2019, Vol.39
2 中国科学院大学, 北京 100049;
3 第四纪科学与全球变化卓越创新中心, 中国科学院地球环境研究所, 陕西 西安 710061)
中国黄土高原沉积了东亚季风区最为典型的黄土，其沉积厚度最大，地层最全。黄土-古土壤序列的代用指标完整记录了第四纪以来在冰期-间冰期尺度上东亚冬、夏季风优势期相互交替的演化[1~3]。由于黄土高原西北部地处气候、植被、地貌等的过渡地带，靠近沙漠物源区，沉积速率大，成壤作用弱，能够较好记录沙漠进退及风尘沉积的快速变化[4~5]；相反，沉积速率总体较低、沉积后期成壤和表层混合作用强的黄土高原中部不利于保留季风快速变化印记[6~7]。黄土高原西部沉积了厚达300~400余米的高分辨率黄土-古土壤序列，沉积速率比黄土高原中东部地区高约3~4倍[8]，代表性剖面有兰州、靖远、西宁、武威和临夏。塬堡剖面位于临夏盆地大夏河的第四级阶地之上，黄土-古土壤沉积序列出露厚度在150 m以上，其中S1以来地层厚度约39.5 m，S1古土壤由稳定的3个主要古土壤层组成，厚度可达8.5 m，可视为典型黄土剖面之一[9~10]。
中国黄土揭示的季风快速变化始于20世纪末，Porter和An[11]利用洛川黄土剖面石英粗颗粒含量变化，首次揭示出末次冰期以来的冬季风多次加强事件与北大西洋变冷事件的动力遥相关。随后，渭南、洛川、西峰、姬塬、靖远、临夏和古浪等地黄土剖面的高分辨率代用指标变化表明北大西洋H事件、Dansgaard-Oeschger(DO)旋回、新仙女木(Younger Dryas)事件等同样蕴含于黄土之中[6, 12~22]，类似的季风快速波动也存在于更老的冰期旋回中[21~26]。近年来，对黄土高原西北地区钻孔和剖面开展了一系列的高分辨多代用指标的研究，包括粒度、磁学、碳酸盐含量、同位素和土壤颜色等，揭示出西部黄土沉积能反演千年尺度季风快速变化[13, 15, 17, 25~29]。例如，沉积速率高、风化成壤弱的靖远和临夏剖面的同位素地球化学研究结果表明，黄土中无机碳酸盐和有机质的碳同位素变化与降水影响的植被发育关系密切，可敏感地揭示夏季风强度变化[6, 27]；古浪剖面的高分辨率元素比值Ca/K、Fe/K以及Si/K记录了东亚冬、夏季风千年尺度上的快速变化特征[23]；通过5个黄土剖面粒度结果的集成，Yang和Ding[25]详细分析了最后两个冰期旋回冬季风的快速变化特征，并与石笋记录对比，探讨了冬、夏季风快速变化的关联。但是，针对同一地质记录提取冬、夏季风代用指标进行对比的工作尚不多见，制约了对季风快速变化特征和机理的理解。因此，本文选取位于黄土高原西北部的临夏钻孔(85 m)，通过分析高分辨率的多代用指标的研究，探讨最后两个冰期旋回东亚冬、夏季风在亚轨道尺度上的变化特征。
2 材料和方法甘肃临夏盆地地处青藏高原与黄土高原过渡地带，地势由西南向东北逐渐降低，盆地呈倾斜状态。在地质构造上临夏盆地属于青藏高原东北部边缘陇中盆地西南隅的一个次级盆地[30]。大部分地区属温带半干旱气候，西南部山区高寒阴湿，东北部干旱，中部的河谷和平川温暖湿润。年均气温6.3 ℃，变化范围为-27.8~32.5 ℃；年均降雨量537 mm，蒸发量1198~1745 mm。2017年7~8月在甘肃临夏实施了黄土钻探和探井取样，钻孔(35.63°N，103.12°E)位于甘肃省临夏市坡头乡寨子村(图 1)，塬面平均海拔约2200 m。分别获取了A孔(LX-A)203.8 m、B孔(LX-B)72 m和上部探井7 m，其中A孔打穿了4级阶地，底部为1.8 m左右的砂砾石层。
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图 1 临夏钻孔剖面位置图 Fig. 1 Map showing locations of Linxia loess core on the Chinese Loess Plateau |
把黄土岩芯从中间切开，用第四代Avaatech高分辨率的XRF岩芯扫描仪进行1 cm分辨率的元素扫描测试。测试时需将样品固定于扫描仪内，确保样品平面保持水平。在上机前，用工具先把岩芯样品表面处理平整光滑，表面覆上一层超薄薄膜，防止分析过程中样品污染检测器影响测试结果。仪器的测试电压50 kv，电流为0.07 mA，计数时长为20 s，测试无效计数时间(dead time)在30 %左右，计数误差 < 5%。50kv电压下测试有效元素为Rb、Sr、Zr和Ba，计数单位cps。对3个钻孔岩芯分别以2 cm分辨率进行粉末样采集，在室内用英制Bartington MS2磁化率仪进行磁化率测量，用英国马尔文(MALVERN)公司生产的Mastersizer 2000激光粒度仪进行粒度测试，所有测试均在中国科学院地球环境研究所技术服务中心岩芯理化测试分析平台完成。
研究过去气候变化的前提和关键是为建立较为可靠的时间标尺，广泛应用的中国黄土时间标尺方法包括以下4种：粒度年代模式[14, 31]、海陆对比法[32]、天文轨道调谐法[33]和磁化率模式[34]等。黄土的粒度[3, 11~14]和磁化率[12, 15, 17~18]通常被认为是可靠的反映冬、夏季风变化的代用指标，由于磁化率只能记录冰期-间冰期尺度上的快速变化，还需寻找敏感响应于千年尺度上夏季风季风快速变化的代用指标。本文先将两个钻孔(其中A孔为85 m)与探井的粒度和磁化率变化的深度统一，重建S2时期以来的古气候变化(图 2)。将粒度和磁化率变化曲线与深海氧同位素阶段对比[35]，表明古土壤层S0、S1和S2分别对应深海氧同位素阶段(MIS)1、5和7。
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图 2 临夏A、B孔以及探井的磁化率、粒度随深度变化曲线(b)及其深海氧同位素(MIS)记录(a)[35]对比 绿色曲线为探井，橙黄色为临夏 Ａ 孔，蓝色为临夏 Ｂ 孔；红点为塬堡剖面光释光测年数据[37] Fig. 2 Variations(b) of magnetic susceptibility and mean grain size of Linxia loess core against depth and their correlation to oxygen isotope record(a) of LR04[35].The green, orange and blue line represent trial pit, LX-A and LX-B core respectively. The red dots are the OSL dating data of Yuanbao profile[37] |
Yang和Ding等[25, 36]通过整合和对比黄土高原西北部多个高分辨率黄土剖面粒度变化曲线发现，最后两个冰期旋回以粒度变化记录的显著变冷千年尺度的气候波动信号与石笋记录有很好的对应关系，大幅度的冬季风增强在最后两个冰期旋回多出现在最冷干的时期。因此，本文以粒度、磁化率分别作为东亚冬、夏季风强度的代用指标，基于海陆古气候记录在轨道时间尺度上的良好可比性，选择深海氧同位素阶段的终止点年龄作为控制点[35]。同时参考临夏塬堡剖面光释光测年数据，在顶部选取1个年龄控制点(0.44±0.06 ka)[37~38](图 2)，对控制点间年龄应用粒度-年代模式线性内插，从而获得了临夏钻孔26万年以来的多代用指标变化的时间序列(图 2和3)。
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图 3 临夏钻孔磁化率(d)、粒度(f)、Rb/Sr (c)及Zr/Rb (e)比值变化及其与北纬65°夏季太阳辐射(a)[48]、三宝-葫芦洞石笋(b)[49~50]、北大西洋OPD983浮冰碎屑记录(g)[51]和深海氧同位素(h)[35]记录对比 (b)和(f)曲线标注数字代表其记录相应的D-O旋回(暖事件)；(g)曲线标注的H1~H7以及6.1~8.3代表(类)H事件(冷事件)，灰色条带代表不同代用指标中北大西洋浮冰碎屑事件的对比；(h)曲线标注的1~7e代表不同时期的深海氧同位素阶段(MIS) Fig. 3 Variations of magnetic susceptibility (d), mean grain size (f), Rb/Sr(c) and Zr/Rb(e) ratios of Linxia core and their comparison with summer solar insolation at 65°N(a)[48], speleothem δ 18 O of Sanbao-Hulu caves (b)[49~50], IRD records of ODP983 (g)[51] and stacked benthic δ 18 O record (h)[35].The numbers of mean grain size of Linxia loess core and speleothem δ 18 O of Sanbao-Hulu caves represent D-O cycles(warm events). The numbers of tagged H1~H7 and 6.1~8.3 of ODP983 curve are Henrich or Henrich-like events, and grey bars indicate the correlation of IRD signals in different proxy records. The numbers of 1~7e of stacked benthic δ 18 O record represent different periods of MIS |
高分辨率多代用指标的研究表明，碳同位素[22~23, 27, 39~41]、化学风化指数[10, 29]、元素比值(Ca/K、Fe/K、Rb/Sr和Sr/Ca等)[23, 29, 42]、游离铁/全铁比值[43]等指标与受降水影响的植被发育和成壤强弱关系密切，可敏感地响应千年尺度上的夏季风强度变化；粒度[7, 12, 33, 44]与不受风化作用影响的某些元素比值(Si/K、Si/Al、Zr/Rb等)[23, 45~46]能够敏感响应冬季风的强度变化。近年来，随着释光测年精度的提高，靖远[4, 21]、古浪[21]和临夏[6]等地具有光释光测年控制的高分辨率粒度变化与石笋和冰芯记录的千年尺度气候波动的良好对应关系。通过研究古浪剖面的高分辨率元素扫描发现元素比值能够很好地记录东亚冬夏季风千年尺度上的快速变化特征[23, 47]，因此，本文选取扫描的Rb/Sr比值及磁化率作为反映夏季风强度的代用指标，粒度和Zr/Rb作为响应冬季风强度变化的替代指标，与石笋进行对比，探讨最后两个冰期旋回(即末期冰期旋回和倒数第二次冰期旋回)东亚冬、夏季风在亚轨道尺度上的变化特征及可能的驱动机制。
在冰期-间冰期尺度上磁化率、Rb/Sr均敏感响应于夏季风的变化，能很好记录轨道尺度上的气候波动(图 3)。在间冰期气候相对暖湿，成岩、化学风化和淋滤作用较强，所以磁化率及Rb/Sr的值高，波动比较大。在S1及S2时期，磁化率值呈阶段性地升高，清晰地记录夏季风3次阶段性增强的事件。在冰期磁化率及Rb/Sr变化存在差异，L1和L2时期磁化率值基本没有波动，表现为弱的成岩及化学风化作用。而Rb/Sr比值在冰期也有较明显的变化，能更精细的记录东亚夏季风的快速变化。高分辨率Zr/Rb比值和粒度记录的东亚冬季风变化与深海氧同位素变化趋势基本一致，古土壤层S0、S1和S2分别MIS1、MIS5和MIS7对应，指示东亚冬季风强度粒度曲线同指示全球冰量变化的深海氧同位素记录有较好的可比性，但表现出更多高频变化的信号。粒度及元素比值在冰期变化幅度小、频率快，但间冰期变化幅度大、频率慢，暗示了亚轨道尺度上的季风突变事件驱动力在冰期-间冰期不同下垫面条件下会有差异[21]。
为了探究东亚季风与北大西洋H事件[52]、D-O旋回[53]等气候突变事件与北半球高纬冰量、太阳辐射变化之间的联系，将粒度、磁化率和元素比值的曲线与三宝-葫芦洞石笋[49~50]、深海氧同位素[38]、北纬65°夏季太阳辐射变化[48]、ODP983钻孔浮冰碎屑含量[51]曲线进行对比。图 3所示，粒度记录的冬季风在冰期-间冰期尺度上的波动与深海氧同位素曲线(指示北半球冰量变化)相似，从而证明东亚冬季风的波动响应于北半球高纬冰量的变化。在每个冰期-间冰期旋回中，高分辨率元素比值和粒度记录分别可以辨别出与石笋相同数量级的变暖事件(图 3)，但粒度记录的D-O旋回明显要多于Rb/Sr及Zr/Rb比值，且这些暖事件并不能很好的一一对应。粒度、Zr/Rb和Rb/Sr所指示东亚季风的快速变化表现出较强的岁差信号，与石笋及65°N太阳辐射具有较好的对应关系，表明太阳辐射对冬、夏季风的变化具有影响。末次间冰期以来的H1~H7事件[54]在黄土和石笋中具有较好的对应关系，虽然在倒数第二个冰期旋回中北大西洋地区记录H6.1~H7.2(与末期冰期中类似的H事件)事件[55]在黄土和石笋中也都有响应，但这些冷事件的变化幅度和持续时间在海洋和陆地的沉积记录中对应关系相对较差(图 3)，可能是由于不同载体和不同指标中的印记之间的差异性和不同时间序列的年代不确定性造成的。黄土多代用指标揭示的季风快速变化与石笋、冰芯记录中气候突变事件高度相似，但黄土中记录的冬季风加强事件明显高于北大西洋H事件的发生频率。临夏地区粒度和Zr/Rb曲线记录的末次冰期3次冬季风增强事件，在北大西洋记录中没有明显的浮冰碎屑事件对应；在倒数第二次冰期旋回中粒度和元素比值曲线记录了2次明显东亚冬、夏季风快速变化事件，与之对应的大西洋浮冰碎屑事件很弱。但总体上，临夏最后两个冰期旋回以粒度及元素比值记录的显著变冷千年尺度的H事件与石笋记录的夏季风明显减弱的事件有很好的对应关系，并且这些大幅度的快速变化H事件多出现在最后两个冰期旋回的最冷干时期。磁化率、粒度、Rb/Sr及Zr/Rb比值等记录指标千年尺度气候事件的变化幅度和频率与石笋、ODP983钻孔的记录有明显差异，归因于这些指标对不同气候要素响应的差异。
4 结论黄土高原临夏钻孔85 m岩芯的粒度和元素等高分辨率多代用指标揭示了东亚冬、夏季风26万以来在轨道尺度和千年尺度上的演化特征。在轨道尺度上，粒度、Zr/Rb、Rb/Sr和磁化率均能响应东亚季风冰期-间冰期的大幅波动。在千年尺度上，磁化率在L1和L2时期值基本没有波动，Rb/Sr比值在冰期也有较明显的变化，能更精细地记录东亚夏季风的快速变化。在冰期-间冰期不同气候背景下，粒度和元素比值揭示的气候快速变化幅度和周期差异较大，冰期幅度小、频率快，间冰期幅度大、频率慢，表明冰量变化对季风快速变化有调制作用。粒度与Zr/Rb记录的冬季风在冰期-间冰期尺度上的波动与深海氧同位素变化一致，表明东亚冬季风的波动与北半球高纬冰量的变化的密切相关。粒度、Zr/Rb和Rb/Sr所指示的东亚季风快速变化表现出较强的岁差信号，与石笋和北纬65°夏季太阳辐射有较好的对应关系，显示东亚季风变化也受到岁差直接驱动的夏季太阳辐射变的控制。总体上，临夏多代用指标揭示的季风快速变化与石笋记录中气候突变事件高度相似，在每个冰期-间冰期旋回中，高分辨率元素比值和粒度曲线分别可以辨别出与石笋相同数量级的D-O旋回，但粒度记录的D-O旋回明显要多于Zr/Rb比值，表明粒度能更敏感地响应于冬季风的快速变化。最后两个冰期旋回以临夏的粒度及元素比值记录显著变冷千年尺度的H事件与石笋记录的夏季风明显减弱的事件有较好的对应关系，表现为黄土记录的末次间冰期以来的H1~H7事件同东亚地区石笋及高纬地区的海洋记录一一对应，在倒数第二个冰期旋回中北大西洋地区记录H6.1~H7.2事件在黄土和石笋中也都有响应。但临夏粒度和元素比值记录的冬季风加强事件高于北大西洋H事件的发生频率，且这些冷事件的变化幅度和持续时间在海洋和陆地的沉积记录中对应关系相对较差可能是由于不同载体和不同指标中的印记之间的差异性和不同时间序列的年代不确定性造成的。
致谢: 感谢两位审稿专家和杨美芳编辑对文章提出的宝贵意见。
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2 University of Chinese Academy of Sciences, Beijing 100049;
3 Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi'an 710061, Shaanxi)
Abstract
Multi-proxies studies of high-resolution loess-paleosoil sequences in the arid northwest Chinese Loess Plateau(CLP) are the key access to revealing the variation characteristics and driving mechanism of Asian monsoon in orbital-suborbital timescale. However, few comparison and sensitivity analysis researches of the multi-proxies obtained from the same geological archive are available for us to refer to, which restricts us to better understand the characteristics and driving mechanism of rapid monsoon changes. In this study, we chose the upper 85 m of two parallel loess cores(35.63°N, 103.12°E) retrieved from Linxia City in Gansu Province to conduct high-resolution mean grain size, magnetic susceptibility(2-cm resolution) and geochemical elements scanning(1-cm resolution) analysis. Ten age control points are selected to match the loess/paleosol boundaries to the glacial/interglacial transitions to reconstruct chronosequence of Linxia loess core since the past 260 ka. Based on the analysis above, the results show that mean grain size and Zr/Rb exhibit significant glacial-interglacial fluctuations, matching well with marine isotope record over the last 260 ka and confirming the strong coupling between the winter monsoon and Northern Hemisphere ice volume. But mean grain size is relatively more sensitive than Zr/Rb in millennial timescales. Rb/Sr ratio and magnetic susceptibility can well document glacial-interglacial variations of summer monsoon. Nevertheless, summer monsoon can be more elaborately recorded by Rb/Sr ratio. The Aisa monsoon implied by mean grain size, Zr/Rb and Rb/Sr ratios exhibits obvious precessional signal, corresponding well with summer insolation at 65°N and speleothem δ18O of Sanbao-Hulu caves, manifesting that summer solar insolation is one of the forcing factors of East Asian monsoon fluctuations. Compared with speleothem curve, mean grain size, Zr/Rb and Rb/Sr ratios can detect the same order of magnitude warm events during the last two glacial cycles. But the number of warm events recorded by mean grain sizes is more than that by Zr/Rb and Rb/Sr ratios, which can not be well correlated with each other. Significantly cooling Heinrich events implied by mean grain size and Zr/Rb ratio are well associated with obviously weakening events of summer monsoon recorded by speleothem. But the frequencies of cooling events are higher than that of marine records. Mismatches of abrupt climate changes in the various proxies are mainly caused by the sensitivity discrepancy among different indicators. In the future, we should conduct more high-resolution and sensitive loess proxies research to help us to better understand the characteristics(such as timing and amplitude) and to analyze the temporal and spatial discrepancy of rapid monsoon changes.