Chemical Weathering Of Silicates Research Articles

Precipitation of short-range order hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) at ambient temperature: Insights into mineral formation pathways, crystal chemistry and solubility-stability relationships

Chemical weathering of silicates on continents and the subsequent formation of clay minerals are important processes within the Earth's critical zone, controlling pH, water-holding capacity and ion exchange properties of soils. Short-range ordered (SRO) hydroxy aluminosilicate (HAS) and hydrous ferric silicate (HFS) phases, such as allophane (∼Al2O3(SiO2)1.3–2·2.5-3H2O) and hisingerite (∼Fe23+Si2O5(OH)4∙2H2O), are such common soil clays, but their crystal-chemical properties, solubilities and formation paths remain disputed. In this study, pure HAS and HFS phases were precipitated at molar [Al]aq/[Si]aq and [Fe]aq/[Si]aq ratios of 1.0, 1.3, 1.5 and 2.0 and ambient temperature using equilibrium-approaching experiments. The formation of HAS-HFS minerals was studied at [(Al + Fe)]aq/[Si]aq = 1 using replacement levels of [Fe]aq for [Al]aq of 10%, 25%, 50%, 75% and 90%. HAS, HFS and HAS-HFS minerals were formed at pH ∼3–6 through condensation of silica tetrahedrons onto Al/Fe-O-OH octahedral templates. The [Al]s/[Si]s, [Fe]s/[Si]s and [(Al + Fe)]s/[Si]s ratios of the precipitated SRO phases ranged from 0.7 for HAS and 0.7–1.0 for HAS-HFS to 1.0–1.3 for HFS minerals, and correlate linearly with the values of the solubility constants (pK) obtained herein and from literature as follows:pKHAS=2.9·Als/Sis+7.9r2=0.96n=6pKHAS−HFS=−23.2·Al+Fes/Sis+24.8r2=0.94n=5pKHFS=23.5·Fes/Sis–26.3r2=0.86n=4The faster formation kinetics and lower solubility of HFS phases (pK = −2.2 to 4.7) and HAS-HFS phases (pK = −1.0 to 6.0) compared to HAS phases (pK = 10.2 ± 0.3) suggests that hisingerite-like and Fe-substituted allophane-like minerals are probably more abundant in the Earth's critical zone than previously thought, thus providing highly reactive substrates for the formation of thermodynamically more stable kaolinite and smectite group minerals.

The Chemical Weathering of Rocks and Its Carbon Sink Effect in the Naqu River Basin of the Nujiang River Source Area, Southwest China

Carbon plays an important role in global climate change. The mechanisms of carbon sources and carbon sinks have also received wide attention from society, and the physical and chemical characteristics of riverine ions can reflect the chemical weathering of rocks and carbon sink capacity of river basins. Based on the data on river, rainwater, and rock samples from 2019, this study used various methods, such as ion ratio diagrams and ternary diagrams, to analyze the chemical characteristics of water; the chemical weathering and carbon sink effects of rocks were also calculated while assuming three scenarios based on the main sources of ions in the Naqu River. The results showed that for the whole catchment, the main ion sources in the river were: carbonate rock chemical weathering > silicate rock chemical weathering > evaporite dissolution > atmospheric precipitation input. According to the calculations, in the three scenarios, the carbonate weathering rates were 16.84, 11.32, and 14.08 t/km2/yr, and the carbon sink capacities were 66.47, 121.13, and 93.80 mol/km2/yr, respectively; the evaporite weathering rates were 2.20, 9.63, and 5.92 t/km2/yr, respectively. The silicate chemical weathering rate and carbon sink capacity did not change significantly in either scenario, with 6.82 t/km2/yr and 248.6 mol/km2/yr, respectively. This study quantified the ion sources in the Naqu River basin and accurately analyzed their chemical genesis, which helps in understanding the role of the rivers of the Qinghai–Tibet Plateau in the global carbon cycle and global climate change, in addition to providing a reference for the scientific development of the Nujing River.

Late Cretaceous erosion and chemical weathering record in the offshore Cape Basin: Source-to-sink system from Hf[sbnd]Nd isotopes and clay mineralogy

Source-to-sink research has often used sediment fluxes as a fundamental parameter when seeking to understand the perturbations caused by tectonics or climate. This parameter is often interpreted only in terms of erosion rates, dismissing the component exerted by chemical weathering. In this study, we characterize sediments from the Deep Surface Drilling Program core 361 (DSDP 361) located in the Cape basin to show the evolution of both, physical erosion and silicate chemical weathering, during the late Cretaceous (100.5–66 Ma) in the source-to-sink system consisting of the South African plateau and the Cape Basin. Transmission Electron Microscopy (TEM) images and trace element analyses indicate that detrital clays predominate within the site and thus reflect variations in surface continental processes. Clay mineralogy shows a predominance of detrital smectite (60–98%) throughout the section. However, an increase in primary clay minerals, in particular illite (20%), indicates a relative enhancement in physical erosion, while an increase in palygorskite (10%) indicates regional climate aridification during the Campanian–Danian interval (ca. 77–62 Ma). Concomitant to this enhancement, we observe both, more radiogenic Hf isotopic compositions and less radiogenic Nd isotopic compositions in the clay fractions (< 2 μm). These variations can be respectively associated to an increase in silicate chemical weathering and the incorporation of unradiogenic material, likely related to the orogenic belts present on the western-south margin of Southern Africa. The regional aridification and the global climate cooling trend observed on δ18O benthic foraminifera records do not appear as suitable mechanisms for the increase in physical erosion and silicate chemical weathering observed. Our results, together with the reported increase in sedimentation rates and the reported tectonic uplift between 80 and 70 Ma, suggest that it is tectonic activity and not climate the main driver in the enhancement of denudation processes. Furthermore, our data suggests that such uplift episode could have impacted the regional climate by causing a rain-shadow effect, with an inland aridification suggested by the increase in palygorskite proportions. The increase in silicate chemical weathering suggested by our new dataset begins in the Campanian (ca. 77 Ma), later than the onset of the global climatic cooling trend that occurs from the Turonian onward. If enhanced weathering of the South African Plateau may not have triggered the recorded climatic cooling, it may still have contributed to maintain cooler conditions in the Campanian-Danian interval.

Paleoenvironmental changes in the coastal zone of the northwest South China Sea during the last 13 kyr

Marine sediments in coastal zones serve as valuable archives for understanding the history of silicate chemical weathering and summer monsoon rainfall in source areas, providing insights into terrigenous climate and environmental evolution. In this study, we investigated the grain size, clay minerals, and geochemistry of sediments retrieved from core KZK01 in the coastal zone of the northwest South China Sea during the past 13 thousand years before present (kyr BP). Our findings demonstrated that the illite crystallinity index served as a reliable proxy for assessing the intensity of chemical weathering in the source area. Moreover, it distinctly recorded significant climatic events such as the Younger Dryas and Bond events during the Holocene. The dominant driver of the regional East Asian summer monsoon was identified as summer solar radiation in the Northern Hemisphere at low latitudes. Cold climate events exhibited global consistency, potentially influenced by the presence of ice sheets at high latitudes. Lastly, our records revealed a distinct transition at 9.0 kyr, highlighting significant impacts of the Qiongzhou Strait and sea level rise on regional climate dynamics.

A relatively warm and humid Oligocene climate in the northeastern Tibetan Plateau based on a high-resolution clay mineralogical and geochemical record

The Oligocene was the earliest cool period in the Cenozoic, and thus its terrestrial climatic evolution is of significant interest but remains poorly constrained for Inner Asia. In this study, we obtained a high-resolution paleoclimate record based on clay minerals and silicate chemical weathering proxies of the clay fraction (<2 μm) in sedimentary rocks from the Xining Basin on the northeastern Tibetan Plateau. We compared these data with those for surrounding basins to reconstruct the Oligocene climatic evolution of the northeastern part of the plateau. Illite, chlorite, smectite (I/S; including irregular mixed-layer illite–smectite), and kaolinite are the four main clay minerals in the Oligocene strata. Multiple proxies indicated three characteristic climatic periods. During the late Eocene to early Oligocene, a weak silicate weathering intensity is indicative of an arid climate in the northeastern Tibetan Plateau. A stronger silicate weathering intensity and enrichment in smectite (I/S) and kaolinite indicate more warm and humid climate conditions during 32–25 Ma. During the Oligocene, before the high topography of the northern and northeastern parts of the Tibetan Plateau had developed, the relatively warm and humid climate conditions since 32 Ma were likely caused by enhanced transport of water vapor in response to relatively warm temperatures. The decrease in the degree of silicate weathering from 25 to 23 Ma was possibly caused by regional tectonism and global climatic changes. These relatively warm and humid conditions would explain the occurrence of large Oligocene mammals in the northeastern Tibetan Plateau.

Limited change in silicate chemical weathering intensity during the Permian–Triassic transition indicates ineffective climate regulation by weathering feedbacks

Vast emissions of volcanogenic greenhouse gases resulted in rapid global warming during the latest Permian mass extinction (LPME, ∼252 Ma), the impact of which on continental chemical weathering conditions was complex. Here, we compile Chemical Index of Alteration (CIA) records from 25 terrestrial and 12 marine sections with a near-global paleogeographic distribution to reconstruct the evolution of chemical weathering intensity of silicates during the LPME. This compilation shows that the CIA profiles, except those from the Australian Sydney and Bowen basins of the high-latitude Southern Hemisphere, exhibit either indiscernible changes or decreases through the LPME interval, contrary to previous inferences of increased silicate chemical weathering intensity. The lack of major changes in silicate chemical weathering intensity is further supported by the near uniformity of clay mineral assemblages through the LPME interval, with only minor increases of illite and chlorite and, if present, decreases in kaolinite content in some terrestrial sections. We infer a shift from dominantly transport-limited to kinetic-limited weathering regimes, likely caused by environmental climate factors such as widespread devegetation, river-flow changes (i.e., toward flashier and more intense discharge), and aridification during the earliest Triassic that effectively reduced water availability on land for water/rock reactions. Combined with limited changes in chemical weathering fluxes from previous estimates, our data imply possibly ineffective climate regulation by silicate chemical weathering at a (near-)global scale, despite high erosion rates, amid the intense global warming that followed the LPME.

Potential accumulation of toxic trace elements in soils during enhanced rock weathering

AbstractTerrestrial enhanced rock weathering (ERW) is a carbon dioxide removal technology that aims at accelerating one of the most powerful negative feedbacks on Earth's climate, the chemical weathering of silicates. To achieve this, ERW proposes to spread ground silicate rock on agricultural soils. According to many models, global application rates of 40 tonnes of ground basaltic rock per hectare and per year would be necessary to sequester a significant amount of CO2, representing up to 24% of the current net annual increase in atmospheric CO2. When assessing the viability of ERW as a global geo‐engineering strategy, a pivotal but overlooked question to address is whether ERW may lead to toxic trace element accumulation in soils at unauthorized and potentially harmful levels. This study evaluates the legal sustainability of ERW with regard to trace element contents in soils. We compare different trace element accumulation scenarios considering a range of rock sources, application rates and national regulatory limits. The results indicate that, at the suggested annual application rate of 40 tonnes per hectare, the first regulatory limits would be exceeded after 6 and 10 years for copper and nickel, respectively. This study argues in favour of close tailoring of ERW deployment to local conditions in order to tap into its climate mitigation potential while preserving long‐term soil uses.Highlights Terrestrial enhanced rock weathering (ERW) is a carbon sequestration technology that consists in applying rock powder to soils. This study evaluates whether rock powder application can lead to trace metals accumulation in soils beyond regulatory limits. ERW can lead to the overrun of some environmental regulatory limits within less than a decade. Specific site and rock selection will be necessary for this method to preserve long‐term soil uses.

Hydrogeochemical factors controlling the occurrence of chronic kidney disease of unknown etiology (CKDu).

Chronic kidney disease of unknown etiology (CKDu) has posed a serious threat to human health around the world. The link between the prevalence of CKDu and groundwater geochemistry is not well understood. To identify the potential geogenic risk factors, we collected 52 groundwater samples related to CKDu (CKDu groundwater) and 18 groundwater samples related to non-CKDu (non-CKDu groundwater) from the typical CKDu prevailing areas in Sri Lanka. Results demonstrated that CKDu groundwater had significantly higher Si (average 30.1mg/L, p < 0.05) and F- (average 0.80mg/L, p < 0.05) concentrations than those of non-CKDu groundwater (average 21.0 and 0.45mg/L, respectively), indicating that Si and F- were the potential risk factors causing CKDu. The principal hydrogeochemical process controlling local groundwater chemistry was chemical weathering of silicates in Precambrian metamorphic rocks. Groundwater samples were mostly undersaturated with respect to amorphous silica and clay minerals such as talc and sepiolite, which was conducive to silicate weathering and elevated Si concentrations in groundwater. Decreased Ca2+ being facilitated by calcite precipitation and cation exchange between Ca2+ and Na+ favored fluorite dissolution and thus led to high groundwater F- concentrations. Competitive adsorption between [Formula: see text] and F- also enhanced the release of F- from solid surfaces. This study highlights the CKDu potential risk factors regarding groundwater geochemistry and their enrichment factors, which helps in preventing the prevalence of CKDu.

Reconstruction of Chemical Weathering Intensity and Asian Summer Monsoon Evolution in the Red River Basin Over the Past 36 kyr

AbstractThe Red River is located at the intersection of the Indian and East Asian monsoons. Understanding the evolution history of the monsoon in the Red River basin helps to comprehend the evolution of Asian Summer Monsoon. Herein, we present the high‐resolution major elements and Sr‐Nd isotopic ratios of the terrigenous sediment component for a well‐dated core retrieved from the northern South China Sea slope, aiming to identify the sediment provenance and reconstruct the silicate chemical weathering intensity since the past 36 kyr. The Red River was confirmed as the major sediment source through Sr‐Nd isotopic ratios. The silicate chemical weathering was affected by the rapid changes of sea level and monsoon, and thus it is difficult to indicate the regional paleoenvironment by the weathering degree. The principal component 1 composed of TFe2O3, Al2O3, K2O and MgO was shown to represent the fine‐grained sediment discharge, and employed as a proxy for the summer monsoon rainfall. Global cooling events/stages were identified through the proxies, including the Last Glacial Maximum, Heinrich events, Younger Dryas, and 8.2 ka, as well as the enhanced summer monsoon stages, including the Bølling/Allerød and early Holocene. Based on the comparison with the neighboring paleoclimate indicators, the region summer monsoon of the Red River basin was found to be governed by the Indian summer monsoon, and the summer insolation at low latitudes might be the primary forcing force on the orbital scale.

High-resolution mapping of the global silicate weathering carbon sink and its long-term changes.

Climatic and non-climatic factors affect the chemical weathering of silicate rocks, which in turn affects the CO2 concentration in the atmosphere on a long-term scale. However, the coupling effects of these factors prevent us from clearly understanding of the global weathering carbon sink of silicate rocks. Here, using the improved first-order model with correlated factors and non-parametric methods, we produced spatiotemporal data sets (0.25°×0.25°) of the global silicate weathering carbon-sink flux (SCSFα ) under different scenarios (SSPs) in present (1950-2014) and future (2015-2100) periods based on the Global River Chemistry Database and CMIP6 data sets. Then, we analyzed and identified the key regions in space where climatic and non-climatic factors affect the SCSFα . We found that the total SCSFα was 155.80±90TgCyr-1 in present period, which was expected to increase by 18.90±11 TgCyr-1 (12.13%) by the end of this century. Although the SCSFα in more than half of the world was showing an upward trend, about 43% of the regions were still showing a clear downward trend, especially under the SSP2-4.5 scenario. Among the main factors related to this, the relative contribution rate of runoff to the global SCSFα was close to 1/3 (32.11%), and the main control regions of runoff and precipitation factors in space accounted for about 49% of the area. There was a significant negative partial correlation between leaf area index and silicate weathering carbon sink flux due to the difference between the vegetation types. We have emphasized quantitative analysis the sensitivity of SCSFα to critical factors on a spatial grid scale, which is valuable for understanding the role of silicate chemical weathering in the global carbon cycle.

Mechanisms controlling the dissolved load, chemical weathering and CO2 consumption rates of Cauvery River, South India: role of secondary soil minerals

Hydrochemical assessment of the Cauvery River Basin (CRB), an east flowing Western Ghats (WG) river, is carried out to understand the dissolved load sources and controlling mechanisms with quantification of source-wise input to the dissolved load. Silicate weathering rates (SWR) and associated CO2 consumption rates (CCR) are evaluated on account of silicate basement of CRB comprising of granulites and supracrustal rocks. The source-wise solute load contributions estimated using the chemical mass balance model signify that 68% of total load is from chemical weathering followed by anthropogenic (18.5%) and atmospheric (13.5%) inputs respectively, indicating that chemical weathering is the major solute load controlling mechanism for CRB. The intensity of silicate chemical weathering occurring in the CRB is measured by degree of weathering index (Re). It is found to be higher (> 3), suggesting an incomplete weathering of drainage rocks (primary minerals) resulting in formation of soils comprising of secondary minerals including oxides, alumino-silicates and clay minerals (kaolinite, montmorillonite and smectite). Detailed understanding of the mechanism of chemical weathering is carried out using Ca/Na and Mg/Na elemental ratios of different end-members including primary minerals from rocks and secondary minerals from soils. The Na-normalized mixing diagram reveals that chemical weathering of secondary minerals is dominating and the solute load contribution to the total dissolved load is significantly higher (35.5%) from secondary minerals than primary minerals (23.5%). The SWR and associated CCR are estimated to be 13 t km−2 year−1 and 3.3 × 105 mol km−2 year−1, respectively, at outlet (Musiri) of the CRB. Results additionally signify that SWR of the east flowing WG river, Cauvery, are several times (~ 4) lower than the average SWR of west flowing WG rivers even though the associated CCR are comparable for both river systems.

East Asian monsoon intensification promoted weathering of the magnesium-rich southern China upper crust and its global significance

The Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO2 consumption process. Here, we performed a first-order estimate of the CO2 consumption induced by silicate chemical weathering and organic carbon burial in subtropical China related to this climatic reorganization. Our results show that an increase in long-term CO2 consumption by silicate weathering varies from 0.06×1012 to 0.87×1012 mol yr−1 depending on erosion flux reconstructions, with an ~50% contribution of Mg-silicate weathering since the late Oligocene. The organic carbon burial flux is approximately 25% of the contemporary CO2 consumption by silicate weathering. The results highlight the significant role of weathering of the Mg-rich upper continental crust in East China, which would contribute to the rapid decline in atmospheric CO2 during the late Oligocene and the Neogene rise in the seawater Mg content. If this climatic reorganization was mainly induced by the Tibetan Plateau uplift, our study suggests that the growth of the Himalayan-Tibetan Plateau can lead to indirect modification of the global carbon and magnesium cycles by changing the regional hydrological cycle in areas of East Asia that are tectonically less active.

Rapid coupling between solid earth and ice volume during the Quaternary

The solid earth plays a major role in controlling Earth’s surface climate. Volcanic degassing of carbon dioxide (CO2) and silicate chemical weathering are known to regulate the evolution of climate on a geologic timescale (> 106 yr), but the relationship between the solid earth and the shorter (< 105 yr) fluctuations of Quaternary glacial–interglacial cycles is still under debate. Here we show that the seawater osmium isotope composition (187Os/188Os), a proxy for the solid earth’s response to climate change, has varied during the past 300,000 years in association with glacial–interglacial cycles. Our marine Os isotope mass-balance simulation reveals that the observed 187Os/188Os fluctuation cannot be explained solely by global chemical weathering rate changes corresponding to glacial–interglacial climate changes, but the fluctuation can be reproduced by taking account of short-term inputs of (1) radiogenic Os derived from intense weathering of glacial till during deglacial periods and (2) unradiogenic Os derived from enhanced seafloor hydrothermalism triggered by sea-level falls associated with increases of ice sheet volume. Our results constitute the first evidence that ice sheet recession and expansion during the Quaternary systematically and repetitively caused short-term (< 105 yr) solid earth responses via chemical weathering of glacial till and seafloor magmatism. This finding implies that climatic changes on < 105 yr timescales can provoke rapid feedbacks from the solid earth, a causal relationship that is the reverse of the longer-term (> 106 yr) causality that has been conventionally considered.

Contrasted Chemical Weathering Rates in Cratonic Basins: The Ogooué and Mbei Rivers, Western Central Africa

Despite the absence of tectonic activity, cratonic environments are characterized by strongly variable, and in places significant, rock weathering rates. This is shown here through an exploration of the weathering rates in two inter-tropical river basins from the Atlantic Central Africa: the Ogooué and Mbei River basins, Gabon. We analyzed the elemental and strontium isotope composition of 24 water samples collected throughout these basins. Based on the determination of the major element sources we estimate that the Ogooué and Mbei rivers total dissolved solids (TDS) mainly derive from silicate chemical weathering. The chemical composition of the dissolved load and the area-normalized solute fluxes at the outlet of the Ogooué are similar to those of other West African rivers (e.g., Niger, Nyong, or Congo). However, chemical weathering rates (TZsil+rate expressed as the release rate of the sum of cations by silicate chemical weathering) span the entire range of chemical weathering intensities hitherto recorded in worldwide cratonic environments. In the Ogooué-Mbei systems, three regions can be distinguished: (i) the Eastern sub-basins draining the Plateaux Batéké underlain by quartz-rich sandstones exhibit the lowestTZsil+rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediateTZsil+rates and (iii) the Southern sub-basins characterized by steeper slopes record the highestTZsil+rates. In region (ii), higher DOC concentrations are associated with enrichment of elements expected to form insoluble hydrolysates in natural waters (e.g., Fe, Al, Th, REEs) suggesting enhanced transport of these elements in the colloidal phase. In region (iii), we suggest that a combination of mantle-induced dynamic uplift and lithospheric destabilization affecting the rim of the Congo Cuvette induces slow base level lowering thereby enhancing soil erosion, exhumation of fresh primary minerals, and thus weathering rates. The study points out that erosion of lateritic covers in cratonic areas can significantly enhance chemical weathering rates by bringing fresh minerals in contact with meteoric water. The heterogeneity of weathering rates amongst cratonic regions thus need to be considered for reconstructing the global, long-term carbon cycle and its control on Earth climate.

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.

Geo-LiM: a new geo-lithological map for Central Europe (Germany, France, Switzerland, Austria, Slovenia, and Northern Italy) as a tool for the estimation of atmospheric CO2 consumption

ABSTRACT We present a new geo-lithological map for Central Europe (Geo-LiM). It was prepared taking into account the chemical and mineralogical composition of the outcropping rocks and paying attention in discriminating metamorphic rocks, that were classified according to the chemistry of protoliths. The map was used for estimating the atmospheric CO2 consumed by the chemical weathering of silicates and carbonates. The map is made available in vector format [Donnini et al,. 2018. A new Geo-Lithological Map (Geo-LiM) for Central Europe (Germany, France, Switzerland, Austria, Slovenia, and Northern Italy) (Version 1.2) [Data set]. Zenodo Retrieved from https://zenodo.org/record/3530257], together with the computer code used to classify the lithologies and to join original maps. As a consequence, researchers can either replicate the product, or alter the code to derive a different lithological classification of the original geological maps, following the concept of Open Science.

Chemical weathering in a moraine at the ice sheet margin at Kangerlussuaq, western Greenland

ABSTRACTWeathering caused by interaction between glacial sediments and water in exposed moraines needs to be studied to evaluate their possible effects on the global carbon cycle. In this study, moraine ponds, moraine porewaters, and till samples were collected at a moraine adjacent to the Greenland Ice Sheet at Kangerlussuaq. Scanning electron microscopy (SEM) studies of the till show limited evidence of silicate chemical weathering, but the moraine waters have substantial solute concentrations. δ34SSO4 and δ18OSO4 data indicate that the origin of dissolved sulfate is the oxidation of sulfides, in agreement with the SEM observations. The dissolved HCO3−/SO42− molar ratios indicate an uneven balance between sulfuric and carbonic acid weathering; C-isotope data indicate that some of the CO2 originates from organic carbon mineralization. Ion–ion plots provide evidence of carbonate weathering and of the formation of secondary gypsum and calcite through evaporation and (or) cryoconcentration. The 87Sr/86Sr ratios in the waters correlate with the corresponding till samples, supporting the local origin of the dissolved strontium, which is higher in the waters than in the till due to the selective weathering of biotite. The data evidence a large degree of chemical weathering in moraines promoted by large rock–water ratios and by the hydraulic isolation created by the frozen till. The high PCO2 in the studied moraine waters indicates that they may represent a previously underestimated CO2 source.

Chemical weathering and associated CO2 consumption in the Mahanadi river basin, India

Chemical weathering of continents is one of the major geochemical processes which consumes atmospheric CO2 and exerts a strong impact on the global climate. The major ion chemistry of the Mahanadi river basin, one of the largest river basins in India, is determined to investigate the sources of major ions and their spatio-temporal variability, chemical weathering rates and associated CO2 consumption in the basin. The results reveal that the ions in the basin are dominated by Ca and HCO3. The total dissolved solids (215 mg L−1 in monsoon and 270 mg L−1 in pre-monsoon) and total cation concentration (2367 µEq in monsoon and 3214 µEq in pre-monsoon) in the basin are relatively higher than the global average. The dissolved loads in the basin are dominantly controlled by rock weathering particularly chemical weathering of silicates and carbonates. The estimated chemical weathering rates based on the forward model are 44.94 t km−2 yr−1 in monsoon and 2.45 t km−2 yr−1 in pre-monsoon with annual average chemical weathering rate of 23.7 t km−2 yr−1. The contribution of silicate weathering rates in the basin is 32 and 1.55 t km−2 y−1 during monsoon and pre-monsoon period respectively. The estimated CO2 consumption rate associated with chemical weathering in the basin is 13.3 × 105 mol km−2 yr−1 during monsoon and 0.66 × 105 mol km−2 yr−1 during pre-monsoon period with an average annual rate of 6.9 × 105 mol km−2 yr−1 which is higher than the global average. The net rate of CO2 consumption by silicate weathering is estimated to be approximately 4.78 × 105 mol km−2 yr−1. It is observed that the runoff and lithology are the major factors influencing chemical weathering in the basin. However, the basin elevation has a good negative correlation with chemical weathering rate.

Chemical Weathering and Riverine Carbonate System Driven by Human Activities in a Subtropical Karst Basin, South China

In the context of climate change, the input of acid substances into rivers, caused by human activities in the process of industrial and agricultural development, has significantly disrupted river systems and has had a profound impact on the carbon cycle. The hydrochemical composition and which main sources of the Lianjiang River (LR), a subtropical karst river in northern Guangdong Province, South China, were analyzed in January 2018. The objective was to explicate the influence on the deficit proportion of CO2 consumption, resulting from carbonate chemical weathering (CCW), driven by nitric acid (HNO3) and sulfuric acid (H2SO4), which is affected by exogenous acids from the industrial regions in north of the Nanling Mountains and the Pearl River Delta. The response of the riverine carbonate system to exogenous acid-related weathering was also discussed. HCO3− and Ca2+, respectively, accounted for 84.97% of the total anions and 78.71% of the total cations in the surface runoff of the LR, which was characterized as typical karst water. CCW was the most important material source of river dissolved loads in the LR, followed by human activities and silicate chemical weathering (SCW). Dissolved inorganic carbon (DIC), derived from CCW induced by carbonic acid (H2CO3), had the largest contribution to the total amount of DIC in the LR (76.79%), and those from CCW induced by anthropogenic acids (HNO3 and H2SO4) and SCW contributed 13.56% and 9.64% to the total DIC, respectively. The deficit proportion of CO2 consumption associated with CCW resulting from sulfuric acid and nitric acid (13.56%), was slightly lower than that of the Guizhou Plateau in rainy and pre-rainy seasons (15.67% and 14.17%, respectively). The deficit percentage of CO2 uptake associated with CCW induced by sulfuric acid and nitric acid, accounted for 38.44% of the total CO2 consumption related to natural CCW and 18.84% of the anthropogenic acids from external areas. DIC derived from CCW induced by human activities, had a significant positive correlation with the total alkalinity, SIc and pCO2 in river water, indicating that the carbonate system of the LR was also driven by exogenous acids, with the exception of carbonic acid. More attention should be paid to the effects of human activities on the chemical weathering and riverine carbonate system in the karst drainage basin.

Water geochemistry of rivers draining karst-dominated regions, Guangxi province, South China: Implications for chemical weathering and role of sulfuric acid

Guangxi province, located in South China, is a typical karst region affected by acid depositions for a couple of decades. Water samples from the major river basins in this region were collected and the chemical compositions were measured with a main purpose to quantify chemical weathering rates and associated CO2 budget. The average total dissolved solids (TDS) of the river waters (232 mg/l) are comparable with other karstic rivers. The major ion compositions of the river waters are characterized by the dominance of Ca2+, Mg2+, HCO3−, and are significantly rich in SO42−. The chemical compositions of the river waters indicate that four major reservoirs (atmospheric, anthropogenic, carbonate and silicate inputs) contribute to the dissolved loads. The carbonate chemical weathering rates range from 61.4 t/km2/yr for the Qingshuihe river basin to 93.3 t/km2/yr for the Zuojiang river basin. The silicate chemical weathering rates range from 2.9 t/km2/yr for the Qingshuihe river basin to 7.9 t/km2/yr for the Liujiang river basin. The CO2 consumption rates by carbonate weathering range from 559 × 103 mol/km2/yr for the Qingshuihe river basin to 817 × 103 mol/km2/yr for the Zuojiang river basin. The CO2 consumption rates by silicate weathering range from 19 × 103 mol/km2/yr for the Zuojiang river basin to 85 × 103 mol/km2/yr for the Liujiang river basin. Sulfuric acid is involved as a proton donor in weathering reactions, and about 9–15% of the chemical weathering rates of the river basins are sulfuric acid-induced. The CO2 consumption rates are only 85–91% of the value when assuming carbonic acid provides all the protons in the weathering reactions. The CO2 released from sulfuric acid-induced carbonate weathering would exceed the CO2 consumed by silicate weathering, making the river basins in Guangxi net CO2 sources on geological timescales. The total net CO2 releasing flux is approximately 15.7 × 109 mol/yr (0.19 × 1012 gC/yr). This result quantitatively highlights the fact that CO2 budget by chemical weathering will be largely modified by sulfuric acid, especially in seriously acid deposition affected region.