Differential weathering of basaltic and granitic catchments from concentration–discharge relationships

It is widely accepted that the consumption of atmospheric CO2 by silicate weathering provides a negative climate feedback mechanism, and that this mechanism has played an important role in moderating global climate over geological timescales (see e.g., Berner1). Extension of this concept to feedbacks between climate change and tectonic uplift, which are discussed in many chapters of this volume, is dependent on the assumption that glaciation and tectonic uplift lead to increased rates of chemical weathering and CO2 consumption. Both glaciation and rapid uplift are accompanied by rapid physical denudation, which exposes fresh bedrock minerals to the agents of chemical weathering. In addition, tectonic uplift may create mountain ranges capable of sustaining alpine glaciers and ice sheets, especially during glacial maxima, thus linking uplift and glaciation on geological timescales. While it is generally accepted that glaciation and uplift will lead to accelerated rates of chemical weathering as compared to regions of nonglaciated or stable topography, the magnitude of this effect has proven difficult to estimate. Taylor and Blum2 investigated the relation between soil age and silicate weathering rates and found a well-defined power law relation for young soils (≤ 138 thousand years old), suggesting that predictions could be made of the effect of glaciation and tectonic uplift on silicate weathering rates by considering the effect of these processes on average soil ages over large geographic areas.

Silicate weathering rate and its controlling factors: A study from small granitic watersheds in the Jiuhua Mountains

Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory

Comparison between Silicate Weathering and Physical Erosion Rates in Andean Basins of the Amazon River

Continental chemical weathering plays a crucial role in the regional and global carbon budgets. However, relevant research on tropical volcanic islands is still limited, particularly the influences of human activities on surficial chemical weathering, which remain scarce. This study analyzed the water geochemistry and dissolved inorganic carbon isotope (δ13CDIC) of the Changhua River draining Hainan Island, aiming to quantitatively elucidate the seasonal CO2 budgets at the catchment scale. We found that the major ions in the river were from atmospheric precipitation (6.79%), anthropogenic input (11.68%), carbonate weathering (41.96%), and silicate weathering (39.56%). The silicate weathering rate and the carbonate weathering rate were calculated as 16.91 t km–2 yr–1 and 30.68 t km–2 yr–1, respectively, resulting in CO2 consumption of 307.82 × 103 mol km–2 yr–1 and 400.95 × 103 mol km–2 yr–1. Considering sulfuric acid- and nitric acid-driven weathering reactions, the catchment became a net source of CO2 on a geological time scale. Furthermore, the drivers of chemical weathering and associated CO2 budget were deciphered. The significant positive correlations among chemical weathering fluxes, acids, and land uses quantitatively emphasized that the chemical weathering of CO2 budgets was largely disturbed by anthropogenic activities. This study showed that integrating river water geochemistry and geographical parameters could achieve a deep understanding of catchment weathering CO2 budgets and potential driving mechanisms.

Estimation of weathering rates and CO2 drawdown based on solute load: Significance of granulites and gneisses dominated weathering in the Kaveri River basin, Southern India

Cenozoic carbon cycle imbalances and a variable weathering feedback

Northern latitude chemical weathering rates: clues from the Mackenzie River Basin, Canada

Silicate weathering on the continents is thought to play a critical part in regulating global climate on geological time scales but determination of the magnitude of silicate weathering fluxes is frustrated by the complexity of the weathering processes. Here we present analyses of stable Sr-isotopic compositions (𝛿88/86Sr) in a suite of river waters and bedloads from the Himalayas in Nepal to establish the lithological controls on 𝛿88/86Sr values and the secondary processes that impact carbonate weathering. A control on 𝛿88/86Sr values is lithology with the rivers in carbonate-dominated catchments marginally lower (∼0.07 ‰) than in silicate-dominated catchments. However, as for 87Sr/86Sr ratios, 𝛿88/86Sr values of carbonates are altered by silicate-carbonate mineral exchange during metamorphism. The major potential secondary control on 𝛿88/86Sr values in Himalayan catchments is precipitation of secondary calcite responsible for the marked elevation of Sr/Ca and Mg/Ca ratios in the waters. We re-evaluate the mechanisms for secondary calcite formation and conclude that a balanced solution and re-precipitation process, driven by the relative instability of the primary Mg-rich calcite, provides a better explanation for the elevated Sr/Ca ratios than simple precipitation from a highly over saturated solution. This solution-re-precipitation process is akin to that invoked to explain diagenesis of deep-sea sediments. The mechanism of secondary calcite formation impacts the distinction of cation inputs from carbonate and silicate minerals. The correlation between 𝛿88/86Sr water-calcite fractionations, Sr/Ca partition coefficients and precipitation rates allows the calcite re-precipitation rates to be inferred from the covariation of water 𝛿88/86Sr values and Sr/Ca ratios. These rates are very low (<10-8 mol m-2 s-1) but are consistent with those inferred from field estimates of the amount of calcite re-precipitated, the surface area of carbonate exposed to weathering and the calcite weathering flux. The low precipitation rates are also consistent with previously reported 𝛥44/40Ca isotope fractionations of ∼−0.2‰. The calcite reprecipitation rates are comparable to silicate weathering rates previously inferred from Li-isotopic compositions which is consistent with calcite re-precipitation taking place very close to equilibrium following the initial rapid saturation of the fluids by calcite.

Major 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.

Chemical weathering in the Krishna Basin and Western Ghats of the Deccan Traps, India: Rates of basalt weathering and their controls

Weathering of ultramafic rocks emplaced at low latitude during arc‐arc and arc‐continent collisions may provide an important sink for atmospheric CO2 over geologic timescales. Accurately modeling the effects of ultramafic rock weathering on Earth's carbon cycle and climate requires understanding mass fluxes from ultramafic landscapes. In this study, physical erosion and chemical weathering fluxes and weathering intensity are quantified in 15 watersheds across the Monte del Estado, a serpentinite massif in Puerto Rico, using measurements of in situ 36Cl in magnetite, stream solute fluxes, and sediment geochemistry. Despite high relief in the study watersheds, erosion fluxes are moderate (22–109 tons km−2 yr−1), chemical weathering fluxes are large (55–143 tons km−2 yr−1), and weathering intensities are among the highest yet reported for silicate‐rock weathering (up to 0.88). We use these data to parameterize power‐law relationships between weathering, erosion, and runoff. We interpret the relative importance of climate versus erosion in setting weathering fluxes and CO2 consumption from the best‐fit power‐law slopes. Weathering fluxes from tropical, montane serpentinite landscapes are found to be strongly controlled by runoff and weakly controlled by the supply of fresh rock to the weathering zone through physical erosion. The strong runoff dependence of weathering fluxes implies that, to the extent that precipitation rates are coupled to global temperature, ultramafic landscapes may be important participants in the negative silicate weathering feedback, increasing (decreasing) CO2 consumption in response to a warming (cooling) climate. Thus, serpentinite landscapes may help stabilize Earth's climate state through time.

The chemistry of major ions and elements in Narmada and Tapti river water was studied based on the monitoring of the drainage basins for monsoon and post‐monsoon seasons in 2004. The results show the influence of rock weathering on the concentration of major ions in the surface water. There is a significant contribution from the catchment geologies in the major constituents of water. The total dissolved solids (TDS) concentrations are reflecting the signature of weathering of rocks in the catchment areas. In the Narmada River,$HCO_{3}^{-}$, Ca2+and Mg2+are dominant ions, whereas in the Tapti River the ions dominating major compositions are$HCO_{3}^{-}$, Na+and Mg2+. The high concentration of Cl−and$SO_{4}^{2-}$is a signature of anthropogenic inputs in addition to the contribution of discharge from springs and/or groundwater. The high concentration of$NO_{3}^{-}$can exclusively be attributed to the anthropogenic activities. The average concentration of dissolved silica being higher than the Indian and world average is reflecting the contribution of silicate weathering. The present study gives the silicate and carbonate weathering rates and the corresponding CO2consumption rates considering the contribution of carbonate rocks present in the selected river basins of the Deccan basaltic province. The mean silicate weathering rates during the investigation period for the Narmada and Tapti Rivers show lower values, but the corresponding mean CO2consumption rates give higher values compared with earlier estimated rates by Dessertet al. (2001.Earth and Planetary Science Letters188: 459). For the study period, the mean carbonate weathering rates and the associated CO2consumption rates for the Narmada and Tapti Rivers are ∼12·67 t km−2year−1and ∼7·32 t km−2year−1and ∼0·36 × 106mol km−2year−1and ∼0·22 × 106mol km−2year−1respectively. Copyright © 2008 John Wiley & Sons, Ltd.

Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO 2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

Controls on riverine magnesium isotopic composition: Evidence from mono-lithological catchments