Carbonate Weathering Rates Research Articles

Chemical weathering and CO2 consumption rates of the Koshi River Basin: modelling and quantifying

The Koshi River is a typical river originating from the Tibetan Plateau that drains into the Indian Plain. To quantify the chemical weathering and CO2 consumption rates of the upstream alpine meltwater-supplied mountainous and downstream rainfall-impacted temperate plain regions, 46 river water samples were collected at 23 sites during monsoon and post-monsoon seasons, and major ions concentrations were measured in the laboratory. The forward model was applied to calculate carbonate weathering rate (CWR), silicate weathering rate (SWR), and their CO2 consumption rates ([CO2]car and [CO2]sil). For investigating the hydrological impacts, correlation analysis was applied between the chemical weathering rates and different runoff components based on the Spatial Process in Hydrology model at each sampling site. Results show that carbonate and silicate rock weathering control the river water chemistry. The average silicate weathering rate is 7.37 tons/km2/yr, consuming 3.40 × 105 mol/km2/yr CO2 while considering sulfide-involved carbonate weathering, the carbonate weathering rate is 26.35 tons/km2/yr, consuming 3.82 × 105 mol/km2/yr CO2. Overall, chemical weathering and CO2 consumption rates are enhanced in monsoon season in both the upstream and downstream regions owing to the intense hydrological cycle. These rates are relatively higher in the alpine upstream region during the post-monsoon season due to intense physical erosion, and higher in downstream during the monsoon season due to the greater volume of water. The significant positive correlations between snowmelt runoff with CWR and [CO2]car indicated that the rising precipitation and temperature leading to enhanced snowmelt would increase the carbonate weathering rate. Hence, this study provides a valuable understanding of the spatiotemporal variations in river hydrochemistry and the role of hydrology in shaping chemical budget within the snow and glacier-fed Himalaya River Basins.

Response of weathering carbon sink effect to anthropogenic sulfuric acid in different lithological catchments: A case study from Southwest China

Accurately quantifying the carbon sink effect resulting from chemical weathering caused by anthropogenic H2SO4 is imperative to improve the assessment of the global carbon budget. Nevertheless, there is still a lack of precise understanding regarding the impact of anthropogenic H2SO4 on CO2 consumption during chemical weathering. Here, spring water samples were collected monthly from three catchments with distinct bedrock lithologies affected by severe acid precipitation in Southwest China for analyses of hydrogeochemistry and δ13CDIC to quantitatively estimate the effect of anthropogenic H2SO4 on the weathering carbon sink budget. The results show that carbonates contribute 97.4 %, 95.0 % and 88.8 % of the total cationic load using a straightforward method in the Beidiping carbonate catchment, as well as in the Shegengyan and Bianyan silicate catchments, respectively. The [Ca2++Mg2+]/[HCO3−] (0.98–1.19) and [Ca2++Mg2+]/[HCO3−+SO42−] (approximately 1) equivalent ratios, and δ13CDIC values (−16.8 to −8.0 ‰) of the samples suggest that besides H2CO3, H2SO4 is involved in carbonate weathering. The stoichiometry of the chemical compositions of spring water indicates that the presence of H2SO4 enhances carbonate weathering rates by 14.8 %, 8.1 % and 7.5 % while decreasing the CO2 consumption by 8.2 %, 4.3 % and 4.0 % in Beidiping, Shegengyan and Bianyan, respectively. Thus the reduced proportion of karst carbon sink in the carbonate catchment is approximately 2 times that in the silicate catchment, suggesting that carbonate weathering in the karst catchment is more sensitive to acid precipitation. The impact of acid precipitation on rock weathering in the silicate catchment is constrained by the soil buffering effect. Our study highlights the important role of anthropogenic H2SO4 in carbonate weathering, which should be critically evaluated in regional and global carbon cycles in future studies.

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.

Erosional modulation of the balance between alkalinity and acid generation from rock weathering

The balance between alkalinity generation by carbonate and silicate weathering and sulfuric acid generation by sulfide weathering controls the effect of weathering on atmospheric pCO2 over geologic timescales. How this balance varies across environmental gradients remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) Headwater Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (δ34SSO4, δ18OSO4, and δ18OH2O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, evaporites, and sulfides to the total weathering budget. Across the TRHR, riverine SO42– is derived mainly from a mixture of an evaporite source with uniform values of δ34SSO4 and δ18OSO4, and a sulfide source that contributes highly variable values of δ34S (−12.2 ‰ to +4.1 ‰) and δ18O (−17.7 ‰ to −1.6 ‰). Contributions of sulfide oxidation to riverine SO42– vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO2 over timescales shorter than marine carbonate burial (∼104 years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (∼107 years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric pCO2, are largely dependent on lithology. However, within comparable lithologic settings, sulfide and carbonate weathering rates rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO2 with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term carbon cycle during mountain building.

Chemical Weathering and CO2 Consumption Inferred from Riverine Water Chemistry in the Xi River Drainage, South China.

Hydrochemistry and strontium isotope data were analysed in water samples from the Xi River Drainage system to reveal the spatial and seasonal variations in chemical weathering, associated CO2 consumption fluxes, and their control factors. The main ions were Ca2+, Mg2+, and HCO3-, which are characteristic of a drainage system on carbonate-dominated bedrock. The dissolved loads were derived from four major end-member reservoirs: silicate, limestone, dolomite, and atmosphere. The silicate weathering rates (SWRs) increased downstream from 0.03 t/km2/year to 2.37 t/km2/year. The carbonate weathering rates (CWRs) increased from 2.14 t/km2/year in the upper reaches, to 32.65 t/km2/year in the middle reaches, and then decreased to 23.20 t/km2/year in the lower reaches. The SWR values were 281.38 and 113.65 kg/km2/month during the high- and low-water periods, respectively. The CWR values were 2456.72 and 1409.32 kg/km2/month, respectively. The limestone weathering rates were 2042.74 and 1222.38 kg/km2/month, respectively. The dolomite weathering rates were 413.98 and 186.94 kg/km2/month, respectively. Spatial and seasonal variations in chemical weathering were controlled mainly by lithology, vegetation, and climate (temperature, water discharge, and precipitation). The CO2 consumption flux by chemical weathering was estimated at 189.79 × 109 mol/year, with 156.37 × 109 and 33.42 × 109 mol/year for carbonate and silicate weathering, respectively. The CO2 fluxes by chemical weathering are substantially influenced by sulfuric acid in the system. The CO2 flux produced by sulfuric acid weathering was estimated at 30.00 × 109 mol/year in the basin. Therefore, the Xi River Basin is a CO2 sink with a net consumption of CO2 flux of 3.42 × 109 mol/year.

Understanding the hydrochemical functioning of glacierized catchments of the Upper Indus Basin in Ladakh, Indian Himalayas.

Recent studies have endorsed that surface water chemical composition in the Himalayas is impacted by climate change-induced accelerated melting of glaciers. Chemical weathering dynamics in the Ladakh region is poorly understood, due to unavailability of in situ dataset. The aim of the present study is to investigate how the two distinct catchments (Lato and Stok) drive the meltwater chemistry of the Indus River and its tributary, in the Western Himalayas. Water samples were collected from two glaciated catchments (Lato and Stok), Chabe Nama (tributary) and the Indus River in Ladakh. The mildly alkaline pH (range 7.3-8.5) and fluctuating ionic trend of the meltwater samples reflected the distinct geology and weathering patterns of the Upper Indus Basin (UIB). Gibbs plot and mixing diagram revealed rock weathering outweighed evaporation and precipitation. The strong associations between Ca2+-HCO3-, Mg2+-HCO3-, Ca2+-Mg2+, Na+-HCO3-, and Mg2+-Na+ demonstrated carbonate rock weathering contributed to the major ion influx. Principal component analysis (PCA) marked carbonate and silicates as the most abundant minerals respectively. Chemical weathering patterns were predominantly controlled by percentage of glacierized area and basin runoff. Thus, Lato with the larger glacierized area (~ 25%) and higher runoff contributed low TDS, HCO3-, Ca2+, and Na+ and exhibited higher chemical weathering, whereas lower chemical weathering was evinced at Stok with the smaller glacierized area (~ 5%). In contrast, the carbonate weathering rate (CWR) of larger glacierized catchments (Lato) exhibits higher average value of 15.7 t/km2/year as compared to smaller glacierized catchment (Stok) with lower average value 6.69 t/km2/year. However, CWR is high in both the catchments compared to silicate weathering rate (SWR). For the first time, in situ datasets for stream water chemical characteristics have been generated for Lato and Stok glaciated catchments in Ladakh, to facilitate healthy ecosystems and livelihoods in the UIB.

Chemical weathering characteristics and controls in the Yarlung Tsangpo River Basin: Evidence from hydrochemical composition

The uplift of the Tibetan Plateau (TP) is thought to be closely related to the global climate. For the Yarlung Tsangpo River (YTR), the largest river on the TP, the hot spring input is an important source of solutes. Without considering hot spring input will lead to an overestimation of chemical weathering and CO2 consumption rate. Two sample series were collected in the basin before and after the monsoon season to evaluate the chemical weathering rates and controlling factors based on the hydrochemical composition. The results of improved forward model indicate that the highest contribution to the total cations comes from rock weathering (59%–99%, average value 93%), of which carbonate dissolution contributes the most (25%–79%, average value 61%), followed by evaporite dissolution (5%–42%, average value 19%) and silicate weathering (1%–29%, average value 13%). The hydrochemistry of tributaries in the middle reaches (Dogxung Tsangpo and Lhasa River) is affected by hot springs significantly, and the maximum contribution reaches 40%. The chemical weathering rates for the sub-basins show a sequence of Lhasa River > Dogxung Tsangpo > Nyang Qu, primarily controlled by lithology and runoff. For the mainstream above the YTR canyon and tributaries in the upper and middle reaches, chemical weathering is controlled by a transport limited regime, and the rest of the basin is governed by a kinetically limited regime. Overall, after deducting the hot spring input from the solutes, the silicate and carbonate weathering rates in the YTR Basin are 1.8 t/km2/yr and 6.3 t/km2/yr, respectively. The atmospheric CO2 consumption by silicate and carbonate weathering in the YTR Basin accounts for ∼0.17% and ∼0.28% of global, respectively, on ∼0.21% of the global land surface area. The carbon sink caused by chemical weathering and its effect on global carbon cycle in the YTR Basin are insignificant.

Seasonal Variations of Chemical Weathering and CO2 Consumption Processes in the Headwater (Datong River Basin) of the Yellow River Draining the Tibetan Plateau

The Yellow River basin covers contrasting tectonics, climate, and vegetation settings. To explore the seasonal chemical weathering differences from the upstream to downstream of the Yellow River basin, we collected weekly river waters from the Datong River draining the Tibetan Plateau in 2017. Our results show remarkably seasonal variations of major ions. A forward model was employed to quantify the contribution of silicate, carbonate, and sulfide oxidation/evaporite and atmospheric input to the cations, which yielded the contributions of 9.21 ± 1.57%, 46.07 ± 1.4%, 21.46 ± 1.03%, and 23.26 ± 1.72%, respectively, indicating a dominated carbonate weathering to the river chemistry. The significant correlation between the carbonate weathering rate and runoff suggests a critical runoff control on chemical weathering in the Datong River catchment. A comprehensive comparison between the upper and middle-lower reaches of the Yellow River basin shows a declined silicate weathering and CO2 consumption rate (ØCO2sil) from the upstream to downstream. In contrast, the physical erosion rate shows an increased trend, with the most prominent increase in the midstream Loess Plateau. A further comparison between the Yellow River draining the Tibetan Plateau and the Loess Plateau shows 4.5 times higher ØCO2sil but 9.5 times lower erosion rate. In conclusion, we propose that the runoff, rather than erosion, plays a central role on chemical weathering in the Yellow River basin, which provides insight for in-depth understanding the surficial weathering and the global carbon cycle.

The effects of hydrological variations on chemical weathering: Evidences from temporal water chemistry, stable carbon and sulfur isotopes

The negative feedbacks between chemical weathering and climate change were hypothesized to regulate the atmospheric CO2 over short-term and geologic timescales. In this study, we collected temporal water samples from the Jialing River, and analyzed water chemistry, stable isotopic compositions of dissolved inorganic carbon (δ13CDIC) and sulfur isotope of sulfate (δ34SSO4) to investigate the effects of hydrological changes on chemical weathering. The results of the inverse model based on the Monte Carlo method showed that major cations were mainly derived from carbonate (63.9%) and silicate (12.0%) weathering, and sulfuric acid plays an important role in carbon biogeochemical processes. Based on water chemistry, δ34SSO4 and δ13CDIC, the carbon biogeochemical processes were investigated, showing the influence of secondary processes. We estimated the weathering rates of carbonate (19.7 t/km2yr) and silicate (3.9 t/km2yr) in the Jialing River, and then estimated CO2 consumption flux by carbonate weathering (24.5 t/km2yr), silicate weathering (8.1 t/km2yr), and total CO2 consumption flux (27.5 t/km2yr) deducting the influence of pyrite oxidation. Our research showed the significant role of pyrite oxidation in carbon cycling. This study highlighted the impacts of hydrological variabilities on the generation and transport of solutes, which have significant implications for understanding carbon cycling under ongoing climate change.

Geochemistry of the Dissolved Load of the Ramganga River, Ganga Basin, India: Anthropogenic Impacts and Chemical Weathering

The Ramganga basin is an important sub-catchment of the Ganga River to study the wide-scale effects of human-induced changes on geochemical processes. The basin inhabits pristine locations in the upstream and dense human establishments in the floodplain region. Furthermore, the entrapment of upstream sediments in the Kalagarh Dam aids in creating different geochemical regimes. To reveal the geochemical heterogeneity over the multi-spatial and temporal scale, controlling factors (natural and anthropogenic), and source end-members, dissolved load samples were collected during the pre-monsoon, monsoon, and post-monsoon season of the year 2014. Major cations and anions data were analyzed using principal component analysis and mass-balancing equations-based forward modeling to quantify the contribution from the atmosphere, rock weathering, and anthropogenic sources. The results show that chemical weathering predominates the dilution effect during the pre- and post-monsoon season. A high level of pollution prevails during the non-monsoon season and particularly in floodplain tributaries. Anthropogenic sources contribute up to 42% of the dissolved load composition, whereas silicate and carbonate weathering predominantly contributes 93 and 82% of the dissolved load. Further, the silicate weathering rate (4.9 t km−2 y−1) is higher than the carbonate weathering rate and efficiently uptakes an average of 3.5 × 105 mol km−2 y−1 of CO2. The findings revealed the extent of geochemical heterogeneity and controlling factors influencing the element flux, weathering rates, and chemical transportation over multi-spatial and temporal scales.

Chemical denudation in a small mountainous coastal river in the tropics: Insights from Kali River, Southwestern India

A study was conducted for the first time to understand the geochemical processes controlling the chemistry of the Kali River in tropical southwestern India and to determine its chemical denudation and net carbon dioxide consumption. The samples were collected from the source to the mouth of the river (15 stations) during monsoon (July 2018), post-monsoon (December 2018), and pre-monsoon (May 2019) seasons. The catchment experiences intense chemical weathering on account of heavy rainfall accompanied by runoff during the summer monsoon. Seasonal variations in silicate weathering processes are significantly controlled by runoff, and their rates are proportional to the discharge. The annual chemical denudation rate (CDR) calculated exclusively from the upstream catchment that are dominated by silicate rocks estimated to be 48.2 tons/km2/yr with a silicate weathering rate (SWR) of 41.3 tons/km2/yr and carbonate weathering rate (CWR) of 6.9 tons/km2/yr. The CDR is two times higher than the global mean average rate. The mean CO2 consumption rate (CCR) for silicate weathering is 2.9 × 105 mol. km−2 y−1, which is three-fold higher than the global average. Silicate rock weathering intensity (Re) values indicate the formation of the gibbsite. The Re does not show variations from upstream to downstream, implying the rapid transport of weathered material from the river catchment area. Intense rain, runoff, and temperature are the dominant climatic factors that accelerate weathering in the study area. This underscores the significance of small mountainous coastal rivers as drivers of intense chemical weathering in humid tropical environments, which removes the atmospheric CO2. This study adds to the existing database on chemical weathering and associated fluxes in granitic catchments across the globe.

Carbonate weathering dominates magnesium isotopes in large rivers: Clues from the Yangtze River

Carbonate weathering regulates the short-term carbon (C) cycle and global climate due to its fast response to hydrological processes. The carbonate weathering flux needs to be well constrained to better understand the climate change at short time scale. Riverine magnesium (Mg) isotopes are sensitive to primary mineral dissolution and so have great potential to trace carbonate weathering. Global large rivers draining continental crust dominate weathering flux to the oceans, but how riverine Mg isotopes respond to carbonate weathering remains unclear. The Yangtze River drainage basin (YRDB) was selected to test the robustness of riverine Mg isotopes (δ26Mg) in tracing continental carbonate weathering because it spans a wide range in lithology, geomorphology and climate. The riverine δ26Mg values within the YRDB show a decreasing trend from the headwater to the mainstream ranging from −1.36‰ to −0.59‰. The dissolved δ26Mg have strong negative correlations with carbonate weathering rate and intensity within the YRDB, indicating a sensitive response of riverine δ26Mg to the carbonate weathering flux. In a compilation of Mg fluxes and δ26Mg in the world's largest rivers, there is similar dominance of carbonate weathering on riverine Mg fluxes and isotopes. Therefore, we propose that riverine δ26Mg in large rivers are a robust tracer of carbonate weathering intensity. Intensifying carbonate weathering under global warming tends to increase riverine Mg and C fluxes to the oceans and thus the atmospheric CO2 sink at the millennial time scale.

Small-catchment perspective on chemical weathering and its controlling factors in the Nam Co basin, central Tibetan Plateau

Very little data is available on chemical weathering and its controlling factors in small catchments of the central Tibetan Plateau (TP) despite being the main part of the plateau. Hydrochemistry of river water in 24 small catchments in the Nam Co basin draining different strata and recharge forms was investigated monthly during the 2018 monsoon season. The results show that carbonate dissolution is the most important geochemical process controlling the evolution of hydrochemistry of most river water in the Nam Co basin (the average contribution to riverine solutes reaches 46%) even though its contribution shows significant spatial distribution: low values occur in some granite regions (e.g. 27% in H10) but is high in catchments where carbonate rocks are prevalent (e.g. 68% in H23). The contribution of silicate weathering is low, with an average of 11% in catchments located on the east and south banks draining sandstone/mudstone, while this value increases to 32% in catchments on the south bank which pass through granite, and on the west bank. The contribution of sulfide oxidation in most rivers on the south bank is unexpectedly high (40%) but is much lower in other rivers (13%). This spatial difference is attributed to the influence of topography, particularly slope. The carbonate weathering rate (CWR) in small catchments of the Nam Co basin shows spatial heterogeneity and is significantly and positively correlated with the lithologic coefficient which proposed to indicate the degree of rock weatherability and negatively correlated with mean slope and mean altitude. The spatial distribution of the silicate weathering rate (SWR) is not as significant as the CWR and is mainly governed by runoff depth. Chemical weathering rates in the glacierised catchments are 1.3 times higher than those in the non-glacierised regions which share similar lithology. Thus, glaciation enhances chemical weathering in the study area. The area-weighted average value of CWR and SWR in the study area is 23.2 t/km2/y and 5.2 t/km2/y, respectively. Chemical weathering rates in the Nam Co basin, central TP are relatively low when compared with those in catchments located on the edge of the TP, indicating the carbon sequestration effect caused by this supergene geological process in this area may be insignificant.

Role of anthropogenic sulfuric and nitric acids in carbonate weathering and associated carbon sink budget in a karst catchment (Guohua), southwestern China

Accurate estimate of carbon sink flux resulted from carbonate weathering in the karst area is of great significance for advancing the current understanding of the global carbon cycle and climate change. However, the carbon sink flux may be overestimated when the natural reaction with carbonic acid is influenced by protons of anthropogenic sulfuric and nitric acids, which can reduce the carbon sink flux during carbonate weathering. Here, we quantitatively evaluated the impact of anthropogenic sulfuric and nitric acids on carbonate weathering and carbon sink flux under base flow condition based on the stoichiometry of chemical compositions of groundwater from a typical karst catchment (Guohua), Guangxi, southwestern China. Seventy groundwater samples were analyzed for the characteristics of hydrochemistry and carbon isotopes of dissolved inorganic carbon (δ13CDIC) during the dry season in 2015. The results show that: (1) Ca2+ and Mg2+ dominate 92.4%–99.7% of the total cations, while HCO3− accounts for 83.0%–97.4% of total anions, indicating that the compositions of groundwater are primarily controlled by carbonate weathering; (2) The [Ca2++ Mg2+]/[HCO3−] equivalent ratios (1.00 to 1.41) and the corresponding δ13CDIC values (−16.8‰ to −8.1‰) of groundwater are all distributed between carbonate weathering by carbonic acid, and carbonate weathering by sulfuric and nitric acids. Meanwhile, the [Ca2++ Mg2+]/[HCO3−+ SO42−+ NO3−] ratios of groundwater are about 1, and the [Ca2++ Mg2+] equivalent concentrations have a good positive correlation with [HCO3−+ SO42−+ NO3−] and [SO42−+ NO3−], suggesting that sulfuric and nitric acids, in addition to carbonic acid, have been involved in carbonate weathering; (3) The contribution of sulfuric and nitric acids involved in carbonate weathering to (Ca2++ Mg2+) and HCO3− in groundwater varies from 0.6% to 58.0% (mean value of 20.9%) and from 0.3% to 40.9% (mean value of 12.2%), respectively. The carbonate weathering rate has increased by 20.9% while the karst carbon sink flux has decreased by 12.2%. Therefore, the carbon sink flux produced by carbonate weathering should be carefully evaluated when anthropogenic sulfuric acid and/or nitric acid are involved and the role of sulfuric and nitric acids in carbonate weathering could not be ignored in the global carbon cycle.

Co-variation of silicate, carbonate and sulfide weathering drives CO2 release with erosion

Global climate is thought to be modulated by the supply of minerals to Earth’s surface. Whereas silicate weathering removes carbon dioxide (CO2) from the atmosphere, weathering of accessory carbonate and sulfide minerals is a geologically relevant source of CO2. Although these weathering pathways commonly operate side by side, we lack quantitative constraints on their co-variation across erosion rate gradients. Here we use stream-water chemistry across an erosion rate gradient of three orders of magnitude in shales and sandstones of southern Taiwan, and find that sulfide and carbonate weathering rates rise with increasing erosion, while silicate weathering rates remain steady. As a result, on timescales shorter than marine sulfide compensation (approximately 106–107 years), weathering in rapidly eroding terrain leads to net CO2 emission rates that are about twice as fast as CO2 sequestration rates in slow-eroding terrain. We propose that these weathering reactions are linked and that sulfuric acid generated from sulfide oxidation boosts carbonate solubility, whereas silicate weathering kinetics remain unaffected, possibly due to efficient buffering of the pH. We expect that these patterns are broadly applicable to many Cenozoic mountain ranges that expose marine metasediments.

Glaciation enhanced chemical weathering in a cold glacial catchment, western Nyaingêntanglha Mountains, central Tibetan Plateau

The uplift of the Tibetan Plateau (TP) not only may enhance physical denudation and chemical weathering but also makes this area contain the largest glacier storage outside the polar regions. There is an argument on whether glaciation favours chemical weathering and carbon sequestration. To explore chemical weathering processes and intensities in the cold glacial region of the central TP, hydrochemistry of the river water in two catchments, i.e. the Qugaqiong and Qugaqie, was monitored from April to September 2018 with high-frequency sampling. These two catchments share similar geographical features except there are some cold glaciers in the upper reaches of the Qugaqie. The results of chemical composition show that the river water of the Qugaqie has a lower proportion of Ca2+ and HCO3− but higher SO42− and K++Na+ compared with the Qugaqiong. The seasonal variation of ionic concentration in the Qugaqie is significant but minor in the Qugaqiong. Those differences highlight the glacial impact in the Qugaqie compared with the non-glacierized Qugaqiong. Solutes in the glacial runoff of the Qugaqie mainly derive from sulfide oxidation. Its contribution to cations in the runoff of the Zhadang glacier and the Hanging glacier reaches 39% and 36%. The second most important contribution is carbonate dissolution (32% for both glacial runoffs). Silicate weathering provides fewer solutes and its contribution is only 20% and 11% in the two glacial runoffs, respectively. River water of the Qugaqiong shares similar hydrochemical characteristics with the non-glacial tributaries draining granodiorite/biotite adamellite in the Qugaqie. The contribution of sulfide oxidation to solutes in those rivers decreases to 10% while carbonate dissolution increases to 50%. Glacial comminution exposed trace sulfide minerals, thus made sulfide oxidation became pretty active due to this reaction is much faster than other mineral dissolution. The carbonate weathering rate (CWR) and silicate weathering rate (SWR) in the Qugaqie catchment are 9.7 t/km2/y and 6.7 t/km2/y. In comparison, values in the Qugaqiong catchment are 6.9 t/km2/y and 3.8 t/km2/y, respectively, which means CWR and SWR is 1.4 and 1.8 times higher in the Qugaqie catchment, respectively. Higher runoff and physical erosion rates are considered as the main reasons for higher chemical denudation rates in the Qugaqie catchment. The drainage system of the glaciers is dominated by the distributed pattern, which caused longer residences time of water, thus further promoted chemical weathering in the glacierized region. Our observation supports the idea that glaciation in the glacierized catchment of the central TP enhances chemical weathering, especially for SWR. Chemical denudation rates in the Qugaqie catchment remain at a moderate level compared with other glacierized catchments in the world and lithology and runoff are important factors to cause this spatial heterogeneity.

Chemical weathering in the upstream and midstream reaches of the Yarlung Tsangpo basin, southern Tibetan Plateau

The Tibetan Plateau (TP) is a focus of chemical weathering research worldwide. However, scant studies on the northern flank of the Himalayas and the vast inland region of the TP hinders understanding of the role the TP has played in the global carbon cycle. The upstream and midstream of the Yarlung Tsangpo (UMRYT) is the longest river in Tibet with the highest elevation in the world. Two sampling campaigns were conducted in December 2012 and August 2013 to estimate the intensity of chemical weathering, CO2 consumption rates, and their controlling factors in the UMRYT basin. Results show that the total dissolved cations and anions have a broad range of changes with an average of 199 mg/L (46–637 mg/L) in December and 196 mg/L (43–523 mg/L) in August. This parameter displays small seasonal variations for most sampling sites; these are just slightly diluted in August even though runoff is 6–12 times higher than that in December. The evidence from TZ+⁎/HCO3−⁎ (⁎indicates after deduction from atmospheric inputs) demonstrates the proportion of rock weathering caused by H2SO4 is surprisingly high in the sub-basins to the extent that it is generally above 50% in both seasons. Dogxung Tsangpo, Nianchu River, and the small tributaries at the southern bank are severely affected by rock weathering involving sulfuric acid (above 90%). The chemical budget shows that the average proportion of TZ+ in river water derives from atmospheric inputs, weathering of silicate, carbonate, and evaporite are 9%, 19%, 50%, and 22%, respectively. The contribution of carbonate dissolution to TZ+ is 8–16% higher during the monsoon season. In contrast, the proportions of silicate weathering and evaporite dissolution are 4–16% and 1–8% larger in the non-monsoon period, respectively. Chemical weathering intensity shows significant spatial differences. Carbonate weathering rates (CWR) and silicate weathering rates (SWR) in the main tributaries vary from 12.6 ± 4.8 to 20.0 ± 5.8 t/km2/y and from 2.9 to 5.6 t/km2/y, respectively. Lithology exerts a profound influence on the spatial heterogeneity of CWR in basins of the main tributaries of the UMRYT, while the spatial distribution of SWR is mainly governed by runoff. The CWR and SWR in the whole basin are 13.2 ± 4.0 t/km2/y and 3.9 t/km2/y, respectively, indicating the chemical weathering intensity and its CO2 sequestration are much lower than most large rivers draining the Tibetan Plateau and other orogenic belts around the world.

Chemical weathering and CO2 consumption rates of rocks in the Bishuiyan subterranean basin of Guangxi, China

To investigate the influence of chemical weathering on CO2 consumption, an analysis was performed of water chemistry by applying water chemistry equilibria methods in the Bishuiyan subterranean basin, SW China. The average value of total ion concentrations (TZ+) was 1,854.97 μEq/L, which was significantly higher than the global average value (TZ+ = 1,250 μEq/L). Ca2+ and HCO3− were the main ionic constituents in the waters. SO42− and NO3− concentrations were relatively higher than other anion concentrations, and Cl− concentrations were consistently the lowest. Dissolved load balance models result showed that carbonate weathering, silicate weathering, and atmospheric input were the primary ionic contributors, wherein the effects of carbonate weathering > silicate weathering > atmospheric input for the whole catchment, with the exception of Taiping, where silicate weathering was prominent over carbonate weathering. In addition, these analyses indicated that the erosion via rock weathering was also affected by atmospherically derived CO2 and allogenic acids. The estimated yield by quantitative calculation for the carbonate weathering rate was 59.7 t/(km2 year), which was 4.40 times higher than that of silicate weathering rate. Further, the carbonate and silicate weathering components of the carbon sink accounted for 71.2% and 28.8%, respectively, of the total basin rock weathering carbon sink.

Chemical weathering and atmospheric carbon dioxide (CO2) consumption in Shanmuganadhi, South India: evidences from groundwater geochemistry.

Chemical weathering in a groundwater basin is a key to understanding global climate change for a long-term scale due to its association with carbon sequestration. The present study aims to characterize and to quantify silicate weathering rate (SWR), carbon dioxide consumption rate and carbonate weathering rate (CWR) in hard rock terrain aided by major ion chemistry. The proposed study area Shanmuganadhi is marked with superior rainfall, oscillating temperature and runoff with litho-units encompassing charnockite and hornblende-biotite gneiss. Groundwater samples (n = 60) were collected from diverse locations and analysed for major chemical constituents. Groundwater geochemistry seems to be influenced by geochemical reactions combining dissolution and precipitation of solids, cation exchange and adsorption along with minor contribution from anthropogenic activities. The SWR calculated for charnockite and hornblende-biotite gneiss was 3.07tonskm-2year-1 and 5.12tonskm-2year-1, respectively. The calculated CWR of charnockite and hornblende-biotite gneiss was 0.079tonskm-2year-1 and 0.74tonskm-2year-1, respectively. The calculated CO2 consumption rates via silicate weathering were 1.4 × 103molkm-2year-1 for charnockite and 5.8 × 103molkm-2year-1 for hornblende-biotite gneiss. Lithology, climate and relief were the key factors isolated to control weathering and CO2 consumption rates.

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.