Chemical Weathering Of Rocks Research Articles

Weathering of rocks induced by lichen colonization — a review

The evidence presented by numerous investigations of the interface between lichens and their rock substrates strongly suggests that the weathering of minerals can be accelerated by the growth of at least some lichen species. The effects of lichens on their mineral substrates can be attributed to both physical and chemical processes. The physical effects are reflected by the mechanical disruption of rocks caused by hyphal penetration, expansion and contraction of lichen thallus, swelling action of the organic and inorganic salts originating from lichen activity. Lichens also have significant impact in the chemical weathering of rocks by the excretion of various organic acids, particularly oxalic acid, which can effectively dissolve minerals and chelate metallic cations. As a result of the weathering induced by lichens, many rock-forming minerals exhibit extensive surface corrosion. The precipitation of poorly ordered iron oxides and amorphous alumino-silica gels, the neoformation of crystalline metal oxalates and secondary clay minerals have been frequently identified in a variety of rocks colonized by lichens in nature. For a better understanding of the impacts of lichens on environments, further work on the comprehensive involvement of the lichen effects on weathering of natural rocks, deterioration of building stones and stonework, and formation of primitive soils should be carried out.

Estimation de la qualité des eaux du Nahr Beyrouth. Utilisation de l'indice diatomique de polluo-sensible (IPS)

The quality of Nahr Beirut's water, supplying water to Beirut and the suburbs, has been studied. A complete algology and chemical study has been conducted. The diatoms are considered one of the best bioindicators of the water's surface because they react directly with organic pollutants and they need a very ecological medium. The use of some diatomic indicators and the polluo-sensible ones (IPS) is widespread in the French hydrographic network. The methodology applied in our biotope showed good results around the head-water and dramatic deterioration around Beirut city. Water samples were collected periodically and analyzed for HCO, Cl, and HNO, and for the major ions/Ca, Mg, Na and K, and for some trace elements. Atomic absorption spectrometry using flame acetylene was used to measure the alkaline metals, whereas graphite furnace was used for the determination of trace elements. It has been found that the major composition in rivers is predominantly influenced by the composition of parent rocks in Lebanon. The slightly elevated values of PH (7.7) in the rivers and the large amount of carbonate HCO, dolomite CaMg (CO) and limestone. These compounds lead to elevated concentration of Ca and Mg in the studied stations. The concentration of the major ions varied with the cationic dominance order of Ca > Mg > Na > K which is compatible with the common natural major ion assemblages established for world rivers. The concentration of the major ions are also predominantly influenced by the Chemical weathering of rocks and minerals. The mean concentration of nitrates increased in urban and agriculture areas. The mean concentration of trace elements in the freshwater increased with increasing distance from the headwater, particularly at Ain el Delbe and the end station of Jisr Bacha, a consequence of anthropogenic inputs. In fact the above mentioned stations are located in industrial and popular areas. (author)

Enhanced chemical weathering of rocks during the last glacial maximum: a sink for atmospheric CO2?

It has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric CO2 on the continents may have at least partly contributed to the low CO2 concentrations during the last glacial maximum (LGM). Variations in continental erosion could thus be one of the driving forces for the glacial/interglacial climate cycles during Quaternary times. To test such an hypothesis, a global carbon erosion model has been applied to a LGM scenario in order to determine the amount of CO2 consumed by chemical rock weathering during that time. In this model, both the part of atmospheric CO2 coming from silicate weathering and the part coming from carbonate weathering are distinguished. The climatic conditions during LGM were reconstructed on the basis of the output files from a computer simulation with a general circulation model. Only the predicted changes in precipitation and temperature have been used, whereas the changes in continental runoff were determined with an empirical method. It is found that during the LGM, the overall atmospheric CO2 consumption may have been greater than today (by about 20%), mainly because of greater carbonate outcrop area related to the lower sea level on the shelves. This does not, however, affect the atmospheric CO2 consumption by silicate weathering, which alone has the potential to alter atmospheric CO2 on the long-term. Silicate weathering and the concomitant atmospheric CO2 consumption decreased together with a global decrease of continental runoff compared to present-day (both by about 10%). Nevertheless, some uncertainty remains because the individual lithologies of the continental shelves as well as their behavior with respect to chemical weathering are probably not well enough known. The values we present refer to the ice-free continental area only, but we tested also whether chemical weathering under the huge ice sheets could have been important for the global budget. Although glacial runoff was considerably increased during LGM, weathering under the ice sheets seems to be of minor importance.

Influence of In Situ Factors on Dynamic Response of Piedmont Residual Soils

The in situ chemical and physical weathering of igneous and metamorphic rocks, identified as the process of formation of Piedmont residual soils, is a fairly well-understood phenomenon. However, the effect this weathering has on the physical, mechanical, and dynamic properties of the rock/soil is not understood fully. This study focuses on the dynamic shear modulus, G, and material damping ratio, D, of this soil formation for low- to midlevel amplitudes of vibration. The paper presents laboratory test results and correlations that demonstrate the effects that the degree of weathering has on these properties for various levels of confining pressure and shear strain amplitude. A total of 12 specimens of Piedmont residual soils from different depths were tested in a Resonant Column device. The specimens tested were SM and ML soils according to the Unified System (USCS) classification. The low-amplitude shear modulus and damping values were found to be similar to those reported in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolidation ratio exert a noticeable influence on the dynamic response as a result of variations in confining pressure. The understanding of these effects will allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on these soil deposits. The response in free field soil deposits compared with that of soils experiencing added confining stresses caused by foundation loading appears to vary significantly in these geologic formations. Threshold strain and the variation of damping, D, with the normalized shear moduli, G/G sub max, fall within the same range as those recently reported by other authors in similar soils.

Soil erosion and atmospheric CO2 during the last glacial maximum: the rôle of riverine organic matter fluxes

Atmospheric CO2 is consumed both by organic matter formation and chemical rock weathering, and subsequently discharged as dissolved organic carbon, particulate organic carbon, and dissolved inorganic carbon to the oceans by rivers. In the long term, varying the ratio of the amount of atmospheric CO2 consumed by continental erosion and the amount of CO2 released during carbonate precipitation and organic matter respiration in the oceans can change the CO2 content in the atmosphere. The purpose of this paper is to determine whether riverine organic carbon fluxes during the last glacial maximum (LGM) may have been different from today in order to assess the potential impact on atmospheric CO2. Previous studies mainly focused on the role of the river fluxes of inorganic carbon in this respect, but none of them examined possible variations in the fluxes of organic carbon, although the erosion of organic carbon actually represents the bulk of the atmospheric CO2 consumption by continental erosion. We therefore applied a global carbon erosion model to a LGM scenario in order to determine the riverine fluxes of organic matter during that time. The climatic conditions during the LGM were reconstructed using a computer simulation with a general circulation model. It is found that during the LGM the riverine organic carbon input into the oceans was at least ˜10% lower than today. Most of the reduction of the total organic matter fluxes is due to the reduction of the fluxes of dissolved organic carbon. The fluxes of particulate organic carbon remained almost unchanged. The oceanic response to the lower carbon input was estimated on the basis of a present-day steady state budget for organic river carbon in the oceans, and implies that the reduction of the river fluxes were more than counterbalanced by lower burial rates due to the smaller shelf area during the LGM. This suggests that both the lower river carbon input and the relatively greater share of this carbon being subjected to oceanic respiration, acted as a negative feedback to the low atmospheric CO2 content during the LGM.

Relative importance of natural and anthropogenic influences on the fresh surface water chemistry of the Hawkesbury–Nepean River, south-eastern Australia

Fresh surface waters from the Hawkesbury–Nepean River, the major river supplying water to the Sydney region in south-eastern Australia, were sampled monthly during 1991 and analysed for major ions (Na, K, Ca, Mg, Cl, SO 4 and HCO 3), nutrients (NO 3 and PO 4), organic carbon and trace metals (Al, Fe, Cu, Zn, Pb, Cd, Ni, Co and Mn). The chemical composition of the river during 1991 was consistent with other studies of the river from 1977 to 1996. The major ion composition in the river is predominantly influenced by sea-salt aerosols in rainwater (headwaters) and connate sea-salt in groundwater (mid-lower reaches), with a cationic dominance order of Na≫Mg>Ca>K (equivalents) and an anionic order of Cl≫HCO 3>SO 4. This is typical of the headwaters of other permanent coastal rivers (freshwater) in south-eastern Australia with a similar catchment lithology. These results differ markedly from the most common natural major ion assemblages established for world rivers (i.e. Ca>Mg>Na>K and HCO 3>SO 4>Cl), which tend to be predominantly influenced by chemical weathering of rocks and minerals. The mean concentrations of major ions, nutrients, organic carbon and trace metals in the freshwater reaches of the Hawkesbury–Nepean River increased by factors of 2.5–4.4, 14–18, 2.2 and 1.6–11, respectively, with increasing distance from the headwaters. Increases in major ion concentrations are attributed mainly to the increasing influence of saline groundwater inflows from regions of Wianamatta shale. Conversely, concentrations of nutrients, organic carbon and trace metals (except Fe and Al) increased as a consequence of anthropogenic inputs, particularly point discharges from sewage treatment plants (i.e. showing distinct, but variable, concentration peaks), as well as diffuse urban and/or agricultural runoff during storm events. The temporal variability of the mean concentrations of all measured parameters in this study was related to variability in water discharge. The mean concentrations of the major ions decreased by a factor of 1.5–3.0 with increasing water discharge, whereas the concentrations of nutrients, organic carbon and trace metals increased by a factor of 2.0–3.0, 1.6 and 1.3–2.0, respectively. This study provides the first survey of trace metal concentrations in the freshwater reaches of a permanent coastal river in Australia using `clean' sampling and handling techniques. The concentrations of Cu, Zn, Pb, Cd and Ni measured in the headwaters of the Hawkesbury–Nepean River were amongst the lowest reported in the literature for riverine (freshwater) systems, and will form a benchmark for assessing the effects of increasing urbanisation in the catchment.

Geochemistry of rare elements in waters and sediments of alkaline lakes in the Sasykkul depression, East Pamirs

The saline lakes (Tuzkul, Chukurkul, Sasykkul Lakes) of the Sasykkul depression located in the semidesert part of the Pamirs provide a prominent example of the evaporative concentration of alkaline waters in a closed basin under arid climate conditions. Chemical evolution of the lake waters has resulted in the development of residual Na–Cl–CO 3–SO 4 and Na–Cl–CO 3 brines. The contents of a number of rare elements were determined in waters and sediments of the Sasykkul depression lakes. Speciation modelling suggests that the main aqueous species of Ag are chloride complexes, Cu, Pb, Zn, Mn are carbonate complexes, Be, Bi, Nb are hydroxocomplexes, Mo, W, As, V are oxyanions. It was established that the salt lake sediments are significantly enriched in B, Ba, Mo, W, U, Th. High contents of trace elements are considered to be due to chemical weathering of rocks within the catchment basin, significant contribution from thermal springs derived from the residual fluids associated with acid igneous and volcanic rocks, and following evapoconcentration in a closed basin. The processes of evaporative concentration, solution complexation, chemical precipitation, and water-sediment interaction lead to the partitioning of trace metals in the salt sediments.

Atmospheric CO 2 consumption by continental erosion: present-day controls and implications for the last glacial maximum

The export of carbon from land to sea by rivers represents a major link in the global carbon cycle. For all principal carbon forms, the main factors that control the present-day fluxes at the global scale have been determined in order to establish global budgets and to predict regional fluxes. Dissolved organic carbon fluxes are mainly related to drainage intensity, basin slope, and the amount of carbon stored in soils. Particulate organic carbon fluxes are calculated as a function of sediment yields and of drainage intensity. The consumption of atmospheric/soil CO 2 by chemical rock weathering depends mainly on the rock type and on the drainage intensity. Our empirical models yield a total of 0.721 Gt of carbon (Gt C) that is exported from the continents to the oceans each year. From this figure, 0.096 Gt C come from carbonate mineral dissolution and the remaining 0.625 Gt C stem from the atmosphere ( F CO 2 ). Of this atmospheric carbon, 33% is discharged as dissolved organic carbon, 30% as particulate organic carbon, and 37% as bicarbonate ions. Predicted inorganic carbon fluxes were further compared with observed fluxes for a set of 35 major world rivers, and possible additional climatic effects on the consumption of atmospheric CO 2 by rock weathering were investigated in these river basins. Finally, we discuss the implications of our results for the river carbon fluxes and the role of continental erosion in the global carbon cycle during the last glacial maximum.

Modeling processes in forest soils—problems, simplifications and caveats

The mechanistic simulations of chemical processes in forest soils require estimates of uptake and release rates of nutrients from trees, as well as rates of microbial transformations of organic matter, chemical weathering and solute transport within the soil profile. Among the processes that affect the chemistry of forest soils chemical inorganic equilibrium processes are best understood. An appropriate presentation of microbiological processes is lacking. Further uncertainty is introduced by simplistic approaches to estimate nutrient uptake by trees and rates of chemical rock weathering. Parameters for processes are derived from studies conducted at different spatial or temporal scales and therefore, may not always be consistent. Since kinetics cannot often be adequately modeled, many processes are described assuming steady-state conditions. Moreover most European temperate forests are in a successional transition phase and considerably off chemical steady-state. Reasons are disturbances due to natural disasters (e.g., wild-fires) and anthropogenic impacts (e.g., litter raking, acidic emmissions and management practices). Changing intensities of anthropogenic land-use and silvicultural methods affect the chemical status of soils for decades. A major constraint to modeling is the inability to properly account for long-term patterns that arise from the longevity of forests. Results of simulation models are rather suitable to establish the boundaries of the status of soil chemistry under defined environmental conditions. Models often fail to accurately describe soil chemistry at particular locations and times.

Modelling the Interactions Between Biospheric and Weathering Processes: Towards a Mechanistic Description of the Land Environment

Recently, some tentative attempts to calculate the distribution of the rock chemical weathering rate over the continents and its variation from the last glacial maximum to the present have been published in the literature (e.g. Gibbs and Kump, 1994). These investigators used empirical relationships between the weathering rate of most important rock types and water runoff. The reconstruction of past weathering can then be performed from the water budget predicted by general circulation models. Although runoff is one of the most important environmental factors contolling chemical weathering at large spatial scales, many complex processes may influence weathering and its response to climatic forcings. For instance, temperature, orogeny or mechanical weathering (and the associated exposure of fresh mineral surface), and vegetation may exert non negligible controls on weathering, as widely discussed in recent years. In particular the effect of vegetation and soil microbial activity is recognized as important in promoting mineral dissolution (e.g. Schwartzman and Volk, 1989), through enhancement of the soil CO2 pressure and through organic secretion from plant roots and fungi. It is unlikely that simple statistical relationships calibrated on the present-day system can be successful in predicting chemical weathering rates under past climatic conditions when the atmospheric CO2 level and the vegetation distribution were very different from today. For this reason, it is necessary to build more mechanistic models of the coupled vegetation-soilrock system, which should describe in some detail the various processes involved in rock weathering, plant growth and soil microbial activity. However, the drawback of such a process-oriented approach is that it must involve an upscaling methodology from small (site, catchment) to large (river basin, continent) spatial units and from short (day, season) to long (103-106 years) time periods, including a validation at each level. Such an initiative has been pursued over the last few years in the biospheric community and global process-oriented models of the land biosphere are currently available. A similar effort may be conducted within the scientific community studying soil formation and weathering. Here we present a preliminary attempt towards a mechanistic description of rock weathering and its interactions with vegetation and soil biogeochemistry. Our approach is based on the chemical weathering model developed by Gwiazda and Broecker (1994) coupled to the CARAIB (CARbon Assimilation In the Biosphere; Warnant et al., 1994) model of the land biosphere. The CARAIB model couples various modules associated with the budget of soil water, photosynthesis, plant growth and respiration, and soil microbial oxidation of organic matter. The most important outputs are the soil water anaount, evapotranspiration, surface runoff, drainage at the bottom of the root zone, net primary productivity (NPP) of vegetation, autotrophic and heterotrophic respirations, and the contents of vegetation, litter and soil carbon reservoirs. The model is designed to be used at the global scale and, in such applications, a resolution of 1~ ~ in longitude-latitude is usually adopted. Nevertheless, in the case of local applications, the spatial resolution may easily be modified or eventually the calculation may only be performed for one single grid cell (test site). The temporal resolution is 1 day for the calculation of every water and carbon reservoirs. If input climatic data are provided monthly, a weather generator can be used to transform these data into daily inputs. This allows to take into account the high degree of non-linearity of

Geochemistry of the rare earth elements in natural terrestrial waters: A review of what is currently known

The range of observed chemical compositions of natural terrestrial waters varies greatly especially when compared to the essentially constant global composition of the oceans. The concentrations of the REEs in natural terrestrial waters also exhibit more variation than what was reported in seawater. In terrestrial waters, pH values span the range from acid up to alkaline. In addition, terrestrial waters can range from very dilute waters through to highly concentrated brines. The REE concentrations and their behavior in natural terrestrial waters reflect these compositional ranges. Chemical weathering of rocks represents the source of the REEs to natural terrestrial waters and, consequently, the REE signature of rocks can impart their REE signature to associated waters. In addition, because of the typical low solubilities of the REEs, both surface and solution complexation can be important in fractionating REEs in aqueous solution. Both of these processes are important in all natural terrestrial waters, however, their relative importance varies as a function of the overall solution composition. In alkaline waters, for example, solution complexation of the REEs with carbonate ions appears to control their aqueous distributions whereas in acid waters, the REE signature of the labile fraction of the REEs is readily leached from the rocks. In circumneutral pH waters, both processes appear to be important and their relative significance has not yet been determined.

Changes in moisture balance between glacial and interglacial conditions: influence on carbon cycle processes

Abstract During the arid Late Glacial and Last Glacial Maximum (between approximately 30 000 and 13 000 calendar years ago), vegetation cover retreated and large areas of the continents were occupied by desert and semi-desert vegetation. The result of this general decrease in biological activity would have been a decrease in the size of the land carbon reservoir, and a decrease in the rate of chemical rock weathering. By contrast, during the early-mid-Holocene, conditions in many areas seem to have been moister than today due to a more active hydrological cycle. All of these processes would have affected the global carbon cycle and altered the amplitude and timing of the climate fluctuations themselves. In effect, the climatic shift between glacial and interglacial conditions creates a very large ‘missing source’ of carbon, perhaps amounting to thousands of gigatonnes, to account for the carbon uptake by the land system during the present interglacial, and thus carbon cycle models of the late Quaternary may need to be revised extensively.

Productivity of pre-vascular continental biota inferred from the Fe(CO3)OH content of goethite

CHEMICAL weathering of rocks by CO2—an important process in the carbon cycle—may have been affected by the appearance of vascular plants during the Silurian period (438–408 Myr ago), because of the enhanced efficacy of these plants in promoting weathering1. The magnitude of this effect is uncertain, however2–4. One key issue is the concentration of CO2 in soils, generated by microbial respiration; this is itself in part a function of soil productivity. We showed recently5 that the CO2 partial pressure in a Late Ordovician soil was 5.6 times that in the atmosphere; Keller and Wood6 have shown, however, that soil CO2 concentrations can be high even for low levels of microbial respiration, as a result of slow diffusion of respired CO2 out of the soil. Here we present an analysis of the Fe(CO3)OH component of goethite (FeOOH) in an Upper Ordovician oolitic ironstone which underwent tropical chemical weathering 440 Myr ago7,8. These data allow us to extract an estimate of the CO2 flux from the soil profile. The high rate that we obtain implies that the productivity of pre-vascular biota was similar to that in modern soils. Thus, attempts to model atmospheric CO2 concentrations over Phanerozoic 9–12 time need not assume that pronounced productivity changes accompanied the development of vascular plants.

Modelling of atmospheric CO 2 consumption by chemical weathering of rocks: Application to the Garonne, Congo and Amazon basins

Modelling of atmospheric CO, consumption by chemical weathering of rocks: Application to the Garonne, Congo and Amazon basins

Microscopic techniques leading to the discovery of endolithic plant communities in a stream ecosystem

The chemical weathering of rock is generally assumed to occur primarily at the water/rock interface and to be dominated by inorganic mechanisms. A previous study aimed at using microscopic techniques to study the weathering of sandstone bedrock in a stream ecosystem suggested the operation of biotic processes inside these rocks. Examination of rock samples by a variety of microscopic techniques led to the discovery of endolithic plant communities. Endolithic microbial communities are known to exist in hot and cold desert, freshwater, and marine environments, occurring in a variety of bedrock types. However, endolithic organisms have not been previously documented in stream ecosystems.

Kinetic control of dissolved phosphate in natural rivers and estuaries: A primer on the phosphate buffer mechanism1

The primary mode of interaction of dissolved phosphate with fluvial inorganic suspended particles is via a reversible two‐step sorption process. The first step, adsorption/desorption on surfaces, has fast kinetics (minutes–hours). The second step, solid‐state diffusion of adsorbed phosphate from the surface into the interior of particles, has slow kinetics (days–months) and is dependent on the time history of the previous surface sorption and the chemistry of the solid diffusional layer. Natural clay particles with a surficial armoring of reactive iron and aluminum hydroxyoxides resulting from chemical weathering of rocks and soils have a high capacity for absorbing phosphate in the second step and for maintaining low “equilibrium phosphate concentrations” in solution. Extrapolation of laboratory sorption and extraction experiments with natural soils and suspended sediments to the environment suggests that the phosphate concentrations of unperturbed turbid rivers (SPM > 50 mg liter−1) are controlled near the dynamic equilibrium phosphate concentration of their particles (EPC0 = 0.2–1.5 µM) and that fluvial suspended particles “at equilibrium” contain up to 10 µmol‐P g−1 that is desorbable. Release of this phosphate from particles entering the sea produces the characteristic shape and magnitude of input profiles of dissolved phosphate observed in unperturbed estuaries. On a global scale, fluvial particulates could transport from 1.4 to 14 × 1010 mol yr−1 of reactive phosphate to the sea, some 2–5 times more than that in the dissolved load alone.

Rates of Chemical Weathering of Rocks and Minerals. 1986

S.M. Colman and D.P. Dethier, An Overview of Rates of Chemical Weathering. Kinetics and Mechanisms of Weathering Reactions: R.A. Eggleton, The Relations between Crystal Structure and Silicate Weathering Rates. D.E. Grandstaff, The Dissolution Rate of Forsteritic Olivine from Hawaiian Beach Sand. G.R. Holdren, Jr., and P.M. Speyer, Stoichiometry of Alkali Feldspar Dissolution at Room Temperature and Various pH Values. J.B. Laronne, Rate Limitation and Dissolution of Highly Soluble Minerals. Rates of Mineral Alteration in Soil Environments: R.D. Hall and R.E. Martin, The Etching of Hornblende Grains in the Matrix of Alpine Tills and Periglacial Deposits. W.W. Locke, Rates of Hornblende Etching in Soils on Glacial Deposits, Baffin Island, Canada. W.C. Mahaney and D.L. Halvorson, Rates of Mineral Weathering in the Wind River Mountains, Western Wyoming. D.B. Nahon, Evolution of Iron Crusts in Tropical Landscapes. F.C. Ugolini, Processes and Rates of Weathering in Cold and Polar Desert Environments. Weathering of Volcanic Ashes and Deposits: R.H. King, Weathering of Holocene Airfall Tephras in the Southern Canadian Rockies. D.J. Lowe, Controls on the Rates of Weathering and Clay Mineral Genesis in Airfall Tephras: A Review and New Zealand Case Study. V.E. Neall and I.K. Paintin, Rates of Weathering of 14C-Dated Late Quaternary Volcaniclastic Deposits in the Western United States. A.F. White, L.V. Benson, and A. Yee, Chemical Weathering of the May 18, 1980, Mount St. Helens Ash Fall and the Effect on the Iron Creek Watershed, Washington. Rates of Formation of Rock-Weathering Features: S.M. Colman, Levels of Time Information in Weathering Measurements, with Examples from Weathering Rinds on Volcanic Clasts in the Western United States. R. Crook, Jr., and A.R. Gillespie, Weathering Rates in Granitic Boulders Measured by P-Wave Speeds. I.E. Whitehouse, M.J. McSaveney, P.L.K. Knuepfer, and T.J.H. Chinn, Growth of Weathering Rinds on Torlesse Sandstone, Southern Alps, New Zealand. Hydrochemical Studies of Rates of Weathering: M.A. Velbel, The Mathematical Basis for Determining Rates of Geochemical and Geomorphic Processes in Small Forested Watersheds by Mass Balance: Examples and Implications. J.L. Clayton, An Estimate of Plagioclase Weathering Rate in the Idaho Batholith Based upon Geochemical Transport Rates. J. Creasey, A.C. Edwards, J.M. Reid, D.A. MacLeod, and M.S. Cresser, The Use of Catchment Studies for Assessing Chemical Weathering Rates in Two Contrasting Upland Areas in Northeast Scotland. D.P. Dethier, Weathering Rates and the Chemical Flux from Catchments in the Pacific Northwest, U.S.A.. T. Pa*afces, Rates of Weathering and Erosion Derived from Mass Balance in Small Drainage Basins. M.J. Pavich, Processes and Rates of Saprolite Production and Erosion on a Foliated Granitic Rock of the Virginia Piedmont. Each chapter includes references. Index.

The role of chemical weathering in the neutralization of acidic deposition

Chemical weathering of rocks and minerals is a key factor which mitigates acidic deposition and affects water chemistry. It supplies cations and alkalinity to the surface water, groundwater, ion-exchange complex, and vegetation in the watershed. The kinetics of chemical weathering have not been determined in the field, but based on laboratory experiments, the rate of weathering has a fractional order dependency on hydrogen ion and organic ligand concentration in bulk solution. Watersheds with the greatest degree of hydrologic and geologic sensitivity can produce only 200–500 eq/ha·yr of cations or alkalinity for export. This is equivalent to 100 cm/yr of precipitation with a pH of 4.3–4.6 or an annual sulfur deposition of 1.0–2.5 g S/m2·yr. When acid and sulfur deposition are greater than these levels, extremely sensitive lakes may become acidified. To illustrate this point, a simple steady-state model is applied to lakes in regions where acidification of lakes has been reported.

Use of 234U/ 238U disequilibrium in measuring chemical weathering rate of rocks

The rate of chemical weathering of rocks has been determined by using uranium as a natural isotopic tracer. The concentration of uranium and 234U/ 238U ratio in natural waters, rocks, and soils of the Preto river basin (Bahia State, Brazil) was measured by alpha-ray spectroscopy. The activity ratio U234/U238 measured in the various samples indicates the uranium fraction which is dissolved from rocks during the weathering process. The results obtained show that 1 m of rock needs 25,000 yr to be weathered in this region under present climatic conditions.

THE SALTS IN ANTARCTIC SOILS, THEIR DISTRIBUTION AND RELATIONSHIP TO SOIL PROCESSES

One of the prominent features of Antarctic soils, and one which shows their relationship with soils in other arid regions of the world, is the high content of soluble salts. These salts consist largely of the sulphates, chlorides, and nitrates of sodium, potassium, magnesium, and calcium. Almost all of the possible crystalline phases that may be formed by combination of these anions and cations are found. The origin of the salts has been a matter of controversy for many years. Various origins have been postulated, including chemical weathering of rocks, deposition of marine salts, and leaching from sedimentary rocks. Soils sampled along the length of the Trans-Antarctic Mountains during the last 17 years have formed from a wide range of soil parent materials, under significantly different climatic conditions and age of weathering surface. From the relationship between these factors and the nature of the salts found in the soils, soil pH, and the distribution of calcite crusts, it is possible to show that the greater part of the salt is derived from marine salts, although there are significant contributions from chemical weathering processes. The chemistry of the salts clearly shows that chemical weathering takes place in Antarctic soils, even in the coldest and most arid regions of the Trans-Antarctic Mountains.