Chemical Weathering Processes Research Articles

Toward process-based modeling of geochemical soil formation across diverse landforms: A new mathematical framework

We present a mathematical model that integrates geochemical and geomorphic processes responsible for soils' elemental and mineralogical evolution on diverse landforms: eroding to depositional to level grounds. This new model combines a hillslope sediment mass balance and a soil geochemical mass balance. We model soils as the sum of a physically disturbed zone (PDZ) and the underlying physically undisturbed but chemically altered zone (CAZ). The model considers processes including the production of PDZ, weathering front propagation, mineral-specific dissolution, colloidal translocation, and colluvial transport. Thus this study serves to (1) review preexisting models of pedogenic processes and to (2) offer a framework to integrate the geochemical and geomorphic processes active in soils. We further discuss the implications of our model in (1) scaling chemical weathering reactions from mineral grains to pedons to soil catenas, (2) understanding the emergence of geochemical soil catenas, and (3) coupling physical and chemical weathering processes.

The model of long-term evolution of the carbon cycle

The weathering processes and their role in the formation of the atmospheric carbon and, as a consequence, on the climate are considered. The model operates in the framework of “active planetary cover”, i.e. considering the interactive role of the biosphere, looking at its development as a non-linear evolutionary system of so-called “virtual biospheres”. First of all, the mechanism of chemical equilibrium in closed system is described, applied to weathering in order to provide a better understanding of reactions in the solution of CaCO 3 and CO 2. The second stage is to describe the chemical transformations of carbon within the system. However here we deal with an open system, and therefore we were compelled to develop some partial case of the theory of chemical equilibrium in open systems, where the values of total amounts of calcium and carbon are slowly changed, described by the equations of slow dynamics of weathering by rain-fall. This relatively slow process of chemical weathering, which is accompanied by an uptake of carbon from the atmosphere and its transportation to the intercontinental and epicontinental seas, is significantly amplified by terrestrial ecosystems, i.e. in the ecosystem covered by vascular plants, that has evolutionary replaced the shallow-water-body system. In accordance with Zavarzin's estimation, the concentration of CO 2 in the soil air is 60–80 times larger than in the atmosphere, caused by root respiration and the activity of micro-organisms. All this leads to a significant acceleration of weathering in soil. It seems that such a concentration gradient cannot be maintained and it will be eroded by diffusion. It is true if we consider bare soil. But in real ecosystems the litter covers soils, so that molecular diffusion becomes prevalent. In other words, the litter can be considered as a semi-permeable membrane for CO 2.

Strontium stable isotopes fractionate in the soil environments?

This study shows that the stable isotopic composition of strontium (the 88Sr/ 86Sr ratio expressed as δ 88/86Sr value relative to the NBS987 standard) varies significantly in sedimentary terrestrial environments. The abundances of 86Sr, 88Sr isotopes were analyzed by MC-ICP-MS “Nu Plasma”. All studied rocks and waters show δ 88/86Sr values that are distinctly different from the measured NBS987 standard (yielding 0.01 ± 0.05‰, all errors are reported as 2 σ). Modern corals from the northern Gulf of Aqaba, Red Sea yielded significantly different value than seawater (δ 88/86Sr = 0.22 ± 0.07‰, compared to 0.35 ± 0.06‰, respectively), in an excellent correlation with the δ 88/86Sr analyses reported by Fietzke and Eisenhauer [Fietzke, J., Eisenhauer, A., 2006. Determination of temperature-dependent stable strontium isotopes ( 88Sr/ 86Sr) fractionation via bracketing standard MC-ICP-MS. Geochm. Geophys. Geosyst. 7 (no. 8)] on other coral samples. All carbonate samples that originated in the marine environment: corals ( porites and acropora from the northern Gulf of Aqaba); Cretaceous limestone and runoff from the Judea Mountains as well as lacustrine evaporitic aragonite (Dead Sea); and Red Sea and Atlantic seawater yield an average δ 88/86Sr value of 0.26 ± 0.1‰. On the other hand, secondary materials (products of chemical weathering) from the terrestrial environment of the Judea Mountain such as terra rossa soil and speleothem calcite (that derives its Sr from the above-lying soil) yielded significantly lower δ 88/86Sr value of − 0.17 ± 0.06‰. This indicates that strontium isotopes fractionate in the soil environment calling for a possible development of strontium isotopes as a tracer for processes of chemical weathering and pedogenesis.

Pedogenesis and weathering rates of a Histic Andosol in Iceland: Field and experimental soil solution study

It is important to study the rate determining processes of chemical weathering and soil formation in volcanic islands since a significant part of the carbon fixed by chemical weathering of silicates on Earth is fixed at the surface of volcanic islands. These soils are fertile and much of the river suspended matter delivered to the ocean stems from these islands. This study determines the factors that drive the pedogenesis of a Histic Andosol in Western Iceland. Soil solutions were extracted from the profile in the field, from undisturbed ex situ mesocosms and from repacked laboratory microcosms. Concentrations of measured and calculated inorganic species in the field and experimental soil solutions were used for thermodynamic and kinetic interpretation, and to calculate the weathering rates. The main primary rock constituent of the 205 cm thick soil profile was basaltic glass, allophane content ranged from 2 to 22% and the soil carbon content ranged from 11 to 42%. Mean soil solution pH value ranged from 4 to 6 with the lowest value at 80 cm depth and highest between 150 to 205 cm. The high solute concentrations in soil solutions in the beginning of the microcosm weathering experiment declined faster for anions than cations. Under field conditions inorganic anions were supplied by marine and anthropogenic rather than pedogenic sources and hence these anions were subsequently leached out during the experimental duration when there was a limited input of anions through experimental precipitation. The factor, which defined the rate at which each ion was depleted from the exchange complex of the soil, decreased down the soil profile. The release sequence at 50 cm depth was Cl > Na > SO 4 > F > Si. The Si and base cations experimental weathering rate when normalized to geographical surface area are similar to or lower than those measured from river catchments in Southwestern Iceland. The dissolved Al flux was much higher from the soil when compared to the river catchments. Field and experimental soil solutions were all highly undersaturated with respect to basaltic glass. Field and mesocosm samples were supersaturated with respect to secondary allophane and imogolite, while samples from the microcosms were often undersaturated with respect to allophane and imogolite. Predicted dissolution rate was dictated by the soil solution a H + 3/ a Al 3+ activity ratio but slowed down by up to 20% and 30% by decreasing undersaturation in field and mesocosms respectively. Predicted dissolution rates according to the a H + 3/ a Al 3+ activity ratio increased up to factor of 7, 30 and 37 by speciating Al 3+ with oxalate in field, mesocosms and microcosms respectively. Speciation with oxalate, which represents the maximum effect of the dissolved organic carbon (DOC) on dissolution rates, generally had more effect near the surface than at deep levels in the soil profile. This study shows that at fixed temperature, reactive surface area, and composition of the volcanic glass in the soil, the chemical weathering rates of Andosols are dictated by: 1) aeolian deposition rates and drainage, which affect the saturation state and the a H + 3/ a Al 3+ activity ratio, 2) the production of organic anions within the soil, and 3) external supply of anions capable of complexing Al 3+.

Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemi- cal analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Soil characteristics on varying lithological substrates in the South Shetland Islands, maritime Antarctica

Soils in ice-free areas of Livingston Island have been forming since the last deglaciation in a maritime warmer climate that is more humid than in interior Antarctica. A soil survey was carried out on Byers and Hurd peninsulas to characterize the soils and investigate the processes. Soils were sampled on two different substrates, mudstones and volcanic rocks in Byers Peninsula and greywackes in Hurd Peninsula. Sampling sites were located on raised beaches, platforms, and on volcanic outcrops across an altitudinal range from few m to 150 m a.s.l. The pH, electrical conductivity, carbonates and organic matter showed similar patterns in each geomorphic unit but their values differed between bedrocks. The sand, silt and clay contents differed greatly in the soils on mudstones and volcanic rocks. The elemental composition was closely related to mineralogy of parent materials. Mg, K, Zn, Mn, and Fe were similar in the soils on all bedrocks. The Al content was highest on the volcanic rocks. The largest difference in the Ca content was between the soils on greywackes and mudstones. The Na, Pb, and Ba contents in soils on the mudstones differed greatly from the rest of bedrocks. Geochemical and mineralogical characteristics suggest that the main process involved in soil development was the mechanical disintegration of bedrock although there was also some leaching. Cryogenic processes play a key role in soil development but chemical weathering processes were also involved in soil evolution although limited in extent due to the restriction of water circulation to summer.

A theoretical model coupling chemical weathering and physical erosion in landslide-dominated landscapes

A current theory that links the uplift of the Himalayas to the drawdown of atmospheric CO 2 assumes that rates of chemical weathering are enhanced by accelerated physical erosion due to the increased exposure of fresh minerals. Although several studies have concluded that weathering rates do increase with erosion rates, there is disagreement over the form of that relationship. Some find a linear relationship between weathering rate and erosion rate; others, however, find that weathering is only approximately proportional to the square root of physical erosion such that changes in erosion rate are not matched by equivalent changes in weathering rate. Defining the nature of the relationship is critical because it determines the sensitivity of changes in weathering rate to changes in erosion rate. In this study, a theoretical model that couples erosion by landslides with chemical weathering processes was derived to explore the nature of this relationship in rapidly eroding landscapes. Results from the model suggest that, where landsliding is the dominant erosional process, the rate of chemical weathering is only proportional to the erosion rate raised to the ∼ 0.6 power. Therefore, even if weathering rate is wholly dependent on erosion rate, increases in erosion will not be matched by equivalent increases in chemical weathering.

Chemical weathering of a marine terrace chronosequence, Santa Cruz, California I: Interpreting rates and controls based on soil concentration–depth profiles

The spatial and temporal changes in element and mineral concentrations in regolith profiles in a chronosequence developed on marine terraces along coastal California are interpreted in terms of chemical weathering rates and processes. In regoliths up to 15 m deep and 226 kyrs old, quartz-normalized mass transfer coefficients indicate non-stoichiometric preferential release of Sr > Ca > Na from plagioclase along with lesser amounts of K, Rb and Ba derived from K-feldspar. Smectite weathering results in the loss of Mg and concurrent incorporation of Al and Fe into secondary kaolinite and Fe-oxides in shallow argillic horizons. Elemental losses from weathering of the Santa Cruz terraces fall within the range of those for other marine terraces along the Pacific Coast of North America. Residual amounts of plagioclase and K-feldspar decrease with terrace depth and increasing age. The gradient of the weathering profile b s is defined by the ratio of the weathering rate, R to the velocity at which the profile penetrates into the protolith. A spreadsheet calculator further refines profile geometries, demonstrating that the non-linear regions at low residual feldspar concentrations at shallow depth are dominated by exponential changes in mineral surface-to-volume ratios and at high residual feldspar concentrations, at greater depth, by the approach to thermodynamic saturation. These parameters are of secondary importance to the fluid flux q h, which in thermodynamically saturated pore water, controls the weathering velocity and mineral losses from the profiles. Long-term fluid fluxes required to reproduce the feldspar weathering profiles are in agreement with contemporary values based on solute Cl balances ( q h = 0.025–0.17 m yr −1). During saturation-controlled and solute-limited weathering, the greater loss of plagioclase relative to K-feldspar is dependent on the large difference in their respective solubilities instead of the small difference between their respective reaction kinetics. The steady-state weathering rate under such conditions is defined as R = q h · m sol M total · 1 S v · b s · The product of q h and the ratio of solubilized to solid state feldspar ( m sat/ M total) define the weathering velocity. The weathering gradient b s reflects the kinetic rate of reaction where S v is the volumetric surface area of the residual feldspar. Both this rate expression and the spreadsheet calculations produce similar plagioclase weathering rates ( R = 5–14 × 10 −16 mol m −2 s −1) which agree with those reported for other environments of comparable climate and age. Weathering-dependent concentration profiles are commonly described in literature. The present paper provides methods by which these data can yield a more fundamental understanding of the weathering processes involved.

Kemungkinan sebaran zirkon pada endapan placer di Pulau Kalimantan

http://dx.doi.org/10.17014/ijog.vol2no2.20073Most alluvium with valued heavy minerals originated from regions of continental shelf, from which recycle process for stable minerals play an important role in transportation and final concentration formation of the minerals. Kalimantan Island as part of continental shelf has a great opportunity to provide condition or environment for placer deposition with zircon content due to: comprising particularly basement rock of calc-alkali to alkali series (granite, granodiorite, tonalite and monzonite) which were suggested as the main supplier sources of zircon; having a long period of regional stability as main contribution in continuation processes of weathering, transportation and forming deposition environment of alluvium and located within a tropical climate with high humidity from which mechanical and chemical weathering processes play an important role in releasing zircon from its source rocks. Continental placer deposit of fluvial sub-environment category is suggested to have possibility to form deposition zone lying between the mountains and the seas, reaching over wide areas and forming huge volume reservoirs with content of zircon. Based on zircon identification (associated with gold or diamond) of placer pan concentrates at some areas of West Kalimantan and Central Kalimantan, these have proved how widespread of zircon deposition within fluvial sub-environments in both regions. This important information inspires an opportunity to do exploration of placer zircon deposits within fluvial sub-environments in entirely Kalimantan Island, without neglecting to the transition placer of strandline sub-environment category and marine environment which enable to have an economic deposit of zircon.

Evolution of Chemistry along the Bagmati Drainage Network in Kathmandu Valley

The Bagmati River in Kathmandu valley, Nepal, was studied to understand the influence of human and geochemical processes on changes in river chemistry (nutrients, organic matter, and major cations and anions) along the drainage network. Population density appeared to drive variation in the chemistry of surface waters at 10 stations in the Bagmati River. For all constituents studied, concentrations increased with distance downstream and many parameters showed strong relationships with human population density adjacent to the river. The composition of river water suggests that sewage effluent entering the river has a major effect on water quality. Concentrations of most solutes were highest during summer and lower during the winter monsoon season. The contribution of chemical weathering processes to water quality of the Bagmati appears to be minor within the Kathmandu valley. Dominant cations and anions when expressed in equivalents per liter were \( {\text{Na}}^{ + } \, > > \,{\text{Ca}}^{{2 + }} \, > {\text{K}}^{ + } \, > > {\text{Mg}}^{{2 + }} \) and \( {\text{Cl}}^{ - } \, > > {\text{SO}}^{{2 - }}_{4} \, > {\text{PO}}^{{3 - }}_{4} \, > > {\text{NO}}^{ - }_{3} \) along the entire Bagmati drainage system. Ammonium contributed almost all nitrogen in the total dissolved nitrogen fraction and the concentration of nitrate was negligible, probably due to rapid denitrification and limited nitrification within the stream channel under conditions of relatively low oxygen. Decreases in sulfate along the stream channel may also be due to the reduction of sulfate to sulfide due to heavy organic matter loading. Water quality is unacceptable for any use and the whole ecosystem is severely affected due to human activities within the urban areas of the drainage basin.

A ∼ 3.5 Ga record of water-limited, acidic weathering conditions on Mars

The secondary mineral budget on Earth is dominated by clay minerals, Al-hydroxides, and Fe-oxides, which are formed under the moderate pH, high water-to-rock ratio conditions typical of Earth's near-surface environment. In contrast, geochemical analyses of rocks and soils from landed missions to Mars indicate that secondary mineralogy is dominated by Mg (± Fe, Ca)-sulfates and Fe-oxides. This discrepancy can be explained as resulting from differences in the chemical weathering environment of Earth and Mars. We suggest that chemical weathering processes on Mars are dominated by: (1) a low-pH, sulfuric acid-rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite (± Fe–Ti oxides) is promoted; and (2) relatively low water-to-rock ratio, such that other silicate phases with slower dissolution rates (e.g., plagioclase, pyroxene) do not contribute substantially to the secondary mineral budget at the Martian surface. Under these conditions, Al-mobilization is limited, and the formation of significant Al-bearing secondary phases (e.g., clays, Al-hydroxides, Al-sulfates) is inhibited. The antiquity of rock samples analyzed in-situ on Mars suggest that water-limited acidic weathering conditions have more than likely been the defining characteristic of the Martian aqueous environment for billions of years.

Multispectral processing of high‐resolution radiometric data for soil mapping

ABSTRACTSoil radioactivity is dominantly a product of the local geology and mineralization, physical and chemical weathering processes within the profile, and soil organic content. Recent studies have shown that local soil patterns can be confidently mapped at scales of 1:10 000 using new high spatial resolution airborne and ground radiometric data acquired with standard 256‐channel gamma‐ray spectrometry systems and sodium‐iodide (NaI) crystal packs. In this study, a new multispectral processing methodology was developed to assess the 256‐channel spectral soil response. The study identified seven distinct gamma‐ray energy peaks, all products of the potassium‐40, uranium‐238 or thorium‐232 decay series, whose spatial and spectral relationships could be used to interpret soil properties and/or soil conditions.In the case studies presented, interpretation of complementary thorium and uranium channels (isolated using the new multispectral processing methodology) reveal additional spatial patterns and spectral information that were unattainable using standard processing methods. In one example, soil conditions attributed to a local, developing clay‐soil unit were resolved by the increased 228Ac (actinium) to 208Tl (thorium) channel response. In a separate example, a change in ratio between 214Bi (uranium) energies 1120 keV and 1764 keV, produced during the same decay emission, was used to differentiate soil conditions in the top 40 cm of an expansive sandy loam on clay duplex soil unit. A decrease in the 1120 keV to 1764 keV ratio was interpreted as an increase in soil density as the lower gamma‐ray energy emissions are preferentially attenuated with increased density. Relative increases in the 1120 keV and 1764 keV channel responses were interpreted as an increase in uranium concentration in the soil profile. With the clay as the dominant uranium contributor, integrated interpretation of the 214Bi channels suggested that in these areas there was an increase in clay percentage within the surface sandy loam, which may result in local increased water‐logging susceptibility.This paper also theorizes that interpretation of the spatial displacement of thorium‐232 decay series 228Ac and 208Tl peaks, produced from decay events approximately 2 years (half‐life) apart, can be used to identify decay series disequilibrium conditions resulting from local soil–water processes or other recent soil activity. This information contributes to an understanding of the soil processes active in the area. In addition, identification of disequilibrium decay conditions may highlight locations where standard radiometric (radioelement) processing, which relies on equilibrium conditions, may be erroneous.

Dissolved constituents and Sr isotopes in river waters from a mountainous island – The Danshuei drainage system in northern Taiwan

Taiwan is a typical active orogenic belt situated at the collision boundary between the Eurasian Continental Plate and the Philippine Sea Plate. Dissolved major and trace constituents, as well as Sr and Sr isotopes in river waters collected from the Danshuei River basin in northern Taiwan have been studied to evaluate chemical weathering processes. The results of principal component analysis show that the ion sources in these river waters can be categorized into 3 major components: chemical weathering, seasalt contribution and local anthropogenic input. The chemical weathering is the most dominant factor that contributes about 85% of total variances. Significantly increased Na/Cl and Ca/Cl, as well 87Sr/ 86Sr, were observed in most upper stream samples. The Na/Cl and Ca/Cl ratios in the Dahan Stream, however, are much higher than the Shindien Stream. Even though average rainfall is stronger in the Shindien drainage basin, chemical evidence from river waters supports less intense weathering in the region. Selective dissolution of secondary calcites explains the observed high Ca/Cl, Sr/Cl and Ca/Na in the Dahan Stream. These results highlight the potential importance of tectonic factors, such as uplift and physical erosion in studying chemical weathering in an active orogenic belt. The variations of 87Sr/ 86Sr in the Danshuei River are quite large, reflecting some strata that released Sr. Most of the upstream waters exhibit more radiogenic 87Sr/ 86Sr, 0.713243–0.714338, due to weathering of ambient low-grade metamorphic rocks, 0.71678–0.72216. The distributions of Cl, Sr and 87Sr/ 86Sr in the main stream were affected by somewhat conservative mixing with sources varing between ambient rocks and seasalt. In the upper Dahan Stream, heavy 87Sr/ 86Sr ratios were coincident with large deviations of Na/Cl from the average seawater value, as high as 40. The degree of chemical weathering in ambient rocks plays a dominant role in affecting the distribution of Sr and 87Sr/ 86Sr in the Danshuei River. This isotopic characteristic makes 87Sr/ 86Sr an invaluable tracer for studying source mixing, migration pathways and chemical weathering in an active orogenic belt.

Interactions of sterile-cultured lichen-forming ascomycetes with asbestos fibres

Sterile cultured isolates of lichen-forming ascomycetes have not yet been used to investigate mycobiont–mineral substrate interactions under controlled conditions. In this study Candelariella vitellina, Xanthoparmelia tinctina and Lecanora rupicola mycobionts were isolated and inoculated with chrysotile fibres in the laboratory, in order to verify whether physical and chemical weathering processes, which were already described in the field, may be reproduced in vitro. Tight adhesion of hyphae to chrysotile fibres was observed in all species. The adhering hyphae affected the chemical composition of asbestos fibres, with the selective depletion of magnesium being a prominent feature, as is the case in field conditions. Oxalic acid and pulvinic acid, mycobiont-derived metabolites of X. tinctina and C. vitellina, were involved in the weathering action. Time and environmental factors and the absence of biological synergisms strongly limited the chemical weathering in vitro compared with what was observed in the field. Nevertheless, the results show that in vitro incubation of sterile-cultured lichen-forming fungi with minerals is a practicable experimental system to investigate the weathering effects of different mycobionts and fungal compounds under controlled conditions.

地圏における重金属類の分布と岩石からの移行プロセスにおける化学形態に関する基礎的検討

We have developed a GIS-based geosphere integration information system named Geosphere Environmental Informatics for evaluating heavy metal distribution on a national-scale. Geosphere Environmental Informatics includes various geosphere environmental information such as geological map, soil map, vegetation topography, locations of mineral deposits, alteration zone, anomalies of heavy metals, satellite images, groundwater information, and concentrations of heavy metals in rocks, soils and river-bed sediments. The system has some new geoprocessing tools. The Map integration tool integrates various geosphere maps. The River catchment tool creates individual stream networks with their features. The Intelligence search refinement tool extracts the objective characteristics of geosphere information from the GIS database.Heavy metal concentrations of several kinds of rock and river-bed sediments distributed in the model field (Kuroko deposits district in Akita Prefecture, Japan) were examined to evaluate heavy metal concentrations caused by mining and those in a natural state. Hydrothermal alteration zones related to are mineralization show the strong effects of heavy metal concentrations in river-bed sediments. It is difficult to distinguish artificial pollution from the natural abundance of heavy metals in altered rocks and river-bed sediments around a mine. A field survey over a wide area and detailed analyses of items including transport and precipitation mechanisms are required to evaluate artificial heavy metal pollution in a mining district.Chemical species of heavy metals in extracted solutions from rocks were examined by successive filtration. Heavy metals extracted from black slate existed in the solution as truly dissolved species. On the other hand, the amounts of truly dissolved species in solutions extracted from weathered slate decreased, and dominant chemical species of heavy metals were adsorbates on fine and coarse colloidal particles. In addition, colloidal particles show different REEs affinities. These results imply possible fractionation among REEs during chemical weathering and transport processes by colloidal particles.

Contrasting zircon Hf and O isotopes in the two episodes of Neoproterozoic granitoids in South China: Implications for growth and reworking of continental crust

The genetic links among rift magmatism, crustal growth and water–rock interaction are an important issue about mass and heat transfer between mantle and crust during supercontinent breakup. A combined study of Hf and O isotopes in zircons from Neoproterozoic granitoids in South China provides evidence for growth and reworking of juvenile and ancient crusts with different styles of water–rock interactions along rift tectonic zones. Two generations of the granitoids show contrasting features in both zircon Hf and O isotope compositions, indicating their distinct petrogenesis. The ∼ 825 Ma granitoids exhibit negative ε Hf( t) values of − 3.4 ± 0.8 to − 1.6 ± 0.8 with old model Hf ages of 1.81 ± 0.07 to 1.92 ± 0.10 Ga, and high δ 18O values of 8.7 to 10.4‰. These indicate that the source material of granitoid magmas was derived from melting of Paleoproterozoic basement that has the Hf isotope signature similar to the enriched mantle but experienced chemical weathering process before anatexis. Reworking of ancient crust is demonstrated to occur at ∼ 825 Ma in the orogenic collapse zone, with overprinting of subsolidus hydrothermal alteration during magma emplacement. In contrast, the 760–750 Ma bimodal intrusives are characterized by positive ε Hf( t) values of 3.5 ± 0.8 to 9.9 ± 0.8 with young model Hf ages of 0.94 ± 0.06 to 1.18 ± 0.06 Ga, and both low and high δ 18O values of 4.2 to 6.2‰ relative to 5.3 ± 0.3‰ for the normal mantle zircon. Prompt reworking of juvenile crust is suggested to occur at ∼ 750 Ma in the rifted tectonic zone, with occurrence of supersolidus hydrothermal alteration and local low- 18O magmatism during supercontinent breakup. Contributions of the depleted mantle to their magma sources are contrasting in the two episodes of magmatism in association with breakup of the supercontinent Rodinia. While the change in melt source from the crust to the mantle keeps pace with the advance from continental rifting to supercontinent breakup, significant transport of both heat and material from the depleted mantle to the continental crust only occurred along the active rifting zone. In either case, the growth and reworking of continental crust are episodically associated with rift magmatism.

Coupling chemical weathering with soil production across soil‐mantled landscapes

AbstractSoil‐covered upland landscapes constitute a critical part of the habitable world. Our understanding of how they evolve as a function of different climatic, tectonic and geological regimes is important across a wide range of disciplines and depends, in part, on understanding the links between chemical and physical weathering processes. Extensive previous work has shown that soil production rates decrease with increasing soil column thickness, but chemical weathering rates were not measured. Here we examine a granitic, soil‐mantled hillslope at Point Reyes, California, where soil production rates were determined using in situ produced cosmogenic nuclides (10Be and 26Al), and we quantify the extent as well as the rates of chemical weathering of the saprolite from beneath soil from across the landscape. We collected saprolite samples from the base of soil pits and analysed them for abrasion pH as well as for major and trace elements by X‐ray fluorescence spectroscopy, and for clay mineralogy by X‐ray diffraction spectroscopy. Our results show for the first time that chemical weathering rates decrease with increasing soil thickness and account for 13 to 51 per cent of total denudation. We also show that spatial variation in chemical weathering appears to be topographically controlled: weathering rate decreases with slope across the divergent ridge and increases with upslope contributing area in the convergent swale. Furthermore, to determine the best measure for the extent of saprolite weathering, we compared four different chemical weathering indices – the Vogt ratio, the chemical index of alteration (CIA), Parker's index, and the silicon–aluminium ratio – with saprolite pH. Measurements of the CIA were the most closely correlated with saprolite pH, showing that weathering intensity decreases linearly with an increase in saprolite pH from 4·7 to almost 7. Data presented here are among the first to couple directly rates of soil production and chemical weathering with how topography is likely to control weathering at a hillslope scale. Copyright © 2006 John Wiley & Sons, Ltd.

Dating of chemical weathering processes by in situ measurement of U-series disequilibria in supergene Fe-oxy/hydroxides using LA-MC-ICPMS

Constraints on the timing of weathering processes in the northern Australian regolith have been obtained by in situ measurement of U-series disequilibria in U-rich supergene Fe-oxy/hydroxides using a laser ablation-MC-ICPMS technique. This approach has permitted the measurement of 234U/ 238U and 230Th/ 238U activity ratios in finely crystalline Fe-oxy/hydroxides from the Ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes. 230Th/ 238U systematics in the finely crystalline Fe-oxy/hydroxides yield 230Th-ages ranging from 60 to 350 ka. The most reliable 230Th-ages cluster between 122 and 216 ka, suggesting Fe-oxy/hydroxides formation and associated weathering, peaked during the previous 2 interglacial periods. This is supported by the 230Th-ages and isotope composition of pisolith (Fe-oxy/hydroxide pedogenic nodules) cores which are demonstrated to behave as closed systems. The U isotopic composition of the Fe-oxy/hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite. Secondary overprinting is evident in some samples as a large range in 234U/ 238U. Our results suggest that weathering intensity varies with global climate cycles and that, together with weathering events dated by 40Ar/ 39Ar of Mn-oxides elsewhere in northern Australia ([Feng, Y.X. and Vasconcelos, P., 2001. Quaternary continental weathering qeochronology by laser-heating 40Ar/ 39Ar analysis of supergene cryptomelane. Geology, 29(7): 635–638.]), weathering rates in this region are orbitally forced.

Analysis of late Palaeozoic glacial to postglacial sedimentary successions in South Africa by geochemical proxies – Response to climate evolution and sedimentary environment

Records of incisive climate changes during the late Palaeozoic Era are best documented in sedimentary basins of South Africa. Glacial (Upper Carboniferous–Early Permian Dwyka Group and equivalents) and postglacial deposits (Early to Middle Permian Ecca and Beaufort Groups) are known from the Main Karoo Basin in South Africa, from the Kalahari Basin (Namibia and Botswana) and other late Palaeozoic Basins of south Gondwana. Glacial deposits comprise diamictites and tillites followed by postglacial shales with variable organic matter contents and intercalated carbonate horizons. This study covers the final glaciation phase (Upper Pennsylvanian–Asselian/Sakmarian) and the transition into the postglacial stage of the Middle to Late Permian. For sedimentary successions from the Karoo and eastern Kalahari Basin element geochemical proxies (Chemical Index of Alteration, Zr/Ti, Rb/K, V/Cr and organic carbon content) supported by mineralogical data record pronounced variation in sedimentary conditions. These took place in the course of glacier retreat at the Dwyka/Ecca boundary where climate and weathering processes are known to have changed. The proxy signals indicate an increase in sea level and salinity, and a shift in redox conditions from oxygenated to temporary anoxic/euxinic conditions. Due to the synchronicity of these events, distant sampling localities in the Karoo Basin can be correlated. Despite facies differences between the Eastern Kalahari and Karoo Basin, the geochemical proxies point to a comparable climatic evolution during the postglacial phase. For the Early Permian, proxy signals indicate warm–humid conditions in the Karoo and Kalahari Basin. During the Middle Permian increased aridity hampered chemical weathering processes as indicated by decreasing CIA values. Results show that: (i) proxies reflect provenance, synsedimentary redox regime and late diagenetic alteration, (ii) by combination of different independent geochemical proxies primary conditions of sedimentation and climate can be deduced, (iii) the geochemical proxies applied are independent of local environments and (iv) the data reveal supra-regional or even hemispherical changes in climate and sedimentary conditions. Thus geochemical proxies can be used as reliable markers for basin wide correlations. Knowledge about synchronicity on basin scale contributes to an improved understanding of SW Gondwana's interaction with and influence on global climatic evolution.

Modeling hydrology and silicon‐carbon interactions in taiga and tundra biomes from a landscape perspective: Implications for global warming feedbacks

We used a simple hydrological‐biogeochemical mixing model to test the hypothesis that export of total organic carbon occurring mainly during spring in taiga and tundra watersheds might be compensated by production and export of bicarbonate (HCO3−) from groundwater during the rest of the year. The investigated watersheds are located in northern Sweden close to the Arctic Circle. An elevated spring flow peak due to snowmelt characterizes the hydrology of boreal and arctic river systems. During this snowmelt, total organic carbon (TOC), previously stored as soil carbon, is flushed and exported from the watersheds, and can be released to the atmosphere via heterotrophic remineralization during riverine transport to the sea, thereby contributing to atmospheric CO2. The TOC yields of the watersheds investigated increased with vegetation and peat cover and ranged across watersheds from 0.5 to 2.8 tons km−2 yr−1. During frozen periods, streamflow is dominated by “old” groundwater. This water has percolated through the soils and is rich in DSi and bicarbonate; that is, atmospheric carbon that has been “consumed” in chemical weathering processes is partly exported as bicarbonate to the sea, where carbon is stored as CaCO3 for geological time. The bicarbonate export of the watersheds investigated was between 0.4 and 1.2 tons C km−2 yr−1 corresponding to 15–73% of the TOC export. Very likely, global warming will affect water flow through the soils in taiga and tundra ecosystems and thus will have an effect on watershed carbon budgets. This bicarbonate export may compensate for significant amounts of the exported TOC, thereby reducing the positive feedback to atmospheric CO2.