Chemical Weathering Rates Research Articles

Seasonal and interannual changes of river chemistry in the source region of Yellow River, Tibetan Plateau

The seasonal and annual processes of river chemistry as well as chemical weathering and controlling factors were examined in the source region of Yellow River (SRYE) during 2013–2015. River discharge exhibited strong seasonality and was dominated by precipitation and/or groundwater. Sediment concentrations were positively related to discharge, and the dominant sediment-producing area was this area from JM to JG owing to large precipitation and strong erosion. The dominant cations and anions were Na+, Ca2+ and HCO3−, and concentrations of most ions displayed obvious seasonality with a negative relationship with discharge, suggesting a hydrological control on river chemistry. Comparison of ion concentrations suggest that the regional climate and land surface affect the soil/rock erosion and solute release. River chemistry was dominated by the weathering of carbonate and silicate. HCO3−, Ca2+ and Mg2+ were primarily derived from calcite and dolomite weathering, and Na+ and K+ mainly came from silicate weathering. CWR and CatWR showed strong seasonality and were closely related to discharge, air temperature, precipitation and PER, suggesting that chemical weathering rates are dominated by lithology, discharge and precipitation. The sediment and solute yields ranged from 25,110 to 97,270 t km−2a−1 and 22,610 to 27,800 t km−2a−1 respectively in the SRYE (at TNH), where CWR and CatWR were 5.42–7.63 t km−2a−1 and 1.20–2.62 t km−2a−1 respectively. Higher CatWR in the SRYE than those in other basins with higher specific discharge implies that silicate weathering is more sensitive to permafrost degradation.

Quantifying the role of chemical weathering rates on soil developed along an altitudinal transect in the mountainous environments, Turkey

Climate and elevations play an important role in controlling rate of weathering and soil formation. The role of chemical weathering rate on soil developed along an altitudinal transect in the mountainous environments in Turkey was investigated to determine the effects of climate on the geochemical characteristics of the soil. The main purposes of this study were: i) To characterize the geochemical characteristics of soils as a function of climate ii) To evaluate the soil formation and decomposition rates in Climosequence depending on the elevation by using geochemical data. For this purpose, four representative profiles were dug at different elevations. The transect of four soils formed in limestone elevations from 1139 to 1809 m. Our results showed that the rate of chemical weathering of CIA, CIW, PIA and MIA indicators decreased with the increase in elevation. In contrast, WIP value increased at higher altitudes and exhibited different weathering directions by deviating from the main trend in the A–CN–K diagram that composition of weathered soils was easily influenced by the quantity of precipitation, degree of gradient and height differences. Therefore, it was concluded that the main factors determining soil development was climate and elevations, and both determine the leaching regime and weathering rates.

Temporal variation of chemical weathering rate, source shifting and relationship with physical erosion in small mountainous rivers, Taiwan

Temporal variation of the chemical weathering rate (CWR) versus physical erosion rate (PER) and the shift of the sources of dissolved solids during rainstorms are explored in two small mountainous rivers, Northern Taiwan. The ionic hydrochemistry of streamwater, rainwater, soil water, and groundwater were collected to quantitatively demonstrate the sources of the dissolved solids and the coupling effect between CWR and PER during typhoon and non-typhoon periods. Results indicate that silicate cation weathering (CWRSC) varies between 33 and 37 t km−2 yr−1, which is ~7 times the global mean, and the annual PER is ~81–166 t km−2 yr−1, similar to the global mean of ~137 t km−2 yr−1. The rainstorm periods only account for 1% of a year but are responsible for 10% of the CWRSC and over 80% of the sediment flux, highlighting the importance of storm events. Silicate cation concentrations present a simple dilution process during rainstorms indicates the high connectivity between hillslopes and streams during rainstorms is established and forces the soil water into streams rapidly. Besides, the coupling between CWRSC and PER becomes less relevant during rainstorms. The coupling between CWRSC and PER is non-linear with flow regime dynamics, and the estimates of CWRSC derived from non-typhoon day are as largely biased as one order of magnitude. High-resolution water chemistry data on the variations in ion production and physical erosion is more valuable than previously thought for characterizing the CWR and PER of active mountain belts.

Integrating Complex Soil Dynamics Using the Non-equilibrium Effective Temperature

Soil dynamics, such as aggregate turnover, play central roles in modulating global cycles of carbon, nitrogen and water. However, understanding soil dynamics, and the role they play in soil system functioning is complicated by the fact that soils naturally exhibit scale-dependent physical and chemical variability across more than a dozen orders of magnitude in both space and time. Herein it is argued that scale-dependent soil variability emerges at large scales in part because soils systems, in the larger sense, are non-equilibrium thermodynamic systems. One driver of soil disequilibrium is pedoturbation, or soil stirring. Soil stirring is widely implicated in affecting long-term processes such as aggregation, horizonation, and rates of chemical weathering. This observation aligns well with advancements recently made in theoretical physics. For a variety of non-equilibrium physical systems , the stirring rate has been shown to be equivalent to an \emph{effective temperature} of the system, and can be used to recover thermodynamic relationships in non-equilibrium settings. This work primarily presents the mathematical basis for calculating the effective temperature, a measurement approach, and discusses the implications of this framework on several outstanding problems within soil science. While effective temperatures have yet to be measured in soils, this framework has the potential to greatly simplify and compliment the modeling of complex soil dynamics, but also potentially provide a new tool to rectify the discordance between small and large scale rates of soil processes.

Cycling phosphorus on the Archean Earth: Part I. Continental weathering and riverine transport of phosphorus

Phosphorus (P) is the key nutrient thought to limit primary productivity on geological timescales. Phosphate levels in Archean marine sediments are low, but quantification of the P cycle and how it changed through a billion years of recorded Archean history remain a challenge, hindering our understanding of the role played by P in biosphere/geosphere co-evolution on the early Earth.Here, we design kinetic and thermodynamic models to quantitatively assess one key component of the early P cycle – continental weathering – by considering the emergence and elevation of continents, as well as the evolution of climate, the atmosphere, and the absence of macroscopic vegetation during the Archean Eon. Our results suggest that the weathering rate of apatite, the major P-hosting mineral in the rocks, was at least five times higher in the Archean Eon than today, attributable to high levels of pCO2,g. Despite this, the weathering flux of P to the oceans was negligible in the early Archean Eon, increasing to a level comparable to or greater than the modern by the end of eon, a consequence of accelerating continental emergence. Furthermore, our thermodynamic calculations indicate high solubilities of primary and secondary P-hosting minerals in the acidic weathering fluids on land, linking to high Archean pCO2,g. Thus, weathering of P was both kinetically and thermodynamically favorable on the Archean Earth, and river water could transport high levels of dissolved P to the oceans, as also supported by the observed P-depletion in our new compilation of Archean paleosols. Lastly, we evaluated the relative rates of physical erosion and chemical weathering of silicates during the Archean Eon. The results suggest that continental weathering on the early and middle Archean Earth might have been transport-limited due to low erosion rates associated with limited subaerial emergence and low plateau elevations; by the late Archean, however, continental weathering would have transited to kinetically-limited state because of continental emergence, increased plateau elevation, and weakening weathering rates. Overall, our weathering calculations together with paleosol evidence indicate an increasing flux of bioavailable P to the oceans through time, associated with late Archean continental emergence, reaching levels comparable to or higher than modern values by the end of the eon. Increased P fluxes could have fueled increasing rates of primary production, including oxygenic photosynthesis, through time, facilitating the irreversible oxidation of the Earth’s atmosphere early in the Proterozoic Eon.

Field biotite weathering rate determination using U-series disequilibria

The chemical weathering rate of silicates is a key parameter of several geochemical processes; however, the long-term field measurement of these rates remains challenging. Currently, there are several methods for quantifying field weathering rates, but each presents it's own unique set of difficulties. In this study uranium-series nuclides were used to investigate long-term biotite weathering in a soil profile. For this purpose, pseudomorph grains of biotites (including weathering products) were hand-picked from four horizons of a soil profile and from underlying granitic saprolite from the experimental Breuil-Chenue site (France).Unexpected behaviors of U and Th nuclides were recorded from these samples. An unambiguous leaching of 232Th occurs during the weathering of biotite, while reduced U release relative to Th was observed, implying an efficient redistribution of U between the primary mineral and weathering products within the pseudomorphs. The measured U-series activity ratios reflect a regular and self-consistent pattern evolution. However, the “U-series-derived-weathered-stage” is not always coherent with the weathering features of major elements and/or with the location of the samples within the soil profile. For instance, biotite from the saprolite displays U-series activity ratios typical of significantly weathered samples, while major elements present rather limited signs of weathering. These results suggest an incongruent leaching of U and Th isotopes and suggest that U- and Th-series nuclides probe some water-mineral interactions that occur before macroscopic mineral tansformation.Using an open-system leaching model, coupled (234U/238U), (234U/230Th), and (226Ra/230Th) disequilibria measured from the samples allow us to calculate a weathering duration range of 6–52 ky for most weathered samples of this soil profile. The biotite weathering rate deduced from these data ranges from 7.8 × 10−17 mol·m−2·s−1 to 6.5 × 10−16 mol·m−2·s−1, which is consistent with the range of field rates previously reported. This rate was compared to the weathering rate of biotites induced by the substitution of vegetation occuring 35 years ago in a nearby soil profile, which was about 2–3 orders of magnitude higher than the long-term rate reported above. This feature is perfectly consistent with the decrease in the weathering rate with time that was previously widely documented but that still remains debated. Our results allude to a valuable use of U-series isotopes for mineral-weathering field rate determination and report a decrease in silicate weathering rates over long- (10,000s of years) and short-term (10s of years) weathering events recorded from the same site. The results show that short term laboratory experiments can be representative of field processes, and suggest that some forms of mineral weathering reactivation, similar to that observed during laboratory experiments, may naturally occur in soils in response to high frequency perturbation processes such as land cover change.

High-frequency monitoring reveals how hydrochemistry and dissolved carbon respond to rainstorms at a karstic critical zone, Southwestern China

Hydrochemical behavior and dissolved carbon dynamics are highly-sensitive to hydrological variations in the monsoon-influenced karstic critical zone which has high chemical weathering rates and experiences strong anthropogenic impact. Continuous high-frequency monitoring in the spring outlet of a karstic catchment in Southwestern China revealed that most hydrochemical variables changed distinctively in response to hydrologic variations, influenced by mixing of different sources and miscellaneous biogeochemical processes. Na+, K+ and SO42− varied significantly with hydrology, showing weak chemostatic behavior controlled by dilution. The flushing effect and random behavior of NO3− and Cl− likely reflect agricultural inputs from high throughflow. Soil CO2 in infiltrated water supports carbonate weathering, enabling DIC (dissolved inorganic carbon) and weathering products (e.g., Ca2+ and Mg2+) to maintain chemostatic behavior. Biogenic DIC exhibited a stronger chemostatic response than carbonate sources and was the foremost control in DIC behavior. Carbon exchange between DIC and DOC (dissolved organic carbon) did not significantly influence DIC concentration and δ13C due to very low DOC concentration. More DOC was exported by flushing from increasing discharge. Hysteretic analysis indicated that the transport processes were controlled by proximal sources mixing and diverse mobilization in various periods responding to rainstorms. NO3− and Cl− presented different hysteresis behavior as sourced from agricultural activities. DOC increased on the hydrograph rising limb and was controlled by a transport-limited regime. However, the hysteresis behavior of most weathering products and DIC were regulated by a process-limited regime in the karstic critical zone. Overall, biogeochemical processes, hydrogeological properties, storm intensity/magnitude and the timing of storms (antecedent conditions) are main factors influencing the response of hydrochemical variables and dissolved carbon to storm events.

Chemical weathering and atmospheric CO2 consumption in the semi-arid Swarnamukhi basin (Peninsular India) estimated from river water geochemistry

A small ephemeral Swarnamukhi River (hard rock) basin, more sensitive to climate change, was sampled and analyzed for various parameters. A total of 66 river water samples (22 of each season) were collected from different sampling stations covering the whole Swarnamukhi River basin in pre- and post- and monsoon seasons. Chemical weathering rate data of the Swarnamukhi River basin is integrated with the available data of other large perennial (Godavari and Krishna) and ephemeral (Kaveri and Periyar) hard rock river basins of Peninsular India. The Peninsular river basins, including Swarnamukhi River basin, are mainly dominated by granitic gneissic terrain. Contribution of inputs through forward modelling shows: silicate > evaporite > carbonate > atmosphere. The mean silicate and carbonate weathering rates in the river basin observed are 27.15 tonnes.km−2.yr−1 and 9.45 tonnes.km−2.yr−1 in pre-monsoon season, whereas 32.73 tonnes.km−2.yr−1and 45.90 tonnes.km−2.yr−1 in post-monsoon season, respectively. The Swarnamukhi river has lower silicate weathering rate (annual average: 30.57 tonnes.km−2.yr−1) than other published river basins data (Narmada: 33.9 tonnes.km−2.yr−1; Nethravati: 42 tonnes.km−2.yr−1; Godavari: 45.6 tonnes.km−2.yr−1) due to different localized geological setting. Mean annual CO2 consumption for the Swarnamukhi River is 6.9 × 105 mol km−2.yr−1 which is comparable to other Peninsular rivers such as the Godavari (6 × 105 mol km−2.yr−1) and Gad (5.7 × 105 mol km−2.yr−1) rivers. The study further provides the inventory for CO2 consumption on river basin scale, which is an important consideration from the global warming point of view.

Hydrochemistry applied to assess the chemical weathering and soil removal rates in the Sorocaba River basin, São Paulo State

ABSTRACT: Chemical weathering and soil removal rates are responsible for the Earth’s landscape, composition of surface and groundwater, producing the soils and buffering the composition of the atmosphere. This study aimed to assess the chemical weathering and soil removal rates in the Sorocaba River basin, Sao Paulo State, Brazil, allowing answering the questions about the dynamics of fluvial transport of dissolved and suspended solids, the chemical weathering processes and associated atmospheric/soil CO2 consumption, and the relationship between chemical weathering and soil erosion rates. The annual specific flux of total suspended solids and total dissolved solids were 49.59 and 60.97 t/km2/yr. The chemical weathering process dominant in the Sorocaba River basin was the monosiallitization (RE = 2.4), with an associated atmospheric/soil CO2 consumption of 2.3 × 105 mol/km2/yr. The chemical weathering and soil removal rates were 7.2 and 29.8 m/Myr, respectively, indicating a soil thickness reduction. Finally, the soil removal rate in the Sorocaba River basin is almost 3-fold higher than the Cenozoic soil removal rates, being this difference related to the current land use which increased the soil removal processes.

The influence of cascade reservoir construction on water chemistry distribution in Karst area

为深入了解河流梯级筑坝对喀斯特地区河流水化学分布的影响，于2017年1、4、7和10月别对乌江干流洪家渡水库（多年调节）、乌江渡水库（季调节）和索风营水库（日调节）3个具有不同滞留时间的水库进行水样采集，分析入库水、坝前剖面水和下泄水的水化学特征，探讨河流梯级筑坝对水化学分布及风化速率估算的影响.研究结果表明：3个水库深层水比表层水HCO₃^-浓度分别高12.9%、5.5%和8.0%，Ca²⁺浓度分别高15.9%、2.4%和8.5%.河流梯级筑坝一定程度上改变了水体水化学组成，从而影响碳酸盐岩风化速率估算.整体上，洪家渡水库、索风营水库和乌江渡水库的全年风化速率变化范围分别为：-1.7%~15.4%、-5.6%~1.1%和-0.3%~3.4%.河流筑坝作用对风化速率估算及主量离子浓度的影响：HCO₃^-与Ca²⁺浓度分布均为：洪家水库 > 乌江渡水库 > 索风营水库，这与水体滞留时间长短规律一致，表明水体滞留时间影响着水化学的组成分布.同时水体离子浓度表现出明显的季节性差异，丰水期各水库变化率明显大于枯水期.上述结论表明喀斯特地区河流连续筑坝后水化学组成及分布特征发生了一定程度改变，影响流域化学风化速率的估算，且影响程度随水体滞留时间延长而增大，并受气温影响.因此，今后在估算流域风化速率及探究水化学空间变化时应对筑坝作用加以考虑，以便更加准确地评估喀斯特流域岩石风化在全球碳循环中的作用.;In order to better understand the effect of cascade damming on river chemistry distribution in the karst region, we collected water samples from Hongjiadu Reservoir (multi-yearly regulation), Wujiangdu Reservoir (seasonal regulation) and Suofengying Reservoir (daily regulation) in January, April, July and October of 2017. We analyzed the water chemistry characteristics of the inflow, outflow and stratified water samples of these three reservoirs and investigated the cascade damming effect on water chemistry. The results show that water chemistry distribution has been highly affected by the damming effect. The HCO₃^- concentration of the hypolimnion is 12.9%, 5.5% and 8.0%, and the Ca²⁺ concentration is 15.9%, 2.4% and 8.5% higher than that of the surface water, respectively, for the Hongjiadu Reservoir, Suofengying Reservoir and Wujiangdu Reservoir. Consequently, the chemical weathering rate estimation of carbonate rock has also been affected. The annual weathering rate variation of Hongjiadu Reservoir, Suofengying Reservoir and Wujiangdu Reservoir is -1.7%-15.4%, -5.6%-1.1% and -0.3%-3.4%. The calculated weathering rate and HCO₃^- and Ca²⁺ concentrations in decreasing order is Hongjiadu Reservoir > Wujiangdu Reservoir > Suofengying Reservoir, which is consistent with the water retention time, indicating that water retention time affects the redistribution of water chemistry. At the same time, the ions concentration showed obvious seasonal variations, and the change rates in the three reservoirs were significantly greater during the flood season than that of the dry season. The above conclusions indicate that the water chemistry of rivers in karst area is highly affected by damming, which affects the estimation of chemical weathering rate. Water residence time and temperature are the main controlling factors on water chemistry redistribution. Therefore, we proposed that the damming effects should be taken into account to calculate the weathering rate accurately.

Controls on granitic weathering fronts in contrasting climates

Granitic weathering profiles display highly diverse morphologies, reflecting the complex relationships between climate and weathering rates. Some profiles exhibit abrupt transitions from fresh bedrock to highly weathered material over short (<1 m) distances, while others exhibit only limited weathering extending 10s of meters into the bedrock. Although granitic weathering processes have been well studied, the controls on profile morphology and weathering rates within granitic, and many other lithologies remain poorly understood; these are likely influenced by a range of both intrinsic and extrinsic factors, which in turn will have crucial implications for understanding, for example, climate-weathering feedbacks. In this study we present multi-scale elemental and mineralogical analyses of a >30 m granitic weathering profile from the cool, temperate, Lysina catchment in the NW Czech Republic, from which we calculated mass transfer, weathering indices, and mineral specific weathering rates. The Lysina profile exhibits limited weathering extending >30 m into fractured bedrock, dominated by albite weathering at a rate of 9.3 × 10−17 mol m2 s−1.To identify environmental and geological controls on weathering front morphology and chemical weathering rates, Lysina was compared to previously published granitic weathering profiles from around the world. Weathering front morphology and weathering rates were calculated for the additional sites from published data and were correlated to mean annual precipitation (MAP), mean annual temperature (MAT), and erosion rates, with MAP having the strongest relationship. Higher MAP likely promotes lower saturation indices in pore waters, allowing weathering reactions to occur further from equilibrium. Comparison of erosion rates amongst the granitic catchments revealed an inconsistent effect on chemical weathering rates, but high erosion rates may promote weathering by reducing the thickness of the regolith and exposing the bedrock to reactive fluids. Mean annual temperatures appear to only have significant impacts on weathering fronts in environments with high precipitation and high erosion rates. Fractured bedrock profiles (Lysina and Río Icacos) have higher weathering intensities, than the other sites studied here. High connected porosity in fractured rocks enhances water movement allowing more efficient removal of weathering products, thus reducing thermodynamic saturation, increasing weathering rates, and producing sharper weathering gradients. These findings indicate that CO2 drawdown on geological timescales is also likely to be governed by precipitation rates, as well as temperature, and that much of the climate-significant weathering may occur within very narrow zones of the Earth's surface.

Weathering Dynamics Under Contrasting Greenland Ice Sheet Catchments

Chemical weathering dynamics in Greenland Ice Sheet (GrIS) catchments are largely unknown, due to a scarcity of field data. This paper presents the most comprehensive study to date of chemical weathering rates from four GrIS catchments of contrasting size. Cationic denudation rates varied greatly between catchments studied (2.6 to 37.6 tons km-2 a-1, world mean = 11.9 tons km -2 a -1), but were of the same order of magnitude to the world non-glacial riverine mean, and are greater than those documented in some major temperate rivers catchments (e.g. Mississippi (1.3 tons km-2 a-1) and Nile (0.4 tons km-2 a-1) rivers). These high chemical denudation rates indicate that the GrIS is a potential source of solute to downstream environments. Dissolved silica yields, indicative of silicate weathering rates, also varied by an order of magnitude, with upper values similar to the world mean (0.2 to 3.8 tons km-2 a-1, world mean = 3.53 tons km-2 a-1). Elevated chemical weathering rates in GrIS catchments are strongly influenced by the specific discharge, which drives flushing of the subglacial environment and physical erosion of the ice sheet bed. The direct relationship between specific discharge and chemical denudation rates supports the hypothesis that Greenland Ice Sheet chemical weathering rates and solute fluxes are likely to increase with enhanced melt rates in a warming climate.

Geochemical and mineralogical records of late Albian oceanic anoxic event 1d (OAE-1d) in the La Grita Member (southwestern Venezuela): Implications for weathering and provenance

In this paper, stratigraphic distributions of major, trace and rare earth element (REE) concentrations as well as the bulk and clay mineralogy of organic-rich calcareous rocks of the La Grita Member (Capacho Formation) in southwestern Venezuela were studied to constrain sedimentary depositional conditions during the late Albian-early Cenomanian oceanic anoxic event 1-d (OAE-1d; 103 to 99.5 Ma). The statistically significant correlations observed between total REEs and Al, total organic carbon (TOC) and phyllosilicates suggest that REEs mainly occur in clay minerals and organic matter. Ce anomalies are mainly controlled by detrital input and cannot be used as redox proxies in this stratigraphic sequence. Detrital proxies, notably phyllosilicate/(K-feldspar + plagioclase), K-feldspar/(K-feldspar + plagioclase), and kaolinite/illite, suggest that greenhouse warming established during the OAE-1d intensified the hydrologic cycle and consequently increased continental chemical weathering rates. The Zr/Sc ratio and Al2O3–TiO2–Zr ternary diagram suggest detrital input dominated by recycling of older sedimentary rocks, while the La–Th-Sc ternary relationship indicates that most sediment samples were deposited between a passive margin and a continental island arc setting. Sedimentary discrimination diagrams (including La–Th-Sc and Co/Th vs. La/Sc), combined with chondrite-normalized REE patterns, suggest that siliciclastic sediments of the La Grita Member were mainly supplied by metasedimentary and intracratonic rocks of the Guayana Craton, notably the Roraima Formation, suggesting that this area was the main source for siliciclastic sediments during the Cretaceous. The Santander Massif and Merida and El Baúl Arches likely acted as secondary provenance areas, confirming previous work using U–Pb detrital zircon geochronology.

Human influences on the present denudation rates of the Paulista Peripheral Depression, Brazil

Chemical weathering and soil erosion in large or small watersheds are used to understand the effect of evolution processes on the Earth's surface and climate. Currently, human-driven land use changes have substantial effects on landscape changes over a range of temporal and spatial scales. In this context, the Sorocaba River basin, São Paulo State, Brazil, is an ideal study area to assess human influence on the present denudation rates of the Paulista Peripheral Depression (PPD). Twelve fluvial water sample collections were carried out at the Sorocaba River mouth, covering one complete hydrological cycle (Jun/2009 to Jun/2010). In the same period, 46 rainfall events were sampled. All samples were analyzed for dissolved cations, anions and silica, and total dissolved solids (TDS). Total suspended solids (TSS) were measured only in fluvial samples. The export of TDS and TSS occurs mostly during the wet season, accounting for ca. 60% and 87% of the total dissolved solids and total suspended sediment transported in the hydrological cycle studied (2009/2010), respectively. The total annual specific flux of TSS was 74.27 t/km2/year (including soil erosion by agricultural activities), with a small portion being derived from the anthropogenic contributions (ca. 2%). The total annual specific flux of TDS (74.43 t/km2/year) was similar to the TSS, but after the correction of the atmospheric inputs and anthropogenic contributions (ca. 18 and 29%, respectively), this value decreased to 39.35 t/km2/year. The chemical weathering rate was 7.1 m/Myear and this process tended towards monosialitization (RE = 2.6), with an atmospheric/soil CO2 consumption rate of 3.3 × 105 mol/km2/year. The difference between the chemical weathering and soil removal rates (7.1 and 49.6 m/Myear, respectively) indicated that the soil thickness reduction occured in the Sorocaba River basin. The climatic control on chemical weathering and soil removal rates was clearly evidenced, with the highest values of denudation occurring in the wet and hot climatic conditions. However, the chemical weathering processes (RE index) was not sensitive to climatic controls. In order to assess human influence on chemical weathering and soil removal rates in the PPD, the results were compared with other works undertaken in this vast geomorphological province. Even considering the uncertainties associated with a number of data points, the chemical weathering and soil removal rates in the PPD observed in this study were approximately 4 and 7.5-fold higher than these natural denudation rates, respectively, evidencing the effect of recent land use changes on the present denudation rates in the PPD. Thus, this study reinforces the complexity of the human-landscape systems in São Paulo State and increases the values of long-term landscape evolution rates.

Geochemical and mineralogical characteristics of calamines from the Olkusz zinc and lead ore district (South Poland)

The mineralogical and geochemical characteristics of calamines developed from chemical weathering within two different tectonic settings (tectonic horst and graben) are described. Calamines collected from the Olkusz-Pomorzany and the Boleslaw zinc and lead mines were analysed geochemically by means of EMP and SEM-EDS and their mineralogical composition was identified by using reflected light microscopy and XRD analysis. The calamines from the Olkusz-Pomorzany mine are composed of marcasite, galena, cerussite, sphalerite, hemimorphite, smithsonite and goethite. Hemimorphite prevails over smithsonite and occurs in the vicinity of primary sulphides. The mineralogical composition of the calamines from the Boleslaw area is less diverse. These are primarily composed of goethite and smithsonite, with a minor content of marcasite and remnant sphalerite. The mineral composition of weathering sites in the Olkusz-Pomorzany and Boleslaw areas may reflect different geochemical conditions (from slightly acidic to alkaline pH values, from lower to higher CO2 partial pressure and solutions of different silica saturation) that could have changed over time. The mineralogical compositions of the calamines differ quantitatively and this could reflect the tectonic setting of the primary ore affecting the rate of chemical weathering.

Middle Permian organic carbon isotope stratigraphy and the origin of the Kamura Event

Large carbon cycle perturbations associated with the Middle Permian (Capitanian) mass extinction have been widely reported, but their causes and timing are still in dispute. Low resolution carbon isotope records prior to this event also limit the construction of a Middle Permian chemostratigraphic framework and global or local stratigraphic correlation, and hence limit our understanding of carbon cycle and environmental changes. To investigate these issues, we analyzed the 13Corg values from the Middle Permian chert-mudstone sequence (Gufeng Formation) in the Lower Yangtze deep-water basin (South China) and compared them with published records to build a chemostratigraphic scheme and discuss the underlying environmental events. The records show increased δ13Corg values from late Kungurian to early Guadalupian, followed by a decrease to the late Wordian/early Capitanian. The early-mid Capitanian was characterized by elevated δ13Corg values suggesting the presence of the “Kamura Event”: an interval of heavy positive values seen in the δ13Ccarb record. We propose that these heavy Capitanian δ13C values may be a response to a marked decline in chemical weathering rates on Pangea and associated reduction in carbonate burial, which we show using a biogeochemical model. The subsequent negative δ13C excursion seen in some carbonate records, especially in shallower-water sections (and in a muted expression in organic carbon) coincide with the Capitanian mass extinction may be caused by the input of isotopically-light carbon sourced from the terrestrial decomposition of organic matter.

Reconciling chemical weathering rates across scales: Application of uranium-series isotope systematics in volcanic weathering clasts from Basse-Terre Island (French Guadeloupe)

To elucidate the factors that contribute to the orders of magnitude difference generally observed in apparent weathering rates across scales, we utilized a U-series isotopic technique to directly determine the duration and rates of chemical weathering recorded in weathering clasts. In this study, we systematically compiled 15 individual volcanic weathering clasts collected from eight andesitic watersheds along a gradient of mean annual precipitation ranging from ∼1600 mm to 6400 mm on the tropical volcanic Basse-Terre Island of French Guadeloupe. We measured U-series isotope compositions to quantify weathering advance rates along the core-rind transects in these weathering clasts. Total rind formation ages, the length of time it took for these weathering clasts to form from andesitic rock fragments or volcanic clasts in the soil zones, range from 60 kyr to 300 kyr. As expected for each individual core-rind weathering transect, the rind ages generally increase almost linearly with distance away from the core-rind boundary. The derived clast weathering rates range from 0.08 ± 0.04 to 0.34 ± 0.01 mm kyr−1, and the rates exhibit a strong positive correlation (R2=0.74) with the annual runoff values of these watersheds. This correlation documents the first direct evidence that weathering over geological timescales on the island is controlled by the amount of precipitation and corroborates the relationship that was previously reported for the riverine weathering fluxes at Basse-Terre Island in Guadeloupe, but measured with much shorter time scales. The clast-scale rates (inferred for the characteristic length scale of mm) are compared to the watershed-scale rates of Basse-Terre Island (length scale of km): constantly higher rates are observed at the watershed scales than the clast scale and the discrepancy equals ∼1800 ± 400 times, despite changes in watershed characteristics such as watershed size, relief, and runoff values. The discrepancy is attributed to the fractal nature of the roughness and surface area across scales. For example, the presence of undulations and fractures (e.g. roughness) at the bedrock-saprolite contact of the watershed, which is generally not assessed in the watershed surface area estimates, is most likely the main contributor to the constant rate difference of ∼1800 times.

Weathering dynamics of Large Igneous Provinces (LIPs): A case study from the Lesotho Highlands

Basaltic terrains contribute a significant component to the global silicate weathering flux despite their limited areal coverage. A strong correlation between chemical weathering flux and climatic factors such as temperature in volcanically inactive basaltic fields implies that weathering of basalt might have played an important role in maintaining the habitability of the Earth and the balance of the geological carbon cycle. However, the low erosion rate of flood basalt provinces on tectonically quiescent cratons, where the clear climate dependence of weathering rate has been observed, implies a ‘supply-limited’ weathering regime so that weathering fluxes are expected to be controlled by physical erosion rather than climate. This work tests the weathering dynamics of flood basalts by investigating the weathering flux of the Karoo LIP on the Lesotho Highlands where an extremely low rate of denudation has been recorded through terrestrial cosmogenic nuclide dating and sediment flux. Stream chemistry of the rivers from the Lesotho Highlands shows typical characteristics of basalt weathering with Ca and Mg as the dominant cations and HCO3− as the dominant anion. Strong spatial and seasonal variability of solute concentration, possibly linked to a dilution effect, is observed. However, the giant Katse Reservoir of the Lesotho Highlands Water Project (LHWP) effectively averages the spatial and seasonal variability of the solute concentration. The average solution concentration and long-term runoff of the Katse Reservoir give an average atmospheric CO2 consumption rate of 0.26 ± 0.02 × 106 mol/km2/yr (mean ± standard deviation). The chemical weathering flux requires complete alteration of primary silicates and, therefore, a ‘supply-limited’ weathering regime is suggested given the low denudation rate. Nevertheless, the chemical weathering rate of the Lesotho Highlands, as well as other low-eroding basaltic plateaus, is still consistent with the global correlation between temperature and the rate of basalt weathering. A production-limited denudation rate, modulated by temperature, is suggested to reconcile the paradox between the observed temperature dependence of chemical weathering rates and ‘supply-limited’ weathering regimes.

Dissolved inorganic carbon export by mountainous tropical rivers of the Western Ghats, India

Large rivers that are known for carrying huge fluxes are few in numbers; nevertheless, the contributions of thousands of small mountainous coastal rivers to global biogeochemical cycles is also measured to be equally important. Small catchment sizes, high channel gradients yet high water discharge are prominent characters of the coastal rivers draining the Western Ghats (WG) of India. Three geo-tectonic divisions, namely, Southern Granulite Terrain (SGT) in southern, Western Darwar Craton (WDC) in central and Deccan Traps (DT) in the north constitute the entire Western Ghats. Based on the seasonal sampling of 70 coastal rivers draining into the Arabian Sea, this study provides a comprehensive discharge-weighted estimate of DIC for the Western Ghats (WG) region. The concentration of bicarbonate measured in-situ using gran plot method. Fluxes and yields were calculated using seasonal discharge and area of respective rivers. The discharge-weighted DIC concentration of WG region is 629 μM, which is slightly higher than the average global DIC concentration for the low latitude (< 30°) rivers. Despite, covering 113×103km2 area, this region yearly exports 175×109 mol DIC to the Arabian Sea. The DIC flux contributed by the WG region is among the highest in peninsular rivers and comparable with several large global rivers. DIC export rates suggest that within the region, the chemical weathering rate of DT is ∼4 times and WDC is ∼2.5 times higher than that of SGT region. The DIC flux of the WG region (covering 0.076% of global land area) is approximately 0.55% of the global riverine DIC flux to the ocean. Thus, in terms of global DIC export to seas, this region is the 28th largest contributor. Due to high annual DIC yield (1547×103 mol km−2) WG region ranks as the 9th most active region of the world. DIC yields of rivers draining granite-dominated (SGT and WDC) and basalt-dominate (DT) regions of WG are remarkably higher than the global rivers draining similar lithology from temperate and arctic regions. The WG rivers display a latitudinal gradient in DIC transport. Type of silicate rocks, rainfall-runoff, temperature range, and sediment yields control the variability in DIC fluxes and export rates of Western Ghats region.

Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate

The rate of chemical weathering has been observed to increase with the rate of physical erosion in published comparisons of many catchments, but the mechanisms that couple these processes are not well understood. We investigated this question by examining the chemical weathering and porosity profiles from catchments developed on marine shale located in Pennsylvania, USA (Susquehanna Shale Hills Critical Zone Observatory, SSHCZO); California, USA (Eel River Critical Zone Observatory, ERCZO); and Taiwan (Fushan Experimental Forest). The protolith compositions, protolith porosities, and the depths of regolith at these sites are roughly similar while the catchments are characterized by large differences in erosion rate (1–3 mm yr−1 in Fushan ≫ 0.2–0.4 mm yr−1 in ERCZO ≫ 0.01–0.025 mm yr−1 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr−1 in Fushan > 1.9 m yr−1 in ERCZO > 1.1 m yr−1 in SSHCZO), so the fastest eroding site experiences nearly twice the mean annual temperature of the other two.Even though erosion rates varied by about 100×, the depth of pyrite and carbonate depletion (defined here as regolith thickness) is roughly the same, consistent with chemical weathering of those minerals keeping up with erosion at the three sites. These minerals were always observed to be the deepest to react, and they reacted until 100% depletion. In two of three of the catchments where borehole observations were available for ridges, these minerals weathered across narrow reaction fronts. On the other hand, for the rock-forming clay mineral chlorite, the depth interval of weathering was wide and the extent of depletion observed at the land surface decreased with increasing erosion/precipitation. Thus, chemical weathering of the clay did not keep pace with erosion rate. But perhaps the biggest difference among the shales is that in the fast-eroding sites, microfractures account for 30–60% of the total porosity while in the slow-eroding shale, dissolution could be directly related to secondary porosity. We argue that the microfractures increase the influx of oxygen at depth and decrease the size of diffusion-limited internal domains of matrix, accelerating weathering of pyrite and carbonate under high erosion-rate conditions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales.