The fluvial geochemistry, contributions of silicate, carbonate and saline-alkaline components to chemical weathering flux and controlling parameters: Narmada River (Deccan Traps), India

The Narmada River in India is the largest west-flowing river into the Arabian Sea, draining through the Deccan Traps, one of the largest flood basalt provinces in the world. The fluvial geochemical characteristics and chemical weathering rates (CWR) for the mainstream and its major tributaries were determined using a composite dataset, which includes four phases of seasonal field (spot) samples (during 2003 and 2004) and a decade-long (1990-2000) fortnight time series (multiannual) data. Here, we demonstrate the influence of minor lithologies (carbonates and saline-alkaline soils) on basaltic signature, as reflected in sudden increases of Ca²⁺-Mg²⁺ and Na⁺ contents at many locations along the mainstream and in tributaries. Both spot and multiannual data corrected for non-geological contributions were used to calculate the CWR. The CWR for spot samples (CWR_spot) vary between 25 and 63 ton km⁻² year⁻¹, showing a reasonable correspondence with the CWR estimated for multiannual data (CWR_multi) at most study locations. The weathering rates of silicate (SilWR), carbonate (CarbWR) and evaporite (Sal-AlkWR) have contributed ∼38-58, 28-45 and 8-23%, respectively to the CWR_spot at different locations. The estimated SilWR (11-36 ton km⁻² year⁻¹) for the Narmada basin indicates that the previous studies on the North Deccan Rivers (Narmada-Tapti-Godavari) overestimated the silicate weathering rates and associated CO₂ consumption rates. The average annual CO₂ drawdown via silicate weathering calculated for the Narmada basin is ∼0.032 × 10¹² moles year⁻¹, suggesting that chemical weathering of the entire Deccan Trap basalts consumes approximately 2% (∼0.24 × 10¹² moles) of the annual global CO₂ drawdown. The present study also evaluates the influence of meteorological parameters (runoff and temperature) and physical weathering rates (PWR) in controlling the CWR at annual scale across the basin. The CWR and the SilWR show significant correlation with runoff and PWR. On the basis of observed wide temporal variations in the CWR and their close association with runoff, temperature and physical erosion, we propose that the CWR in the Narmada basin strongly depend on meteorological variability. At most locations, the total denudation rates (TDR) are dominated by physical erosion, whereas chemical weathering constitutes only a small part (<10%). Thus, the CWR to PWR ratio for the Narmada basin can be compared with high relief small river watersheds of Taiwan and New Zealand (1-5%) and large Himalayan Rivers such as the Brahmaputra and the Ganges (8-9%).     

Dissolved rhenium in the Yamuna river system and the Ganga in the Himalaya: role of black shale weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere

Extensive measurements of dissolved Re and major ion abundances in the Yamuna River System (YRS), a major tributary of the Ganga, have been performed along its entire stretch in the Himalaya, from its source near the Yamunotri Glacier to its outflow at the foothills of the Himalaya at Saharanpur. In addition, Re analysis has been made in granites and Precambrian carbonates, some of the major lithologies of the drainage basin. These data, coupled with those available for black shales in the Lesser Himalaya, allow an assessment of these lithologies’ contributions to the Re budget of the YRS.The Re concentrations in the YRS range from 0.5 to 35.7 pM with a mean of 9.4 pM, a factor of ∼4 higher than that reported for its global average concentration in rivers. Dissolved Re and ΣCations∗ (= Na∗+K+Ca+Mg) are strongly correlated in the YRS, indicating that they are released to these waters in roughly the same proportion throughout their course. The Re/ΣCations∗ in most of these rivers are one to two orders of magnitude higher than the (Re/Na+K+Mg+Ca) measured in granites of the Yamuna basin. This leads to the conclusion that, on average, granites/crystallines make only minor contributions to the dissolved Re budget of the YRS on a basin-wide scale, though they may be important for rivers with low dissolved Re. Similarly, Precambrian carbonates of the Lesser Himalaya do not seem to be a major contributor to dissolved Re in these rivers, as their Re/(Ca+Mg) is much less than those in the rivers. The observation that Re concentrations in rivers flowing through black shales and in groundwaters percolating through phosphorite-black shale-carbonate layers in phosphorite mines are high, and that Re and SO₄ are significantly correlated in YRS, seems to suggest that the bulk of the dissolved Re is derived from black shale/carbonaceous sediments. Material balance considerations, based on average Re of 30 ng g⁻¹ in black shales from the Lesser Himalaya, require that its abundance in the drainage basin of the YRS needs to be a few percent to yield average Re of 9.4 pM. Furthermore, the positive correlation between Re and ΣCations∗ would require that these Re-rich sediments (e.g., black shales) and Re-poor lithologies (e.g., crystallines, Precambrian carbonates) contribute Re and cations in roughly the same proportion throughout the drainage basin. The available data on the abundance and distribution of black shales in the basin are not adequate to test if these requirements can be met.The annual fluxes of dissolved Re at the base of the Himalaya from the Yamuna are ∼150 mol at Batamandi and ∼100 mol at Saharanpur, compared to ∼120 mol from the Ganga at Rishikesh. The total flux from the Yamuna and the Ganga account for ∼0.4% of the global riverine Re flux, much higher than their contribution to global water discharge. This is also borne out from the mobilization rate of Re: ∼1 to 3 g km⁻² y⁻¹ in the Ganga and Yamuna basins in the Himalaya, compared to the global average of ∼0.1 g km⁻² y⁻¹.Black shale weathering can also significantly influence the budgets of Os and U in rivers and CO₂ in rivers and the atmosphere. Using dissolved Re in rivers as a proxy, it is estimated that ∼(6-9) × 10⁸ kg y⁻¹ of black shales are being weathered in the Ganga and Yamuna basins in the Himalaya. Weathering of such amounts of black shales can account for the reported concentrations of Os and U in these rivers. Furthermore, if the weathering results in the conversion of organic carbon in the black shales to CO₂, it would release ∼2 × 10⁵ mol of CO₂ km⁻² y⁻¹ in the Yamuna and Ganga basins in the Himalaya, comparable to the CO₂ consumption from silicate weathering.     

Chemical erosion in the eastern Himalaya: Major ion composition of the Brahmaputra and δC of dissolved inorganic carbon

Major ion composition of waters, δ¹³C of its DIC (dissolved inorganic carbon), and the clay mineral composition of bank sediments in the Brahmaputra River System (draining India and Bangladesh) have been measured to understand chemical weathering and erosion and the factors controlling these processes in the eastern Himalaya. The time-series samples, collected biweekly at Guwahati, from the Brahmaputra mainstream, were also analyzed for the major ion composition. Clay mineralogy and chemical index of alteration (CIA) of sediments suggest that weathering intensity is relatively poor in comparison to that in the Ganga basin. This is attributed to higher runoff and associated physical erosion occurring in the Brahmaputra basin. The results of this study show, for the first time, spatial and temporal variations in chemical and silicate erosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude. The Eastern Syntaxis basin dominates the erosion with a rate of ∼300 t km⁻² y⁻¹, one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains. In contrast, the flat, cold, and relatively more arid Tibetan basin undergoes much slower chemical erosion (∼40 t km⁻² y⁻¹). The abundance of total dissolved solids (TDS, 102-203 mg/L) in the time-series samples collected over a period of one year shows variations in accordance with the annual discharge, except one of them, cause for which is attributable to flash floods. Na* (Na corrected for cyclic component) shows a strong positive correlation with Si, indicating their common source: silicate weathering. Estimates of silicate cations (Na_sil+K_sil+Ca_sil+Mg_sil) suggest that about half of the dissolved cations in the Brahmaputra are derived from silicates, a proportion higher than that for the Ganga system. The CO₂ consumption rate due to silicate weathering in the Brahmaputra watershed is ∼6 × 10⁵ moles km⁻² y⁻¹; whereas that in the Eastern Syntaxis subbasin is ∼19 × 10⁵ moles km⁻² y⁻¹, similar to the estimates for some of the basaltic terrains. This study suggests that the Eastern Syntaxis basin of the Brahmaputra is one of most intensely chemically eroding regions of the globe; and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern Himalaya.     

Geochemical fluxes and weathering of volcanic terrains on high standing islands: Taranaki and Manawatu-Wanganui regions of New Zealand

Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J. D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol.100, 525-544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K⁺, and depleted in Ca²⁺ and Sr²⁺ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca²⁺ and Sr²⁺ depletions may reflect the relative absence of CaCO₃ in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate chemical weathering yields of 60-240 tons km⁻² a⁻¹. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93-480 tons km⁻² a⁻¹) and East Cape (62-400 tons km⁻² a⁻¹). Calculated silicate weathering yields of 12-33.6 tons km⁻² a⁻¹ and CO₂ consumption of 852-2390 × 10³ mol km⁻² a⁻¹ for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO₂ consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580-2450 × 10³ mol km⁻² a⁻¹) and Réunion Island(1300-4400 × 10³ mol km⁻² a⁻¹). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.     

Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes

Concentrations of major ions, Sr and ⁸⁷Sr/⁸⁶Sr have been measured in the Gomti, the Son and the Yamuna, tributaries of the Ganga draining its peninsular and plain sub-basins to determine their contribution to the water chemistry of the Ganga and silicate and carbonate erosion of the Ganga basin. The results show high concentrations of Na and Sr in the Gomti, the Yamuna and the Ganga (at Varanasi) with much of the Na in excess of Cl. The use of this ‘excess Na’ (Na∗ = Na_riv − Cl_riv) a common index of silicate weathering yield values of ∼18 tons km⁻² yr⁻¹ for silicate erosion rate (SER) in the Gomti and the Yamuna basins. There are however, indications that part of this Na∗ can be from saline/alkaline soils abundant in their basins, raising questions about its use as a proxy to determine SER of the Ganga plain. Independent estimation of SER based on dissolved Si as a proxy give an average value of ∼5 tons km⁻² yr⁻¹ for the peninsular and the plain drainages, several times lower than that derived using Na∗. The major source of uncertainty in this estimate is the potential removal of Si from rivers by biological and chemical processes. The Si based SER and CER (carbonate erosion rate) are also much lower than that in the Himalayan sub-basin of the Ganga. The lower relief, runoff and physical erosion in the peninsular and the plain basins relative to the Himalayan sub-basin and calcite precipitation in them all could be contributing to their lower erosion rates.Budget calculations show that the Yamuna, the Son and Gomti together account for ∼75% Na, 41% Mg and ∼53% Sr and ⁸⁷Sr of their supply to the Ganga from its major tributaries, with the Yamuna dominating the contribution. The results highlight the important role of the plain and peninsular sub-basins in determining the solute and Sr isotope budgets of the Ganga. The study also shows that the anthropogenic contribution accounts for ?10% of the major ion fluxes of the Ganga at Rajmahal during high river stages (October). The impact of both saline/alkaline soils and anthropogenic sources on the major ion abundances of the Ganga is minimum during its peak flow and therefore the SER and CO₂ consumption rates of the river is best determined during this period.     

Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and ⁸⁷Sr/⁸⁶Sr. Cationic fluxes from the volcanic basins are dominated by Ca²⁺ and Mg²⁺ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg²⁺. The areally weighted average CO₂ export flux from our study area is 3.89 ± 0.21 × 10⁶ mol/km²/yr, or 5.99 ± 0.64 × 10⁶ mol/km²/yr from ophiolites and 3.58 ± 0.23 × 10⁶ mol/km²/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO₂ export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO₂ export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.     

The role of sulfur in chemical weathering and atmospheric CO2 fluxes: Evidence from major ions, δCDIC, and δSSO4 in rivers of the Canadian Cordillera

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, K⁺, Na⁺), δ¹³C of dissolved inorganic carbon (δ¹³C_DIC), and δ³⁴S of dissolved sulfate (δ³⁴S_SO4) to quantify chemical weathering rates and exchanges of CO₂ between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ³⁴S_SO4 (−8.9 to 14.1‰_CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ¹³C_DIC values (−9.8 to −3.7‰ _VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰).δ³⁴S_SO4 is negatively correlated with δ¹³C_DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ¹³C_DIC of rivers. The negative correlation between δ³⁴S_SO4 and δ¹³C_DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas.Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10³ mol C · km⁻² · yr⁻¹) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10³ mol C · km⁻² · yr⁻¹). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO₂ to carbonate rocks, sulfide-carbonate weathering can liberate CO₂ from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO₂ drawdown by silicate weathering in the region.     

Chemical weathering in the Three Rivers region of Eastern Tibet

Three large rivers - the Chang Jiang (Yangtze), Mekong (Lancang Jiang) and Salween (Nu Jiang) - originate in eastern Tibet and run in close parallel over 300 km near the eastern Himalayan syntaxis. Seventy-four river water samples were collected mostly during the summer season from 1999 to 2004. Their major element compositions vary widely, with total dissolved solids (TDS) ranging from 31 to 3037 mg/l, reflecting the complex geologic makeup of the vast drainage basins. The major ion distribution of the main channel samples primarily reflects the weathering of carbonates. Evaporite dissolution prevails in the headwater samples of the Chang Jiang in the Tibetan Plateau interior, as evidenced by the high TDS (928 and 3037 mg/l) and the Na-Cl dominant major element composition. Local tributary samples of the Mekong and Salween, draining the Lincang Batholith and the Tengchong Volcano, show distinctive silicate weathering signatures. We used five reservoirs - rain, halite, sulfate, carbonate, and silicate - in a forward model to calculate the contribution from silicate weathering to the total dissolved load and to estimate the consumption rate of atmospheric CO₂ by silicate weathering. Carbonate weathering accounts for about 50% of the total cationic charge (TZ⁺) in the samples of the Mekong and the Salween exiting the Tibetan Plateau. In the “exit” sample of the Chang Jiang, 45% of TZ⁺ is from halite dissolution inherited from the extreme headwater tributaries in the interior of the plateau, and carbonates contribute only 26% to the TZ⁺. The net rate of CO₂ consumption by silicate weathering is (103-121) × 10³ mol km⁻² year⁻¹, lower than the rivers draining the Himalayan front. GIS-based analyses indicate that runoff and relief can explain 52% of the spread in the rate of atmospheric CO₂ drawdown by silicate weathering, but other climatic (temperature, precipitation, potential evapotranspiration) and geomorphic (elevation, slope) factors also show collinearity. Only qualitative conclusions can be drawn for the significance of lithology due to lack of digitized lithologic information. The effect of the peculiar drainage pattern due to tectonic forcing is not readily apparent in the major element composition or in increased chemical weathering rates. The ⁸⁷Sr/⁸⁶Sr ratios and the silicate weathering rates are in general lower in the Three Rivers than in the rivers draining the Himalayan front.     

The CO2 system in rivers of the Australian Victorian Alps: CO2 evasion in relation to system metabolism and rock weathering on multi-annual time scales

The patterns of dissolved inorganic C (DIC) and aqueous CO₂ in rivers and estuaries sampled during summer and winter in the Australian Victorian Alps were examined. Together with historical (1978–1990) geochemical data, this study provides, for the first time, a multi-annual coverage of the linkage between CO₂ release via wetland evasion and CO₂ consumption via combined carbonate and aluminosilicate weathering. δ¹³C values imply that carbonate weathering contributes ∼36% of the DIC in the rivers although carbonates comprise less than 5% of the study area. Baseflow/interflow flushing of respired C3 plant detritus accounts for ∼50% and atmospheric precipitation accounts for ∼14% of the DIC. The influence of in river respiration and photosynthesis on the DIC concentrations is negligible. River waters are supersaturated with CO₂ and evade ∼27.7 × 10⁶ mol/km²/a to ∼70.9 × 10⁶ mol/km²/a CO₂ to the atmosphere with the highest values in the low runoff rivers. This is slightly higher than the global average reflecting higher gas transfer velocities due to high wind speeds. Evaded CO₂ is not balanced by CO₂ consumption via combined carbonate and aluminosilicate weathering which implies that chemical weathering does not significantly neutralize respiration derived H₂CO₃. The results of this study have implications for global assessments of chemical weathering yields in river systems draining passive margin terrains as high respiration derived DIC concentrations are not directly connected to high carbonate and aluminosilicate weathering rates.     

Basalt weathering in Central Siberia under permafrost conditions

Chemical weathering of basalts in the Putorana Plateau, Central Siberia, has been studied by combining chemical and mineralogical analysis of solids (rocks, soils, river sediments, and suspended matter) and fluid solution chemistry. Altogether, 70 large and small rivers, 30 soil pore waters and groundwaters and over 30 solids were sampled during July to August 2001. Analysis of multiannual data on discharge and chemical composition of several rivers of the region available from the Russian Hydrological Survey allowed rigorous estimation of mean annual major element concentrations, and dissolved and suspended fluxes associated with basalt weathering. For the rivers Tembenchi and Taimura that drain monolithologic basic volcanic rocks, the mean multiannual flux of total dissolved cations (TDS_c = Ca + Mg + Na + K) corrected for atmospheric input is 5.7 ± 0.5 t/km²/yr. For the largest river Nizhniya Tunguska—draining essentially basic rocks—the TDS_c is 6.1 ± 1.5 t/km²/yr. The overall CO₂ consumption flux associated with basalt weathering in the studied region (∼700,000 km²) achieves 0.08 × 10¹² mol/yr, which represents only 2.6% of the total CO₂ consumption associated with basalt weathering at the Earth’s surface. The fluxes of suspended matter were estimated as 3.1 ± 0.5, 9.0 ± 0.8, and 6.5 ± 2.0 t/km²/yr for rivers Taimura, Eratchimo, and Nizhniya Tunguska, respectively. Based on chemical analyses of river solutes and suspended matter, the relative dissolved versus particulate annual transport of major components is C_inorg ≥ C_org > Na + K > Ca > Mg > Si > Fe ≥ Mn ≥ Ti ≥ Al which reflects the usual order of element mobility during weathering.According to chemical and mineralogical soil and sediment analyses, alteration of basalt consists of (1) replacement of the original basaltic glass by Si-Al-Fe rich amorphous material, (2) mechanical desegregation and grinding of parent rocks, leading to accumulation of “primary” hydrothermal trioctahedral smectite, and (3) transformation of these trioctahedral (oxy)smectites and mixed-layer chlorite-smectite, into secondary dioctahedral smectite accompanied by removal of Ca, Mg, and Fe, and enrichment in Al. No vertical chemical differentiation of fluid and solid phases within the soil profile was identified. All sampled soil pore waters and groundwaters were found to be close to equilibrium with respect to chalcedony, gibbsite, halloysite, and allophanes, but strongly supersaturated with respect to goethite, nontronite, and montmorillonite.Over the annual cycle, the contribution of atmospheric precipitation, permafrost melting, underground reservoirs, litter degradation, and rock and soil mineral weathering for the overall TDS_c transport in the largest river of the region (Nizhniya Tunguska) is 9.3 ± 3, 10 ± 5, 10.5 ± 5, 25 ± 20, and 45 ± 30%, respectively. In the summertime, direct contribution of rocks and soil mineral weathering via solid/fluid interaction does not exceed 20%. The main unknown factors of element mobilization from basalt to the river is litter degradation in the upper soil horizon and parameters of element turnover in the vegetation.     

Chemical weathering in the Upper Huang He (Yellow River) draining the eastern Qinghai-Tibet Plateau

We examined the fluvial geochemistry of the Huang He (Yellow River) in its headwaters to determine natural chemical weathering rates on the northeastern Qinghai-Tibet Plateau, where anthropogenic impact is considered small. Qualitative treatment of the major element composition demonstrates the dominance of carbonate and evaporite dissolution. Most samples are supersaturated with respect to calcite, dolomite, and atmospheric CO₂ with moderate (0.710-0.715) ⁸⁷Sr/⁸⁶Sr ratios, while six out of 21 total samples have especially high concentrations of Na, Ca, Mg, Cl, and SO₄ from weathering of evaporites. We used inversion model calculations to apportion the total dissolved cations to rain-, evaporite-, carbonate-, and silicate-origin. The samples are either carbonate- or evaporite-dominated, but the relative contributions of the four sources vary widely among samples. Net CO₂ consumption rates by silicate weathering (6-120 × 10³ mol/km²/yr) are low and have a relative uncertainty of ∼40%. We extended the inversion model calculation to literature data for rivers draining orogenic zones worldwide. The Ganges-Brahmaputra draining the Himalayan front has higher CO₂ consumption rates (110-570 × 10³ mol/km²/yr) and more radiogenic ⁸⁷Sr/⁸⁶Sr (0.715-1.24) than the Upper Huang He, but the rivers at higher latitudes are similar to or lower than the Upper Huang He in CO₂ uptake by silicate weathering. In these orogenic zones, silicate weathering rates are only weakly coupled with temperature and become independent of runoff above ∼800 mm/yr.     

Major ion chemistry of the Yarlung Tsangpo-Brahmaputra river: Chemical weathering, erosion, and CO2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

The Yarlung Tsangpo-Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo-Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO₂ consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo-Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km⁻² yr⁻¹ and the silicate cation flux more than 34 tons km⁻² yr⁻¹. In total, chemical weathering in this area consumes 15.2 × 10⁵ mol CO₂ km⁻² yr⁻¹, which is twice the Brahmaputra average. These data show that 15-20% of the total CO₂ consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na⁺ and Cl⁻ fluxes throughout southern Tibet, comprising more than 50% of all Na⁺ in some stream systems. Carbonate weathering generates 80-90% of all dissolved Ca²⁺ and Mg²⁺ cations in much of the Yarlung Tsangpo catchment.     

Stream geochemistry, chemical weathering and CO2 consumption potential of andesitic terrains, Dominica, Lesser Antilles

Recent studies of chemical weathering of andesitic-dacitic material on high-standing islands (HSIs) have shown these terrains have some of the highest observed rates of chemical weathering and associated CO₂ consumption yet reported. However, the paucity of stream gauge data in many of these terrains has limited determination of chemical weathering product fluxes. In July 2006 and March 2008, stream water samples were collected and manual stream gauging was performed in watersheds throughout the volcanic island of Dominica in the Lesser Antilles. Distinct wet and dry season solute concentrations reveal the importance of seasonal variations on the weathering signal. A cluster analysis of the stream geochemical data shows the importance of parent material age on the overall delivery of solutes. Observed Ca:Na, HCO₃:Na and Mg:Na ratios suggest crystallinity of the parent material may also play an important role in determining weathering fluxes. From total dissolved solids concentrations and mean annual discharge calculations we calculate chemical weathering yields of (6-106 t km⁻² a⁻¹), which are similar to those previously determined for basalt terrains. Silicate fluxes (3.1-55.4 t km⁻² a⁻¹) and associated CO₂ consumption (190-1575 × 10³ mol km⁻² a⁻¹) determined from our study are among the highest determined to date. The calculated chemical fluxes from our study confirm the weathering potential of andesitic-dacitic terrains and that additional studies of these terrains are warranted.     

Geochemistry of the dissolved load of the Changjiang Basin rivers: Anthropogenic impacts and chemical weathering

This study focuses on the chemical and Sr isotopic compositions of the dissolved load of the rivers of the Changjiang Basin, one of the largest riverine systems in the world. Water samples were collected in August 2006 from the main tributaries and the main Changjiang channel. The chemical and isotopic analyses indicated that four major reservoirs (carbonates, silicates, evaporites and agriculture/urban effluents) contribute to the total dissolved solutes. The overall chemical weathering (carbonate and silicate) rate for the Changjiang is approximately 40 ton/km²/year or 19 mm/kyr, similar to that of the Ganges-Brahmaputra system, and the basin is characterized by carbonate and silicate weathering rates ranging from 17 to 56 ton/km²/year and from 0.7 to 7.1 ton/km²/year, respectively. In the lower reach of the Changjiang main channel, the weathering rates are estimated to be 36 and 2.2 ton/km²/year for carbonates and silicates, respectively. It appears that sulphuric acid may dominate chemical weathering reactions for some sub-basins. The budgets of CO₂ consumption are estimated to be 646 × 10⁹ and 191 × 10⁹ mol/year by carbonate and silicate weathering, respectively. The contribution of the anthropogenic inputs to the cationic TDS of the Changjiang is estimated to be 15-20% for the most downstream stations. Our study suggested that the Changjiang is strongly impacted by human activities and is very sensitive to the change of land use.     

Barium in Deccan Basalt Rivers: Its Abundance, Relative Mobility and Flux

The concentration of dissolved Ba in a number of rivers having their drainage almost entirely in Deccan Trap basalts has been measured. These results along with available data on the abundances of major elements in these waters, and on Ba and major elements in bed sediments of these rivers provide a measure of (i) the relative mobility of Ba during chemical weathering and erosion of basalts, particularly with respect to alkaline earths, Mg, Ca and Sr, and (ii) the flux of Ba out of the Deccan and its global significance. The concentration of dissolved Ba ranges from 8 to 105 nM. The average Ba/Mg*, Ba/Ca* and Ba/Sr (* is concentration corrected for atmospheric contribution) in waters is lower than the corresponding mean ratios in Deccan basalts, though they overlap within errors. Majority of the water samples, however, have ratios less than that in basalts. These findings can be interpreted as a cumulative effect of limited release/mobility of Ba during chemical weathering and erosion of basalts and its reactive behaviour in waters which promote its association with clays and oxy-hydroxides of Fe. These results also indicate that during chemical erosion of Deccan basalts, Ba is the least mobile among the alkaline earth elements. The abundance of Ba in sediments and their Ba/Al ratios relative to basalts are consistent with the above conclusion. Ba/Mg and Ba/Ca ratios in water and in sediments from the same location are strongly correlated; however, the mean ratios in waters are far less than those in sediments. This is a result of limited Ba mobility, effectively 5–6 times lower than that of Mg. The annual flux of dissolved Ba out of the Deccan Traps is ~1 × 10⁷ moles, ~ 0.2% of its global riverine transport to oceans. The contribution of dissolved Ba from Deccan Traps, seem lower than its aerial coverage, ~ 0.5% of the global drainage area; the potential causes for this could be the lower abundance of Ba in basalts relative to “average continental crust”, and its behaviour during chemical weathering and erosion.     

Chemical weathering of carbonates and silicates in the Han River basin,South Korea

A detailed investigation of the fluvial geochemistry of the Han River system allows to estimate the rates of chemical weathering and the consumption of CO₂. The Han River drains approximately 26,000 km² and is the largest river system in South Korea in terms of both water discharge and total river length. It consists of two major tributaries: the North Han River (NHR) and the South Han River (SHR). Distinct differences in basin lithology (silicate vs. carbonate) between the NHR and SHR provide a good natural laboratory in which to examine weathering processes and the influence of basin geology on water quality. The concentrations of major elements and the Sr isotopic compositions were obtained from 58 samples collected in both summer and winter along the Han River system in both 2000 and 2006. The concentrations of dissolved loads differed considerably between the NHR and SHR; compared with the SHR, the NHR had much lower total dissolved solids (TDS), Sr, and major ion concentrations but a higher Si concentration and ⁸⁷Sr/⁸⁶Sr ratio. A forward model showed that the dissolved loads in the NHR came primarily from silicate weathering (55 ± 11%), with a relatively small portion from carbonates (30 ± 14%), whereas the main contribution to the dissolved loads in the SHR was carbonate weathering (82 ± 3%), with only 11 ± 4% from silicates. These results are consistent with the different lithologies of the two drainage basins: silicate rocks in the NHR versus carbonate rocks in the SHR. Sulfuric acid derived from sulfide dissolution in coal-containing sedimentary strata has played an important role in carbonate weathering in the SHR basin, unlike in the NHR basin. The silicate weathering rate (SWR) was similar between the NHR and SHR basins, but the rate of CO₂ consumption in the SHR basin was lower than in the NHR basin due to an important role of sulfuric acid derived from pyrite oxidation.     

Carbonate and silicate weathering in two presently glaciated, crystalline catchments in the Swiss Alps

We present a weathering mass balance of the presently glaciated Rhône and Oberaar catchments, located within the crystalline Aar massif (central Switzerland). Annual chemical and physical weathering fluxes are calculated from the monthly weighted means of meltwater samples taken from July, 1999 to May, 2001 and are corrected for precipitation inputs. The meltwater composition issuing from the Oberaar and Rhône catchments is dominated by calcium, which represents 81% and 55% of the total cation flux respectively (i.e. 555 and 82-96 keq km⁻² yr⁻¹). The six to seven times higher Ca²⁺ denudation flux from the Oberaar catchment is attributed to the presence of a strongly foliated gneissic zone. The gneissic zone has an elevated calcite content (as reflected by the 4.6 times higher calcite content of the suspended sediments from Oberaar compared to Rhône) and a higher mechanical erosion rate (resulting in a higher flux of suspended sediment). The mean flux of suspended calcite of the Oberaar meltwaters during the ablation period is 7 times greater than that of the Rhône meltwaters. Taking the suspended calcite as a proxy for the total (including sub-glacial sediments) weathering calcite surface area, it appears that the available surface area is an important factor in controlling weathering rates. However, we also observe an increased supply of protons for carbonate dissolution in the Oberaar catchment, where the sulphate denudation flux is six times greater. Carbonic acid is the second important source of protons, and we calculate that three times as much atmospheric CO₂ is drawn down (short term) in the Oberaar catchment. Silica fluxes from the two catchments are comparable with each other, but are 100 kmol km² yr⁻¹ lower than fluxes from physically comparable, non-glaciated basins.     

Hydrochemistry of the Amur River: Weathering in a Northern Temperate Basin

We report the dissolved major element, organic carbon, and δ¹³C_DOC, δ¹³C_POC, δD, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr composition of 19 summer samples from the Amur River. The Amur transported 2.6 Tg C/year of total organic carbon to the Sea of Okhotsk. The physical weathering rate (PWR) based on suspended particulate material was 13 (1.4–14) tons/(km² year), and the chemical weathering rate based on total dissolved solids was 7 (4.3–46) tons/(km² year). We further quantified the sources of the dissolved cations using an inverse model: rain accounted for 2 (0.6–5)%, evaporite 3 (0.7–7)%, carbonate 51 (29–74)%, and silicate 45 (25–64)%. The silicate weathering rate (SWR) in the Amur basin was 23 (15–98) × 10³ mol/(km² year) or 0.67 (0.40–2.81) tons/(km² year), comparable to those of the Siberian rivers and the Mackenzie at higher latitudes. The SWR of the Amur was negatively correlated with elevation and relief, and positively correlated with runoff.     

Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO₂ consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO₂ consumption to silicate weathering than methods utilizing Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and ⁸⁷Sr/⁸⁶Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P_CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca²⁺-Mg²⁺-HCO₃⁻ waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca²⁺ and Sr²⁺, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P_CO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO₂. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg²⁺, Ca²⁺, and HCO₃⁻ mass balances support interpretations of preferential Ca²⁺ removal by calcite precipitation. On the basis of patterns of saturation state and P_CO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca²⁺ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca²⁺ during riverine transport brings the Ca/Sr and ⁸⁷Sr/⁸⁶Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr²⁺ budget and suggest that the conventional application of two-component Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations has overestimated silicate-derived Sr²⁺ and HCO₃⁻ fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.