陕西洛川黄土剖面的Rb/Sr值及其气候地层学意义

本文系统地测试了陕西洛川黄土-古土壤剖面中的Rb,Sr含量和磁化率。通过对比该剖面2．50MaB．P．以来Rb／Sr曲线和磁化率曲线,发现两者的形状和变化的相位十分相似,然而两者的相对振幅却有明显的差别。Rb／Sr曲线记录了0．80MaB．P．左右一次重要的气候转型界面,揭示了黄士和古土壤内部次一级的气候波动旋回,反映了早更新世黄土高原较温暖潮湿的特点以及更新世以来气候由湿向干的变化趋势。研究表明,Rb／Sr值可以作为衡量东亚夏季风环流强度变化的较为敏感的替代性指标。     

Variation of Rb/Sr Ratios in the Loess-Paleosol Sequences of Central China during the Last 130,000 Years and Their Implications for Monsoon Paleoclimatology

Rb, Sr, and magnetic susceptibility have been measured in the last interglacial–glacial loess profiles at Luochuan and Huanxian, central China. A high degree of similarity between the parameters in both profiles suggests that variations of Rb/Sr ratios in the sequence can be regarded as an indicator of East Asian summer monsoon strength. Matching the Rb/Sr record with the SPECMAP δ¹⁸O curve suggests that the Rb/Sr ratio responds sensitively to changes of the East Asian monsoon induced by global ice-volume variation.     

Impacts of grain size sorting and chemical weathering on the geochemistry of Jingyuan loess in the northwestern Chinese Loess Plateau

Major and trace elemental compositions of loess samples collected from the Jingyuan section in the northwestern Chinese Loess Plateau (CLP) were analyzed to investigate the potential impacts of grain size sorting and chemical weathering on the loess geochemistry and to extract appropriate geochemical indices for better evaluating the East Asian monsoon variability. Based on variations of major and trace elements in different grain size fractions, seventeen elements were classified into three types: (1) Si and Na display higher contents with the increased particle sizes; (2) Al, Fe, Mg, K, Mn, Zn, Rb, Cr, V are mainly enriched in fine size fractions; (3) Ti, Ba, Zr, P, Ca and Sr show irregular variations among different size fractions. Comparison of Al-normalized elemental ratios with Zr/Rb and Rb/Sr ratios (two commonly employed indicators for grain size sorting and pedogenic weathering) indicates that Si/Al, Zr/Al, Ti/Al variations match well with Zr/Rb and grain size results, whilst Ca/Al, Sr/Al, P/Al ratios display similar variability as that of Rb/Sr ratio. Comparison of loess based proxies (e.g., elemental ratios, magnetic susceptibility, grain size) of Jingyuan section with speleothem and ice-core records confirms that elemental ratios of high-resolution loess sequences developed in the northwestern CLP can be employed to address fluctuations of the winter monsoon-induced grain size sorting and summer monsoon-related weathering and pedogenesis at glacial–interglacial and millennial timescales.     

临夏黄土记录的26万年来季风快速变化

黄土高原西部高分辨率黄土序列开展的多代用指标研究,有助于揭示轨道-亚轨道尺度上东亚冬、夏季风的变化特征及动力机制。但针对同一地质记录提取冬、夏季风多代用指标进行对比和敏感性分析的工作尚不多见,因此制约了对季风快速变化特征和机理的理解。本文利用甘肃临夏85 m黄土岩芯开展高分辨率的粒度、磁化率和元素扫描分析,发现在轨道尺度上粒度和Zr/Rb比值显示的大幅冰期-间冰期波动与深海氧同位素记录有很好的对应关系,表明东亚冬季风的波动响应于北半球高纬冰量的变化。但在千年尺度上,粒度能更敏感地响应冬季风的快速变化;Rb/Sr比值和磁化率均能很好记录轨道尺度上的东亚夏季风波动,但在亚轨道尺度上Rb/Sr的敏感性明显高于磁化率。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,表明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。在最后两个冰期-间冰期旋回中(即末期冰期旋回和倒数第二次冰期旋回)Rb/Sr及Zr/Rb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,但粒度记录的暖事件明显要多于Rb/Sr及Zr/Rb比值,且这些暖事件并不能一一对应。粒度及元素比值记录显著变冷的千年尺度的Heinrich(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,但其发生频率明显高于海洋记录。这些指标记录的千年尺度上波动信号的不一致性主要归因指标对不同气候要素敏感性的差异。未来需要进一步开展高分辨率多代用指标敏感性分析,选取对季风快速变化的敏感指标,有助于我们理解季风快速变化的特征(如周期和幅度等)和分析季风快速变化的时空差异性。     

Decou pled Summer and Winter Monsoon in Northeast Tibet and Northwest Loess Plateau During the Last Interglaciation

DECOUPLED SUMMER AND WINTER MONSOON IN NORTHEAST TIBET AND NORTHWEST LOESS PLATEAU DURING THE LAST INTERGLACIATIONtheNationalKeyProjectforBasicResearchonTibetPlateau (G19980 4 80 0 )andtheNationalNaturalSci encesFoundationofChina (No.4 990     

洛川黄土记录的最近2500ka东亚冬夏季风变化周期

对厚约１４０ｍ的陕西洛川坡头村黄土剖面进行间距３～１０ｃｍ的系统采样,测量了全部样品的磁化率和粒度,选择〉３０μｍ颗粒百分含量和磁化率分别作东亚冬,夏季风强度变化的替代性指标,以新建立的时间标尺为基础,分析了最近２５００ｋａ以来东亚季风变化的周期特征。结果表明,第四纪东亚冬,夏季风变化时间序列包括含有１００ｋａ,４１ｋａ和２３ｋａ地球运动轨道要素变化的周期,同时包含有约８０ｋａ,５６ｋａ和３０ｋａ     

Changing color of Chinese loess: Geochemical constraint and paleoclimatic significance

Two loess–paleosol sequences on the central Chinese Loess Plateau were investigated to understand spatial and temporal variations in the soil color (e.g., lightness and redness) and factors that control those variations. Color difference between the original samples and pretreated samples suggests that loess lightness is influenced by the pedogenic matter (e.g., iron oxides, organic matter, and calcium carbonate) to varying degrees, depending on the concentrations. Iron oxides and organic matter darken the loess lightness, whereas carbonate lightens the loess lightness. By contrast, the redness is dominantly controlled by the types and concentrations of iron oxides. Variations in magnetic susceptibility and redness are associated with different magnetic minerals that formed mainly during post-depositional processes, and therefore both proxies can be employed to reconstruct the history and variability of the East Asian summer monsoon. Since the sensitivity of these two proxies to the summer monsoon variation is different, we generate a stacked summer monsoon index by combining these two records and interpret the stacked index as most indicative of precipitation variability. The fidelity of this new index is supported by an independent ratio, estimated from the first-derivative values of the color reflectance spectra. Our loess-based proxies provide a new understanding of the East Asian summer monsoon variability as a two-phase strengthening of summer monsoon intensity during the penultimate deglaciation.     

南京老虎山黄土剖面中铷锶地球化学和磁化率与古气候变化的关系

宁镇地区的下蜀黄土记录了古气候变化的丰富信息，作为中国北方风尘堆积的南部边缘相，下蜀黄土在全球环境变化研究中占有重要的地位。本文以10cm间距系统分析了老虎山剖面的Rb、Sr含量和频率磁化率。结果显示，磁化率和Rb含量可以较好地指示东亚冬季风强度变化；Rb/Sr比可以作为下蜀黄土化学风化强度的替代性指标；剖面的Rb/Sr比由下至上升高是南京地区12万年以来化学风化作用增强的结果。     

大别山北麓黄土-古土壤Rb、Sr与磁化率及其古环境意义

对大别山北麓0.78 Ma以来的黄土-古土壤样品的磁化率及Rb和Sr的地球化学特征研究表明,相对高的w(Rb)和w(Rb)/w(Sr)值及低w(Sr)值指示夏季风占优势的湿润时期,对应古土壤层;相对低的w(Rb)和w(Rb)/w(Sr)值及高w(Sr)值指示冬季风占优势的干旱期,磁化率相对较低,对应黄土层。剖面的w(Rb)/w(Sr)值自下至上降低反映大别山北麓地区的风化作用逐渐减弱,也揭示自0.78 Ma以来气候具有由湿向干的趋势;磁化率与w(Rb)/w(Sr)值的对比分析表明,它们的变化曲线存在显著不同,原因可能是由于磁化率在古土壤中的增强机制比较复杂所致,w(Rb)/w(Sr)值作为气候替代指标比磁化率更灵敏。与洛川剖面相比,大别山黄土-古土壤的w(Rb)较高而w(Sr)较低及Rb、Sr较强的分异能力特征反映了大别山气候整体较为温湿,沉积物遭受的风化程度更强。     

黄土高原南部全新世黄土-古土壤 序列若干元素分布特征及意义*

选择黄土高原南部的XJN,XMC和JYC全新世剖面为研究对象。通过对地层中Ca,Ba,Rb和Sr元素分布的研究,发现Ba/Sr和Rb/Sr比值在不同地层中含量差异十分明显,并与成土作用强度显著正相关,可作为良好的气候替代指标;表生环境中Rb和Ba较稳定,Sr和Ca元素十分活跃,易于迁移,其中Ca的迁移能力高于Sr元素;从XJN→XMC→JYC剖面,Rb和Ba元素含量增加,而Sr和Ca元素含量降低,这与区域环境差异有关;全新世中期暖湿的亚热带气候主要出现关中盆地地区,六盘山以西则是暖温带气候。     

Variability of East Asian Winter Monsoon in Quaternary Climatic Extremes in North China

In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess–paleosol sequences in the Chinese Loess Plateau were investigated. Sandy layers in the loess deposits, the “Upper sand” and “Lower sand” (layers L9 and L15, respectively), which represent a high-resolution record of paleomonsoon changes, have been sampled at intervals of 5–6 cm from sections at Luochuan and Xifeng. The grain size and magnetic susceptibility was measured for all samples. The grain-size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. However, the winter monsoon was relatively stable during warm periods. The magnetic susceptibility signal (a proxy of summer monsoon intensity) is practically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr. The detection of this oscillation in older glacial stages strongly suggests that it may be a pervasive cycle of the cold climatic phases of the Quaternary. Millennial-scale variations of the winter monsoon may be caused by instability of the westerly jet, which is determined by temperature differences between the polar and the equatorial regions.     

Zr/Rb ratio in the Chinese loess sequences and its implication for changes in the East Asian winter monsoon strength

The long-term dust accumulation sequences on the Chinese Loess Plateau (CLP) provide a valuable opportunity to study the evolution of East Asian winter monsoon strength. Grain size of bulk samples of loess deposits, though widely used, is a modified measure of the strength of winter monsoon wind due to modification by post-depositional weathering and pedogenesis. This study proposes a new and reliable geochemical climate proxy that records the variability of the East Asian winter monsoon on the CLP. Six loess-paleosol sections on the Plateau were selected for measuring concentrations of the trace elements Zr and Rb as well as grain size. Variations of the Zr/Rb ratio in all the sections encompassing the last 130 ka display a generally similar pattern to that of mean grain size of bulk samples (MGSB). Though a positive correlation exists between the Zr/Rb ratio and MGSB, the correlation coefficient decreases in southern and eastern sections where intense pedogenesis occurred in the relatively warm and humid climates. Long-term Zr/Rb variation in the Lingtai section exhibits amplitudes and frequencies similar to those in MGSB and the mean grain size of quartz particles (MGSQ, accepted as a more reliable proxy than MGSB) in the upper loess-paleosol sequence over the past 2.6 Ma. However, for the underlying Red Clay formation, in the interval from about 7-2.6 Ma B.P., the MGSB record is relatively stable, whereas both the Zr/Rb ratio and MGSQ show distinct variability and display amplitudes similar to those observed in the overlying loess-paleosol sequence. These results demonstrate that the Zr/Rb ratio reflects original eolian grain size and may serve as a reliable index of the strength of East Asian winter monsoon winds.     

浑善达克沙地南缘晚更新世黄土-古土壤序列的高分辨率磁性气候记录

浑善达克沙地处于季风边缘区,目前对该区域末次间冰期以来的古气候演化历史及其动力学机制尚不清楚。文章以浑善达克沙地南缘山间低洼地带北沟剖面总厚度4.72 m（未见底）的黄土-古土壤序列为研究对象,开展了光释光测年和精细的岩石磁学、环境磁学、地球化学和漫反射光谱分析。结果表明：1）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χ_ARM/χ）与深海氧同位素曲线的对比建立了13万年来该黄土-古土壤序列精细的年龄模式;2）在黄土高原上常规的用来指示成壤强度和夏季风强度的磁参数并不完全适用于干旱-半干旱地区（400 mm等降水线附近）的风成黄土,而地球化学指标在指示季风边缘区的地球化学风化过程上表现出一定的合理性,北沟剖面复杂的磁信号可能是对末次冰期季风边缘区显著减弱的夏季风的阈值响应;3）在末次间冰期东亚夏季风可以延伸到内蒙古中东部,北沟剖面记录了浑善达克沙地南缘末次间冰期以来区域古季风的演化历史。

     

郑州邙山黄土碳酸盐和白云石含量记录的80 ka以来季风降水变化及驱动机制

揭示东亚季风气候精细变化过程和区域差异,是第四纪古气候学研究的重要内容。本文以季风影响显著的中原郑州黄土堆积上部31 m为研究对象,基于高密度光释光独立定年和619份样品的碳酸盐、白云石含量、磁化率测试,重建了约80 ka以来东亚季风降水/季风强度变化过程。黄土碳酸盐、白云石含量时间序列显示,末次间冰期末期以来,中原地区发生过数次季风降水增强事件,每次降水增强期持续时间短于岁差、偏心率等轨道周期,表明存在亚轨道时间尺度季风快速变化现象。中原黄土碳酸盐/白云石含量主要受土壤呼吸CO₂的影响,土壤呼吸CO₂含量由季风降水和土壤温度控制,因此,碳酸盐/白云石含量是东亚夏季风强度的良好替代指标。中原地区千年时间尺度的季风湿润和干旱事件与其他地区黄土、石笋记录具有同步性,证实在东亚季风降水核心区、中心区和边缘区千年时间尺度干湿变化的同步性,支持季风环流大空间同步增强的模式。这一季风降水型式与现代观测到的"南涝北旱"的"+-+"三极型季风降水分布不同,可能表明不同时间尺度季风降水型式有明显区别。进一步分析表明,季风降水增强/减少事件存在约22 ka和约1.6 ka的周期,表明低纬太阳辐射、大洋环流和高纬度冷事件对东亚季风的联合驱动作用。

     

2.5Ma以来大陆风化强度的演变

本次研究得出了２．５ＭａＢＰ以来洛川剖面黄土－古土壤序列的酸不深物＾８７Ｓｒ／＾８６Ｓｒ、Ｒｂ／Ｓｒ、磁化率和沉积速率的变化曲线。黄土和古土壤酸不深物＾８７Ｓｒ／８６Ｓｒ、Ｒｂ／Ｓｒ和磁化率的变化具有类似的阶段性和周期性,这与晚新生代以来古气候的主要变化以及行星轨道参数相一致。研究证明,黄土和古土壤酸不深物＾８７Ｓｒ／＾８６Ｓｒ值得化学风化强度的替代指标,沉积速率是物理风化强度的替代指标。自２．５     

Intensification of the East Asian Monsoon in Southern China at about 300–400 kaBP Inferred from Eolian Deposits in the Middle-lower Reaches of the Yangtze River

In the East Asian monsoon region,eolian deposits widely distributed in the middle-lower reaches of the Yantgze River are among the best materials available for studies on Quaternary climate change in the subtropical zone of Southern China.Typical eolian deposits in this region include upper Xiashu Loess(XL) and underlying Vermiculated Red Soil(VRS) layers.In this paper,chronological and paleoclimatic studies are conducted on an eolian deposit sequence near Jiujiang(JJ) city in northern Jiangxi province.A magnetostratigraphic study,combined with optically stimulated luminescence(OSL) dating,is conducted on the JJ section and provides further evidence that eolian deposits in the middle-lower reaches of the Yangtze River have been formed since the late Early Pleistocene,and that the boundary age between the XL and VRS layers is about 300-400 kaBP.In grain-size records of the JJ section,the median grain-size and content of the 30 μm size fraction increase sharply after 300-400 kaBP,representing an East Asian winter monsoon intensification event.Further pollen analysis reveals differing pollen assemblages before and after 300-400 kaBP:there is an evident increase in plants adapted to grow in a warm humid environment after 300-400 kaBP,implying an increase in precipitation caused by intensification of the East Asian summer monsoon.Global ice volume and uplift of the Tibet Plateau(TP) are regarded as crucial factors influencing variations of the East Asian monsoon on a long-term scale.The deep-sea δ~(18)O record,which reflects variations in global ice volume,shows no obvious change after 300-400 kaBP.Moreover,the influence of global ice volume changes on the East Asian summer and winter monsoons is inverse;the global ice volume increase(decrease) implies a strengthened(weakened) winter monsoon and weakened(strengthened) summer monsoon.We therefore interpret the coupled intensifications of the East Asian summer and winter monsoons at about 300-400 kaBP to the uplift of the TP in the Middle Pleistocene.This climate event is also documented in eolian deposits from the southern margin of the Chinese Loess Plateau(CLP) and from the desert-loess transitional belt.However,it is not recorded in the loess-paleosol sequences from the central part of the CLP,thereby indicating differing climate responses to TP uplift in different regions,which requires further study.     

Roles of sorting and chemical weathering in the geochemistry and magnetic susceptibility of Xiashu loess, East China

Uncertainties in paleoenvironmental interpretations for traditional chemical analysis of bulk samples result from different grain-size sub-populations of sediments containing variable distributions of elements and minerals. Therefore, it is important to understand the elemental and mineral distribution in different grain sizes in determining the quantitative relationship between chemical weathering and climatic change. We sieved a series of Xiashu loess samples into three sub-populations of different grain sizes (<2, 2–45 and >45 μm, respectively), and then analyzed each population for rubidium (Rb), strontium (Sr), rare earth elements and magnetic susceptibility. In comparison with elemental concentrations of bulk samples, clay mineralogy and illitic crystallinity, our results show that distinct elemental distributions and magnetic susceptibilities for different grain-size sub-populations are controlled by sorting and/or chemical weathering, although we also suggest that the Xiashu loess may have the same provenance as the Central Chinese Loess. Maximum concentrations of Rb and fine-grained magnetic minerals in the less than 2 μm sub-population, coupled with our finding of maximum Sr in the larger than 45 μm fraction, indicate that Sr was lost during chemical weathering. Grain-size sub-population analysis is, therefore, an effective method for extracting paleoenvironmental information, because individual sub-populations show minimal variations in initial Rb/Sr ratios compared to bulk analysis of all sizes together. Furthermore, a negative correlation between Rb/Sr ratios and Sr concentrations for the <2 μm fraction (R² = 0.97) may indicate that clay is a sensitive indicator of intensity of chemical weathering and is an ideal sub-population for determining Rb/Sr ratios, but not for magnetic susceptibility.     

末次盛冰期北美风成环境变化及其在全球气候突变事件中的意义

本文用末次冰期北美大冰盖南缘的风尘堆积记录来重建末次盛冰期的亚极地环境变化.磁化率、有机碳含量和潜育化程度的资料表明,末次盛冰期的前期(约21000～15000aB P.)植被覆盖较差,而后期(15000～10000aB.P.)植被覆盖较好,15 000aB.P.环境发生了急剧的变化,磁化率曲线显示了大约1000a的氧化-还原周期.这种大约1000a的环境变化周期很可能由低纬地区的海洋和大气的扰动所驱动,这种扰动的能量通过北美大冰盖及时地传递到北大西洋.     

汉江上游I级河流阶的形成及对东亚季风 变化的响应

对郧县—白河段汉江Ⅰ级河流阶地上风成黄土的沉积学、理化性质、地球化学和年代学进行了系统研究。结果表明,汉江Ⅰ河流阶的形成不晚于25 ka BP;黄土具有马兰黄土(L1)→过渡黄土(Lt)→古土壤(S0)→全新世黄土(L0)→表土(TS)的地层序列,与渭河谷地的黄土地层序列完全可比;25~11.5 ka BP,冬季风强盛,气候冷干,从11.5 ka BP开始,冬季风逐渐减弱,气候开始向暖湿方向逐步转化,从8.5 ka BP开始,夏季风达到了末次冰期结束后的鼎盛时期,3.1 ka BP前后,东亚季风格局发生变化,夏季风减弱,重新进入一个相对干冷的时期,而人类活动对地表的影响形成了表土;汉江上游谷地黄土记录的末次冰期后季风逐渐加强、中全新世季风强盛、随后季风衰退和气候变干的夏季风演变模式与渭河谷地黄土的记录高度一致,与邻区石笋和泥炭记录的季风变化趋势也有良好的可比性,但与石笋/泥炭记录的夏季风强盛期的起始时间(9.3~4.2 ka BP)并不完全一致。     

黄土高原记录的MIS 6.5期东亚夏季风信号及其古气候意义

深海氧同位素(MIS)6.5期是古气候演化的一个特殊时期。其太阳辐射强度显著高于全新世, 但当时冰量较大, 全球气候整体处于冰期环境; 此现象支持全球气候变化主要受控于全球冰量。然而, 来自中国石笋氧同位素异常偏负, 指示东亚季风区湿润程度已达间冰期水平, 支持东亚夏季风直接受太阳辐射所驱动。为理解以上冲突, 本研究选取黄土高原沉积序列为研究对象, 考察东亚夏季风是否可能在MIS 6.5期发生扩展, 理解东亚季风区对太阳辐射的响应关系。研究结果发现:MIS 6.5期黄土高原水分条件与MIS 3期类似, 显著低于典型间冰期气候。MIS 6.5期与典型间冰期之间降水量的差别, 由季风边界区向核心区呈现逐渐降低的趋势。东亚夏季风演化与太阳辐射变化并不完全一致。黄土高原西北部沉积序列记录的东亚夏季风演化包含有强烈的岁差周期和100ka周期, 可能受全球冰量和低纬地区太阳辐射的共同影响。