The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust

The terrestrial surface, the “skin of the earth”, is an important interface for global (geochemical) material fluxes between major reservoirs of the Earth system: continental and oceanic crust, ocean and atmosphere. Because of a lack in knowledge of the geochemical composition of the terrestrial surface, it is not well understood how the geochemical evolution of the Earth’s crust is impacted by its properties. Therefore, here a first estimate of the geochemical composition of the terrestrial surface is provided, which can be used for further analysis. The geochemical average compositions of distinct lithological classes are calculated based on a literature review and applied to a global lithological map. Comparison with the bulk composition of the upper continental crust shows that the geochemical composition of the terrestrial surface (below the soil horizons) is significantly different from the assumed average of the upper continental crust. Specifically, the elements Ca, S, C, Cl and Mg are enriched at the terrestrial surface, while Na is depleted (and probably K). Analysis of these results provide further evidence that chemical weathering, chemical alteration of minerals in marine settings, biogeochemical processes (e.g. sulphate reduction in sediments and biomineralization) and evaporite deposition are important for the geochemical composition of the terrestrial surface on geological time scales. The movement of significant amounts of carbonate to the terrestrial surface is identified as the major process for observed Ca-differences. Because abrupt and significant changes of the carbonate abundance on the terrestrial surface are likely influencing CO₂-consumption rates by chemical weathering on geological time scales and thus the carbon cycle, refined, spatially resolved analysis is suggested. This should include the recognition of the geochemical composition of the shelf areas, now being below sea level.     

Solute geochemical mass-balances and mineral weathering rates in small watersheds: Methodology,recent advances,and future directions

Solute-based geochemical mass balance methods are commonly used in small-watershed studies to estimate rates of a variety of geochemical processes at the Earth’s surface, including primary-mineral weathering and soil formation, and the quantitative contribution of these elemental transfer processes to cation budgets, nutrient cycling, and landscape susceptibility to acid deposition. Weathering rates of individual minerals in watershed mass-balance studies are determined by solving a system of simultaneous linear geochemical mass-balance equations with constant (stoichiometric) coefficients. These equations relate the measured net fluxes to the (known) stoichiometries and (unknown) rates of weathering reactions for multiple minerals in the weathering profiles. Solving the system of equations requires petrologic, mineralogic, hydrologic, botanical, and aqueous geochemical data. The number of mineral-weathering rates that can be determined is limited by the number of elements for which solute mass-balance equations can be written. In addition to calculating mineral weathering rates, elemental transfer into or out of the biomass may also be calculated. Elemental uptake by aggrading forest vegetation can act as an intrawatershed sink for at least some mineral-derived cations, producing mineral weathering rates higher than would be estimated from solute fluxes alone; similarly, element release from decaying forest biomass can result in higher solute fluxes than are produced by weathering alone. The mathematics of, significant contributions from, role of biomass in, and recent advances in, watershed geochemical mass-balance methods are discussed using examples from the Appalachian headwaters watersheds of the Coweeta Hydrologic Laboratory in the southern Blue Ridge Physiographic Province of North Carolina, USA.     

Effect of source integration on the geochemical fluxes from springs

Geochemical fluxes from watersheds are typically defined using mass-balance methods that essentially lump all weathering processes operative in a watershed into a single flux of solute mass measured in streamflow at the watershed outlet. However, it is important that we understand how weathering processes in different hydrological zones of a watershed (i.e., surface, unsaturated, and saturated zones) contribute to the total geochemical flux from the watershed. This capability will improve understanding of how geochemical fluxes from these different zones may change in response to climate change. Here, the geochemical flux from weathering processes occurring solely in the saturated zone is investigated. This task, however, remains exceedingly difficult due to the sparsity of subsurface sampling points, especially in large, remote, and/or undeveloped watersheds. In such cases, springflow is often assumed to be a proxy for groundwater (defined as water residing in fully saturated geologic formations). However, springflow generation may integrate different sources of water including, but not limited to, groundwater. The authors’ hypothesis is that long-term estimates of geochemical fluxes from groundwater using springflow proxies will be too large due to the integrative nature of springflow generation. Two conceptual models of springflow generation are tested using endmember mixing analyses (EMMA) on observations of spring chemistries and stable isotopic compositions in a large alpine watershed in the San Juan Mountains of southwestern Colorado. In the “total springflow” conceptual model, springflow is assumed to be 100% groundwater. In the “fractional springflow” conceptual model, springflow is assumed to be an integration of different sources of water (e.g., groundwater, unsaturated flow, preferential flow in the soil, etc.) and groundwater is only a fractional component. The results indicate that groundwater contributions in springflow range from 2% to 100% overall and no springs are consistently composed of 100% groundwater; providing support for the fractional springflow conceptual model. Groundwater contributions are not strongly correlated with elevation, spring contributing area, spring discharge, or seasonality. This variability has a profound effect on long-term geochemical fluxes. The geochemical fluxes for total springflow overestimate long-term solute release by 22–48% as compared to fractional springflow. These findings illustrate that springflow generation, like streamflow generation, integrates many different sources of water reflecting solute concentrations obtained along many different geochemical weathering pathways. These data suggest that springs are not always ideal proxies for groundwater. Springs may be integrating very distinct portions of the groundwater flow field and these groundwater contributions may become mixed at the spring emergence with much younger sources of water that have never resided in the groundwater system.     

Cosmogenic nuclide methods for measuring long-term rates of physical erosion and chemical weathering

Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as ¹⁰Be and ²⁶Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.     

Kinetic modelling of geochemical processes at the Aitik mining waste rock site in northern Sweden

A steady-state geochemical model has been developed to study water-rock interactions controlling metal release from waste rock heaps at the Aitik Cu mine in northern Sweden. The Cu release in drainage waters from the site is of environmental concern. The waste rock heaps are treated as single completely mixed flow-through reactors. The geochemical model includes kinetices of sulphide and primary silicate mineral weathering, heterogeneous equilibrium with secondary mineral phases and speciation equilibrium. Field monitoring of drainage water composition provides a basis for evaluation of model performance.The relative rate of oxidative weathering of sulphides and dissolution of primary silicate minerals, using published kinetic data, are consistent with net proton and base cation fluxes at the site. The overall rate of Fe²⁺ oxidation within the heap is three orders of magnitude faster than that which could be explained by surface-catalysed reaction kinetics. This suggests significant activity of iron-oxidizing bacteria. The absolute weathering rates of sulphides and silicate minerals, normalized to a measured BET surface area, are approximately two orders of magnitude lower at field scale than published rates from laboratory experiments. Because of the relative absence of carbonate minerals, the weathering of biotite and plagioclase feldspar are important sources of alkalinity.     

特殊地质地貌区填图物化探技术应用

基于物化探技术方法在特殊地质地貌区填图中的应用实验,介绍从区域物化探数据提取地质填图信息的方法,指出重磁与氡-汞气测联合应用是覆盖区探测隐伏岩体、断裂的高效低成本物化探技术组合。运用基于地球化学理论方法的元素地球化学判别技术,在强烈风化区定量划分风化等级为:基岩-弱风化-中度风化-强风化-全风化-残积土等;指出风化壳元素地球化学行为对原生矿物分解、次生矿物形成具有示踪效应;不同风化层稀土元素富集分异显著,强烈富集于全风化层（岩土界面或风化岩石顶界）中的稀土元素分布模式,可作为界定风化壳分层的重要地球化学判别指标。      

Investigation of the weathering layer using seismic refraction and high-resolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16?m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18?m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.     

The control of Phanerozoic atmosphere and seawater composition by basalt–seawater exchange reactions

We present results from a long term geochemical cycling model, with a focus on the sensitivity of atmospheric carbon dioxide, oxygen, and the major element composition of seawater to seafloor spreading rates. This model incorporates rock weathering, basalt–seawater exchange reactions, and the formation and destruction of chemical sediments and organic matter. Hydrothermal reactions between seafloor and seawater involving calcium, magnesium, sodium, potassium, sulfate and carbon are the high temperature counterparts to low temperature redox, weathering, precipitation and diagenetic reactions. A major source of uncertainty is the extent to which these exchange fluxes are controlled by seafloor spreading rate. In addition, the return fluxes of these components to the atmospheric and primary silicate reservoirs reflect not only the overall rates of subduction and metamorphism, but the distribution of the overlying sedimentary burden and authigenic minerals formed during basalt alteration as well. In particular, we show how the stoichiometry of exchange fluxes (Mg/Ca and SO₄/Ca) may buffer atmospheric CO₂ and O₂ concentrations.     

Evaluation of weathering-resistance classes in clastic rocks on the example of Polish sandstones

The scope of the paper is an attempt at the identification of the weathering-resistance classes within clastic rocks by means of analysis of capillary pressure saturation curves. The porosimetric parameters corresponding to the cementing character and the grains’ mineralogical content are very important features of stone building materials, because of the weathering processes. The analysed rocks were Polish sandstones and muddy sandstones used for building purposes, collected from different geological units of Poland (i.e. Sudety Mts. Carpathian Mts. and Holy Cross Mts.) The results indicate the usefulness of sandstone materials for building purposes. They could also be used in conservation procedures and for the reconstruction of existing buildings and monuments. Basing on the parameterisation, with the van Genuchten function, of cumulative capillary pressure saturation curves, it was possible to distinguish four groups of the sampled rocks. The lithological features and weathering sustainability within the groups are quite uniform, what allow identifying the weathering resistance classes. Taking into account the complicated nature of all the factors influencing weathering processes, it is supposed that the presented parameterisation could be a useful tool for weathering-resistance classification of clastic rocks. The classification could be useful in building industry and in conservation of historical stone monuments.     

Weathering potential index for rocks based on density and porosity measurements

Rock samples belonging to ten lithological types under different stages of weathering, were collected from different stratigraphical horizons at Bhagalpur. Their densities and porosities were determined experimentally and the data obtained were fitted empirically in a linear equation for each lithological type. The slopes of the curves, which were negative in each case, showed that the increase in porosity for the same decrease in density were in the order, white sandstone > ferruginous sandstone > white claystone > porphyritic gneiss > quartzite > pegmatite > amphibolite ≅ biotite gnejss > basalt ≅ dolerite. A new weathering potential index based on the density-porosity data was proposed and the values for a specific stage of weathering for all the lithological types studied fall within the same range.     

Paleosols and their relevance to Precambrian atmospheric composition

Various paleosols have been reported from within the Witwatersrand and Ventersdorp Supergroups, South Africa. They were studied in an attempt to constrain the amount of oxygen available in the atmosphere during deposition of the gold- and uranium-bearing Witwatersrand conglomerates. The majority of these horizons do not have any physical characteristics of paleosols, and none of them have a chemistry consistent with weathering, suggesting they have been subjected to modification by later alteration processes. A similar chemistry, indicating overprinting of any original soil chemistry, has been reported from paleosols elsewhere (Elliot Lake region, Canada; Hekpoort basalt, South Africa), but it does not appear that significant cognizance has been taken of this fact when using these paleosols to determine the composition of the Precambrian atmosphere. It is concluded here that characteristics previously attributed to Precambrian weathering in an oxygen-deficient atmosphere are better explained by post-burial, hydrothermal alteration along lithological contacts.     

Defining element associations and inferring geological processes from total element concentrations in Australian catchment outlet sediments: Multivariate analysis of continental-scale geochemical data

In this paper, the geochemical composition of surficial regolith is statistically analysed and compared to independent geoscientific datasets to infer processes governing regolith composition. Surface (0–10 cm depth) and sub-surface (∼60–80 cm depth) transported sediment samples from the National Geochemical Survey of Australia were analysed for total element content in both coarse (<2 mm) and fine (<75 μm) grain-size fractions. Multi-element total content data was obtained from mainly XRF and total digestion ICP-MS analysis, of which the 50 elements satisfying data quality criteria, plus Loss on Ignition, are used herein.Censored data (<lower limit of detection) was imputed using a nearest neighbour-based analysis. The compositional data was transformed using centered log ratios (clr) to circumvent closure issues. A Principal Component Analysis (PCA) was then performed on the dataset. The first four PCs account for 59% of the variance in the dataset. Both negative and positive loadings of each of these PCs relate to geological processes consistent with the element associations they represent as well as the spatial distribution patterns they produce. The positive loadings of PC1 represent the accumulation of resistant minerals rich in Rare Earth Elements (REEs) that results from intense weathering, except in southeastern Australia where they reflect REE-enriched igneous and sedimentary rocks. Negative PC1 loadings represent secondary minerals formed during weathering (carbonates, sulfates, Fe-oxyhydroxides). Negative PC2 loadings are a mixture of elements (e.g., Co, Mn, Zn, V) characterising mafic and ultramafic minerals; conversely negative PC3 loadings (e.g., K, Rb, Na, Sr, Ca) represent more felsic minerals. Spatial distributions of the PCs are compared with independent spatial information from geological maps, airborne radiometric and spaceborne spectroscopic datasets. The differences between surface and sub-surface and between coarse and fine grain-size fractions are analysed. The implied processes (e.g., lithological control, weathering, transport, secondary mineral precipitation) overall match well with this new geochemical evidence. Future work directions with this dataset include lithological prediction and mineral prospectivity analysis.     

四川盆地北缘铜金成矿特征及找矿前景的初步认识

从所处的大地构造环境及其形成的地质环境着手 ,并结合其地球化学信息反映特征 ,分析了铜金矿受控的地层、岩石及构造等因素。指出本区铜金成矿具有矿化普遍但零散 ,难以聚集形成大规模工业矿床的特点 ,并分析了区内铜金矿的找矿前景     

Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone. ⁸⁷Sr/⁸⁶Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm⁻² ka⁻¹. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater ⁸⁷Sr/⁸⁶Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified.     

The correlation between geochemical data and SPOT satellite imagery of lateritic terrain in southern Mali

Detailed geochemical mapping of superficial lateritic formations is compared with recent high-resolution SPOT satellite images in the Dagadamou prospect in southern Mali.The two main landscape features standing out in SPOT images are expressed by thematic indices derived from multispectral data. They reflect the distribution of vegetation cover and the distinction between silty clay soils in the valleys and ferruginous duricrust exposed on the plateaus. Geochemical differentiation factors are closely related to the nature of sampled materials and reflect the relative amounts of major constituent minerals estimated by normative calculation.Duricrust samples with high Fe and Al content are also enriched in trace elements, P, V, Cr, As, Mo, Nb and Cu, immobilized in the weathering profile with secondary oxihydroxide minerals.Soils of flats are composed mainly of quartz and kaolinite; they also concentrate heavy minerals characterized by high contents of Zr, Ti, Ce and Y.Accumulation of detrital material at the periphery of duricrust plateaus is marked by a geochemical halo of higher contents in Zr and quartz at their periphery. This feature is clearly visible on SPOT imagery as a zone of high reflectance, devoid of vegetation.The density of vegetation on duricrust is related to its kaolinite content. Its distribution is characterized by a striped pattern, which is probably controlled by bedrock lithological structures preserved in the lateritic cover.This study shows the relationships, both direct and indirect, existing between spectral reflectance and geochemical composition of superficial lateritic formations. Many other landscape features identified with high resolution on SPOT images provide complementary information, which could be very helpful at various stages of geological and mineral exploration in lateritic terrain.     

亚洲主要河流的沉积地球化学示踪研究进展

发源于喜马拉雅—青藏高原的亚洲几条大河的河流地球化学研究揭示了高原隆升、流域风化剥蚀、大气CO2消耗和亚洲季风气候变化之间的耦合关系。研究认为南亚主要河流流域的化学风化对全球大气CO2消耗和海洋化学通量变化贡献较大,河流沉积地球化学研究反映的高原阶段性隆升过程、流域剥蚀速率以及亚洲季风演化信息也明显比东亚主要河流的记录清晰;尤其是最近几年运用河流碎屑单矿物化学和年代学方法来示踪流域构造演化、沉积物从源到汇过程以及河流演化历史等,取得了许多重要的研究成果。比较而言,我国的河流在元素地球化学和水化学组成方面虽然开展了大量基础研究工作,但目前急需进一步提炼科学目标,与国际性的研究计划结合,综合多学科的研究力量,在研究思路和关键方法上需要突破和深入,加强研究的广度和深度。长江更可以作为一个突破口和研究平台,来开展深入的沉积地球化学示踪研究。     

Stream geochemistry, chemical weathering and CO2 consumption potential of andesitic terrains, Dominica, Lesser Antilles

Recent studies of chemical weathering of andesitic-dacitic material on high-standing islands (HSIs) have shown these terrains have some of the highest observed rates of chemical weathering and associated CO₂ consumption yet reported. However, the paucity of stream gauge data in many of these terrains has limited determination of chemical weathering product fluxes. In July 2006 and March 2008, stream water samples were collected and manual stream gauging was performed in watersheds throughout the volcanic island of Dominica in the Lesser Antilles. Distinct wet and dry season solute concentrations reveal the importance of seasonal variations on the weathering signal. A cluster analysis of the stream geochemical data shows the importance of parent material age on the overall delivery of solutes. Observed Ca:Na, HCO₃:Na and Mg:Na ratios suggest crystallinity of the parent material may also play an important role in determining weathering fluxes. From total dissolved solids concentrations and mean annual discharge calculations we calculate chemical weathering yields of (6-106 t km⁻² a⁻¹), which are similar to those previously determined for basalt terrains. Silicate fluxes (3.1-55.4 t km⁻² a⁻¹) and associated CO₂ consumption (190-1575 × 10³ mol km⁻² a⁻¹) determined from our study are among the highest determined to date. The calculated chemical fluxes from our study confirm the weathering potential of andesitic-dacitic terrains and that additional studies of these terrains are warranted.     

一种研究区域化探资料的新方法及其应用

介绍了一种对区域化探资料进行二次开发研究的方法。这个方法综合运用了常规物化探数据处理的方法技术、经验公式等 ,对元素的原始含量数据进行处理 ,将地球化学信息与地质背景信息融为一体 ,形成一种新型的地球化学综合信息图示资料 ,可屏蔽三分之二强的非矿及大量岩性异常 ,使其与矿床和矿化相关的异常信息一目了然 ,达到快速、有效的提取和利用有用信息的目的     

运用扫描电镜和压汞法研究江汉盆地古新统—白垩系砂岩储层孔喉结构及定量参数特征

通过储集层的孔喉结构来评价储层已广泛运用于油气开发,但目前尚未运用于钾盐储层的评价。本文利用扫描电镜和压汞法获得的大量储层孔隙、孔喉实验数据,研究了江汉盆地古新统—白垩系富钾卤水砂岩储层的孔喉类型和结构、毛细管压力曲线,以及孔喉定量特征参数。研究表明,古新统沙市组砂岩储层以Ⅱ类孔隙结构为主,排驱压力平均值为2.72 MPa,孔隙度平均值为6.32%。而白垩系红花套组砂岩储层以Ⅰ、Ⅱ类孔隙结构为主,Ⅰ类排驱压力平均值为0.28 MPa,孔隙度平均值为16.99%,渗透率为87.79×10~(-3)μm~2;Ⅱ类排驱压力平均值为1.103 MPa,孔隙度平均值为10.02%,渗透率为18.83×10~(-3)μm~2。通过综合分析孔喉定量特征参数(如最大汞饱和度、最大孔喉半径、中值半径、退汞效率等)来评价储层质量,认为古新统沙市组砂岩类型以粉砂岩为主,储层质量较差,属于特低孔低渗储层;白垩系红花套组砂岩类型以细砂岩为主,储层质量较好,属于低孔低渗储层。本研究为该区富钾卤水储层评价以及进一步勘探开发提供了可靠的地质依据。     

准噶尔盆地隆起区石炭系顶面不整合及其意义及其意义

石炭系风化壳是准噶尔盆地火山岩油气储集层的主要类型,物性明显好于同时代原生型火山岩。通过对陆梁隆起滴西地区的铸体薄片与岩心观测,发现风化淋滤对火山岩储集性能的改造主要体现在孔隙上,对裂缝的改造作用不明显。由于火山岩形成的构造环境与古地貌存在差异,不同的海拔高度决定了其接受风化时间的长短。准噶尔盆地内石炭系不整合上覆地层时代存在明显差异,下二叠统至白垩系均有分布。最长风化时长在东部隆起约为150Ma,在陆梁隆起约为100Ma,在西部隆起约为55Ma;而3处隆起最少则仅缺失部分上石炭统-下二叠统。风化时长的长短与火山岩孔隙度的大小呈近波峰状对应关系,火山岩孔隙度初始随风化时长的增大(至约30Ma)逐渐增大,在到达峰值(对应风化时长约40Ma)后随时长的增大(55～100Ma)逐渐减小。最大的孔隙度均出现在火山角砾岩和玄武岩中,安山岩的孔隙度次之,而凝灰岩的孔隙度最小。