Carbonate Weathering Rates Research Articles

Impact of the atmospheric deposition of major acid rain components, especially NH4, on carbonate weathering during recharge in typical karst areas of the Lijiang River basin, southwest China

We analysed precipitation and river water samples from the Lijiang River basin with the aim of understanding the important role of acid rain components, especially NH4, in chemical weathering of karst areas. First, analysis of the hydrochemical characteristics revealed the following: In the precipitation samples, the volume-weighted mean pH was 4.91, and the acid rain rate was 65%. The major ion composition was characterized by a dominance of NH4, Ca, SO4 and NO3, accounting for 40.0%, 35.3%, 46.3% and 30.5%, respectively, of the total cation and anion concentrations. In the river water samples, the pH ranged from 6.75 to 9.16, with an average of 7.55. The major hydrogeochemical species in the river water samples were Ca–HCO3 and Mg–HCO3. Second, by employing chloride-normalized ratios of the other major ions in precipitation, we estimated that the contributions of SO4 and NO3 from the atmosphere to the river were53.6% and 35.0%, respectively. Third, combining the hydrochemical index with the value of δ13CDIC and using the hydrochemical composition method showed that the proportion of carbonate weathered (R) by NH4, H2SO4 and HNO3 from acid rain accounted for 9.3%, 9.4% and 3.1% of the total weathering rate, respectively, and the proportions in the upper reaches were significantly higher than those in the lower reaches of the river. The average values of the carbonate weathering proportion (R) and the carbonate weathering rate (CWR) due to acid rain were approximately 22.0% and 2.1 t/km2/yr, respectively. Taken together, our results revealed that carbonate weathering by acid rain can significantly increase the carbonate weathering rate, which is essential information for accurate estimation of the carbon cycle (namely, inorganic carbon sources and sinks).

Major ion chemistry and atmospheric CO2 consumption deduced from the Batal glacier, Lahaul–Spiti valley, Western Himalaya, India

This study mainly focuses on the source identification of various ions in meltwater and estimation of CO2 consumption rate by chemical weathering in the Batal glacier basin on the basis of 2 years of study (2015 and 2017). The glacier meltwater has been monitored as slightly acidic in nature having mean pH value of 6.6. Ca2+ was observed as the most dominant cation contributing about 76% of TZ+ (total cations), whereas SO42− was observed as the most dominant anion contributing about 70% of TZ− (total anions) in the stream meltwater. High ratios of (Ca + Mg) versus TZ+ (mean value: 0.89 ± 0.02) and (Ca + Mg) versus (Na + K) (mean value: 8.51 ± 2.07) elucidate that stream water chemistry of the Batal glacier is largely controlled by carbonate weathering. Concentration of total dissolved solid in the glacial stream water was higher during the low-melt season (September) and lower during the high-melt period (July). The average value of daily mean TDS flux of the study area was calculated to be 12.4 t/day. The mean values of CWR (carbonate weathering rate) and SWR (silicate weathering rate) for the Batal glacier basin were calculated to be 97.4 and 22.8 t/km2/year, showing higher contribution of CWR as compared to SWR in the investigation area. CO2 consumption rate by the combined silicate and carbonate (chemical) weathering was estimated to be 11.1 × 105, 28.8 × 105 and 35.5 × 105 mol/km2/year during the study period September 2015, June 2017 and July 2017, respectively. The annual CO2 drawdown by the Batal glacier basin on the basis of CO2 consumption rate by chemical weathering is much lower as compared to the Gangotri glacier, which may be due to bigger size and higher meltwater runoff of the Gangotri glacier as compared to the Batal glacier.

Impact of anthropogenic sulfate deposition via precipitation on carbonate weathering in a typical industrial city in a karst basin of southwest China: A case study in Liuzhou

An analysis was performed of the hydrochemical characteristics of precipitation and river water samples collected from the Liujiang River Basin from June 2014 to May 2015, SW China to determine the carbon source and sink of carbonate weathering by anthropogenic sulfate deposition via precipitation in a typical Karst industrial city. Our findings revealed that (1) the pH of the precipitation samples has a volume-weighted mean (VWM) of 5.78 and that the acid rain rate is 12%. The enrichment factor (EF), sea salt fraction (SSF), crustal fraction (CF) and anthropogenic fraction (AF) were calculated, and the results indicated that precipitation was characterized by high concentrations of SO4, NH4, Ca and NO3. Na and Cl, Ca and NH4, and SO4 (98.2%) and NO3 (99.8%) were mainly derived from sea salt, earth crust, and anthropogenic activities, respectively. Combined with the air mass trajectory (HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory)) and the concentration of non-sea salt (NSS) SO4 and δ34S–SO4nss, the δ34S–SO4sam ranged from −2.6–12.6‰ and atmospheric sulfate was mainly associated with local coal combustion, fossil fuels and the transmission distance of pollution sources from the northwest and southwest in the interior of the continents. (2) An analysis of the river hydrochemical characteristics showed that the cations are mainly Ca and Mg, and they accounted for approximately 90% of the total cations. The anions were mainly HCO3 and SO4, and the hydrochemical composition of the river was mainly controlled by the weathering of carbonate minerals in the river basin. An analysis of the correlation between the changes in the chemical equivalence concentration of [Ca + Mg]/[HCO3+SO4] showed that the ratio of most river water samples was approximately 1, indicating that Ca and Mg in the river need HCO3 and SO4 for chemical equilibrium and that SO4 in the river may be involved in the dissolution of carbonate minerals. (3) Corrections were performed based on the chloride-normalized ratios and the contribution of each terminal element was calculated to estimate the major ions of precipitation input; the results showed that the average contribution ratio of SO4 transported to the river by anthropogenic sulfuric acid in precipitation reached 85.57%. (4) According to the proportional relationships, carbonate weathering accounts for 31.73% of the anthropogenic sulfuric acid via precipitation and would cause an approximate 6.27% increase in carbonate weathering rates but a 19.48% decrease in CO2 consumption rates. Taken together, our results revealed that even in high-carbonate areas of Southwest China, chemical weathering by sulfuric acid can reduce the carbon sink effect caused by carbonate weathering.

Seasonal variation of chemical weathering and its controlling factors in two alpine catchments, Nam Co basin, central Tibetan Plateau

To investigate the seasonal variation of chemical weathering and its controlling factors in the central Tibetan Plateau (TP), hydrochemistry of stream water in two alpine catchments, the Niyaqu and Qugaqie, was studied at an unprecedented temporal resolution from July 2016 to September 2017. Chemical compositions of river water in the two catchments were dominated by Ca2+ and HCO3−. SO42− in the stream water mainly derived from sulfide oxidation but sulfuric acid played a minor role in rock weathering compared with carbonic acid. The contributions of atmospheric input, silicate weathering, carbonate weathering and sulfide oxidation to riverine solutes of the Niyaqu, which overlies tuff, sandstone, shale, and carbonate and is mainly recharged by groundwater, were about 5%, 12%, 65%, and 18%, respectively; these contributions to dissolved loads of the Qugaqie, where lithology consists of granite and sandstone and nearly half the stream water originates from glaciers meltwater, were 10%, 35%, 34% and, 20%, respectively. Contribution of different sources to riverine solutes in the Niyaqu were almost invariant between different seasons, while the percentage of these sources in the stream water of the Qugaqie showed significant seasonal variations, especially for the weathering of silicate and carbonate: average value of the former increased from 29% in the monsoon season to 45% during the nonmonsoon period, on the contrary, average value of the latter decreased from 37% to 29% in the corresponding periods. Carbonate weathering rates (CWR) and silicate weathering rates (SWR) in the study areas exhibited distinct seasonal variability. During the monsoon season, the average CWR and SWR values in the Niyaqu were 34.3 t/km2/y and 6.8 t/km2/y, respectively, and their corresponding values in the Qugaqie were 16.7 t/km2/y and 11.0 t/km2/y, respectively, which means that the average value of CWR in the Niyaqu during the monsoon season is comparable with the CWR (average for one year) in catchments located in the eastern TP, and the SWR in the Qugaqie during the monsoon season is close to the SWR (average for one year) of some basins at the south slope of the Himalaya. Chemical denudation rates decreased sharply during the nonmonsoon period, indicating little chemical exhumation within that time due to cold and arid weather. Runoff was the most important factor that controls chemical weathering in both the Niyaqu and Qugaqie, followed by physical erosion rates, and the influence of temperature seems to be insignificant in the Niyaqu while at the same time crucial in the Qugaqie.

Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin

The exposure of organic carbon in rocks to oxidative weathering can release carbon dioxide (CO2) to the atmosphere and consume atmospheric oxygen. Alongside volcanism, metamorphism, and the weathering of carbonate minerals by sulfuric acid, this is a major source of atmospheric CO2 over million year timescales. The balance between CO2 release and CO2 drawdown by silicate weathering and organic carbon burial sets the net geochemical carbon budget during weathering and erosion. However, the rates of rock-derived organic carbon (petrogenic organic carbon, OCpetro) oxidation remain poorly constrained. Here, we use rhenium as a proxy to trace and quantify CO2 release by OCpetro oxidation in the Mackenzie River Basin, Canada, where the other carbon fluxes have been well constrained previously. River water and sediment samples were collected between 2009 and 2013 at gauging stations along the Mackenzie River and its main tributaries (Liard, Peel and Arctic Red). To assess rhenium inputs from silicate, sulfide and OCpetro mineral phases we normalize dissolved rhenium concentrations, [Re]diss, to sodium and sulfate ion concentrations. This approach suggests that >85 percent of [Re]diss is derived from OCpetro in the main river channels. [Re]diss and water discharge measurements are used to quantify dissolved Re yields. River sediments provide a measure of the Re to OCpetro ratio of materials undergoing weathering in the basin, and agree well with published rock samples. Dissolved Re yields are combined with river sediment [Re]/[OCpetro] ratios to estimate the CO2 emissions by OCpetro weathering. These are 0.45 +0.19/−0.11 metric tonnes of carbon, tC km−2 yr−1for the Mackenzie River at Tsiigehtchic (3.8 +1.5/−0.9 × 104 moles km−2 yr−1), and 0.94 +0.41/−0.26 tC km−2 yr−1, 0.78 +0.35/−0.21 tC km−2 yr−1 and 1.01 +0.42/−0.25 tC km−2 yr−1 for the Peel, Arctic Red and Liard catchments, respectively. When considered alongside published silicate and carbonate weathering rates and the sedimentary burial of biospheric organic carbon, these data suggest that the upper part of the Mackenzie River Basin presently acts as an atmospheric CO2 sink of ∼1 tC km−2 yr−1 (∼8 × 104 moles km−2 yr−1) as a result of the carbon transfers by weathering and erosion. During the Last Glacial Maximum, it is possible that the net geochemical carbon balance may have been very different: potential increases in CO2 emissions from oxidative weathering of OCpetro and carbonate minerals, coupled with reduced biospheric carbon burial, may have tipped the balance to a net source of CO2.

Hydro‐Geochemical and Sr Isotope Characteristics of the Yalong River Basin, Eastern Tibetan Plateau: Implications for Chemical Weathering and Controlling Factors

AbstractMajor ions and Sr isotope compositions of waters in the Yalong River basin in the eastern Tibetan plateau were investigated to explore the sources of dissolved loads and the controls on chemical weathering rates. Measured radiogenic 87Sr/86Sr ratios of water samples are most likely derived from the weathering of metamorphic carbonates or the Precambrian silicate rocks. Quantitative analysis of water geochemistry shows that carbonate weathering dominates the chemical characteristics of the river water. The chemical weathering rate of the whole Yalong River basin is calculated to be 7.6 ton·km−2·year−1 by silicate weathering and 44.7 ton·km−2·year−1 by carbonate weathering, which consumes 158.5 × 103 and 885.0 × 103 mol·km−2·year−1 of atmospheric CO2, respectively. The highest carbonate weathering rate (116.3 ton·km−2·year−1) in the Yalong River basins is observed from the basin with the steepest catchment average slope. This highest magnitude of carbonate weathering is comparable to the rates from catchments in the southern Himalayas. Positive relationships are observed between silicate weathering rates and both climatic (precipitation and temperature) and topographic (relief and slope) controls. The combined climatic and tectonic effects increase silicate weathering and counterbalance the dilution effect. However, the relationship between silicate weathering rate and climatic and topographic controls varies with hydrologic conditions. The rate for low‐discharge catchments is more closely related to climatic parameters, while the rate for high‐discharge catchments is more closely related to local topography.

Hydrochemical Changes and Influencing Factors in the Dongkemadi Region, Tanggula Range, China

In order to detect the source and controlling factors of hydrochemical ions in glacier meltwater-recharged rivers, the chemical characteristics of the river water, precipitation, and meltwater of the Dongkemadi River Basin, China, in 2014 (from May to October) were systematically analyzed, and combined with the hydrological and meteorological data. The results show that the hydrochemical pattern of the typical river was HCO3−-Ca2+. The most cations were Ca2+ and Mg2+, and the predominant anions were HCO3− and SO42−, in the river. The concentration of major ions and total dissolved solids (TDS) in the river water were much larger than that in the precipitation and meltwater. The TDS concentration was ordered: River water > precipitation > meltwater. The water-rock interaction and the dilution effect of the precipitation and meltwater on the runoff ions resulted in a negative correlation between the ion concentration of the river water and the river flow. The chemical ions of the river runoff mainly originated from rock weathering and the erosion (abrasion) caused by glacier movement. In addition, the contributions of different sources to the dissolved components of the Dongkemadi River were ordered: Carbonate (75.8%) > silicate (15.5%) > hydatogenic rock (5.7%) > atmospheric precipitation (3%), calculated by a forward geochemical model. And the hydrochemical weathering rates of carbonate and silicate minerals were 12.30 t·km−2·a−1 and 1.98 t·km−2·a−1, respectively. The CO2 fluxes, consumed by the chemical weathering of carbonate and silicate, were 3.28 × 105 mol·km−2·a−1 and 0.91 × 105 mol·km−2·a−1, respectively.

Hydrogeochemical Processes in a Small Eastern Mediterranean Karst Watershed (Nahr Ibrahim, Lebanon)

Watersheds located in semiarid areas such as the eastern Mediterranean are particularly sensitive to the impact of climate change. To gain knowledge on the hydrogeochemical processes occurring in the Nahr Ibrahim watershed, a Critical Zone Observatory in Lebanon, we analyze the isotopic composition of the river water as well as the concentrations of the major ions exported (Ca2+, Mg2+, HCO3−, Na+, Cl−, K+, SO42−). Sampling campaigns were conducted from March 2014 to August 2016 to capture contrasting hydrological conditions. The results indicate that the carbonate lithology of the watershed is the predominant source of Ca2+, Mg2+ and HCO3−, whereas the low contents of Na+, Cl−, K+, SO42− mainly originate from sea spray. Except in the headwaters, the Nahr Ibrahim River is oversaturated with respect to calcite and dolomite. During wet seasons, calcite weathering and dolomite weathering contribute in an equivalent manner to the solute budget, whereas during dry seasons, calcite precipitates in the river. The isotopic composition of the river water reveals little seasonal dependency, the groundwater recharge by snowmelt infiltration leading to spring waters depleted in heavier isotopes during the dry seasons. A carbonate weathering rate of about 176 t/km2/year was determined at the outlet of the Nahr Ibrahim watershed. The calculated values of CO2 partial pressure, on average twice the atmospheric pressure, suggest that the river is a significant source of CO2 to the atmosphere (111 t/year).

Major Ionic Features and Possible Controls in the Groundwater in the Hamatong River Basin

In order to study the major ion chemistry and controls of groundwater, 59 groundwater samples were collected and their major ions measured in the Hamatong River Basin. The hydrogeochemical characteristics of groundwater in this basin were analyzed by means of mathematical statistics, Piper triangular diagrams, Gibbs figures, and ionic relations, and the water chemical evolution and ion sources of the Hamatong River Basin were determined. The results showed that Ca2+ was the main cation in the groundwater, accounting for 22.1% to 72.4% of the total cations, with an average value of 48.7%. HCO3- was the main anion, accounting for 35.3% to 97.5% of the total anions, and with an average value of 80%. Total dissolved solids concentration ranged from 93.3 mg·L-1 to 521.1 mg·L-1 with a median value of 219.1 mg·L-1. The hydrochemical types of groundwater are HCO3-Ca, HCO3-Ca·Mg, and HCO3-Ca·Na. Chemical weathering rates of carbonates and silicates were estimated, and the chemical composition of groundwater samples located in the middle of Gibbs model indicated that the major chemical process of groundwater was controlled by rock weathering. Silicate weathering is believed to significantly contribute to dissolved solute compositions, and carbonate weathering played an important role as the source of dissolved ions.

Seasonal Variations in River Water Chemical Weathering and Its Influence Factors in the Malian River Basin

In order to discern temporal variations, sources, and controlling factors of river water chemistry in the Malian River Basin, time series samples were collected from the Yuluoping hydrological station in 2016. The compositions of major cations and anions were analyzed and a forward model was used to calculate the weathering rates of evaporite, silicate, and carbonate. Results showed that river water was brackish with average total dissolved solids of 1154.0 mg·L-1, indicating significant differences from other main rivers in China. Na+, Ca2+, Mg2+, and SO42- were the major ions present in water, with mean concentrations of 202.8, 86.0, 78.6, and 431.2 mg·L-1 respectively. Water chemistry exhibited distinct seasonal variations, with major ions gradually declining during the pre-monsoon period and increasing in the post-monsoon period. An abrupt rise in concentrations of major ions during the ice melting interval was observed, as well as a sharp drop during stormy events. Dissolved loads were mainly derived from chemical weathering, with the contribution ratios of evaporite, silicate, and carbonate being 67.1%, 13.7%, and 19.2% respectively. Chemical processes showed different responses to climate forcing, attributed to variations in mineral content in the watershed and dissolution kinetics. The dominant contribution of evaporite in the monsoon season was due to its rapid dissolution, while silicate weathering increased during the pre-monsoon period, with longer water rock interaction times when water discharge was lower. During the post-monsoon season, carbonate weathering was enhanced due to its high content in loess and due to more CO2 absorption by rain from soil. The average chemical weathering rates of evaporite, silicate, and carbonate were 30.6, 6.2, and 8.7 kg·(km2·d)-1, respectively. A strong correlation between evaporite weathering rates and river discharge was evident; a correlation was also observed between carbonate weathering rates and river discharge, indicating that water discharge played a dominant role in chemical weathering, rather than temperature or precipitation.

Chemical balance of the Yellow River source region, the northeastern Qinghai-Tibetan Plateau: Insights about critical zone reactivity

Rivers play a vital role in geochemical processes of the critical zone. As the Third Pole, the geochemical processes of the Qinghai-Tibetan Plateau are vital to global element cycles. The Yellow River source region is located in the northeastern Qinghai-Tibetan Plateau, yet there has been little research concerning water chemistry in this region. In this study, river water and rain samples were collected to analyze the stable isotopes and major ion chemistry in the Yellow River source region. The δ18O and δD values ranged from −12.9‰ to −4.5‰ and from −95.7‰ to −36.7‰, respectively. The stable isotopic values of tributaries were lower than those in the main stream, and the construction of the dam could result in the enrichment of δ18O and δD. The stable isotopic values showed that precipitation was the main source of river water. The total dissolved solids (TDS) in river water ranged from 214.9 to 563.2 mg/L, and calcium and bicarbonate were the dominant ions. The average contributions of atmospheric input, evaporites, silicates, and carbonates to the dissolved load of river water were 1.8%–12.5%, 12.4%–15.9%, 18.1%–18.4%, and 56.7%–64.3%. The silicate and carbonate weathering rates were 2.3–2.5 t/km2/yr and 17.4–19.6 t/km2/yr, respectively. The chemical denudation rate in the study area was about 23.0–26.8 t/km2/yr, comparable to the world average value. The CO2 consumption rates by silicate and carbonate weathering were 91–94 × 103 moL/km2/yr and 185–208 × 103 moL/km2/yr, respectively. The TDS flux in the study area was 6.1 × 109 kg/yr, which accounted for 38–47% of that of the Yellow River into the sea. This study showed that although as an alpine arid area, the Yellow River source region had a comparable chemical denudation rate to the world average value, and even higher CO2 consumption rates by silicate and carbonate weathering than some of the large rivers in the world.

Temporal and spatial variations of rock weathering and CO2 consumption in the Baltic Sea catchment

This study provides the first estimate of silicate and carbonate weathering rates and derived CO2 consumption rates for the Baltic Sea catchment using river chemistry data of 78 rivers draining into the Baltic Sea. The silicate weathering rates (denoted as total dissolved solids) of individual river basin range from 0.014Tg/year to 4.55Tg/year and the carbonate weathering rates range from 0.079Tg/year to 6.49Tg/year. The total CO2 consumption across the Baltic catchment is approximately 3.9TgC/year and is almost equally shared by silicates and carbonates. Uncertainty associated with the weathering estimate is around 32%, which is mainly caused by incomplete pollution correction for a few major rivers in the south. The calculations for the boreal river basins have higher certainties because of less human impacts. The CO2 consumption rate of individual river basin vary between 0.53 and 5.66gC/m2/year with an average of 2.97gC/m2/year, in which carbonates consume CO2, 1.4gC/m2/year and silicates take 1.5gC/m2/year. This is in the same range as has been reported for the Mackenzie River and Siberian river basins, but at the lower range of tropical rivers, suggesting the Baltic Sea catchment holds solid weathering signals for high-latitude systems, especially in the pristine boreal silicate-dominated areas. The amount of CO2 consumed by weathering in the Baltic Sea catchment accounts for approximately 3–30% of the net ecosystem carbon exchange (10–100gC/m2/year), implying that weathering contributes as a significant sink of atmospheric CO2. Although many studies have shown the positive relation between temperature and weathering rates in various river catchments, multiple regression analysis using the 40-year continuous records of river chemistry in the boreal area of the Baltic Sea catchment reveals a strong correlation between weathering flux and precipitation, but no statistically significant correlation between weathering and temperature. This suggests not only that temperature is not necessarily to be primary controlling factor for weathering rates, but also besides precipitation, other factors, like increased soil organic matter contents and water path changes may have high impact on weathering rates. The 40-year data analysis also shows generally increasing weathering fluxes by 10–20% in the pristine boreal area over the past decades. This indicates that increased CO2 consumption by weathering and the resulting elevated dissolved inorganic carbon delivery to the ocean act as a negative feedback for ocean acidification, such as the Arctic Ocean that has become more acidic due to high terrestrial organic carbon delivery together with increased river water input.

A probabilistic analysis of meteorically altered δ13C chemostratigraphy from late Paleozoic ice age carbonate platforms

Abstract The stratigraphic expression of meteoric diagenesis in carbonates is a glimpse into the weathering, fluid transport, and biological productivity of the ancient near-surface terrestrial environment. To infer this environmental information, we use a probabilistic approach to merge an isotope-based reactive transport model with chemostratigraphic data from carbonates that were subaerially exposed during an expansion of the late Paleozoic ice age in the middle Carboniferous. The rate of carbon flowing through the carbonate platform relative to the rate of carbon reacting between mineral and fluid phases controls the length scale, curvature, and magnitude of the diagenetic carbon isotope profiles. The ratio of advection to reaction is determined for seven stratigraphic sections, and the advection rate is used to estimate the minimum carbonate weathering associated with each profile. These carbonate weathering rates extrapolated over the expansive shallow carbonate platforms of the middle Carboniferous indicate that glacioeustatic fall may have caused a 20%–50% increase in the dissolved CaCO3 flux to the ocean. The complex feedbacks among carbonate weathering and accumulation, atmospheric pCO2, glacioeustasy, and passive margin subsidence may have played an unexplored role in the glacial-interglacial climate dynamics of the late Paleozoic ice age.

Geochemistry of the dissolved loads of the Liao River basin in northeast China under anthropogenic pressure: Chemical weathering and controlling factors

This study focuses on the chemical and Sr isotopic compositions of the dissolved load of the rivers in the Liao River basin, which is one of the principal river systems in northeast China. Water samples were collected from both the tributaries and the main channel of the Liao River, Daling River and Hun-Tai River. Chemical and isotopic analyses indicated that four major reservoirs (carbonates (+gypsum), silicates, evaporites and anthropogenic inputs) contribute to the total dissolved solutes. Other than carbonate (+gypsum) weathering, anthropogenic inputs provide the majority of the solutes in the river water. The estimated chemical weathering rates (as TDS) of silicate, carbonate (+gypsum) and evaporites are 0.28, 3.12 and 0.75t/km2/yr for the main stream of the Liao River and 7.01, 25.0 and 2.80t/km2/yr for the Daliao River, respectively. The associated CO2 consumption rates by silicate weathering and carbonate (+gypsum) weathering are 10.1 and 9.94×103mol/km2/yr in the main stream of the Liao River and 69.0 and 80.4×103mol/km2/yr in the Hun-Tai River, respectively. The Daling River basin has the highest silicate weathering rate (TDSsil, 3.84t/km2/yr), and the Hun-Tai River has the highest carbonate weathering rate (TDScarb, 25.0t/km2/yr). The Raoyang River, with an anthropogenic cation input fraction of up to 49%, has the lowest chemical weathering rates, which indicates that human impact is not a negligible parameter when studying the chemical weathering of these rivers. Both short-term and long-term study of riverine dissolved loads are needed to a better understanding of the chemical weathering and controlling factors.

Water geochemistry of the Qiantangjiang River, East China: Chemical weathering and CO2 consumption in a basin affected by severe acid deposition

The chemical composition of the Qiantangjiang River, the largest river in Zhejiang province in eastern China, was measured to understand the chemical weathering of rocks and the associated CO2 consumption and anthropogenic influences within a silicate-dominated river basin. The average total dissolved solids (TDS, 113mgl−1) and total cation concentration (TZ+, 1357μeql−1) of the river waters are comparable with those of global major rivers. Ca2+ and HCO3− followed by Na2+ and SO42−, dominate the ionic composition of the river water. There are four major reservoirs (carbonates, silicates, atmospheric and anthropogenic inputs) contributing to the total dissolved load of the investigated rivers. The dissolved loads of the rivers are dominated by both carbonate and silicate weathering, which together account for about 76.3% of the total cationic load origin. The cationic chemical weathering rates of silicate and carbonate for the Qiantangjiang basin are estimated to be approximately 4.9tonkm−2a−1 and 13.9tonkm−2a−1, respectively. The calculated CO2 consumption rates with the assumption that all the protons involved in the weathering reaction are provided by carbonic acid are 369×103molkm−2a−1 and 273×103molkm−2a−1 by carbonate and silicate weathering, respectively. As one of the most severe impacted area by acid rain in China, H2SO4 from acid precipitation is also an important proton donor in weathering reactions. When H2SO4 is considered, the CO2 consumption rates for the river basin are estimated at 286×103molkm−2a−1 for carbonate weathering and 211×103molkm−2a−1 for silicate weathering, respectively. The results highlight that the drawdown effect of CO2 consumption by carbonate and silicate weathering can be largely overestimated if the role of sulfuric acid is ignored, especially in the area heavily impacted by acid deposition like Qiantangjiang basin. The actual CO2 consumption rates (after sulfuric acid weathering effect deduction) is only about 77% of the value calculated with the assumption that carbonic acid donates all the protons involved in the weathering reaction.

Major ion geochemistry of the Nansihu Lake basin rivers, North China: chemical weathering and anthropogenic load under intensive industrialization

To explore the chemical weathering processes and the anthropogenic disturbance of weathering, 20 water samples were collected from the tributaries in the Nansihu Lake basin, a growing industrial area. The major ions in river waters were analyzed to identify and quantify the contributions of the different reservoirs. Based on stoichiometric analyses and end-member determination, the contributions of individual reservoirs were calculated for each tributary. In the study region, the averaged contributions of atmospheric inputs, anthropogenic inputs, evaporite weathering, carbonate weathering and silicate weathering were 2, 37, 28, 25 and 8 %, respectively. Combined with information regarding runoff and drainage area, the annual average contribution of TDS to waters was estimated to be 1.90 ± 0.95 ton/km2 from silicate weathering, 5.68 ± 2.84 ton/km2 from carbonate weathering. Furthermore, the associated consumption of CO2 was calculated to be approximately 7.50 × 109 mol/a. The industrial and mining activities were the main sources for anthropogenic inputs, and they produced non-CO2 acids (NCA). Of all protons involved in chemical weathering, 34 % was presumed to be originated from NCA, causing 2.74 × 109 mol/a of CO2 degassing. Moreover, industrial inputs could play a major role in the modification of the chemicals in the water system, and they could even change the carbonate weathering rate in such an intensively industrializing region. In North China, the chemical weathering associated with NCA was found to be significant for the first time.

Riverine hydrochemistry and CO2 consumption in the tropic monsoon region: a case study in a granite-hosted basin, Hainan Island, China

To evaluate the carbon sink capacity caused by rock chemical weathering processes and its controlling factors in the tropical monsoon region, the hydrochemistry of the granite-hosted Changhuajiang River (CHJR) basin in Hainan Island, China, were systematically investigated by collecting water samples from the river mouth to headwater in the dry season and the wet season, respectively. The results show that Na+, Ca2+ and HCO3 − are dominant in the chemical runoff of the CHJR. The spatial and temporal variations of major ions imply the influence of the multi-sources. The concentrations of Ca2+, Mg2+, HCO3 − and DSi, mainly sourcing from rock (silicates and carbonates) weathering, are lower in the wet season than those in the dry season. However, the concentrations of K+, Cl−, SO4 2−and NO3 −, mainly sourcing from the atmospheric precipitation and human activities, are significantly controlled by the monsoon rainfall and hydrological stage in the CHJR basin. The contributions of the silicates chemical weathering, human activities, atmospheric input, evaporites dissolution and carbonates chemical weathering to the chemical runoff are 82.62, 9.05, 5.24, 2.05 and 1.05 % in the CHJR, respectively. The chemical weathering rates of silicates, carbonates and evaporites are 16.25, 0.40 and 0.62 t km−2 year−1, respectively. Chemical weathering rates in global granite-hosted basins significantly correlate with temperatures (P 10 °C). On a regional scale, we find that the chemical weathering rates in the southern China appear a weak positive correlation with normalized differential vegetation indexes (NDVI), which need to be confirmed by enough data. The flux of atmospheric CO2 consumption from rock weathering in the CHJR basin is 2.71 × 105 mol km−2 year−1 by the discharge water of 2014, with being close to that (2.9 × 105 mol km−2 year−1) in the other tropical basins and far from that (5.06 × 105 mol km−2 year−1) calculated by the averaged discharge water of over years in the CHJR. The silicates weathering intensity lies at the moderate phase, implying that the weathering processes are still developing in global tropical granite basins. Hence, how the atmospheric CO2 consumption from rock weathering will change with increasing the atmospheric CO2 level and dam construction altering water cycle in the tropical monsoon area is expected to be deserved.

Hydrochemistry of inland rivers in the north Tibetan Plateau: Constraints and weathering rate estimation

The geographic region around the northern and northeastern Tibetan Plateau is the source of several inland rivers (e.g. Tarim River) of worldwide importance that are generated in the surrounding mountains systems of Tianshan, Pamir, Karakorum, and Qilian. To characterize chemical weathering and atmospheric CO2 consumption in these regions, water samples from the Tarim, Yili, Heihe, Shule, and Shiyang Rivers were collected and analyzed for major ion concentrations. The hydrochemical characteristics of these inland rivers pronouncedly distinguish them from large exorheic rivers (e.g., the Yangtze River and the Yellow River), as reflected in very high total dissolution solids (TDS) values. TDS was 115–4345mgl−1 with an average of 732mgl−1, which is an order of magnitude higher than the mean value for world rivers (65mgl−1). The Cheerchen River, Niya River, Keliya River and the terminal lakes of the Tarim River and the Heihe River have TDS values higher than 1gl−1, indicating saline water that cannot be directly consumed. Therefore, the problem of sufficient and safe drinking water has become increasingly prominent in the northwestern China arid zone. According to an inversion model, the contribution from evaporite dissolution to the dissolved loads in these rivers is 12.5%–99% with an average of 54%. The calculated silicate and carbonate weathering rates are 0.02–4.62tkm−2y−1 and 0.01–11.7tkm−2y−1 for these rivers. To reduce the influence of lithology, only the silicate weathering rates in different parts of the Tibetan Plateau are compared. A rough variation tendency can be seen in the rates: northern regional (0.15–1.73tkm−2y−1)<northeastern regional (0.74–4.62)≈western regional (1.75)<eastern regional (0.18–16.4)≈southeastern regional (3.5–10.6)<southern regional (13.5–38.0). The weathering rates did not show a noticeable correlation with a single influencing factor, such as temperature, elevation, vegetation, and physical erosion rates. Rainfall and runoff, however, seems to have a positive correlation with silicate weathering rates.

Chemical weathering and CO2 consumption of a high-erosion-rate karstic river: a case study of the Sanchahe River, southwest China

The Sanchahe River in southwest China is a tributary of the Wujiang River and experiences high erosion rates. Geochemical analysis was conducted on Sanchahe River basin samples collected in the wet and dry seasons of 2014 in order to better understand local chemical weathering processes, anthropogenic influences, and associated CO2 consumption. The samples’ total dissolved solid concentrations were found to be significantly higher than that of the global river average. Ca2+ was the dominant cation in the samples and accounted for 64 % and 73 % of the total cations in the dry and wet seasons, respectively. HCO3 − and SO4 2− were the dominant anions, accounting for 92 % of the total anions. Stoichiometry analyses of the river waters suggested that the water chemistry is controlled by carbonate dissolution by both carbonic and sulfuric acid. The chemical weathering rates of carbonate and silicate evaporites in the Sanchahe River basin were estimated to be approximately 109.2 and 11.0 t/(km2 a), respectively, much higher than both the global mean values and the Wujiang River, a typical karstic river. The CO2 consumption by carbonate and silicate weathering are estimated to be 597.4 × 103 and 325.5 × 103 mol/(km2 a), which are much higher than corresponding values in the Wujiang River, indicating a high erosion rate in the Sanchahe River basin.

Hydrochemistry of the middle and upper reaches of the Yarlung Tsangpo River system: weathering processes and CO2 consumption

The study focuses on the chemical compositions of the river waters in the middle and upper reaches of the Yarlung Tsangpo river system. Samples were collected in two periods along the river system to analyze the spatio-temporal variation characteristics and to determine the weathering processes and CO2 consumption. The results show that the chemical facies of river waters are dominated by Ca-HCO3 type and the TDS values cover a wide range of 98.2–619.8 mg/l with an average value of 268.6 mg/l, which is higher than that of the global river waters. There are three major reservoirs (carbonates, evaporites, and silicates) contributing to the major ions, and the predominances of the reservoirs of different rivers show spatial heterogeneity. The chemical weathering rates of silicates and carbonates are 3.43 and 4.49 t/km2/year, respectively. Overall, the middle and upper reaches of Yarlung Tsangpo are responsible for 0.3 and 0.16 % of global atmospheric CO2 consumption by silicates (1.7 × 105 mol/km2/year) and carbonates (1.27 × 105 mol/km2/year), respectively. In addition, the chemical weathering fluxes of the catchments consume atmospheric CO2 of 0.5456 Tg C/year, accounting for 0.14 % of the total organic carbon flux to the oceans by rivers globally.