甘肃灵台黄土-红粘土序列中坡缕石的分布及其古气候意义

坡缕石是典型的干旱、强蒸发环境的指示矿物,黄土-红粘土序列中坡缕石的分布对古气候和古环境重建具有重要的指示意义。本文对灵台任家坡剖面中黄土、古土壤和红粘土代表性样品进行了场发射扫描电镜和高分辨透射电镜观察,结果发现,在3.6Ma以前沉积的红粘土中存有大量的自生坡缕石,而在3.2Ma以来沉积的红粘土及其上覆黄土古土壤序列没有坡缕石。剖面样品扫描电镜原位观察形貌特征和微观结构显示,坡缕石属于成壤作用自生矿物,系由伊蒙矿物在富镁孔隙液作用下转化形成。灵台剖面中坡缕石消失的层位与红粘土磁化率升高和粉尘通量及沉积速率增加的层位相一致。坡缕石在剖面中的分布指示3.6～3.2Ma前后是东亚季风的重要转型期,东亚古气候格局发生改变,由连续干暖的夏季风占主导向干冷冬季风和暖湿夏季风高频强波动交替环境演化。     

洛川黄土-红粘土序列铁氧化物组成及其古气候指示

对洛川黄土、古土壤和红粘土中磁性矿物组成、成因和相关系进行了研究。结果表明:黄土磁性矿物以风尘磁铁矿为主,少量的成土赤铁矿和成土磁赤铁矿;古土壤磁性矿物以成土磁赤铁矿为主,成土赤铁矿次之,少量的风尘磁铁矿和赤铁矿;红粘土磁性矿物以成土赤铁矿为主,风尘磁铁矿和成土磁赤铁矿次之,少量风尘赤铁矿。黄土、古土壤和红粘土磁性矿物组成差异,反映了其形成期不同的古气候特性以及不同气候条件下生物地球化学作用强度的差异。干冷的冰期,黄土弱成土作用形成了以粗颗粒的风尘磁铁矿核 赤铁矿边的磁化率载体。间冰期的温暖湿润的古气候最有利于生物活动,强烈生物活动导致古土壤中大量纳米超细磁赤铁矿/磁铁矿产生,形成以磁赤铁矿为主,风尘磁铁矿核 赤铁矿边为辅的磁化率载体。红粘土成壤期,强降雨强蒸发的长干短湿的高温炎热的古气候使得红粘土化学风化强烈,生物地球化学活动较弱,形成以磁铁矿核 赤铁矿边和磁赤铁矿核 赤铁矿边的磁化率载体。黄土、古土壤和红粘土磁性矿物组成、磁性矿物相关系是其形成期独特的古气候指示。     

灵台风尘堆积中钙质结核的地球化学研究

通过对甘肃灵台风成黄土－古土壤－红粘土序列中钙质结核的野外分布及形态特征观察,并对主元素和微量元素进行化学分析,发现结核中氧化物活动给分和惰性组分含量的比值在剖面中自下而上逐渐变大,反映了结核形成时其上覆土层所经历的淋溶程度逐渐增强;而ＭｇＯ含量校正后的ＭｇＯ／Ｃａｏ比值则逐渐减小,批示结核形成时环境温度逐渐降低。灵台剖面自下而上不同层位结核中Ｓｒ／Ｂａ比值及ＣａＣＯ３含量的变化同氧化物含量及比值     

黄土高原风尘序列的碳酸盐成因及其风化过程

碳酸盐是黄土高原风尘序列的主要组成部分,其含量与原生碳酸盐溶解、再沉淀,以及含钙硅酸盐化学风化强度等密切相关。碳酸盐在表生环境下极易移动,且其迁移变化受大气降水、温度等古气候所制约,因此黄土次生碳酸盐记录了古气候的重要信息。甘肃西峰赵家川剖面碳酸盐含量、磁化率的系统测定和风尘序列地质特征研究表明,在整个剖面中,碳酸盐含量和磁化率曲线呈显著的负相关,并从剖面上部到红粘土底部,碳酸盐含量有增加趋势。黄土、古土壤和红粘土沉积分别处在不同风化阶段:黄土化学风化最弱,处于钙质残积阶段,古土壤处于强烈钙质淋溶阶段和硅铝残积阶段,红粘土化学风化最强烈,处于硅铝残积阶段或红土化阶段;不同的化学风化阶段古气候性质有别。     

西峰黄土-红粘土序列有机质记录及其对磁化率古气候意义启示

大量研究证实成壤磁铁矿和磁赤铁矿是黄土-红粘土磁化率变化主要原因,但是对这些磁性矿物起源于无机沉淀还是生物作用尚需深入研究.为此,本文选择黄土高原200余米厚的西峰黄土-红粘土序列为主要研究对象,采用化学分析和透射电镜手段,对其中有机质含量和磁性矿物成因进行研究,结果表明:S5以来的沉积层有机质含量相对较高,集中在0.25％ ～0.50％之间,有机质含量变化与黄土和古土壤层磁化率变化基本一致,即有机质高值对应磁化率高值,有机质与磁化率值成正相关;S5以下至午城黄土底部不仅有机质含量降低而且变化幅度减小;红粘土沉积层中,虽然不能完全遵循磁化率高值对应有机质高值,但基本符合峰-峰和谷-谷对应规律.有机质和磁化率的相关性以及透射电镜研究表明,生物成因磁性矿物是风尘序列磁化率变化的主要原因,有机质含量高低反映风尘序列成壤过程中生物地球化学强度变化,生物将磁化率和古气候密切联系起来.黄土和红粘土磁化率古气候意义相似,但是由于黄土和红粘土形成于不同气候背景下,不能仅仅利用二者磁化率大小对比反演古气候,利用表土磁化率重建古气候要考虑气候载体形成的气候背景差异.     

佳县红粘土磁学性质

黄土/古土壤序列和红粘土均已被证明是风积成因,然而红粘土研究的程度远低于黄土/古土壤序列,比如强发育红粘土获得比较低磁化率值的原因并没有得到清楚解释,红粘土磁化率代用指标问题和红粘土搬运路径等问题还存在较大的争议,红粘土记录古气候意义研究也相对薄弱,因此加强对红粘土的研究,有助于对古气候的理解.佳县位于黄土高原与毛乌素沙漠交接地区,季风区与非季风区过渡地带.对佳县厚60m的红粘土样品进行系统磁学参数测量分析发现,佳县红粘土以软磁性矿物(磁铁矿、磁赤铁矿)为主,同时含有一定量硬磁性矿物(赤铁矿、针铁矿,主要是赤铁矿);强发育古土壤层含有较多软磁性矿物,弱发育古土壤层含有较多硬磁性矿物;磁颗粒粒径基本在0.2μm以下,x＞64×10-8m3/kg时,磁颗粒粒径小于0.1μm.剖面底部磁学参数变化存在异常,原因可能是:赤铁矿含量增加,超顺磁(SP)亚铁磁性颗粒含量急剧减少,单畴(SD)颗粒相对含量增加.该异常原因解释符合前人提出的成壤路径:水铁矿→SP磁赤铁矿→SD磁赤铁矿→SD赤铁矿.     

中国黄土磁性矿物特征及其古气候意义

最直观、最直接的古气候记录莫过于陆相黄土地层,它由许多黄土和古土壤层叠覆而成,详细地记录了第四纪以来的古气候变化。黄土高原中部黄土和古土壤的风化和发育程度可以用磁学参数,如磁化率等来描述。本文作者对甘肃西峰剖面约13万年以来的黄土地层做了详细采样,测量其磁滞回线各参数,频率磁化率、低场和高场磁化率、温度磁化率、磁组构等参数。研究表明,黄土与古土壤中的磁性矿物组分相似而且稳定。磁铁矿、赤铁矿和磁赤铁矿(主要是磁铁矿)的含量及其颗粒大小决定了它们的磁学特征。黄土和古土壤的磁学性质差别主要受成土过程中形成细小磁铁矿的影响。     

黄土高原风尘沉积物中纳米矿物

由黄土-红黏土序列组成的风尘沉积是全球最完整、最连续的陆相古气候载体之一,蕴含了丰富的东亚古季风信息。解读黄土记录的信息以及透彻理解黄土替代指标与古气候的内在联系,都需要从纳米尺度揭示黄土成壤作用过程中碎屑矿物、成壤自生矿物形成与演化。本文利用高分辨透射电镜和场发射扫描电镜,研究了风尘沉积物中磁铁矿氧化的粒径制约、自生磁赤铁矿形成机制,建立了古土壤磁化率增强的成因矿物学理论,为磁化率与古气候的内在联系奠定了矿物基础;发现了第四纪黄土中广泛分布纳米棒状方解石及其含量和微结构显著的规律变化,从生物诱导矿化理论阐述了其形成机制;查明了凹凸棒石在黄土-红黏土序列中的分布规律,提出了古气候变化的黏土矿物响应机制。     

西昆仑山黄土的岩石磁学特征及其磁化率增强机制

成壤过程中形成的细颗粒的软磁性矿物被认为是导致古土壤磁化率增加的主要原因。但近来的研究表明,在一些地区,尤其是靠近沙漠边缘的黄土-古土壤序列,源区对黄土磁化率的影响要远大于成壤作用。因此,有必要对不同地区、不同环境条件下的典型黄土堆积进行详细的岩石磁学研究。日前,我们在西昆仑山北侧钻取了一根长达671m的岩芯,这为研究极端干旱区黄土的岩石磁学性质提供了难得的契机。本文对第一期黄土钻探得到的207m岩芯进行了详细的岩石磁学研究,结果表明: 昆仑山黄土的主要载磁矿物为磁铁矿和磁赤铁矿,同时还含有少量的针铁矿、赤铁矿;  该地区磁化率的变化主要受源区粗颗粒的软磁性矿物含量的影响,成壤作用形成的细颗粒磁性矿物对磁化率的贡献极小;  磁化率、粒度在0.5Ma左右急剧升高和变粗,主要与气候干旱化加剧有关。     

三门峡会兴沟剖面黄土—古土壤序列的岩石磁学研究

岩石磁学是古地磁学和环境磁学研究的基础,是鉴定岩石和沉积物中磁性矿物种类、粒度和含量的有效途径。对黄土高原东南部三门峡盆地的水沟—会兴沟旧石器遗址会兴沟剖面黄土—古土壤序列(S_0~S_8)进行系统的岩石磁学研究表明:本剖面沉积物的主要载磁矿物为磁铁矿、磁赤铁矿和赤铁矿,显示准单畴(PSD)磁性颗粒特征。所有磁学参数曲线均表现出基本一致的变化特征,与深海氧同位素曲线能够很好的对应,反映了第四纪以来的冰期—间冰期旋回中,东亚季风影响下的风尘黄土堆积中磁性矿物种类、粒度和含量的周期性变化特征。黄土中高矫顽力磁性矿物的相对含量要高于古土壤中的,而随着成土作用的加强,在古土壤中细粒的低矫顽力磁性矿物显著增加的同时,其中高矫顽力磁性矿物的绝对含量也相应增加。质量磁化率(χ)与非磁滞剩磁磁化率(χ_(ARM))和饱和等温剩磁(SIRM)及磁粒度参数χ_(ARM)/SIRM和χ_(ARM)/χ均呈明显的正相关关系,表明由成土作用产生的单畴(SD)颗粒和较小PSD颗粒对磁化率增强有显著的贡献。     

西峰剖面高分辨记录指标研究及古气候重建

对甘肃省西峰黄土- 红粘土剖面的夏季风替代性指标作了全面、系统的测试,包括磁化率、频率磁化率、碳酸盐、有机质和游离氧化铁的测定,并通过碳酸盐和磁化率值来初步恢复古气候变化特征。结果表明, 频率磁化率、有机质、游离铁和磁化率具有正相关关系,而碳酸盐变化与磁化率呈负相关,它们都可以很好地作为夏季风替代性指标。通过数学模型认为整个第四纪古气候变化大致可分为三个大的阶段：0~1.1 Ma BP,1.1~1.9 Ma BP和1.9~2.6 Ma BP。从0.1 Ma 以来,降水量和年均温都有降低趋势,说明气候向着越来越干冷的方向演化。     

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.     

中国第四纪黄土地球化学研究进展

在研究第四纪时期发牛的古环境变化的过程中,对矿物在表生环境下的地球化学行为的研究是正确理解一些十分重要的替代性气候指标环境意义的基础。在我国著名地质学家、地球化学家侯德封先生生前竭力倡导下,我国第四纪沉积,特别是黄土的地球化学研究这些年来取得了举世瞩目的成果。本文试图对我国这一领域发展的现状作一系统的综述,特别对存在的问题提出一些看法,以其推动我国第四纪地球化学的研究工作,并表示对侯老的怀念之情。     

最近2.5Ma黄土高原环境变化研究进展——来自洛川黄土地层的证据

洛川黄土—古土壤序列记录了最近2.5Ma黄土高原自然环境的演化和变迁。国内外研究人员运用地层学、古生物学、土壤学、磁学、沉积学、岩石学、矿物学、地球化学等多学科的研究方法，统一地层划分，明确成因机制，建立时间标尺，分析气候指标，提取环境信息，逐步揭示了自早更新世以来该区气候由暖湿而冷干、频繁剧烈波动的历史以及古生物的演替过程，为全球变化研究提供了重要资料。     

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.     

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。     

Particle size contributions to bulk magnetic susceptibility in Chinese loess and paleosol

The magnetic enhancement of paleosol is observed in the Chinese loess. The origin of this magnetic enhancement is still very uncertain. It is, however, a key problem in correctly understanding the paleoclimatic significance of changes of magnetic susceptibility and in transferring the magnetic signals to paleoclimatic parameters. Two main models have been proposed to explain the mechanism of magnetic enhancement in paleosols: a depositional model and a pedogenic model. Together with composition and concentration, grain size distribution of magnetic minerals also plays an important role to the magnetic enhancement of paleosols. Systematic susceptibility measurements were carried out on the samples of the upper part (S₀ to top of L₂) of three loess sections from Jixian, Xifeng, and Xining, along an east–west transect in the loess plateau, China. The samples with the highest value of magnetic susceptibility in S₁ and the lowest value in L₂ of each section were selected as the representatives. These representatives were separated into different grain size fractions based on Stokes’ law for coarse grains and by centrifuge for fine grains. Measurements of magnetic susceptibility and mass have been carried out on these fractions. Results show that for the loesses magnetic susceptibility changes little in the fractions with different grain size and for the paleosols it increases with decreasing grain size. The magnitude of changes is bigger in the east (Jixian and Xifeng) than that in the west (Xining). The fraction with the finest particle size in paleosols does not show very high magnetic susceptibility values. A new approach is used to estimate the contribution of each fraction to total magnetic susceptibility. The contribution comes mainly from coarse grains (>10 μm) for loess samples. It reaches about 90%. The main contribution comes, however, from the particles with medium size (10–0.2 μm). The very fine grained particle (<0.3 μm), which is considered to be with the pedogenic origin, contributes little to the bulk magnetic susceptibility, no more than 3% because of their very little amount. This approach provides a sounder basis for the study of the origin of the magnetic susceptibility enhancement in paleosols and of the paleoclimatic significance of magnetic susceptibility of loess and paleosols.     

The paleoenvironmental-magnetic record of the Gold Hill Steps loess section in central Alaska

In contrast to the loess-paleosol sequences of China and Central Europe, paleosols of the Halfway House and Gold Hill Steps loess sections in central Alaska show no enhancement in magnetic susceptibility. Evidence for maghemitization and ultrafine superparamagnetic minerals is found in the topsoil and in the lowest weathered loess of the sequence. These minerals are not detected in the three paleosols within the profiles. Rock-magnetic characteristics also indicate a smaller magnetic grain size in the loess with the lowest magnetic susceptibility values. This finding supports the interpretations of Begét et al. (1990) that magnetic concentration and corresponding susceptibility variations may be related to changes in wind intensity.     

最近13万年来黄土高原黄土剖面中绿泥石的化学风化与古气候变迁

对黄土高原不同纬度地区的4个黄土—古土壤剖面和lO个现代土壤样品中绿泥石的化学风化进行研究，发现黄土—古土壤剖面中的绿泥石在最近13万年发生了明显的化学风化，其风化程度受剖面位置和气候控制；黄土—古土壤剖面中ω(绿泥石 高岭石)／ω(伊利石)的比值与磁化率之间具有良好的负相关关系，绿泥石的化学风化与古土壤中铁氧化物矿物的形成和磁化率的增强之间有成因联系；黄土高原现代地表样品中ω(绿泥石 高岭石)／ω(伊利石)比值与现代年平均温度和年平均降水量有着良好的相关关系。认为ω(绿泥石 高岭石)／ω(伊利石)的比值可作为新的指示夏季风变化的替代性指标，对于定量重建古气候的变迁历史具有重要意义。     

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.