Chemical Weathering Of Carbonate Rocks Research Articles

Concentration, speciation, and fractionation of rare earth elements in alluvial soils in contiguous karst landform, southwestern China

Karst is a unique landform formed by strong chemical weathering of carbonate rocks. This study investigated the concentration and speciation of rare earth elements and yttrium (REY) in alluvial soils and REY fractionation from carbonate rocks and alluvial soils in the world's largest contiguous karst landform in southwestern China. The results showed that the REY of alluvial soils ranged from 73.77 to 1251.56 μg⋅g−1, with a median value of 230.30 μg⋅g−1, which was higher than that of alluvial soils across China (178 μg⋅g−1) and the upper continental crust (168 μg⋅g−1), revealing a high background of REY in these karst areas. Meanwhile, the average REY in alluvial soils (244.96 μg⋅g−1) was close to 14 times that in carbonate rocks (17.61 μg⋅g−1) in the study area, indicating that REY was significantly enriched in the pedogenesis. Correlation analysis showed that the REY concentration was not significantly related to any single factor (i.e., Fe2O3, MnO, Al2O3, TOC, pH, or CIX). BCR speciation analysis revealed a declining trend in the proportion of REY across the residual (F4, 73.21 %), reducible (F2, 19.40 %), oxidizable (F3, 6.89 %), and weakly acid-extracted (F1, 0.50 %) phases. Additionally, significant REY fractionations from carbonate rocks to alluvial soils were observed. The bedrock-normalized REY pattern revealed an enrichment of light rare-earth elements (LREE) and middle rare-earth elements (MREE), a depletion of heavy rare-earth elements and yttrium (HREY), and positive Ce and Gd anomalies. Secondary mineral phases that formed during weathering—such as clay minerals, oxyhydroxides, phosphates, and organic matter—played an important role in the retention and fractionation of the REY in the soils. Besides these natural factors, anthropogenic inputs also contributed to the REY fractionation, as indicated by the slight positive Gd anomaly in the karst alluvial soils. Furthermore, the differences of δCe and δEu in the soils developed from three types of pedogenic rocks—including carbonate, basalt, and granite—were discussed. The different δEu anomalies could help to identify the parent rocks of alluvial soils.

Geochemistry of Weathering Cover and the Main Influencing Factors in Karst Area of Guilin, Southwest China

The chemical weathering of carbonate rocks is the main form of interaction between earth spheres in the karst critical zone. The karst weathering cover, which is composed by residua from carbonate rocks weathering, contains important information about the climate environment and material cycle. We present the chemical composition of weathering covers in karst area of Guilin, Guangxi province, analyze their weathering process and strength, and compare them with the other weathering covers in other karst area in China, including Yunnan, Guizhou, Hunan, and Qinghai Tibet Plateau. The results showed: (1) the chemical composition of Guilin weathering covers were similar to that of carbonate weathering covers in other areas of China, and had the common characteristics of uniform distribution of chemical composition in the profile and obvious enrichment of Fe, Al and trace elements. During the formation of carbonate weathering cover and the residua, the rapid dissolution of Ca and Mg had an important impact on the migration and enrichment of other elements. (2) The chemical index of alteration (CIA) of carbonate weathering covers in Guilin and other karst areas of China was much larger than that in the upper crust (UCC) (60.13), which showed strong chemical weathering characteristics of the humid and hot climate. (3) The weathering process of carbonate rock was different from that of silicate and loess. In the early stage of carbonate rock weathering, soluble components (calcite and dolomite) had been rapidly dissolved and leached. All the carbonate weathering residua was measured to be in the stage of K2O weathering and Al2O3 increasing. Therefore, the weathering degree of carbonate weathering cover was mainly determined by the leaching of K2O and the increase of Al2O3. As a result, there was no correlation between CIA and Na/K (molar ratio), but was a significant negative correlation between CIA and K/Al (molar ratio) in the carbonate weathering cover. (4) The CIA values of weathering residua and carbonate weathering cover in southern China were negatively correlated with latitude, reflecting the influence of climate factors. From the influence of lithology, the chemical compositions of parent rock can affect the CIA of weathering cover, and the content of insoluble matter in carbonate rock was negatively correlated with CIA. From the influence of topography, the CIA value of weathering residua decreased from the high to the low position and from the shallow to the deep part of the profile.

Chemical weathering of small watersheds in the upper reaches of Jinsha river on the eastern edge of the Qinghai–Tibet Plateau

Land chemical weathering is a crucial and common geological process on the Earth’s surface. Studying the surface chemical weathering process in basins is important to the net consumption rate of atmospheric CO2, climate change in geological history, and the global carbon cycle. In this study, a small watershed in the upper reaches of Jinsha River on the eastern edge of the Qinghai-Tibet Plateau is selected as the study area. By measuring the physical and chemical properties and the main ion content of the water in the watershed and by using the factor analysis method, the source of the main ion components in the water in the alpine basin and the characteristics of the chemical weathering of the basin rocks are examined. Results show that the pH value of surface water in the study area varies from 7.4 to 8.2, and the overall performance is moderately to weakly alkaline. The total salinity in the basin is between 52.42 and 398.19 mg/L, with an average value of 180.06 mg/L. The concentration of main cations in surface water follows the order Ca2+>Mg2+>Na+>K+. The concentration relationship of the main anions is in the order HCO3−>SO42−>Cl−, and the hydrochemical type is HCO3_Ca. The main source of ions in the study area is rock weathering, which is controlled by the chemical weathering of carbonate rocks. The contribution rates of carbonate (mainly dolomite and calcite dissolution), evaporite, and silicate to the dissolved matter in the river are 59.40%, 7.30%, and 3.00% respectively, and the contribution rate of CO2 in the atmosphere to the dissolved matter in the river is 30.30%. The total chemical weathering rate in the study area is 29.09 t/km2 ⋅a, which is at a low level, indicating that weak hydrothermal conditions slow down the chemical weathering process of rock minerals.

Hydrochemical Evolution in the Yarlung Zangbo River Basin

In order to explore the hydro-chemical evolution law of the Yarlung Zangbo River Basin from 1973 to 2020, the hydro-chemical characteristics and major ion sources were studied using a Piper diagram, Gibbs diagram, ion ratio, and correlation analysis, and the irrigation applicability of the Yarlung Zangbo River was evaluated using the sodium adsorption ratio (SAR), sodium percentage (Na+%), and permeability index (PI). The results showed that the mean value of TDS was (208.30±58.26) mg·L-1, which increased with time. Ca2+ was the dominant cation, accounting for (65.49±7.67)% of the total cations. HCO-3 and SO2-4 were the dominant anions, accounting for (68.56±9.84)% and (26.85±9.82)% of the main anions, respectively. The annual growth rates of Ca2+, HCO-3, and SO2-4 were respectively 2.07, 3.19, and 4.70 mg·(L·10 a)-1. The hydro-chemical type of the Yarlung Zangbo River was HCO3-Ca type, and the main ionic chemistry was controlled by the chemical weathering of carbonate rocks. The weathering of carbonate rocks during the period of 1973 to 1990 was mainly controlled by carbonation, whereas from 2001 to 2020 it was mainly controlled by both carbonation and sulfuric acid. The main ion concentrations in the mainstream of Yarlung Zangbo River were within the range of drinking water standards, with SAR between 0.11-0.93, Na+% between 8.00-36.73, and PI values between 0.39-0.87, demonstrating that the waters were suitable for drinking and irrigation. The results were of great significance to the protection and sustainable development of water resources in the Yarlung Zangbo River Basin.

Carbonate Mineral Dissolution and Its Carbon Sink Effect in Chinese Loess

The relationship between the source and sink of atmospheric CO2 has always been a widely discussed issue in global climate change research. Recent studies revealed that the chemical weathering of carbonate rocks contributed to 1/3 (~0.5 Pg C/yr) of the missing carbon sinks (MCS) globally, and there are still 2/3 of MCS (~0.5 Pg C/yr) that need to be explored. As one of the main overburdened parts of the earth, loess is one of the important driving factors for atmospheric CO2 consumption. Here, we elaborated on the dissolution process and the carbon sink effect from carbonate and silicate minerals in loess. The relationship between carbonate dissolution and carbon source/sink is elucidated, and the mechanism of carbon sink formation from secondary carbonates in loess is clarified. Additionally, the commonly used methods for the identification of primary and secondary carbonates are summarized, and the methods for the study of loess carbon sinks and the influencing factors of loess carbon sinks are also revealed. Based on the research results and progress interpretations, the prospects of loess carbon sinks are discussed to provide a scientific basis for further research on loess carbon sinks.

Impact of chemical weathering on microstructures and mechanical properties of karstic limestone

Dissolution and chemical weathering of carbonate rocks have complex effects on their mechanical properties, accelerating their rate of deterioration that can lead to early failure under external stresses. To investigate the effects of acidic solutions on the mechanical properties of inherently low-porosity karstic limestone, compression tests were performed on sound and weathered samples exposed to different corrosive environments and for different durations. The tested samples exhibited a general increase in porosity and decrease in peak strength and Young's modulus for higher acidity and exposure duration. To establish links between these mechanical changes and those at the micro-scale, microstructures and mechanisms of deformation were also studied. As indicators of the sample degradation, the distribution, orientation and intensity of the fractures in the tested samples and their twinning density were used. Based on these parameters, it was found that chemical erosion influences microcrack propagation, orientation, distribution and intensity, due to modification of the competence contrast between the constituents of the rock. These effects significantly influence the semi-brittle behaviour of the samples, including the distribution and intensity of microcracking and crystal plasticity.

Carbon Isotopic and Lithologic Constraints on the Sources and Cycling of Inorganic Carbon in Four Large Rivers in China: Yangtze, Yellow, Pearl, and Heilongjiang

AbstractTransport of terrigenous carbon by rivers has been affected extensively by climate change and anthropogenic activities in China over the last few decades. Here, we present results on carbon isotopes (13C, 14C) of dissolved and particulate inorganic carbon (DIC and PIC) and combined with major lithologic ions measured in the four largest rivers in China, namely, the Yangtze, Yellow, Pearl, and Heilongjiang rivers, to reveal the sources and transport of terrigenous inorganic carbon in the rivers. The DIC concentrations showed large variations in the four rivers and ranged from 253 to 3,122 μM. The Yangtze, Yellow and Pearl rivers transported high DIC contents that had low Δ14C values of millennium‐aged DIC and very old PIC; however, the Heilongjiang River presented a lower DIC concentration with much younger 14C ages than the global average (1,100 μM). The strong correlations between the DIC isotope values and major lithological ions (Ca2+ and Mg2+) suggest that chemical weathering played important but variable roles in controlling the production and fate of DIC in the rivers. Using dual isotopes and the MixSIAR model, we calculated that the chemical weathering of carbonate rocks contributed 95 ± 5% of the riverine DIC to the headwater of the Yangtze River while silicate rock weathering and riverine organic matter respiration contributed 62 ± 25% and 5 ± 5% of the DIC in the middle and lower reaches of the river, respectively. In contrast, chemical weathering of silicate rocks contributed the dominant fraction of DIC in the Yellow (55 ± 17%), Pearl (61 ± 20%) and Heilongjiang (83 ± 29%) rivers.

Hydrochemical characteristics and karst carbon sink estimation under the influence of allogenic water

The water originating from non-karst area, i.e., allogenic water (AW), has low pH and low calcite saturation. After entering the karst area, the chemical weathering of carbonate rocks would be accelerated and the karst carbon sink flux would be increased. Yufuhe River Basin, located in Ji’nan, Northern China, was selected to clarify the mechanism of the karst carbon sink and its flux under the influence of AW; both the surface water and groundwater were detected in situ and their hydrochemical characteristics were analyzed in the laboratory. To estimate the influence of AW on the karst carbon sink exactly, a modified Galy model was adopted. The results showed that the hydrochemical type of Yufuhe River Basin is HCO3·SO4–Ca, and its ions were derived mainly from carbonate rock and less from silicate rock. Water saturation model demonstrated that AW became saturated after they mixed with the karst water (KW) as indicated by dolomite saturation index (SId) and calcite saturation index (SIc). The dissolution capacity of KW and karst carbon sink flux was then enhanced. The estimation showed that the weathering rate of silicate rocks is 8.06 mm/ka, and the weathering rate of carbonate rocks is 18.51 mm/ka. When AW mixed with KW, the dissolution rate of carbonate erosion rate increased to 20.12 mm/ka. There was also a significant promoting effect of AW on the karst carbon sink. The CO2 sink rate of metamorphic rock (gneiss-dominated area) in the upper reaches of the basin is 3.87 t/km2a. In the karst area, the average CO2 sink rate of the KW is 7.51 t/km2a, while the value became 13.03 t/km2a when it mixed with AW, nearly doubled compared with KW. Based on the above analysis, the annual CO2 consumption of Yufuhe River Basin is 5500 t, contributing to an important approach of the regional CO2 sink.

Role of sulfuric acid in chemical weathering of carbonate rocks for evaluating of carbon sinks in the Yangtze River Basin, China

Chemical weathering of rock by river systems is an important process in the global carbon cycle. Sulfuric acid produced from anthropogenic sources in the Yangtze River basin of China has the potential to change rock weathering processes and the carbon cycle. Monitoring and analysis of the dissolved constituents of the main channel and major tributaries of the Yangtze River indicate that the sulfuric acid has enhanced the carbonate rock weathering rate by an average of 28% and reduced the CO2 consumption rate by 12%. Analysis of dissolved sources for SO42- in the Yangtze River indicates that 36% of SO42- can be attributed to rainwater, 26% to dissolution of evaporitic rocks, and 38% to input from coal. Calculations indicate that the annual output flux of CO2 from the Yangtze River Basin to the sea is 3495×104 tons, 80% of which is attributed to the weathering of carbonate rocks. Whilst the average consumption rate of atmospheric CO2 is 20.6 t/km2 y-1.

Rock Weathering Characteristics and the Atmospheric Carbon Sink in the Chemical Weathering Processes of Qingshuijiang River Basin

Carbon sink produced during rock weathering is critical to global carbon cycles. In this work, the major ion chemistry and ion sources of Qingshuijiang River Basin were investigated. The principal component analysis, mass balance approach and deduction method were applied for estimating the weathering rate and atmospheric CO2 consumption via the chemical weathering of rocks. The results demonstrated that the chemical weathering of carbonate and silicate rocks within the drainage basin was the main source of the dissolved chemical substances in the Qingshuijiang River Basin, prior to carbonate rock weathering. Some 58.28% of the total dissolved chemical substances were derived from the chemical weathering of carbonate rock, 17.38% from the dissolution of silicate rock, and 17.74% from atmospheric CO2 contribution rates. The chemical weathering rate of this catchment was estimated to be 109.97t·(km2·a)-1, which was comparable to Wujiang River Basin, but higher than the average of global rivers. Furthermore, the atmospheric CO2 consumption rate was estimated to be 7.25×105 mol·(km2·a)-1. The CO2 flux consumed by the rock chemical processes within this catchment was 12.45×109 mol·a-1, of which about 63.13%(7.86×109 mol·a-1) was resulted from carbonate weathering and 36.87%(4.59×109 mol·a-1) from silicate weathering. The CO2 consumed by rock chemical weathering in the Qingshuijiang River reduced the atmospheric CO2 level and constituted a significant part of the global carbon budget. Correlation and spatial distribution analysis of SO42-, F-, NO3- showed that anthropogenic activities contributed remarkably to dissolved solutes and associated CO2 consumption worldwide, and anthropogenic inputs probably contributed some 4.87% to the dissolved solutes in the Qingshuijiang River.

Water geochemistry and dissolution rates of the karst-dominated Houzhai River basin, China

ABSTRACTHouzhai (Guizhou Province, Southwest China) is a typical karstification area, with dolomite and limestone constituting the major rock types. The aim of this study was to utilize major element mass-balance and multivariate statistical methods to analyze the chemical composition of water in the Houzhai River catchment area with regard to seasonal variations in ion content, dissolution reactions, and rates. Data demonstrated that the Houzhai's hydrochemical composition is dominated by calcium (Ca2+), magnesium (Mg2+), bicarbonate (HCO3−), and sulfate (SO42−) which predominantly originated from chemical weathering of carbonate rocks like limestone and dolomite. Carbonate weathering contributes 83% of the dissolved Ca2+. Sulfuric acid also plays an important role in rock weathering. Our analysis identified three principal sources of hydrochemical variation in the study area, including dolomite dissolution, limestone dissolution, and anthropogenic activities. In addition, the identified two main hydrochemical facies were characterized by the predominance of Ca-Mg-HCO3 and Ca-Mg-HCO3-SO4. In terms of temporal variation, all of the examined parameters showed strong inter-annual and seasonal variations of concentration distribution, except pH levels and potassium and sodium (K++Na+). The dissolution rate of carbonate was estimated using the concentration of Ca2+ and discharge rate, which constitutes a simple, practical, and innovative method. Compared with other research methods, our strategy was found to be simple, feasible, readily adaptable, and less demanding of data inputs.

Drivers of the carbonate system seasonal variations in a Mediterranean gulf

The effects of different physical and biogeochemical drivers on the carbonate system were investigated in a semi-enclosed coastal area characterized by high alkalinity riverine discharge (Gulf of Trieste, Northern Adriatic Sea, Mediterranean Sea). Our 2-year time-series showed that a large part of the seasonal carbonate chemistry variation was controlled by the large seasonal change of seawater temperature, though air-sea CO2 exchange, biological activity (primary production-respiration), and riverine inputs also exerted a significant influence.With the exception of summer, the Gulf of Trieste was a sink of atmospheric carbon dioxide, showing a very strong CO2 fluxes from atmosphere into the sea (−16.10 mmol m−2 day−1) during high wind speed event of north easterly Bora wind. The CO2 influx was particularly evident in winter, when the biological activity was at minimum and the low seawater temperature enhanced CO2 solubility. During spring, the drawdown of CO2 by primary production overwhelmed the CO2 physical pump, driving a significant decrease of dissolved inorganic carbon (DIC), [CO2], and increase of pHT25 °C. In summer the primary production in surface waters occurred with the same intensity as respiration in the bottom layer, so the net biological effect on the carbonate system was very low and the further reduction of seawater CO2 concentration observed was mainly due to carbon dioxide degassing induced by high seawater temperature. Finally, during autumn the respiration was the predominant process, which determined an overall increase of DIC, [CO2], and decrease of pHT25 °C. This was particularly evident when the breakdown of summer stratification occurred and a large amount of CO2, generated by respiration and segregated below the pycnocline, was released back to the whole water column. Local rivers also significantly affected the carbonate system by direct input of total alkalinity (AT) coming from the chemical weathering of carbonate rocks, which dominate the river watershed. Our finding clearly demonstrates a high AT concentration in low salinity surface waters (AT max = 2742.8 μmol kg−1) and a negative AT-salinity correlation. As a result the Gulf of Trieste revealed a low Revelle factor (10.1) and one of the highest buffer capacities of the Mediterranean Sea (ßDIC = 0.31 mmol kg−1), which allows the system to store a significant amount of atmospheric CO2 with a small decrease of seawater pH.

Determining CO2 consumption from elemental change in soil profiles developed on carbonate and silicate rocks

To further understand the roles of carbonate and silicate rocks in regulating the atmosphere/soil CO2 level, the flux of CO2 consumed by the chemical weathering of silicate and carbonate rocks was determined from the elemental change in soil profiles. Results showed that the chemical weathering of carbonate rocks mainly occurred at the rock-regolith interface, and that the further weathering of the residua soil on the carbonate rocks was similar to that of the granite profile. Chemical weathering of the silicate rocks occurred through the whole profiles. Therefore, CO2 consumed per volume by the silicate profiles [M sr (CO2)] and the residues on carbonate rocks [M cr (CO2)] were calculated based on the elemental weathering gradients. CO2 consumed by carbonate protolith [M cp (CO2)] was calculated from the elemental change at the rock-regolith interface. The M sr (CO2) were about tens to thousands orders of magnitude greater than M cr (CO2). Even so, this demonstrated that the residues on carbonate rocks could be a sink of CO2 on long-term scales. The M cp (CO2) was about four times larger than M sr (CO2), which demonstrated that carbonate rocks played a more important role in regulating the CO2 level than the silicate rocks did during the pedogenic process of the profiles.

Alteração experimental de rochas carbonáticas, Caverna das Pérolas, Iporanga (SP)

Trabalhos experimentais foram realizados com o intuito de estudar a alteração química em rochas carbonáticas provenientes da Caverna das Pérolas, Município de Iporanga, Estado de São Paulo. Extratores soxhlet foram utilizados para acelerar o processo de lixiviação de duas formas: contínua e intermitente. A solução de lixiviação obtida no ensaio contínuo foi analisada quimicamente no final do período de lixiviação (cerca de 454 horas), enquanto os precipitados foram separados e analisados mineralogicamente por difração de raios X (DRX). No ensaio de alteração intermitente, as soluções foram recolhidas após períodos pré-determinados e analisados quimicamente junto com os precipitados, para determinar o total de íons remobilizados da rocha alterada. A composição química das rochas alteradas experimentalmente também foi analisada e comparada com a da rocha sã, a partir das frações molares de seus constituintes maiores. Verificou-se que os principais íons lixiviados no início do processo de alteração foram Na+ e Mg2+; no entanto, com o aumento da alteração química, os íons Sr2+, Ba2+ e Ca2+ passaram a ser, juntamente com o Na+, os íons mais remobilizados, dando início também à precipitação de calcita. Observou-se que no experimento contínuo as concentrações de Sr2+ e Ba2+ ficaram inferiores em relação ao experimento intermitente, revelando que esses íons possivelmente co-precipitaram com a calcita. A baixa razão Mg/Ca pode ter promovido a precipitação preferencial da calcita, em relação à aragonita, nas soluções de alteração experimental. Verificou-se que houve maior taxa de remobilização dos constituintes da rocha carbonática na zona em que as amostras foram submetidas a flutuações do nível freático no compartimento do soxhlet. Entre os minerais secundários observados na rocha alterada, destaca-se a caulinita.

An atomic force microscopy study of calcite dissolution in saline solutions: The role of magnesium ions

In situ Atomic Force Microscopy, AFM, experiments have been carried out using calcite cleavage surfaces in contact with solutions of MgSO 4, MgCl 2, Na 2SO 4 and NaCl in order to attempt to understand the role of Mg 2+ during calcite dissolution. Although previous work has indicated that magnesium inhibits calcite dissolution, quantitative AFM analyses show that despite the fact that Mg 2+ inhibits etch pit spreading, it increases the density and depth of etch pits nucleated on calcite surfaces and, subsequently, the overall dissolution rates: i.e., from 10 −11.75 mol cm −2 s −1 (in deionized water) up to 10 −10.54 mol cm −2 s −1 (in 2.8 M MgSO 4). Such an effect is concentration-dependent and it is most evident in concentrated solutions ([Mg 2+] >> 50 mM). These results show that common soluble salts (especially Mg sulfates) may play a critical role in the chemical weathering of carbonate rocks in nature as well as in the decay of carbonate stone in buildings and statuary.

Geochemistry of surface and ground water in Guiyang, China: Water/rock interaction and pollution in a karst hydrological system

The chemical compositions of the surface/ground water of Guiyang, the capital city of Guizhou Province, China are dominated by Ca 2+, Mg 2+, HCO 3 - and SO 4 2 - , which have been derived largely from chemical weathering of carbonate rocks (limestone and dolomite). The production of SO 4 2 - has multiple origins, mainly from dissolution of sulfate evaporites, oxidation of sulfide minerals and organic S in the strata, and anthropogenic sources. Most ground water is exposed to soil CO 2 and, therefore, the H 2CO 3 which attacks minerals contains much soil C. In addition, the H 2SO 4 produced as a result of the oxidation of sulfides in S-rich coal seams and/or organic S, is believed to be associated with the chemical weathering of rocks. The major anthropogenic components in the surface and ground water include K +, Na +, Cl −, SO 4 2 - and NO 3 - , with Cl − and NO 3 - being the main contributors to ground water pollution in Guiyang and its adjacent areas. The seasonal variations in concentrations of anthropogenic components demonstrate that the karst ground water system is liable to pollution by human activities. The higher content of NO 3 - in ground water compared to surface water during the summer and winter seasons, indicates that the karstic ground water system is not capable of denitrification and therefore does not easily recover once contaminated with nitrates.

Characteristics of U-pattern distribution of high-concentration 210Pb in surface air at central Guizhou, China

A better understanding of the variation of 210Pb concentrations in the surface air is the key to trace lake sedimentation and catchment erosion, to establish and validate the global diffusion model of contaminants, and to assess the impact of natural radiation on the ecological system. Basing on the study progresses of the sedimentation, the atmospheric model and the unique environmental background in the Yunnan-Guizhou Plateau, and starting from December 20, 2001, we made use of a 500 NE Type aerosol sampler to collect filter samples, once a week in the past consecutive two years, in the surface air at Mt. Guanfeng, Guiyang City, China. Synchronous detailed γ-energy spectrometric observations of 210Pb concentrations at both Chinese and U.S. laboratories have shown that the average monthly 210Pb concentration displays a regular U-patterns distribution of annual variations, with high values appearing in winter and low values in late spring and summer. The average annual 210Pb concentrations are estimated to be 2.77±0.63 mBq/m3, about 4 times the average maximum concentrations reported from quite a number of monitoring stations throughout the world. The possible factors affecting the sources of 210Pb in the surface air in the central part of Guizhou Province are: Release of 222Rn from the U-Ra series enriched in the soils; chemical weathering of carbonate rocks; the exploitation, processing and utilization of coal and phosphorous resources. The principal factor controlling the high 210Pb concentrations in the surface air in the central Guizhou and their annual U-pattern distribution is the release of 222Rn. The average monthly 210Pb concentrations show a good negative powerfunction relationship with increasing precipitation and also show a good negative linear-function relationship with the rise of air temperature in month grouping. Group No.1 (June to November) reflects a relatively low release of 222Rn from soils in relatively high temperature seasons; Group No.2 (December to May) mirrors a relatively high release of 222Rn from soils in relatively low temperature seasons. The average monthly 210Pb concentrations simulated in terms of the precipitation and air temperature are in good consistency with the measured values, revealing that the regular U-pattern distribution characteristics of 210Pb concentrations in the surface air in the central Guizhou are controlled predominantly by regional precipitation and air temperature. Meanwhile, it is also indicated that the magnitude of variation of air temperature and precipitation within the same year also constrains the ratio of high values over low values of average monthly 210Pb concentrations. On a month time scale, precipitation and air temperature are a main factor affecting the release of 222Rn from soils (rocks). In this aspect strong evidence has been obtained.

The role of chemical weathering of carbonate rocks in the geochemical cycle of CO2

The role of chemical weathering of carbonate rocks in the geochemical cycle of CO₂