南黄海北部晚更新世以来常量元素记录的化学风化作用

以南黄海北部高沉积速率柱状样DLC70-3孔作为研究对象,对沉积物的黏土矿物和常量元素地球化学组成进行了综合分析。结果显示,DLC70-3孔沉积物中黏土矿物组合以伊利石为主,其次为蒙皂石,绿泥石和高岭石含量较低;绝大部分样品中伊利石与蒙皂石含量的比值<6,表明沉积物主要来源于黄河物质的输送。研究认为DLC70-3孔沉积物的化学风化指标CIA值受到海平面变化和源区气候变化共同控制,其中源区的气候变化为主要控制因素,而海平面变化造成的机械沉积分异作用主要影响27.80~38.00 m（MIS 4）层位沉积物的CIA值。CIA值显示在MIS 5和MIS 3期大陆化学风化作用较强,与内陆黄土高原地区夏季风和化学风化指标的变化趋势一致,尤其是在MIS 3早期（40~60 ka）记录的化学风化作用非常强,反映了黄河流域地区出现强夏季风降雨过程。     

成都平原风尘堆积的化学风化特征及其古气候意义

对成都平原胜利红土剖面进行了系统的常量元素分析,并与川西典型黄土、古土壤样品的地球化学特征进行对比,研究本区风尘堆积所经历的化学风化作用过程,并对该序列化学风化特征的古气候意义进行探讨.研究结果表明,成都红土的常量元素组成与川西典型黄土、古土壤样品相似,与上地壳的平均化学成分也基本一致,说明成都红土与川西黄土一样,在堆积以前也经历了多次充分混合作用.在成都红土的形成过程中,Ca、Na元素大量淋失,含量很低,K、Mg也出现了不同程度的淋失,说明成都红土经历了斜长石和钾长石的化学风化过程.进一步的研究表明,成都黏土已达到中等风化的末期阶段,而网纹红土已进入强烈风化作用阶段.从整个序列的化学风化参数来看,距今1.13 Ma以来化学风化强度不断减弱,表明四川盆地及周围地区更新世中期以来有逐渐变干的趋势,与全球气候变化具有一致性.     

化学风化及水动力分选对河流入海沉积物组成的影响

化学风化作用是引起河流沉积物相对其源岩发生地球化学分异的重要因素。在化学风化的初始阶段,长石含量减少,转化为黏土矿物,主要形成2:1型层状黏土矿物,如伊利石。随着风化作用的进行,伊利石脱K转化为1:1型层状黏土矿物,如高岭石(Nesbitt and Young,1982)。水动力分选同样显著影响沉积物的矿物和元素地化组成,粒度较粗、密度较大的石英、长石与重矿物一般富集于表层沉积物中,细粒级的,具片层状结构的黏土矿物则富集于悬浮物中(Garzanti et al,2011)。本研究讨论了长江、椒江、瓯江、木兰溪浊水溪等河流沉积物(包括悬浮物与表层沉积物)的化学风化与水动力分选。这些河流分属不同的源汇体系,长江属于"大河-大三角洲-宽广陆架"的源汇体系,作为世界性的大河,其下游干流悬浮物代表整个流域上陆壳细颗粒风化物质的混合,反映整个流域的化学风化强度。台湾河流属构造隆升背景下的山溪性小河流,有"瞬时大通量—极端气候影响—快速物质转换"的特点。其河流沉积物的源岩为多旋回的沉积岩和变质沉积岩,反映出强物理风化,弱化学风化的特点。浙闽河流同属于山溪性小河流,但其流域区域构造较稳定,地形起伏也较小,加之气候炎热多雨,所以化学风化较强,反映出流域受季风气候的控制。沉积物的Al2O3/Si O2值可作为一个可靠的矿物分选指标,Al2O3/Si O2与CIA,WIP,αAl值等化学风化指标的相关关系可以指示分选与风化作用对沉积物组成的影响。WIP受石英稀释作用影响较大,而CIA几乎不受石英稀释作用影响,二者的相关关系可以指示沉积多旋回性对沉积物组成的影响(Garzanti et al,2013)。沉积物地球化学组成的"粒度控制",是由于不同粒级沉积物中矿物组成差异所致,"粒度分异"的实质是"矿物分异"。本研究从矿物入手,讨论了水动力分选及化学风化作用对河流入海沉积物的影响,揭示出分选和化风化作用共同影响沉积物的矿物及元素地化组成。元素的表生地球化学行为,本质上是由元素在不同矿物中富集及迁移的规律决定的。化学风化和水动力分选都影响碎屑沉积物矿物组成的变化,继而引起沉积物元素地球化学组成的变化。     

红河盆地的化学风化作用:主要和微量元素地球化学记录

河流沉积物的元素含量有助于反映其流域的自然风化过程。红河是世界上重要的河流之一,但其沉积物的地球化学研究却几乎没有。本文通过开展红河盆地干流和主要支流40个样品的主要和微量元素地球化学分析,发现红河流域硅酸盐岩的化学风化作用为中等强度,与长江及亚马逊河的风化强度相近,而高于黄河,低于珠江;且化学风化作用受该区域的气候和构造作用控制。     

塔中地区志留系砂岩元素地球化学特征与物源判别意义

通过分析塔中地区志留系砂岩的某些常量元素和微量元素含量的变化,研究了该地区砂岩的地球化学以及其物源特征。结果表明塔中地区志留系的沉积构造环境为活动大陆边缘和被动大陆边缘,物源主要来自再循环造山带。砂岩的矿物成分主要在石英、钾长石、斜长石、伊利石、绿泥石等矿物之间变化。化学风化作用指标(CIW)和化学蚀变作用指标(CIA)反映了该地区砂岩的碎屑成分受到了强烈的风化作用,风化作用强度为:依木干他乌组>塔塔埃尔塔格组>柯坪塔格组。化学组分变化指标(ICV)反映了有些砂岩中含有第一次旋回沉积物。A-CN-K三角图反映了砂岩的碎屑源岩中斜长石含量要高于钾长石含量,主要在花岗闪长岩和花岗岩之间变化,砂岩的碎屑成分在风化过程中受到了钾的交代作用,长石以及长石中的高岭石发生了伊利石化。稀土元素及其比值(La/Yb)_N、(La/Lu)_N等也反映了该地区砂岩来源于花岗闪长岩和花岗岩的混合物。     

贵州高原地区典型风化剖面地球化学特征及其对比研究

选取了贵州高原喀斯特地区的典型硅酸盐风化剖面(毕节邓家湾剖面)和碳酸盐岩风化剖面(安顺平坝剖面)作为研究对象,以贵州地区的铝土矿剖面和中国黄土作为对比,对其元素分布特征及化学风化作用和强度与进行了研究。结果表明:SiO2、Fe2O3、Al2O3均为两个剖面含量最高的组分,但由于成土母质的化学组分不同,元素分布差异比较明显;同时由于经历的化学风化阶段并不相同,两个剖面的化学蚀变指数(CIA)值相差较大,邓家湾剖面经历了中等强度的风化作用,而平坝剖面的风化强度则十分强烈,CIA反映了土壤剖面的化学组成的差别,以及成土母质、风化成土过程和程度上的差异;虽然两个剖面稀土元素总量都较高,但配分模式存在着明显的差异,剖面的形成基本上是由于各自基岩的风化作用形成的,受中国黄土的影响并不明显。      

硅酸盐化学风化强度评估的沉积物指标与方法研究进展*

风化作用是源-汇沉积体系中的重要环节,气候、构造、地形、植被和岩性在不同程度上控制着地表硅酸盐化学风化,量化硅酸盐化学风化强度有助于开展全球性的实例研究对比。本研究归纳总结了基于碎屑沉积物的化学风化强度指标,包括岩石学和矿物学指标、元素地球化学指标和非传统稳定同位素指标,并指出了指标在应用中存在的问题。这些指标中:砂质沉积物骨架颗粒组成、粉砂级碎屑的矿物组成和矿物表面结构特征从矿物组成和结构上直观地反映了化学风化强度,运用不多但值得关注;黏土矿物组合和主微量元素指标如CIA、Rb/Sr、α^AlE等在实例研究中运用广泛,同时也显现出易受物源和水动力分选影响等弊端;新近开发的利用锂、硼、钾、镁、硅等同位素示踪化学风化强度的方法具有广阔的应用前景,同位素的分馏机理有待完善。源-汇体系中的其他过程如物源供给、水动力分选、成岩作用、再旋回作用、成土作用、物理淋滤和生物利用等会影响化学风化指标的使用效果,通过多指标的综合运用和相互验证,可以有效提升化学风化强度评估的准确性。     

江西武宁县石门寺钨铜矿区风化作用过程中W、Cu等元素的表生地球化学行为(之一)

以江西省武宁县石门寺钨铜矿区燕山期和晋宁期岩体风化作用过程中W、Cu等元素的表生地球化学行为为研究对象,通过反映风化程度的化学风化指数CIA、风化作用过程中组分得失的估算、不同介质中W、Cu等成矿元素的含量变化及大离子亲石元素表生地球化学行为等方法的研究表明,石门寺钨铜矿区2个时期岩体元素淋失与风化强度有关,土壤剖面中碱金属、碱土金属元素大量淋失,而W、Cu等成矿元素在土壤B层中显著富集;从岩石到土壤、再到水系沉积物风化过程中,W、Cu等成矿元素含量逐渐升高,为此,一级水系沉积物和B层土壤是赣北九岭W、Sn、Cu、Mo多金属矿集区地球化学找矿的绝佳采样介质,是矿集区内找矿新突破的有效手段。     

江西武宁县石门寺钨铜矿区风化作用过程中W、Cu等元素的表生地球化学行为（之一）

以江西省武宁县石门寺钨铜矿区燕山期和晋宁期岩体风化作用过程中W、Cu等元素的表生地球化学行为为研究对象,通过反映风化程度的化学风化指数CIA、风化作用过程中组分得失的估算、不同介质中W、Cu等成矿元素的含量变化及大离子亲石元素表生地球化学行为等方法的研究表明,石门寺钨铜矿区2个时期岩体元素淋失与风化强度有关,土壤剖面中碱金属、碱土金属元素大量淋失,而W、Cu等成矿元素在土壤B层中显著富集;从岩石到土壤、再到水系沉积物风化过程中,W、Cu等成矿元素含量逐渐升高,为此,一级水系沉积物和B层土壤是赣北九岭W、Sn、Cu、Mo多金属矿集区地球化学找矿的绝佳采样介质,是矿集区内找矿新突破的有效手段。     

黄土高原黄土和红粘土~(10)Be地球化学特征

充分认识元素和同位素在不同环境条件下的地球化学行为,是运用元素和同位素示踪环境变化的前提。对来自于黄土高原的黄土、古土壤和红粘土样品的宇宙成因核素10Be测量和化学成分分析,以及各种化学淋溶实验表明：10Be主要以吸附状态赋存于粉尘沉积物粘粒矿物的表面,部分已结合进自生的粘土矿物中;在粉尘沉积物风化过程中10BC与9Be和Al的活动性相似,基本没有发生迁移,其原因是连续沉积的粉尘含有大量的碱性物质,阻止了10Be的解吸附和淋滤;沉积和风化作用导致了10Be浓度与化学指标在黄土－古土壤和红粘土剖面中的协同变化。     

化学蚀变指数(CIA)及其在新元古代碎屑岩中的应用

化学蚀变指数(CIA)首先是作为一个判断源区化学风化程度的化学指标被广泛应用的,它的表达式通常表示为:CIA={x(Al_2O_3)/[x(Al_2O_3)+x(CaO~*)+x(Na_2O)+x(K_2O)]}×100,主成分均指摩尔分数,caO~*仅为硅酸盐中的caO。利用CIA分析湖南石门杨家坪剖面的新元古代地层,发现冰期沉积物(南沱组、东山峰组)的CIA值(60～70)和间冰期沉积物(湘锰组)CIA值(70～75)分别反映了冰期和间冰期的化学风化特点,并且根据渫水河组CIA值(55～70)可以判断渫水河组可能也是冰期的沉积产物。另一方面,可以利用A-CN-K图解判断沉积岩源岩成分,因为样品点在三角图解中投影点的趋势线与Pl—Ksp连线的交点代表的是源岩斜长石和钾长石的比率。据此初步判断渫水河组源岩成分是南秦岭—扬子地区元古宙TTG岩石。此外,钾交代作用在前寒武纪普遍发育,在A-CN-K图解中表现为样品点的趋势线向右偏离预测的风化趋势线,渫水河组的样品点就表现出这样的特征,可以判断渫水河组钾交代作用发育。样品的选取是进行CIA研究的一个重要的步骤。首先样品的岩性最好是细屑岩,这样可以排除分选作用对源岩成分的改造;其次,需要考虑去除成岩作用过程中加入的K_2O;再次,利用ICV这个化学指标来判断沉积再循环作用对沉积物成分的改造,ICV大于1意味着沉积再循环作     

Modelling Compositional Change: The Example of Chemical Weathering of Granitoid Rocks

Perturbation on the simplex is an operation which can be used to numerically describe changes in the composition of, for example, soils, sediments, or rocks. The combination of perturbation and power transformation provides a strong tool for analyzing compositional linear processes in the simplex. When the process is constrained in the sense of a well-known starting (or final) composition, noncentred principal component analysis can be used to estimate the leading perturbation vector of the process. Applying these mathematical tools to chemical major element data from a weathering profile developed on granitoid rocks allows us to model the compositional changes associated with the process of chemical weathering. The comparison of these results with the compositional linear trend defined by erosional products of several of the world's major drainage systems yields close similarities. The latter observation allows for a mathematical formulation of a global mean weathering trend within the system Al₂O₃–CaO– Na₂O– K₂O. We further demonstrate the usefulness of the approach for validating processes behind individual trends and for combining the effects of different processes which modify the composition of soils, sediments, and rocks. Alternatives to the Chemical Index of Alteration (CIA) are discussed to obtain a translation-invariant scale for the process of chemical weathering.     

Pervasive autocorrelation of the chemical index of alteration in sedimentary profiles and its palaeoenvironmental implications

The chemical index of alteration has been used widely for reconstruction of the palaeoclimate. However, the mechanisms and environmental factors controlling the chemical index of alteration of sediments are not yet fully understood. In this study, autocorrelations of the chemical index of alteration in nine sedimentary profiles, from both the land and the sea, spanning different geological times, are discussed. The sediments of these profiles have different origins (dust, fluvial or ocean sediments) and are from various climate situations and sedimentary environments. Autocorrelations of chemical index of alteration series are ubiquitously evident in all profiles. It is suggested here that autocorrelations may be caused by post‐depositional changes such as persistent weathering and diagenesis. As a result, the chemical index of alteration may not reflect climatic conditions during the time of sediment deposition. This study strongly recommends the confirmation of the reliability and veracity of the chemical index of alteration before it is adopted to evaluate the weathering degree of parent rocks and to reconstruct the past climate. Significant autocorrelations in loess profiles were specifically observed, suggesting that the existing understanding of loess deposition in terms of climate conditions requires re‐examination, and that previous reconstructions of rapid climate changes (for example, in centennial‐millennial scales) should be treated with caution.     

Geochemistry of large river suspended sediments: silicate weathering or recycling tracer?

This study focuses on the major and trace element composition of suspended sediments transported by the world’s largest rivers. Its main purpose is to answer the following question: is the degree of weathering of modern river-borne particles consistent with the estimated river dissolved loads derived from silicate weathering?In agreement with the well known mobility of elements during weathering of continental rocks, we confirm that river sediments are systematically depleted in Na, K, Ba with respect to the Upper Continental Crust. For each of these mobile elements, a systematics of weathering indexes of river-borne solids is attempted. A global consistency is found between all these indexes. Important variations in weathering intensities exist. A clear dependence of weathering intensities with climate is observed for the rivers draining mostly lowlands. However, no global correlation exists between weathering intensities and climatic or relief parameters because the trend observed for lowlands is obscured by rivers draining orogenic zones. An inverse correlation between weathering intensities and suspended sediment concentrations is observed showing that the regions having the highest rates of physical denudation produce the least weathered sediments. Finally, chemical and physical weathering are compared through the use of a simple steady state model. We show that the weathering intensities of large river suspended sediments can only be reconciled with the (silicate-derived) dissolved load of rivers, by admitting that most of the continental rocks submitted to weathering in large river basins have already suffered previous weathering cycles. A simple graphical method is proposed for calculating the proportion of sedimentary recycling in large river basins. Finally, even if orogenic zones produce weakly weathered sediments, we emphasize the fact that silicate chemical weathering rates (and hence CO₂ consumption rates by silicate weathering) are greatly enhanced in mountains simply because the sediment yields in orogenic drainage basins are higher. Hence, the parameters that control chemical weathering rates would be those that control physical denudation rates.     

Geochemical and mineralogical characteristics of recent clastic sediments from lower Godavari river: Implications of source rock weathering

The major, trace and rare earth elements geochemistry and clay mineral compositions in the river bed sediments from lower reaches of Godavari river suggest that they are derived from weathering of felsic rocks. Trace and rare earth elemental compositions indicate evidence of sedimentary sorting during transportation and deposition. Lower concentrations of transition elements, such as V, Ni and Cr imply enrichment of felsic minerals in these bed sediments. The REE pattern in lower Godavari sediments is influenced by the degree of source rock weathering. The light rare earth elements (LREE) content are indicating greater fractionation compared to the heavy rare earth elements (HREE). A striking relationship is observed between TiO₂ and gZREE content suggesting a strong control by LREE-enriched titaniferous minerals on REE chemistry. Shale-normalized REE pattern demonstrate a positive Eu anomaly, suggesting weathering of feldspar and their secondary products, which are enriched in Eu. Chondrite-normalised REE pattern is characteristic of felsic volcanic, granites and gnessic source rocks. Trace elemental compositions in sediments located near urban areas suggest influence of anthropogenic activity. Chemical Index of Alteration (CIA) is high (avg. 65.76), suggesting a moderate chemical weathering environment. X-ray diffraction analysis of clay fraction shows predominance of clay minerals that are formed because of the chemical weathering of felsic rocks.     

新生代气候变化与陆地硅酸盐岩风化和海洋Sr同位素研究

新生代全球气候自始新世55Ma以来在逐渐变冷。陆地硅酸盐岩在造山带和高原地区通过加速化学风化，消耗了大气中的CO2温室气体造成了新生代气候变冷。目前关于陆地硅酸盐岩风化与海洋Sr同位素关系的研究，主要存在两种观点：一种认为海水Sr同位素变化可作为全球地表化学风化的替代性标志；另一种观点认为全球并不存在平均地表风化这种说法，海水Sr同位素在地质历史中的演化是主要与某种地质构造事件或岩石类型的风化有关。我国应当不失时机地加快对源于喜玛拉雅山地的恒河－布拉马普特拉河进行研究，以进一步揭示陆地硅酸盐岩风化与海洋Sr同位素变化的关系。     

湖光岩玛珥湖非残渣态Rb/Sr比值研究及其古气候意义

湖泊沉积物Rb/Sr比值变化主要受控于Sr含量的变化,而真正反映流域化学风化强度的主要是迁移至集水盆地(湖泊)的溶解态Sr2+,在沉积物中主要以碳酸盐结合态和铁锰氧化物结合态等形式存在.通过对不同赋存状态的Rb/Sr比值与相应的Sr含量之间相关关系的对比分析发现,沉积物非残渣态Rb/Sr比值更好地反映了流域化学风化.根...     

Elemental and Sr–Nd isotopic compositional disparity of riverine sediments around the Yellow Sea: Constraints from grain-size and chemical partitioning

To obtain a better understanding of the source compositions of the river sediments around the Yellow Sea and their relationship with source rocks, elements and strontium-neodymium (Sr–Nd) isotopes of different grain-sizes (silt and clay populations) and chemical (labile and residual phases) fractionations in riverine sediments were studied extensively. These results clearly revealed a systematic compositional disparity between Korean river (KR) and Chinese river (CR) sediments, especially in the residual (detrital) fraction. The geochemical dissimilarity between these might reflect inherited signatures of their source rocks but with minor control from chemical weathering. In particular, the remarkable enrichment of some elements (iron (Fe) and magnesium (Mg)) and the behavior of large ion lithophile elements (e.g., barium (Ba), potassium (K) and Sr) during weathering as well as less-radiogenic Sr isotopic compositions implies that CR sediments might be weathering products of relatively more mafic rocks, with abundant ferromagnesian and plagioclase feldspar minerals, compared with KR sediments derived from silicic granites with relatively higher quartz and potassium feldspar contents. This different petrological rationale is clearly evident in an A–CN–K diagram, which estimated the source rock of CR sediments as granodioritic, a composition that reflects accurately the average composition of weathered continental crust in China. The recognition of such geochemical systematics in two river sediments, especially in grain-size and chemically partitioned data, may contribute to the establishment of provenance tracers for the Yellow Sea and East China Sea sediments with multi-sources as well the dust deposition in the western Pacific.