宁夏长城塬全新世黄土—土壤元素地球化学特征与成壤环境演变研究

【作者】 杨红瑾；

【导师】 黄春长；

【作者基本信息】 陕西师范大学 ， 自然地理学， 2010， 硕士

【摘要】 当今,一系列前所未有的重大的全球性环境问题对世界各国的经济发展和人类社会的生存产生了深远的影响,全球气候变化及其产生的影响更是与人类的生存息息相关。全新世作为与人类社会发展密切相关的一个时期,是国际过去气候变化研究中的重点,研究全新世气候变化对预测未来环境和气候演变具有重要的参考价值。至今许多研究方法和信息载体已经引入全新世环境演变的研究中,并且取得了很大的进展。黄土高原的黄土一土壤序列以其堆积完整、沉积连续的特点成为第四纪环境变迁研究中公认的良好信息载体之一,通过对黄土-土壤序列的高分辨率研究,学者们己经弄清楚了中国黄土高原地区风成黄土的形成机制、物质来源及其所反映的气候变迁历史。本文在了解了国际国内地学界和环境学界第四纪环境演变进展的基础上,经过缜密的野外观察,选定宁夏彭阳长城塬地区一个典型的全新世黄土-土壤剖面进行高分辨率采样,确定磁化率、化学元素、粒度、烧失量、碳酸钙和吸湿水含量等环境指标进行室内测定分析,获得了宁夏长城塬全新世环境演变的高分辨率数据信息。实验数据分析表明磁化率、烧失量、吸湿水和粘粒高值均出现在土壤层,低值出现在黄土层。而碳酸钙、粗粉沙和沙级颗粒却在土壤层中含量低,黄土层中含量高。活动性较强的元素Na、Ca、Sr在土壤层中发生迁移淋失,从而在黄土层中出现高值。其他微量元素和大部分重金属元素(除Pb、V外)的含量分布特征反映了受气候变化的成土过程的影响,在土壤层中表现为高值,在黄土层中表现为低值。钾钠比、残积系数,Rb／Sr比值作为良好的气候代用指标在彭阳CCY剖面中呈现出有规律的变化：即最大值出现在古土壤层,最小值出现在黄土层；硅铝系数、钙镁比(CaO/MgO)、(CaO+Na2O+K2O)/Al2O3则表现出相反的变化规律。这些化学参数与磁化率、频率磁化率曲线形成很好的对比,表明黄土风化成壤过程中元素的迁移变化可以间接反映全新世以来的气候变化和成壤环境的变化。元素对环境条件变化的响应表现在地球表生作用过程中化学元素的迁移富集。土壤发生的实质是各种化学元素的重新组合及其迁移淋失。因此,黄土-土壤序列中元素含量的动态变化、化学参数等地球化学特征都对环境变化有直接的指示意义,代表了研究区域土壤的化学风化强度及时空规律。本文结合数据分析对彭阳地区CCY黄土-土壤剖面的元素地球化学特征和风化成壤强度以及成壤环境演变进行了研究讨论,主要得出以下结论：(1)相对马兰黄土层,其他黄土层中的常量元素和微量元素均没有发生明显的分异,反映了黄土物质组成的均一性和物质来源的统一性；而在古土壤层(So)中常量元素和微量元素发生了明显的分异,全新世大暖期,CCY剖面在土壤风化成壤过程中,常量元素中Ca, Mg、Na在化学风化过程中发生了迁移淋失,Si、Fe、K、则呈现出富集特征。常量元素的活动性大小为CaO>MgO>Na2O>SiO2Fe2O3>K2O。元素的迁移特征表明宁夏长城塬黄土只经历了早期的化学风化,应处在脱Ca、Na阶段,尚未进入去K风化阶段。微量元素中Sr发生了很明显的迁移淋失,大部分微量元素呈现富集特征,元素迁移序列由强至弱依次为Sr>Ba>V>Cu>Zn>Ni>Mn>Ti>Rb>Cr。因生物生命活动特别是植物生长吸收而应在地表大量富集的微量元素Cu、Zn,在CCY剖面中富集很弱,反映全新世宁夏长城塬地区风化成壤时期植物生命活动弱,植被生长状况较差。(2)化学蚀变指数CIA和A-CN-K风化趋势图指示在全新世大暖期,黄土高原地区由北到南黄土化学风化程度逐渐增强,彭阳长城塬地区经历了低等的化学风化作用,与平凉地区黄土风化相似,而弱于关中扶风地区的黄土风化作用。彭阳地区黄土的化学风化过程及其与黄土高原其他地区黄土化学风化强度的差异主要受到气候条件的控制,气候条件通过年平均气温和年降水量对化学风化的地球化学环境起着重要的影响,其中降水在整体比较干旱的黄土高原地区的黄士化学风化过程中可能起着更为重要的制约作用。(3)CCY全新世黄土-土壤剖面各项环境代用指标和元素地球化学特征揭示了全新世以来气候环境的演变,相应地宁夏长城塬地区的成壤环境表现出由全新世早期干冷气候条件下的沉积期(形成过渡层Lt)向全新世中期温湿气候条件下的强成壤期(形成古土壤S0)演变,再向全新世晚期冷干气候条件下的沉积期(形成全新世黄土L0)的演变规律。1500a B.P.以来,气候条件有所改善,在人类农业耕作扰动下,形成成壤作用较强的表土层(TS)。(4)与黄土高原关中地区的磁化率、Rb／Sr值和CIA指数的比较发现,相对于南部关中地区的亚热带暖湿气候,彭阳地区在全新世大暖期可能为暖温带的气候环境。(5)重金属元素的相关性分析与成壤环境演变研究表明在万年时间尺度上,彭阳长城塬地区土壤中重金属元素含量变化,从表面上看,受到土壤粒度成分和碳酸钙含量变化影响。深入分析发现,其中最根本的控制因素,还是季风气候的变化导致沙尘暴强度和堆积速率以及降水量变化。更多还原

【Abstract】 Nowdays a series of global environment problems which have never occurred before, especially global changes in climate have deep impacts on both economic development of all the coutries and survival of human beings. The Holocene which is closely related to the development of human society is an essential part of PAGES. And study of the Holocene climate change has a viallty inportmant reference value to forcasting future environment changes. Many methods were introduced into the studies and great progress was made. The loess-paleosol sequence in the Loess Plteau which is located in Northwest China was accepted as a wonderful information carrier in Quaternary research. The formative mechanism, source of loess-paleosol and history of climatic changes in the Loess Plateau was made clear through the studies of the Loess-paleosol profile.Through field exploration, systematical sampling and experimenting, we gained a lot of high-definition datum of magnetic susceptibility, partical size, chemical elements, content of calcium carbonate, loss-on-ignition and hygroscopic water which have an important indicative significance to paleoclimate evolution. The results show that the content of magnetic susceptibility, loss-on-ignition, hygroscopic water and clay in soil are higer than those in loess. But the content of calcium carbonate, coarse fraction and sand-size particles in soil are lower than those in loess. The elemet Na, Ca, Sr were obviously migrated and leached during the chemical weathering progress so that they had a high value in loess.The distribution characteristics of heavy metals (except Pb, V) and other trace elements reflect the effect of soil formation, so that they have a high value in soil and low value in loess. K2O/Na2O ratio, eluvial coefficient and Rb/Sr, as good proxy climate indexes,varied regularly in the CCY Holocene loess-soil profile, i.e. they peaked in the paleosol,and were lower in the loess,and reduced along with the intensity of pedogenesis decreasing.Whereas CaO/MgO ratio, SiO2/Al2O3 ratio and leaching coefficient went the other way in the profile,indicating that during the paleosol formation,the climate was warm and humid with abundant precipitation;while during the loess accumulation.the climate was relatively cold and arid.There is a good comparisons between these chemical parameters and magnetic susceptibility, frequency dependent susceptibility, showing that during the process of loess chemical weathering.elements seems to indirectly reflect the change of climate and pedogenic environment.The migration and enrichment of chemical elements were related to the environment changes. The Holocene loess-soil in the Loess Plateau constituted an excellent record of climate evolution, aeolian dust and soil formation. The essential of soil was that the chemical elements reset and mignation. Therefore, the distribution of elements content in soil and the chemical parameter all can show the variety of environment and intension of chemical weathering in Pengyang. In paper, the element geochemical characteristics, intensity of chemical weathering and pedogenetic environment changes were studied and discussed. The conclusions maily are as follows:(1)In relation to Malan loess layer, the major elements and trace elements in other loess layers were not obviously differentiated, which reflect the homogeneity and uniformity of loess material. But in Holocene Megatherma, the major elements and trace elements were differentiated in paleosoil (So). During the pedogenetic progress, Ca, Mg, Na were leached but Fe, Si, K were enriched. According to the migration ability, the major elements are ranked in the following order: Ca>Mg>Na>Mn>Si>Ti>Fe>K. The migration features of the macro-elements reveal that Holocene loess in Pengyang only experienced primary process of chemical weathering characterized by leaching of Ca、Na,and not reached the secondary process characterized by leaching of K.Most of the trace elements were enriched except the element Sr was obviously migrated and leached during the chemical weathering process. According to the migration ability, the major elements are ranked in the following order:Sr>Ba>V>Cu>Zn>Ni>Mn>Ti>Rb>Cr. Cu, Zn was faintly enriched in CCY profile, which indicated that the vegetation is poor during the chemical weathering progress in Pengyang.(2) CIA and A-CN-K ternary diagram indicate that intensity of chemical weathering of loess in Loess Plateau was gradually increased from northwest to southeast. In Holocene Megatherma, the Pengyang loess underwent primary chemical weathering, which is similar to that of Pingliang loess, much weaker than that of Guangzhong loess. The chemical weathering differences among the studied Aeolian-dust deposits in Loess Plateau were mainly induced by the distinction of climate condition, which imposed important influence on the geochemical environment through the mean annual temperature and annual precipitation. It seems that the annual precipitation has more important influence on the chemical weathering process in arid Loess Plateau.(3)The environmrnt proxy indicators and element geochemical characteristics in CCY Holocene loess-soil profile reveal the evolution of climatic environment. Pedogenetic environment in Pengyang underwent such process:the early Holocene with dry and cold climate deposited trasitional loess Lt, the middle Holocene with warm and humid climate developed paleosoil So, the late Holocene with dry and cold climate deposited Holocene loess Lo. Since 1500a B.P.,the climate condition had improved, and a potent soil layer TS was developed under the human farming perturbations.(4)Comparison with magnetic susceptibility, Rb/Sr, CIA of JYC profile at Guangzhong basin, these values of CCY profile was obviously lower, showing that paleosol’s intensity of pedogenesis in the CCY profile was significantly weaker than that in JYC profile, precipitation was on the low side, which indicating that in relation to the subtropical climate in Guangzhong basin, Changcheng loess tableland in Pengyang may only belong to warm temperate climate in Holocene Megathermal.(5)Research in pedogenetic environment changes and correlation analysis between heavy metals and climatic index indicate that at a long-time scale particle-size distribution and content of calcium carbonate are the important factors that directly affect the distribution of heavy metals in the profile. However, these are ultimately affected by the changes in the intensity of dust storms and precipitation that controlled by monsoonal climatic changes during the Holocene.更多还原