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1.
Diamondoids are commonly found in petroleum and sediments and have an inherent resistance to thermal and biological destruction, which means they can provide useful information in situations where conventional biomarkers cannot. Here, we present the results of an investigation of the effects of atmospheric evaporation on the concentration and distribution of low molecular weight diamondoids in four petroleum fractions (gasoline, condensate, diesel and fuel oil). These experiments indicate that both adamantanes and diamantanes evaporate with the other light hydrocarbons from oils and that variations in the concentrations of these compounds during evaporation are controlled by the type of petroleum fraction, the extent of evaporation and the boiling point of the diamondoid compounds within the oil. Evaporation has a significant effect on adamantane concentration ratios, whereas no changes in diamantane concentration ratios occur, suggesting that diamantane-based concentration and distribution indices can be used for the correlation of oils and determination of maturity even if oils have undergone evaporation. Some diamondoid concentration ratios, such as adamantane/1-methyladamantane, 1-methyladamantane/2-methyladamantane, 1-methyladamantane/1-ethyladamantane, 1-methyladamantane/4-methyldiamantane, adamantane/diamantane and 1,3-dimethyladamantane/4,9-dimentyldiamantane, progressively decrease with ongoing evaporation and are independent of petroleum fraction type, indicating that given the original unaltered index value, these indices can be used to deduce the relative extent of oil evaporation. The study also indicates that slight to moderate evaporation of oils leads to an increase in diamantane concentrations that is nearly proportional to the extent of oil evaporation, indicating that these compounds can be used as indices to estimate the extent of oil evaporation. 相似文献
2.
Lithostratigraphy, mineralogy, major and trace element concentrations, carbon and sulphur contents are investigated from a shallow depth profile from Pachapadra paleo-lake, Thar Desert, north-western India, to understand the phases of paleo-hydrology and paleo-limnology. Based on the geochemical proxies (Na/Al, Si/Al, Zr/Al and Ca/Mg) and evaporite mineralogy, the depth profile is divided into three geochemical zones of variable sediment-water interaction, evaporation and aeolian activity. The sub-recent zone (I) enriched in halite (NaCl) indicates low chemical weathering and higher aeolian input. The intermediate relatively humid zone II is enriched in major elements, trace elements and calcite (CaCO3) and reflects higher chemical weathering in the catchments. Zone III is enriched in gypsum (CaSO4·2H2O) and characterised by lower chemical weathering, higher aeolian activity and evaporation. 相似文献
3.
峡东地区南华纪-寒武纪地层风化过程元素及Sr-Nd同位素演化特征及其地球化学意义 总被引：1，自引：0，他引：1
以峡东地区南华纪、震旦纪和寒武纪标准地层泥岩、冰碛泥岩、砂岩、灰岩和白云质灰岩及对应的风化土壤为研究对象, 分析了地层风化成土过程中不同元素的迁移行为, 根据剖面样品的Sr-Nd同位素组成变化, 探讨了其同位素体系的封闭性特征与应用意义.结果表明: (1) 不同岩性基岩在成土过程中的蚀变强度有明显的差异, 在相似地表条件下, 碳酸岩风化剖面的风化程度高于泥质岩和砂岩; (2) 通过对比稳定高场强Ti元素在基岩和风化剖面中的含量变化, 计算出土壤样品在风化过程中体积相对基岩发生的改变量, 进而计算出不同岩性基岩在风化过程中微量元素的绝对含量变化以探讨这些元素的活动规律.结果表明, 灰岩和白云质灰岩的风化剖面元素含量变化明显, 而在泥质岩风化过程中大多数元素保持了相对稳定, 说明沉积岩风化过程中元素的活动性特征明显地受到了原岩矿物组成的制约.风化过程中, 不同性质的元素的活动性差异明显, 其中亲硫元素(Cu、Zn、Pb、Mo) 和大离子亲石元素(Rb、K、Sr、Ba) 在不同岩性的风化剖面中均表现出明显的元素含量变化, 而高场强元素(Zr、Hf、Nb、Ta) 含量则相对稳定; (3) 泥质岩风化形成的土壤层REE含量变化较小, 而碳酸岩风化土壤层REE含量发生了明显下降, 且其风化形成的土壤表现出LREE和HREE相对于MREE的富集.无论是碳酸岩或泥质岩风化形成的土壤, 均出现了明显的Eu负异常和Ce的正异常, 但在其原岩中这些异常并不存在或不明显; (4) 基岩与土壤剖面间Sr同位素组成和Rb/Sr比值存在明显差异, Rb-Sr同位素组成发生了明显的开放.所形成土壤层的Sr同位素组成受到2种因素的约束: 原岩性质和外来组成的Sr同位素比值.因此在总体上, 风化土壤的Sr同位素组成已不能代表基岩的Sr同位素组成; (5) 沉积岩风化过程中, 碳酸岩和泥质岩形成的风化土壤基本保持了原岩的Sm-Nd同位素组成特点, 由其组成所获得的Nd同位素亏损地幔模式年龄等能反映其原岩信息, 而近源沉积形成的砂岩和含砾冰碛泥岩所形成的土壤, 其Nd同位素组成则存在不同程度的改变. 相似文献
4.
塔城盆地地下水氟分布特征及富集机理 总被引：2，自引：0，他引：2
塔城盆地位于新疆维吾尔自治区西北部,干旱少雨,蒸发强烈。但相对于新疆其他盆地,塔城盆地地下水水质相对较好,溶解性总固体和F-含量相对较低。为解译这种差异及盆地内高氟地下水的成因,本文在对盆地地下水样品水化学组分系统分析的基础上,结合多种水文地质调查数据,利用数理统计、离子比及主成分分析等手段,研究高氟水的成因及其分布规律。结果表明:受气候以及地质等因素控制,研究区地下水氟浓度总体较低,高氟水主要分布于扇前洼地及盆地中部的低洼地带;受承压含水层的顶托补给,地下水氟浓度呈现出上高下低的垂向分带特征。研究区地下水径流途径短,水循环快,水岩相互作用时间较短,且山区地下水以深径流形式循环补给平原区深层承压含水层,再顶托补给潜水,避免了强烈的蒸发浓缩作用。山前洪积扇地下水氟富集主要受控于沉积地层中含氟矿物的风化溶解,而岩石风化、蒸发浓缩、阳离子交换、竞争吸附为平原区地下水氟浓度的主要影响因素。 相似文献
5.
Statistical empirical index of chemical weathering in igneous rocks: A new tool for evaluating the degree of weathering 总被引：6，自引：0，他引：6
Chemical weathering indices are useful tools in characterizing weathering profiles and determining the extent of weathering. However, the predictive performance of the conventional indices is critically dependent on the composition of the unweathered parent rock. To overcome this limitation, the present paper introduces an alternative statistical empirical index of chemical weathering that is extracted by the principal component analysis (PCA) of a large dataset derived from unweathered igneous rocks and their weathering profiles. The PCA analysis yields two principal components (PC1 and PC2), which capture 39.23% and 35.17% of total variability, respectively. The extent of weathering is reflected by variation along PC1, primarily due to the loss of Na2O and CaO during weathering. In contrast, PC2 is the direction along which the projections of unweathered felsic, intermediate and mafic igneous rocks appear to be best discriminated; therefore, PC1 and PC2 represent independent latent variables that correspond to the extent of weathering and the chemistry of the unweathered parent rock. Subsequently, PC1 and PC2 were then mapped onto a ternary diagram (MFW diagram). The M and F vertices characterize mafic and felsic rock source, respectively, while the W vertex identifies the degree of weathering of these sources, independent of the chemistry of the unweathered parent rock.
The W index has a number of significant properties that are not found in conventional weathering indices. First, the W index is sensitive to chemical changes that occur during weathering because it is based on eight major oxides, whereas most conventional indices are defined by between two and four oxides. Second, the W index provides robust results even for highly weathered sesquioxide-rich samples. Third, the W index is applicable to a wide range of felsic, intermediate and mafic igneous rock types. Finally, the MFW diagram is expected to facilitate provenance analysis of sedimentary rocks by identifying their weathering trends and thereby enabling a backward estimate of the composition of the unweathered source rock. 相似文献
6.
新疆伊犁黄土化学风化特征及其控制因素 总被引：1，自引：0，他引：1
通过对伊犁盆地昭苏黄土剖面元素地球化学分析,结合与黄土高原典型第四纪风成堆积洛川剖面的对比,对伊犁黄
土常量元素分布规律及化学风化特征进行初步研究发现,伊犁昭苏剖面黄土和古土壤主要元素含量具有较好的一致性,暗
示着沉积后两者所经历的风化作用相似。与黄土高原相比,伊犁黄土和古土壤除CaO,MgO和Na2O,P2O5分别出现较大程度的
富集和较小程度的亏损外,其它常量元素含量的变化趋势基本一致;CIA指数、Na/K比值及A-CN-K图解显示伊犁黄土和古土
壤均经历了低等强度的化学风化作用,弱于黄土高原的洛川黄土及古土壤,黄土和古土壤风化分异作用不明显。气候条件
尤其水热组合模式（降水、温度和蒸发量等）对伊犁黄土化学风化的起着重要的制约作用。 相似文献
7.
Characterisation of groundwater chemistry in an eastern coastal area of Cuddalore district,Tamil Nadu 总被引：1，自引：0，他引：1
K. Srinivasamoorthy M. Vasanthavigar S. Chidambaram P. Anandhan V. S. Sarma 《Journal of the Geological Society of India》2011,78(6):549-558
The groundwater quality detoriation due to various geochemical processes like saline water intrusion, evaporation and interaction
of groundwater with brines is a serious problem in coastal environments. Understanding the geochemical evolution is important
for sustainable development of water resources. A detailed investigation was carried out to evaluate the geochemical processes
regulating groundwater quality in Cuddalore district of Tamilnadu, India. The area is entirely underlined by sedimentary formations,
which include sandstone, clay, alluvium, and small patches of laterite soils of tertiary and quaternary age. Groundwater samples
were collected from the study area and analyzed for major ions. The electrical conductivity (EC) value ranged from 962 to
11,824 μS/cm, with a mean of 2802 μS/cm. The hydrogeochemical evolution of groundwater in the study area starts from Mg-HCO3 type to Na-Cl type indicating the cation exchange reaction along with seawater intrusion. The Br/Cl ratio indicates the evaporation
source for the ion. The Na/Cl ratios indicate groundwater is probably controlled by water-rock interaction, most likely by
derived from the weathering of calcium-magnesium silicates. The plot of (Ca+Mg) versus HCO3 suggests ions derived from sediment weathering. The plot of Na+K over Cl reflects silicate weathering along with precipitation.
Gibbs plot indicates the dominant control of rock weathering. Factor analysis indicates dominance of salt water intrusion,
cation-exchange and anthropogenic phenomenon in the study. 相似文献
8.
Weathering effects on the strength and deformational behaviour of crystalline rocks under uniaxial compression state 总被引：1，自引：0，他引：1
Uniaxial compression tests were performed on different categories of weathering of three lithological units: Malanjkhand granite; Nagpur basalt; and Delhi quartzite, occurring in central and northern parts of India. The deformational behaviour is studied in terms of variation in tangent modulus (Et50) and initial modulus (Ei) due to weathering. The power relationship between uniaxial compressive strength (σc) and Et50 shows strong correspondence for weathering sequence of common rock types. This relationship has been established by regression analysis and significant correlation parameter (coefficient of determination, r2=0.87) for crystalline rocks. It is shown that there is a systematic decrease in stiffness ratio, that is, ratio of tangent modulus and uniaxial compressive strength with increased weathering state. Comparison of Et50 and Ei values has shown that Et50 decreases more gradually than Ei, and reduction is more drastic for Ei values with an increased degree of weathering in all the three rock types. The mode of failure has been found to be influenced by weathering extent in rocks. A brief account is given of the intrinsic characteristics of fresh and weathered rocks and mineralogical changes produced by weathering investigated quantitatively. Correlation drawn between the petrographical and mechanical indices has shown that mechanical properties are apparently dependent on the intrinsic characteristics of weathered rocks. 相似文献
9.
在自然营力和人为作用下,平遥古城墙和民居的青砖产生了严重的风化病害,亟须保护和修复。通过现场调查,发现风化病害发生部位都在毛细水上升高度范围内（距地面1~3m）,主要风化形式为等厚状剥落、鳞片状剥落、颗粒脱落、盐分结晶。通过室内测试和实验,得到了古砖的矿物、化学成分,物理、力学性质以及耐盐、耐冻性能。本文认为毛细水在墙体中上升时携带的盐分是古砖风化的主要原因,冻融循环则加剧了风化的进程。由于水分减少和/或温度降低,古砖孔隙中的可溶盐会“捕获”自由水使其变成带有若干结晶水的结晶盐。盐分结晶后体积增大在古砖孔隙中产生的膨胀力致使古砖破坏。根据古砖的风化机理,提出了“阻水、脱盐、强砖”的防风化方法。本文提出的古砖风化机理和防风化方法的技术流程和分析方法,可以为相关砖石文物的保护和修复提供参考。 相似文献
10.
11.
在相同的物理化学(常温、常压、氧化)环境中,用大气降水分别对两块相似的硫化物矿石样品进行蒸发或淋漓实验,模拟干旱地区和潮湿地区的岩矿石风化作用。实验共进行603d。实验结果显示:经蒸发实验的样品无论是硬度、颜色和矿物组成等都发生了明显的变化,而经淋漓实验的样品无论硬度、颜色和矿物成分等基本保持实验前的特征;作蒸发实验浸泡液的pH值先降后升,最后稳定在6.5～7,而作淋漓实验淋漓液的pH先降,然后稳定于5左右,浸泡液的阳离子浓度却比淋漓液的高。实验结果表明蒸发作用比淋漓作用更容易使岩矿石风化。蒸发作用主要是化学反应。所以,这一实验结果向干旱地区主要是物理风化的传统认识提出了的挑战。 相似文献
12.
Weathered and fresh samples of metamorphic rocks from Sri Lanka were collected from various localities and analysed for major elements by XRF method (RIGAKU, KG-X System, Japan). The content of water was determined by the ignition method.
The XRF results, obtained from these samples form the basis of a new index of chemical weathering, particularly for Sri Lanka, which is named the Silica-Titania Index, and is calculated as follows: Silica-Titania Index = {(SiO2/TiO2)/[(SiO2/TiO2) × (Al2O3/TiO 2) × (SiO2/Al2O3)]} × 100(molecular proportions).
The index can be used to determine the degree of weathering in chemically weathered silicate rocks of other countries in tropical regions. A triangular diagram plots the position of this index. The point load strengths of fresh rocksand weathered rocks with different degrees of weathering were correlated with the values of this new chemical index. The relative variation in strengths of fresh rocks and weathered rocks clearly indicates its suitability and usefulness for engineering geologists. 相似文献
13.
Major element chemistry of the Huai River basin, China 总被引：3，自引：0，他引：3
Liang Zhang Xianfang Song Jun XiaRuiqiang Yuan Yongyong ZhangXin Liu Dongmei Han 《Applied Geochemistry》2011,26(3):293-300
The chemistry of major ions (Ca, Mg, Na, K, HCO3, SO4, Cl and Si) in the water of the Huai River basin was studied, based on samples from 52 sites from nine different water bodies in July 2008. Ions and total dissolved solids (TDS) displayed clear spatial patterns with lower concentrations in the south and higher in the north of the basin; the same conditions were also found in the East Line of South-North Water Transfer Project (SNWTP) in this region. The Huai River main channel and Hongze Lake have moderate ion concentrations relative to the whole basin. TDS concentrations versus the weight ratios of Na/(Na + Ca) and ternary ions demonstrate that the southern rivers (Shi R. and Pi R.) are mainly controlled by the weathering of carbonates, whereas the northern water systems (Guo R., Shaying R., Nansi Lake and its tributaries) are dominated by the weathering of evaporites. The Huai River main channel, Hongze Lake and the East Line of SNWTP are synergistically influenced by weathering of evaporites and carbonates, yet Hongze Lake and the East Line of SNWTP are mainly controlled by evaporation and crystallization processes. This study also confirmed that the Huai River is the geographic division between southern and northern China. Most rivers of this basin have very high ionic composition relative to the global median and other world rivers. The spatial patterns and ionic composition also suggest that intensive anthropogenic activities in northern areas of this basin are well characterized. A comparison with WHO and Chinese standards for drinking water indicates that the northern water systems of this basin are not suitable for use as drinking water sources, and pollution control should be improved and enhanced in northern areas of the basin. 相似文献
14.
N. Subba Rao 《Environmental Geology》2002,41(5):552-562
Hydrogeochemical investigations, which are significant for the assessment of water quality, have been carried out to study the sources of dissolved ions in groundwaters of some rural areas of Guntur district, Andhra Pradesh, India. Groundwaters in the area are mostly brackish. High contents of SiO2, and Na+ and Cl- ions in groundwater, in comparison with those of seawater, suggests a meteoric origin of groundwater. The high concentration of SiO2 and various geochemical signatures reflect the weathering of minerals. However, the Na++K+ vs Cl- ratio suggests weathering, has occurred only to some extent. The chemistry of groundwater favours the formation of clay minerals (montmorillonite, illite and chlorite), because of evapotranspiration. The positive saturation index of CaCO3 and the high signatures of Ma2+:Ca2+ and Na+:Ca2+ reveals the occurrence of evaporation. The evaporation enhances the concentration of ions (which occurred originally in the water) in the soils during summer. The very high % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr % 0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8 % frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaqabeaadaaakeaacqqG % tbWucqqGpbWtdaqhaaWcbaGaeeinaqdabaGaeeOmaiJaeeyla0caaa % aa!2EC5! SO42 - {\rm SO}_{\rm 4}^{{\rm 2 - }} and Cl- contents in some groundwaters and the occurrence of kankar (CaCO3) in the area suggest a long history of evaporation. Greater ionic concentration in the groundwaters of post-monsoon compared with pre-monsoon indicates the increasing addition of leachates into the groundwater from the soils in the monsoon and anthropogenic activities, which leads to a deteriorating quality of groundwater. According to the Gibbs' diagrams, rock weathering, to some extent, and evaporation are the dominant phenomena responsible for the higher ionic concentrations found in groundwater. Measures that benefit sustainable management of groundwater quality are suggested in this study. 相似文献
15.
Weathered whole rock samples of five lithic units from Maine, Virginia, North Carolina and South Carolina were analyzed to evaluate the effect of chemical weathering on the Rb-Sr ages of the original rocks. Major element and turbidity analyses, as well as standard X-ray diffraction and petrographic studies were made on selected samples. All weathered granitic rocks are friable; several are almost saprolites. Volcanic rocks are powdery or are pitted as a result of the weathering of plagioclase phenocrysts. The weathering profiles studied all are young relative to the age of the unaltered rock. Considering the extent of weathering, however, ages of the weathered samples are remarkably close to the published ages for unaltered samples of the same units. Weathered samples have ages ranging from essentially the same as fresh samples to no more than 20% younger than the age obtained from the fresh samples. In one profile, we have attempted to quantify the extent of weathering: there is a positive correlation between our measure of the degree of weathering and per cent ‘lowering’ of the age. Weathering of plagioclase apparently has little effect on whole rock ages; ages are lowered significantly only where biotite and K-feldspar have undergone considerable alteration. 相似文献
16.
The present paper deals with the biomarker characteristics of crude oils and source rocks from different environments(fresh,fresh-brackish and salt waters)of nonmarine depositional basins of different ages in China.Their characters are summarized as follows:1)Souce rocks and crude oils derived from fresh-water lacustrine facies have an odd/even predominance of n-alkanes and high pristine/phytane ratios.Oils from the fresh-water lacustrine facies differ from typical marine oils in the relative contents of total steranes and terpanes,the concentrations of hopanes and organic sul-phur compounds and the values of methylphenanthrene indices and C,H,S stable isotopes.2)The source rocks and crude oils derived from saline lacustrine facies possess an even/odd predominance of n-alkanes and high phytane/pristine ratios.There are also some differences between saline lacustrine oils and freshwater lacustrine oils in the concentrations of steranes,tricyclic terpanes and organic sulphur compounds,as well as in the values of methylphenanthrene indices and C,H,S stable isotopes.3)Oils derived from fresh-brackish water lake facies differ from oils from fresh-water lacustrine or samline lacustrine environments in respect of some biomarkers.According to the various distributions of these biomarkers,a number of geochemical parameters can be applied synthetically to differentiating and identifying the nature of original depositional environments of crude oils and source rocks and that of organisms-primary source materials present in those environments. 相似文献
17.
A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO2 consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ34S values of 16 to 20‰CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS2. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO2 fluxes from silicate weathering range from ~ 0.012 × 106 to 0.039 × 106 mol/km2/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO2 consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO2 consumption rates in particular, based on major ion abundances, should be treated with skepticism. 相似文献
18.
《Geochimica et cosmochimica acta》1999,63(13-14):1939-1953
Accessory calcite, present at concentrations between 300 and 3000 mg kg−1, occurs in fresh granitoid rocks sampled from the Merced watershed in Yosemite National Park, CA, USA; Loch Vale in Rocky Mountain National Park CO USA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico. Calcite occurs as fillings in microfractures, as disseminated grains within the silicate matrix, and as replacement of calcic cores in plagioclase. Flow-through column experiments, using de-ionized water saturated with 0.05 atm. CO2, produced effluents from the fresh granitoid rocks that were dominated by Ca and bicarbonate and thermodynamically saturated with calcite. During reactions up to 1.7 yr, calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates, principally biotite. Mass balance calculations indicate that most calcite had been removed during this time and accounted for 57–98% of the total Ca released from these rocks. Experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excess Ca and alkalinity indicated that calcite had been previously removed by natural weathering.The extent of Ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in the Yosemite, Loch Vale, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the western United States. This Ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or less than the Ca/Na ratios for plagioclase in the subtropical Panola and tropical Rio Icacos watersheds, in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport-limited weathering. These results indicate that the weathering of accessory calcite may strongly influence Ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces. 相似文献
19.
David Domínguez-Villar Laura Razola Rosa M. Carrasco Carrie E. Jennings Javier Pedraza 《Quaternary Research》2009,72(2):218-228
The morphometrical analysis of gnammas (weathering pits) in granite landscapes has been used to establish the relative chronology of recent erosive surfaces and to provide the weathering history in a region. To test the validity of gnammas as relative chronometer indicators, and the reliability of the obtained weathering record, two sites have been studied in Serra da Estrela, Portugal. The first site is within the limits of the glacier that existed in these mountains during the last glaciation, whereas the second site is located in an unglaciated sector of the mountains, which preserves a longer record of weathering in the bedrock surface. The number of gnamma weathering phases recorded in the latter site (8) is larger than those from the former (6). Correlation between both measurement stations based on morphometrical criteria is excellent for the younger six weathering phases (1 to 6). Consequently, the parameter used for relative chronology (δ-value) has been verified to be age dependent, although absolute values are modulated by microclimate due to altitude variations. The weathering record was essentially duplicated once the surfaces at both sites were exposed, demonstrating the reliability of gnamma evolution as a post-glacial environmental indicator for the region. 相似文献