |
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表 3 汉江上游TJW剖面磁化率、粒度、色度特征 Table 3 Magnetic susceptibility, grain-size and color characteristics of TJW profile in the upper Hanjiang River valley |
黄土的粒度分布特征常被广泛用作指示东亚季风变化的替代性指标[17, 63]。一般常用>63μm、>50μm等粗颗粒含量反映冬季风的盛衰，< 2μm、< 5μm等粘粒含量指示风化成壤作用及夏季风的强弱[20, 64]。结果表明，TJW剖面的粒径组成以粉砂(5~50μm)为主，整个剖面含量范围介于35.1 % ~61.8 %。总体来看，粘粒( < 5μm)含量的高峰值出现在古土壤S0中(均值24.4 %)，黄土L1层的均值含量(13.2 %)较低；而砂粒(>50μm)含量的变化趋势与粘粒呈一定的反镜像关系，古土壤S0和黄土L1均值分别为21.7 %和41.9 %。这些特征说明古土壤S0经历了强烈的风化成壤改造，不稳定矿物被分解形成大量细小的次生粘土矿物，而黄土L1受到的成壤作用微弱。同时，粒度变化曲线显示在马兰黄土L1的228~260cm和294~370cm深度出现两个明显的峰区(图 3)，对应于弱古土壤层L1-2-S1和L1-2-S2。这两层弱古土壤的粘粒含量(平均17.2 %和19.2 %)明显高于黄土L1，而低于古土壤S0，砂粒含量(平均35.7 %和25.9 %)则呈相反特征(表 3)，这也证明弱古土壤(L1-2-S1和L1-2-S2)经历了明显的风化成壤改造，但其风化成壤强度弱于古土壤S0。
5.1.3 色度证据土壤中不同色度的记录能深刻地反映气候和环境信息[65~69]，目前描述土壤颜色的系统包括Munsell系统和CIE(Commission Internationale de lEclairage)系统，而后者能更好地定量颜色变化信息[67]。本文选用CIE1976表色系统，L *、a *和b *分别代表土壤亮度、红度和黄度。相同母质基础条件下，L *值的变化主要受控于成壤改造作用的影响(尤其是有机质的形成)，且成壤改造作用越强，对应的L *值就越低[68]；铁是土壤中主要染色杂质，红度a *变化主要受控于赤铁矿(以Fe2O3为主)的含量多少，黄度b *则受控于黄铁矿、褐铁矿等铁矿物的影响[69]，且风化成壤作用越强，土壤颜色加深，a *值相应升高，b *值相应降低[67~69]。图 3显示，TJW剖面的L *值变化与烧失量变化几乎呈完全的反相关关系，其低峰值(平均55.8) 出现在古土壤S0，而黄土L1层均值含量(60.0) 较高；a *值变化曲线与Fe2O3的变化趋势一致，高峰值(6.3) 出现在古土壤S0，黄土L1层(4.4) 较低；b *值的变化则与a *值呈一定的反相关关系，古土壤S0的均值含量(16.2) 较低，黄土L1层含量(17.6) 较高。这些色度参数变化说明古土壤S0受到的成壤改造作用强烈，形成大量有机质和次生铁磁性矿物；而黄土经历的成壤作用微弱。同样，马兰黄土L1中228~260cm和294~370cm深度的两次变化在其颜色(L *、a *和b *)变化曲线上均有显示(图 3)。弱古土壤L1-2-S1和L1-2-S2层的a *值(平均5.6和5.7) 明显高于马兰黄土L1，而低于古土壤S0，L *值(平均57.9和56.5) 和b *值(平均17.5和17.0) 则表现出相反特征(表 3)，这样的色度参数分布说明它们经历了明显的成壤改造作用，形成大量有机质和次生铁氧化物，也指示了其成壤改造强度弱于古土壤S0。
5.2 弱古土壤记录的气候变化特征上述分析显示，不同气候替代性指标在228~260cm和294~370cm深度的分布明显高/低于上下马兰黄土L1，磁化率值(136.9×10-8 m3/kg和161.3×10-8 m3/kg)、< 5μm粘粒含量(17.2 %和19.2 %)、红度a *(5.6和5.7) 值相对升高，而>50μm粗颗粒含量(35.7 %和25.9 %)、亮度L *(57.9和56.5)、黄度b *(17.5和17.0) 相对降低，说明其经历了一定程度的风化成壤改造，属于弱古土壤(L1-2-S1和L1-2-S2)，它们记录了末次冰期中两次较温湿的气候变化特征。
OSL测年数据显示，弱古土壤L1-2-S1(250~255cm深度)和L1-2-S2(350~355cm深度)的测年结果分别为21.59±1.20ka和27.26±1.44ka(图 2和表 2)，考虑到样品位置及测年误差，可推测这两层弱古土壤的形成时间约为28.5~21.0ka B.P.，即说明汉江地区在该期间发生两次气候较温湿事件，且具有明显冷暖波动，其中，弱古土壤L1-2-S2的形成时间(27.26±1.44ka)与全球范围内的MIS 3晚期大致相当，说明MIS 3晚期的温湿气候特征在汉江地区亦有表现；弱古土壤L1-2-S1(21.59±1.20ka)的形成可能暗示了末次盛冰期内也存在较温湿的气候波动，但具体问题还有待更深入的研究。从风化成壤强度看，这两层弱古土壤的成壤强度低于古土壤S0，这暗示汉江地区28.5~21.0ka B.P.期间气候温湿程度弱于中全新世大暖期[62]。
5.3 TJW剖面记录的MIS 3气候事件及区域对比MIS 3时期相对温湿的气候变化特征在我国不同区域不同载体中均有记录。例如，古里雅冰芯δ 18 O的研究说明MIS 3时期气候呈异常温暖特征[6, 7]；黄土高原多处黄土剖面的研究认为MIS 3时期的气候比末次冰期中的MIS 2、MIS 4更温湿[21, 24, 25]；川西高原的黄土记录认为末次冰期中存在两个显著湿润期(31~28ka B.P.和45~42ka B.P.)[30]；下蜀黄土研究记录了MIS 3阶段明显的特殊暖湿特征[29]；青藏高原、阿拉善高原、新疆等地的湖泊沉积物研究认为这一时期湖泊呈现高湖面特征[9~11]，但也有争议[49, 70]；南京和黔贵地区石笋δ 18 O的研究表明MIS 3时期气候偏暖偏湿且波动频繁[71, 72]；在我国渤海湾[12, 73]、长江三角洲[13, 14]、珠江三角洲[15, 16]、东南沿海[74]等地区的证据也记录到MIS 3时期海平面上升特征；同时，MIS 3时期较温湿气候在南极Vostok冰芯[75]和北极GRIP冰芯[76]中也有清楚记录。显然，这一时期较温湿气候尽管仍有争议，但整体是全球性的气候变化事件。TJW剖面马兰黄土中弱古土壤L1-2-S2的形成表明汉江流域MIS 3晚期存在气候较温湿的阶段，且这一特征在汉江流域其他地点(如：汉中盆地军王村(JWC)剖面1)和其他地点或指标(神农架三宝洞石笋[77])均有记录。这说明TJW黄土剖面记录的MIS 3时期气候较温湿事件是该地区对全球气候变化的具体响应。
1)杨 丹，庞奖励，黄春长等. 汉中盆地军王村黄土-古土壤剖面色度特征及机理研究. 中山大学学报(待发表)
已有研究表明MIS 3时期气候具有高度不稳定性和周期性波动特征[21~22, 50, 78, 79]，其在全球不同区域表现的气候变化幅度存在显著差异[80~82]。SPECMAP认为MIS 3时期的气候比MIS 2和MIS 4时期偏温湿，但其变化幅度较小，温湿程度尚不及末次间冰期和全新世时期[5](图 4g)；古里雅冰芯的研究认为青藏高原MIS 3晚期(40~30ka B.P.)的气候异常温暖湿润，已达到间冰期的程度[6, 7](图 4f)。将TJW剖面与其他黄土剖面对比，发现不同地点黄土剖面记录的MIS 3时期气候温湿变化幅度也存在一定差异(图 4)。渭南、洛川和宜川等地黄土剖面的复合古风化曲线可看出MIS 3时期气候发生了3次增温增湿的变化，但其温湿程度要弱于全新世时期[21](图 4b)；临夏塬堡剖面表明MIS 3时期气候变化幅度很弱，其温湿程度远远低于全新世时期[24](图 4c)；甜水沟剖面的研究发现MIS 3晚期的气候要明显优越于末次冰期MIS 2、MIS 4，但其温湿程度仍弱于末次间冰期和全新世时期[25]。汉江TJW剖面的磁化率在MIS 3晚期(27.26±1.44ka左右)发生显著升高，但其变化幅度低于全新世时期，暗示其气候温湿程度弱于全新世时期(图 4a)。显然，不同地点黄土记录的MIS 3时期气候温湿程度均弱于全新世时期。
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图 4 汉江上游TJW剖面马兰黄土OSL年龄数据 (a)汉江上游TJW黄土剖面；(b)渭南、洛川、宜川等黄土剖面复合古风化曲线[21]；(c)临夏塬堡黄土剖面[24]；(d)甘肃会宁黄土剖面[26]；(e)陕西金丁黄土剖面[27]；(f)古里雅冰芯[6]；(g)SPECMAP[5] Fig. 4 Climate records of MIS 3 period during the Last Glacial in different regions. (a)The loess profile at the TJW site in the upper Hanjaing River valley; (b)The stacked paleo-weathering curve of the loess profiles at Weinan, Luochuan and Yichuan sites[21]; (c)The loess profile at Yuanbao site in Linxia City[24]; (d)The loess profile at Huining site in Gansu Province[26]; (e)The loess profile at Jinding site in Shaanxi Province[27]; (f)Guliya ice core[6]; (g)SPECMAP[5] |
进一步对比发现，不同地点黄土记录的MIS 3时期气候冷暖变化并不完全相同。洛川等地黄土剖面的复合古风化曲线可看出MIS 3时期气候状况大概可以分为3次温湿和2次冷干阶段，但其晚期阶段的温湿程度要弱于早期[21](图 4b)；临夏塬堡剖面(图 4c)的研究表明这一阶段气候变化可划分为5个阶段，强温湿(56 1~42 2ka B.P.)—冷干(42 2~39 3ka B.P.)—弱温湿(39.3~33.1ka B P.)—冷干(33 1~31 0ka B.P.) —中等温湿(31 0~25 0ka B P.)，即MIS 3晚期气候温湿程度弱于早期阶段[24]；甘肃会宁剖面(图 4d)记录的MIS 3时期气候特征可分为3个阶段：弱温湿(55. 5~45. 9ka B.P.) —干冷(45.9~27.3ka B.P.)—温湿(27.3~23.8ka B P.)，其晚期气候的温湿程度要高于早期[26]；金丁剖面(图 4e)记录的MIS 3时期气候则经历了弱温湿(51.6~44.9ka B.P.)—冷干(44. 9~42. 0ka B.P.)—中等温湿(42. 0~39. 9ka B P.)—冷干(39.9~37.4ka B.P.)—强温湿(37. 4~27. 8ka B.P.)的5个时期，MIS 3晚期的气候要比其早期更温湿[27]。TJW剖面在对MIS 3早期的温湿气候记录并不显著，但记录到其晚期有明显的温湿变化。以上分析说明MIS 3时期气候变化在不同区域的表现存在一定差异性，具有明显的区域性特征。
6 结论(1) TJW剖面马兰黄土L1中228~260cm和294~370cm深度呈暗红棕、粘土粉砂质地、比较紧实坚硬的特征，与上下黄土L1明显不同，且其磁化率(136.9×10-8 m3/kg和161.3×10-8 m3/kg)、< 5μm粘粒含量(17.2 %和19.2 %)和红度a *值(5.6和5.7) 偏高，>50μm粗颗粒含量(35.7和25.9)、亮度L *(57.9和56.5) 和黄度b *值(17.5和17.0) 均偏低，具有明显成壤的特征，属于两层弱古土壤(L1-2-S1和L1-2-S2)。
(2) OSL测年数据显示，弱古土壤L1-2-S1和L1-2-S2的测年结果分别为21.59±1.20ka和27.26±1.44ka，结合样品位置及测年误差，认为弱古土壤的形成时期介于28.5~21.0ka B.P.，暗示汉江地区该期间发生两次气候较温湿事件，且有明显冷暖波动。其中弱古土壤L1-2-S2的形成时间(27.26±1.44ka)与MIS 3晚期阶段大致相当，说明MIS 3晚期气候较温湿变化特征在汉江流域亦有表现。各气候替代指标显示其成壤强度弱于古土壤S0，指示TJW剖面记录的MIS 3晚期气候温湿程度弱于中全新世大暖期。
(3) TJW剖面弱古土壤L1-2-S2记录的MIS 3晚期气候较温湿事件是汉江区域对全球气候变化的具体响应。对比研究发现，不同地区黄土剖面均记录到MIS 3时期气候增温增湿的变化特征，且其温湿程度均弱于全新世时期，但不同剖面记录的气候变化幅度及冷暖变化阶段存在明显差异。TJW剖面对MIS 3早期温湿记录并不显著，而其晚期阶段气候发生明显温湿变化。
致谢 非常感谢南京大学鹿化煜教授、编辑部杨美芳老师和匿名专家在百忙之中对完善本文提出的宝贵意见，在此深表谢意。
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National Demonstration Center for Experimental Geography Education(Shaanxi Normal University), Xi'an 710119)
Abstract
Marine Isotope Stage 3(MIS 3) is a relatively warm-wet period during the Last Glacial. This special climate change has also been recorded by different carriers(e.g. loess, stalagmites, lake sediments, ice cores)in different regions of China. But there are still some controversies surrounding the climate characteristics during MIS 3 recorded by Chinese loess profiles. The upper Hanjaing River valley located in the southern margin of Qinling Mountains, is more sensitive to the climate change. Recently, loess deposits were founded in the first to fourth river terraces. Several studies have shown that the loess profiles in the first terrace recorded a continuous climate evolution information since 55.0ka B.P. in the region. However, whether the climate change during MIS 3 has been recorded by the loess profiles in the upper Hanjiang River valley?Relevant researches have seldom been reported. In this paper, the Tuojiawan(TJW)profile(32°05'47″N, 110°22'51″E; 168m a.s.l.)at the first river terrace of the Yunxi reach of the upper Hanjiang River was studied in detail. This profile is 8m thick and consists of Holocene loess-paleosol(0~178cm)and Malan loess(178~818cm)of the Late Pleistocene. The stratigraphic sequence is quasi-complete, continuous, mostly undisturbed and well comparable with other loess profiles in the area, indicating that the TJW profile has also recorded a continuous climate change information since 55ka B.P. On the basis of optically stimulated luminescence(OSL)ages from 11 samples and the proxy indexes of magnetic susceptibility, grain-size and color variation from 347 samples in the profile, we can drawing the following results:(1) Two soil layers(228~260cm and 294~370cm)with dark reddish brown, clay-silt textures, dense and solid structure were found inserted in the Malan loess L1 layer during the field investigations. The experimental results show that their magnetic susceptibility(136.9×10-8 m3/kg and 161.3×10-8 m3/kg), clay( < 5μm) contents(17.2% and 19.2%), a* values(5.6 and 5.7) are higher and sand(>50μm) contents(35.7% and 25.9%), L* (57.9 and 56.5), b* values(17.5 and 17.0) are lower than the loess L1. These suggest that they were two weak paleosol layers(L1-2-S1 and L1-2-S2), which have experienced a certain degree of pedogenic modification. (2) Samples from the two weak paleosol layers(L1-2-S1 and L1-2-S2)were OSL dated to 21.59±1.20ka and 27.26±1.44ka, suggesting the formation period of the two weak paleosol layers can be confined into approximately 28.5~21.0ka B.P. The forming time of weak paleosol layer L1-2-S2(27.26±1.44ka)can approximately correspond to the late stages of MIS 3 period, suggesting that the warmer and wetter climate change during MIS 3 have also been reflected in the upper Hanjiang River valley. And the climate is not as warm and wet as that during the Mid-Holocene Climatic Optimum as indicated by weaker pedogenic intensity than the paleosol S0. (3) The relatively warm-wet and unstable climate during the late stages of MIS 3 period in the upper Hanjiang River valley is similar with the climate records from different carriers(e.g. loess, ice cores, stalagmites)in the world and considered to be regional responses to global climate change. Compared with the loess records in the Chinese Loess Plateau, the paleoclimate during MIS 3 recorded by different loess profiles are not as warm and wet as that of the Holocene. However, the rangeability and the warm-wet intervals of the paleoclimate during MIS 3 are significantly different in these loess profiles, presenting an obvious regional discrepancy. This paper plays an important role in better understanding the climate characteristics during MIS 3 in China.