2019, Vol.39
2 重庆市地质矿产勘查开发局607地质队, 重庆 400056)
东亚季风是全球大气环流的重要组成部分，揭示其变率特征及其时空差异对于认识亚洲内陆的自然和人类生存环境具有重要意义[1~3]。广泛分布在黄土高原的中国风成黄土-红粘土序列以其连续性好、沉积分辨率高和时间跨度长的特点被成功用于重建构造-轨道-千年尺度上的东亚季风历史和亚洲内陆干旱化过程[4~10]。与黄土高原所代表的东亚季风区相比，处于东亚季风区边缘地带的内蒙古中东部干旱-半干旱地区具有可靠年龄约束的较长时间尺度的地质记录则颇为匮乏[11~12]。以浑善达克沙地为例，虽然区域内湖泊和风成砂-古土壤沉积序列较为发育[13~14]，遗憾的是，这些地质记录大多在末次冰消期以来才逐渐形成[12, 15~18]，且不同沉积记录间所指示的全新世适宜期发生和持续时间存在差异[15~20]，这些都严重制约了对该地区冰期-间冰期尺度上古气候演化历史的准确和完整理解。最近，我们在野外地质调查中发现浑善达克沙地南缘的山间低洼地带保存了末次间冰期以来沉积连续的黄土-古土壤序列，这些风成黄土为深入理解我国北方季风边缘区在轨道尺度上的古季风演化过程及其动力学机制提供了良好素材。
风成黄土中磁性矿物的形成、搬运、沉积以及后期改造过程都受到气候变化所控制，而磁学参数可查明磁性矿物的种类、含量和粒径等信息，且磁参数的测试具有样品用量少、灵敏度高、方法简单快捷等优点，因而各种磁学参数及其组合被广泛用来提取黄土-古土壤序列中的古气候信息[21~26]。在黄土高原腹地，黄土沉积后的成壤过程会形成大量超顺磁(SP)和单畴(SD)亚铁磁性矿物从而导致磁性显著增强[23, 27~30]，因而磁化率(χ)、频率磁化率(χfd)、非磁滞剩磁(ARM)等主要反映与成壤相关的亚铁磁性颗粒(包括含量和粒度)变化的磁参数被广泛用于指示东亚夏季风强度[31~32]。随着研究深入，近年来发现这些磁参数在西部黄土高原和明显受西风环流影响区风成沉积的环境指示意义颇为复杂，在个别层位甚至整个沉积记录中的磁信号明显受风力强度的影响或控制[33~37]，即磁性增强是粗粒碎屑磁性矿物含量增加而非成壤过程的单一结果[38]。风力作用与成壤作用叠加的磁性增强过程会导致部分磁学参数的古气候意义表现出不确定性和多解性。考虑到黄土-古土壤序列的地球化学特征亦与古气候变化密切相关[39~40]，而且前人根据黄土中不同元素的地球化学行为成功论证了颗粒分选和化学风化对黄土地球化学特征的显著影响，并发现一些地球化学比值可指示东亚夏季风和东亚冬季风的强度[39~42]。为此，我们对浑善达克沙地南缘新发现的北沟黄土剖面开展了精细的岩石磁学、环境磁学、地球化学和漫反射光谱(DRS)分析，探讨了该区域黄土与黄土高原黄土磁学性质的异同及其深层次原因；根据光释光测年结果并通过提取出的冬季风和夏季风指标与深海氧同位素曲线对比建立了该剖面13万年来精细的年代学框架，进而揭示末次间冰期以来浑善达克沙地的古季风演化历史及其动力学机制。
1 采样与测试北沟剖面(42.40°N，115.71°E)位于内蒙古正蓝旗西北方向约30 km处(图 1)。该地区年平均降水为359 mm，其中夏季降水量最多，约占全年降水的85 %；年平均气温为2.5 ℃，7月平均温度19.4 ℃为全年最高，1月平均温度-17.2 ℃为全年最低(数据源自中国气象数据网)。该剖面总厚度为4.72 m(未见底)，包含了一个完整的黄土-古土壤旋回，上部黄土层和之下古土壤层的分界出现在深度2.8 m上(图 2)。本次研究对该剖面以4 cm等间距进行采样，共采集了118个散样。此外，在剖面新鲜面上采集了3个光释光测年样品，采样深度分别为0.3 m、1.57 m和2.53 m。
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图 1 北沟剖面位置示意图 红色和绿色星形符号分别代表北沟剖面和中国黄土代表性剖面，红色和蓝色箭头分别代表东亚夏季风和东亚冬季风的推进方向，虚线为年平均降雨量(mm)等值线；左下图蓝点和黑点分别表示内蒙古中东部主要湖泊和城市A~K代表中国主要沙漠和沙地：A，塔克拉玛干沙漠；B，古尔班通古特沙漠；C，库木塔格沙漠；D，柴达木沙漠；E，巴丹吉林沙漠；F，腾格里沙漠；G，毛乌素沙地；H，库布齐沙漠；I，浑善达克沙地；J，科尔沁沙地；K，呼伦贝尔沙地 Fig. 1 Schematic map showing the locations of the Beigou loess section(red star), typical loess sections(green stars), deserts and mean annual precipitation isohyets(mm, dashed lines)in China. The red and blue arrows indicate the East Asian summer monsoon(red)and the East Asian winter monsoon(blue)respectively. The inset map shows the locations of the Beigou section, representative lakes(blue dots)and cities(black dots)in middle-eastern Inner Mongolia. Deserts:A, Taklimakan; B, Gurbantunggut(Junggar); C, Kumtag; D, Qaidam; E, Badain Jaran; F, Tengger; G, Mu Us; H, Hobq; I, Hunshandake; J, Horqin; K, Hulun Buir |
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图 2 北沟剖面野外露头(a)和深度-年龄模式(b) Fig. 2 Photograph of the outcrop of the Beigou section (a) and the age-depth model (b) |
首先将野外采集的散样装入2 cm×2 cm×2 cm的无磁立方体塑料盒中，在中国地质科学院地质力学研究所自然资源部古地磁与古构造重建重点实验室由2 G 755-4K超导磁力仪依次完成了ARM和饱和等温剩磁(SIRM)的测试。ARM由2 G交变场退磁仪施加最大场为100 mT的交流场和0.05 mT的直流场来获得；SIRM由IM10-30型脉冲磁力仪对样品施加2 T的直流场来获得，并反向施加300 mT的直流场获得IRM-0.3 T，然后得到用来表征硬磁组分含量的磁参数HIRM和S-0.3，这里HIRM和S-0.3分别代表(SIRM+IRM-300 mT)/2和-IRM-0.3 T/SIRM。频率磁化率(χfd)和磁滞回线测试在中国地震局地球物理研究所岩石磁学实验室进行，其中低频、中频和高频磁化率测试由AGICO MFK 1-FB卡帕桥磁化率仪完成，测试频率分别为976 Hz、3904 Hz和15616 Hz，这里χfd由χ976Hz-χ15616Hz来表示。磁滞回线由MicroMag 3900振动磁力仪来完成。磁滞回线测试所加饱和直流场为1 T，在进行顺磁性校正后，获得饱和剩磁(Mrs)、饱和磁化强度(Ms)、矫顽力(Bc)和剩磁矫顽力(Bcr)值。
DRS分析在中国科学院地质与地球物理研究所古地磁与年代学实验室由Cary 5000 UV-vis-IR分光光度计来完成，将得到的反射率值转换为Kubelka-Munk(K-M)函数，计算K-M函数的二阶导数，在二阶导数图谱中，针铁矿和赤铁矿分别在425 nm和535 nm波长附近有一个特征峰，进而根据特征峰的数值运用公式Y=-0.133 + 2.817X-1.709X2和Fed=GI425 + HI535，定量计算赤铁矿和针铁矿的含量[43]，其中I425为425 nm波长附近特征峰振幅，I535为535 nm波长附近特征峰振幅，X为I535/(I425 + I535)，Y为Hm/(Hm + Gt)，G和H是回归系数，分别为8.56×103和22.2×103，Fed可视作赤铁矿与针铁矿的总和。地球化学主、微量元素分析在中国地质科学院国家地质实验测试中心由PW4400 X射线荧光光谱仪和ICP-MS-PE300D等离子质谱仪来完成。光释光年龄测试在北京光释光实验室科技有限公司采用丹麦Ris公司生产的TL/OSL-DA-20释光测年仪完成，辐照源为(90 Sr/90 Y)β源，激发光源选择强度为90 %的蓝光发光二极管(λ=470±20 nm)。
2 结果与讨论 2.1 北沟剖面磁学参数的古气候意义在剖面顶部至下0.3 m、1.57 m和2.53 m处采集的3个光释光样品年龄分别为16.15±0.95 ka、41.37±2.69 ka和49.99±2.40 ka。结合野外土壤学观察(图 2)、光释光测年以及与黄土高原上典型剖面的磁化率对比结果(图 3)，可判定该剖面地表出露的黄土-古土壤序列的沉积历史可追溯至末次间冰期，即上部出露的粒度偏粗的黄土层对应L1黄土层，而剖面下部的红色古土壤层为S1古土壤层。该剖面末次间冰期以来形成的黄土-古土壤不仅在沉积厚度上远低于黄土高原腹地的同期沉积[10, 44~46](图 3)，同时其磁化率值也整体上明显低于黄土高原南部和中部的典型剖面(如渭南和洛川[44])，而略大于(如九州台[45])或相当于六盘山以西的陇西黄土(如白草塬[46])(图 3)。在末次间冰期，该剖面记录的3个磁化率高值分别对应3个古土壤亚层(S1-1、S1-2和S1-3)，其他指示成壤强度的磁参数(如χfd、ARM及其比值χARM/χ)在S1中亦呈完全一致的变化特征(图 4)，以上特征与黄土高原典型剖面[44~46]相符合。由于χfd主要反映成壤形成的SP和SD临界区间内的亚磁性矿物含量[47~48]，而ARM对细粒SD磁性颗粒反应灵敏，二者均对成壤作用十分敏感，高/低值反映较强/弱的成壤作用[49~50]。这表明了内蒙古中东部地区的古土壤虽然成壤作用较弱，但其磁性特征总体上受成壤强度所控制，即较小/较大值代表成壤强度较弱/强的层位[29]。S-0.3用于判断高、低矫顽力磁性颗粒的相对含量[51~52]，S-0.3趋近于1表明低矫顽力磁性矿物占主导，而S-0.3值减小表明高矫顽力磁性矿物的贡献在增加[53]。对于磁组分参数S-0.3来说，洛川和渭南段家坡等典型剖面S1的S-0.3值均大于0.85[54~55]，而北沟剖面该古土壤的S-0.3值变化范围在0.77与0.85之间(图 4)，表明高矫顽力磁性矿物相对含量较高。中国黄土中高矫顽力矿物主要为赤铁矿和针铁矿[56]，而HIRM值[51]和DRS分析可以定量化以上反铁磁性矿物的含量。HIRM值和DRS分析得出的北沟剖面赤铁矿含量均在S1中较高，而在L1中相对较低(图 4)，证实了与成壤作用相关的反铁磁性矿物的形成和含量的增加。
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图 3 末次间冰期以来我国典型黄土剖面[44~46]与北沟剖面磁化率对比 灰色条带分别代表指示S1-1、S1-2、S1-3三个古土壤亚层；图中白草塬、九州台和洛川剖面磁化率单位虽为国际单位(SI)，但通常采用的磁化率测试方法是选取10 g样品进行磁化率测试，这样得出的质量磁化率χ值即为我们引用的国际单位制(SI)下的数值，而且单位为10-8 m3/kg，磁化率(χ)的换算公式为χ=10*κ/样品质量(m)(单位：10-8 m3/kg)，κ为国际单位制下未校正的磁化率 Fig. 3 Comparisons of magnetic susceptibility records between Beigou and other typical Chinese loess sections[44~46]. The gray bars denote three sub-paleosol layers S1-1, S1-2and S1-3, respectively. Although the International System of Units(SI)are used in susceptibility records of Baicaoyuan, Jiuzhoutai and Luochuan sections, the measurements of magnetic susceptibility are generally to select 10 grams of samples, thus the mass susceptibility is the value under the international system of units we quoted, and the unit is 10-8 m3/kg. The conversion formula of magnetic susceptibility is χ=10×κ/m(10-8 m3/kg), κ and m in the formula indicate the uncorrected susceptibility in International System of Units and quality of sample, respectively |
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图 4 北沟剖面岩性划分、主要磁参数及基于漫反射光谱分析的赤铁矿含量随深度变化 Fig. 4 Stratigraphic variations of selected magnetic parameters of the Beigou section |
前人研究表明，由于Rb和Sr在成壤过程中分别相对富集和相对亏损，因而黄土沉积中的Rb/Sr常被用作东亚夏季风强度指标[42, 57~59]。而来自西北内陆干旱区粉尘在搬运过程中，Zr和Rb分别相对富集在粗颗粒和细颗粒中，且二者在风化和成壤过程中不易发生迁移，因而Zr/Rb常被作为东亚冬季风强度指标[60]。如图 5所示，S1样品的χ与χfd、χ与ARM以及Rb/Sr与ARM均呈显著的正相关关系(图 5a、5c和5d)(χ和χfd的相关系数R2=0.93)，χ与SIRM具有较明显的正相关关系(RS2=0.67)(图 5e)，而SIRM与Zr/Rb呈较弱的负相关关系(RS2=0.12)(图 5f)。所有样品的ARM与χfd都具有显著的正相关关系，相关系数R2=0.91(图 5b)。可见，在末次间冰期χ、χfd、ARM以及SIRM都对成壤过程中生成的亚铁磁性矿物反应敏感，即可以指示东亚夏季风变化。总的来说，末次间冰期形成的古土壤的磁学特征符合中国黄土高原磁性增强模式[21, 28~30]，即末次间冰期的磁信号主要受控于成壤作用生成的磁性颗粒。
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图 5 代表性磁参数及其与地球化学参数线性关系图 (a)χ-χfd散点图(χ0是χ轴的截距值)，(b)ARM-χfd散点图，(c)χ-ARM散点图，(d)Rb/Sr-ARM散点图，(e)χ-SIRM散点图，(f)Zr/Rb-SIRM散点图蓝点和红点分别代表黄土样品和古土壤样品，RL和RS分别代表黄土和古土壤样品的相关系数 Fig. 5 Selected interparametric scatter plots. (a)χ versus χfd(χ0 is intercept χ); (b)ARM versus χfd; (c)χ versus ARM; (d)Rb/Sr versus ARM; (e)χ versus SIRM; (f)Zr/Rb versus SIRM. Loess and paleosol samples are indicated by blue and red cycles, respectively. RL and RS indicate the regression coefficients of loess and paleosol samples, respectively |
如图 3所示，在末次冰期的马兰黄土L1堆积期，黄土高原腹地的洛川和渭南等典型剖面[44]普遍存在一个对应于MIS 3的弱古土壤层L1SS1。由于这一弱古土壤层磁化率相对较高，因而可以通过其磁化率与深海氧同位素曲线[61]的对比来建立末次冰期中国黄土的年代模式。而在北沟剖面的L1地层中，用来指示成壤强度的χ、χfd和ARM的整体变化幅度较小，无法通过磁参数与MIS曲线的直接对比来获得L1的年龄框架。因而，本文利用对东亚夏季风变化响应灵敏的地球化学指标Rb/Sr与MIS曲线的精细对比，来建立该段黄土-古土壤序列13万年来精细的年代模式(图 6)。
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图 6 北沟剖面代表性磁参数和地球化学参数及其与深海氧同位素记录对比 (a)橘色部分为S1古土壤，灰色条带代表末次冰期时冬季风显著增强时段，蓝色和红色箭头分别代表冬季风和夏季风变化趋势；(b)虚线为北沟黄土的χARM/χ和Rb/Sr在深度上的变化及其与深海氧同位素记录对比 Fig. 6 Stratigraphic variations of typical magnetic and geochemical parameters of the Beigou section(a), and comparisons (b) of χARM/χ and Rb/Sr with MIS record. (a)The orange band represents S1 paleosol, and the gray bars denote the intervals of relatively strong East Asian winter monsoon; (b)The blue and red arrows indicate the trends of winter monsoon strengthening and summer monsoon weakening, respectively |
北沟剖面的χ、ARM及其比值χARM/χ在剖面上的总体变化趋势与我国典型黄土[22, 25~26, 62]基本一致的同时，SIRM、Ms和S-0.3的变化则比较复杂。如图 3所示，L1的S-0.3值整体上大于S1，而SIRM和Ms基本上同步变化并在S1/L1界线以上显著增强且呈大幅波动的特点，这些特征与黄土高原古土壤层比相邻黄土层具有更大的SIRM、Ms和S-0.3值的这一特点不一致[22, 25~26, 29]。从野外土壤学观察来看，该剖面L1的成壤强度明显弱于黄土高原同时期形成的马兰黄土[63]，而L1整体上低的χ和ARM特征也表明在L1堆积期与成壤相关的SP/SD亚铁磁性矿物对SIRM的贡献应明显弱于S1。因而，L1的SIRM和Ms显著增强和整体高于S1的S-0.3值特征显然不是成壤作用所致。前人对微米级磁铁矿的磁学性质研究表明，粒径处于SP/SD临界区间之上的细粒SD磁铁矿和粒度明显偏粗的假单畴(PSD)-多畴(MD)磁铁矿对SIRM的贡献最大[64]。在Day图[65]上，L1样品中的磁性颗粒接近于MD区间，而S1样品更接近于PSD区间，二者的粒径区别明显(图 7)。重要的是，L1样品的SIRM与冬季风指标Zr/Rb呈明显的正相关关系(图 5f)，并且从末次间冰期-末次冰期过渡期开始，SIRM、Ms与Zr/Rb表现出同步变化特征(图 6)。以上证据均表明，该剖面L1样品的SIRM和Ms磁参数总体上反映了处于季风边缘区的浑善达克沙地在末次冰期粉尘携带的碎屑成因磁粒度的显著变粗和冬季风的显著增强的特点。
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图 7 北沟剖面L1黄土(蓝色)和S1古土壤(红色)样品的Day氏图 Fig. 7 Day plot of L1(blue)and S1(red) samples of Beigou section |
值得注意的是，L1样品的SIRM与χ亦呈较好的正相关关系(图 5e)，而且χ与Zr/Rb及Ms呈一定的正相关(图 6)，表明χ明显受到碎屑成因的粗粒亚铁磁性矿物的影响。由于χ与SIRM类似，不仅对成壤作用形成的SP/SD转换区间附近的亚铁磁性颗粒反应灵敏，同时当碎屑来源的MD磁性颗粒占主导时，其对χ的贡献会显著增加[64]。与χ、SIRM不同，χfd与ARM分别反映SP/SD转换区间和粒度偏细的SD亚铁磁性矿物的贡献[29, 49~50]。因而，当黄土的磁学性质由成壤作用占主导时，χ、χfd和ARM会呈同步变化特征[22]。而北沟剖面L1的χ与χfd以及χ与ARM之间的相关性都较差，且χ与χfd、ARM相比在该层位上变化幅度较大(图 5a和5c)，表明在末次冰期夏季风显著减弱条件下，χ主要反映了与冬季风相关的粗粒亚铁磁性矿物输入通量的变化，即部分服从wind vigor模式[38]。此外，与S1相比，L1中指示成壤强度的ARM与Rb/Sr的正相关性显著减弱，在Rb/Sr值(介于0.28~0.45之间)呈一定幅度变化情况下，ARM值基本保持在0.04×10-3 Am2/kg附近(图 5d)。这也可能说明，Rb/Sr和Ba/Sr等地球化学参数与用来指示成壤强度的磁参数(χfd和ARM等)相比，在表征季风边缘区东亚夏季风演化上具有一定的优越性(图 6)。
前人研究表明，降水和温度是控制土壤磁学性质的两个基本气候因素[66~70]。与气候冷湿地区相比，有效降雨量低且气候温暖地区形成的土壤中赤铁矿含量相对较高[71]。位于浑善达克沙地南缘北沟剖面处在400 mm等降水线以外的半干旱-干旱气候区(图 1)，尽管该地区的夏季温度与黄土高原相当，但年均降雨量(365 mm/a)明显低于处于黄土高原腹地的洛川(592 mm/a)(图 8)。在更干旱的环境下，浑善达克沙地一带的潜在蒸发量超过降水量，即该地区的有效降雨量远低于黄土高原腹地，而更干旱的环境下可能有利于赤铁矿的形成。最近对黄土高原不同地区现代土壤样品的磁学研究也表明，黄土高原土壤中赤铁矿生成的主控气候要素为年均温度，而亚铁磁性矿物的生成主要受控于年均降水量的变化[72]。与黄土高原相比，北沟剖面S1中反铁磁性矿物(赤铁矿和针铁矿)的含量更高，可能反映了在季风边缘区有效降雨量较低条件下的成壤过程更有利于弱磁性矿物的形成和保存。
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图 8 北沟剖面(a)与洛川剖面(b)的温度和降水对比(数据源自中国气象数据网) Fig. 8 Variations of precipitation and temperature at Beigou (a) and Luochuan (b)(the data derived from http://data.cma.cn) |
如前所述，SIRM、Ms和冬季风指标Zr/Rb从末次间冰期-末次冰期过渡期开始呈同步变化并逐渐增强，表明冬季风开始增强(图 6)。与此同时，夏季风指标Rb/Sr和χARM/χ则在这一气候过渡期表现出逐渐减小的趋势，即夏季风的逐渐减弱。以上参数的变化明确指示了在末次间冰期向末次冰期转换过程中东亚冬季风的阶段性增强而夏季风逐渐减弱的特点，这与目前中国黄土研究中利用东亚冬季风指标(如粒度)和夏季风指标(如磁化率)在冰期-间冰期尺度上的反相关所揭示的冬季风与夏季风在轨道尺度上的反相位变化特点相一致[73~74]。前人研究中常用的东亚冬季风指标(如黄土粒度)具有较明显的41 ka和100 ka周期，暗示了全球冰量变化对冬季风具有很大影响[73~74]；而东亚夏季风指标(如石笋δ18O)则具有明显的23 ka周期以及23 ka、19 ka与41 ka周期外差产生的其他周期(如35.4 ka、52.4 ka)[75]，表明示夏季风主要受太阳辐射变化控制。东亚冬季风和夏季风在23 ka与41 ka周期相位上变化特征并不相同，可能导致了冬、夏季风在气候过渡期具有不同方式的反相位变化。
基于北沟剖面夏季风指标Rb/Sr和χARM/χ与深海氧同位素曲线对比并结合光释光测年结果，我们给出了该剖面精细的年龄模式(图 2和图 6)。按照这一年龄模式，将末次冰期以来该剖面的Rb/Sr和Ba/Sr曲线与西部黄土高原的夏季降雨历史、粒径记录、中国葫芦洞石笋δ18O记录、格陵兰冰芯δ18O记录以及北半球夏季太阳辐射变化等区域和全球古气候记录进行了对比[76~79](图 9)。由于Ba/Sr在黄土沉积成壤过程中与Rb/Sr具有类似的元素亏损/富集行为，二者均可用来指示风化成壤作用的强度并可作为夏季风强度替代性指标[80]。如图 9所示，Rb/Sr和Ba/Sr的高值时段对应于夏季风增强时期，而低值时段则反映夏季风减弱时期，二者与北半球夏季太阳辐射呈相似的变化趋势，证实了该地区的夏季风至少在亚轨道尺度上响应北半球太阳辐射变化[81]。前人对全球末次冰期古气候记录的研究显示，Dansgaard-Oeschger(D-O)旋回和Heinrich事件等千年尺度古气候波动广泛存在于北半球高分辨率的格陵兰冰芯、石笋和海洋沉积中[76, 82~83]。在具有高沉积分辨率的西部黄土高原的黄土剖面上也记录了较完整的D-O旋回和Heinrich事件等短尺度全球气候事件[6, 78, 84]。如图 9所示，北沟剖面中的数个Ra/Sr、Ba/Sr高值段可分别对应MIS3阶段、末次冰消期和其他几个较短尺度暖期。不可否认的是，在末次冰期，北沟剖面黄土的沉积速率(约4.6 cm/ka)远低于西部黄土高原典型剖面(如北郭塬40.6±7.5 cm/ka)[85]，因而该剖面只记录了北半球上气候波动较剧烈的短尺度气候事件，而并没有像西部黄土高原典型剖面那样记录了较完整的D-O旋回。尽管如此，Ra/Sr和Ba/Sr曲线还是呈现出数个低值时段，对应了其他古气候记录中的Heinrich事件(图 9)。总的来说，北沟黄土虽然沉积分辨率较低，但仍表现出记录亚轨道尺度上的夏季风变化特征和个别短尺度干冷事件的初步潜力。
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图 9 末次冰期时北沟黄土剖面的Rb/Sr、Ba/Sr与区域和全球古气候记录的对比 红色和蓝色条带分别指示亚轨道尺度上的暖湿事件和Heinrich事件：(a)Rb/Sr比，(b)Ba/Sr比，(c)深海氧同位素记录[61]，(d)格陵兰冰芯氧同位素记录[76]，(e)葫芦洞石笋氧同位素记录[77]，(f)基于西部黄土高原塬堡黄土剖面的有机碳同位素重建的夏季降雨量变化[78]，(g)塬堡剖面粒径变化[78]，(h)北纬65°夏季太阳辐射[79] Fig. 9 Comparisons of Rb/Sr (a) and Ba/Sr (b) at Beigou with MIS record[61](c), NGRIP δ18O record from Greenland[76](d), Hulu stalagmite δ18O record[77](e), reconstructed summer precipitation variation[78](f)and grain size record[78](g)from Yuanbao section in western Chinese loess plateau, and Northern Hemisphere solar insolation in July[79](h)during the last interglacial. Red bars indicate the sub-orbital warm events, and blue bars denote the Heinrich-like events identified in these records |
本文对浑善达克沙地南缘新近发现的末次间冰期以来形成的厚度为4.72 m的北沟黄土剖面地层开展了光释光测年和精细的环境磁学、地球化学和漫反射光谱分析。岩石地层、磁化率和光释光测年结果表明，该剖面记录了末次间冰期以来完整的风成堆积历史，并通过东亚夏季风指标(Rb/Sr及χARM/χ)与深海氧同位素曲线对比建立了该剖面13万年来黄土-古土壤序列精细的年龄模式。高分辨率的岩石磁学结果显示：在末次间冰期形成的古土壤层S1由3个亚层(S1-1、S1-2和S1-3)组成，同时常规的用来指示成壤强度的磁参数(如χ、χfd和ARM)在该段地层中表现出与中国黄土高原一致的磁性增强模式；而在马兰黄土L1堆积期，以上被用来指示东亚夏季风强度的磁参数(如χfd、ARM及χARM/χ)整体上数值低且变化幅度小，符合末次冰期季风边缘区受夏季风影响显著减弱的特点。在该剖面的以上磁学参数与我国典型黄土的总体表现特征较一致的同时，SIRM和S-0.3的变化趋势则不尽相同，其中磁组分参数S-0.3在古土壤S1中相对较小，表明成壤时期形成的高矫顽力磁性矿物相对含量较高，可能反映了在季风边缘区有效降雨量较低条件下成壤过程更有利于弱磁性矿物(赤铁矿和针铁矿)的形成和保存；而在L1堆积期，S-0.3整体上数值较大，磁粒度显著变粗达PSD-MD粒级，而SIRM表现出显著增强和大幅度波动的特点，并与指示冬季风强度的地球化学指标(Zr/Rb)呈完全正相关关系，表明末次冰期的冬季风强度控制了碎屑亚铁磁性矿物所携带的磁信号。此外，从末次间冰期向末次冰期过渡期开始，χ与Zr/Rb亦呈较一致的变化，表明该时期χ主要受控于冬季风所携带的粗粒亚铁磁性颗粒的通量变化。可见，在黄土高原上常用来表征成壤强度和夏季风强度的磁浓度参数(χ和SIRM)并不完全适用于干旱-半干旱地区(400 mm等降水线附近)的北沟黄土。与磁参数相比，通常使用的东亚夏季风指标(Rb/Sr和Ba/Sr)和冬季风指标(Zr/Rb)的古气候指示意义更为确切，可分别准确反映马兰黄土堆积时期极微弱的成土作用和冬季风演化过程。总的来看，在末次间冰期东亚夏季风可以延伸到内蒙古中东部，这与现今气象观测相一致；而在末次冰期，北沟剖面复杂的磁信号则可能是对季风边缘区显著减弱的夏季风的阈值响应。通过磁参数与地化参数的综合对比研究发现，在末次间冰期-末次冰期过渡期，冬季风阶段性增强的同时夏季风逐渐减弱，这与目前冬季风与夏季风反相位变化的观点相一致。此外，在末次冰期北沟黄土Rb/Sr和Ba/Sr与其他区域黄土、石笋、冰芯中的古气候记录具有较好的一致性，表明北沟黄土虽然沉积分辨率较低，但仍表现出记录亚轨道尺度上夏季风快速变化和千年尺度干冷事件的潜力。
致谢: 真诚地感谢审稿专家以及编辑部老师建设性的修改意见，使文章得以完善。
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2 Chongqing Bureau of Geology and Mineral 607 Geological Team, Chongqing 400056)
Abstract
Because of spatial scarcity of reliable sedimentary archives, paleoclimatic history in eastern Inner Mongolia since the last interglacial remains unclear. Although extensive lakes, eolian sands, and soils within and surrounding the Hunshandake Sandy Land(HSL), located in the current northern margin of the East Asian summer monsoon(EASM), provide ideal paleoarchives for retrieving paleoenvironmental variations in the arid and semi-arid eastern Inner Mongolia, these records are commonly confined to the last deglaciation, hampering a complete understanding of the exact climatic fluctuations on glacial-interglacial time scales. Recently, we found a complete Late Pleistocene loess-paleosol sequence with a total thickness of 4.72 m in southern extremity of the HSL. This unique record provides an excellent opportunity to reconstruct regional climatic variations in the monsoonal margin. We therefore carried out detailed rock magnetic, geochemical and diffuse reflectance spectrometer analyses of this section.The 4.72 m-thick Beigou section(42.40°N, 115.71°E) was sampled at 4 cm intervals, and a total of 118 bulk samples were collected. The mass-specific magnetic susceptibility (χ) was measured using an AGICO MFK 1-FB Kappabridge at frequencies of 976 Hz, 3904 Hz, and 15616 Hz, respectively. Both anhysteretic remanent magnetization(ARM) and saturation isothermal remanent magnetization(SIRM) were measured using a 2G 755-4K cryogenic magnetometer. DRS were performed by a Cary 5000 UV-vis-IR spectrophotometer. Concentrations of major and trace elements were determined by PW4400 X-ray Fluorescence spectrometer and ICP-MS-PE300D, respectively.We constructed the depth-age model based on linear interpolation of three OSL dates and detailed correlations between the variations of our Rb/Sr ratio, a faithful proxy of EASM strength, with the LR04 benthic δ18O stack. This compelling land-ocean resemblance suggests that the sedimentation of the Beigou loess commenced since the last interglacial. During the last interglacial, our magnetic record clearly defines three sub-paleosol layers of S1-1, S1-2 and S1-3 with enhanced pedogenesis corresponding to MIS 5a, MIS 5c and MIS 5e, and all concentration dependent magnetic parameters(e.g., χ, ARM and SIRM) exhibit concordant variations with higher values reflecting stronger pedogenesis, similar with those of typical loess in the hinterland of the Chinese Loess Plateau(CLP). Contrary to higher S-0.3 values of paleosol on the CLP, a case of lower S-0.3 values in paleosol is found here, suggesting that higher proportions of pedogenesis-derived high-coercivity magnetic minerals are formed during the interglacial. During the last glacial, the commonly-used grain-size dependent magnetic parameters(e.g., χfd, ARM and χARM/χ), which have been successfully adopted to denote EASM variations, show muted variations that are less correlative with loess records from the CLP, reflecting the much weaker influence of EASM on the monsoon-marginal region. By contrast, Rb/Sr and Ba/Sr records exhibit broad comparability with summer precipitation variations and the grain size record from the western CLP, Chinese speleothem δ18O record, the ice-core δ18O record in Greenland, and the northern hemisphere summer insolation, demonstrating that eolian deposits in the southern HSL have the good potential in recording sub-orbital EASM variations as well as several millennial-scale cooling events, regardless of limited sedimentation rates. Interestingly, during the last glacial, both SIRM and Saturation magnetization(Ms) display high-amplitude variations with higher values, and are positively correlative with the variations of Zr/Rb ratio, a robust geochemical indictor of East Asian winter monsoon(EAWM), reflecting a strong control of winter vigor on magnetic signals dominated by lithological ferrimagnetic minerals. The consistently high S-0.3 values during the last glacial also suggest the general dominance of coarse-grained ferrimagnetic minerals. We also found that the climatic conditions during the last interglacial-glacial transition are characterized by gradual retreat of EASM but stepwise propagation of EAWM, indicating an anti-phased pattern of EASM and EAWM during this climatic transition.In conclusion, our combined mineral magnetic and geochemical results demonstrate that during the last interglacial the front of the EASM can extend to middle-eastern Inner Mongolia, consistent with modern observations of climate change. A negative correlation between SIRMs and ZR/Rb during the last interglacial but a positive correlation during last glacial suggest that SIRMs are controlled by pedogenesis during interglacial and by wind vigor during glacial, respectively. We attribute this complex magnetic record to a threshold response to the East Asian monsoon variations in eastern Inner Mongolia.