我国南方岩溶区和北方黄土区的大气CO2效应

我国南方岩溶区与北方黄土区都是巨大的碳库。碳酸盐的溶蚀及再结晶是两个碳库与大气ＣＯ^２交换的重要过程。碳的区域平衡是评价化学风化消耗或逸散ＣＯ^２的基础。岩溶区与黄土区在地球化学风化的环境背景、溶蚀过程、产物运移和归宿等差异很大。黄土区化学风化消耗大气ＣＯ^２通量较岩溶区小。目前评价两类地区土壤与大气ＣＯ^２的源汇关系尚不成熟,需要定量认识土壤ＣＯ^２与下伏碳酸盐岩溶蚀或与下伏黄土次生碳酸盐化作用。岩溶区湖泊沉积物中有机质分解产生的ＨＣＯ^３－制约外源及内生碳酸盐溶解和自生碳酸盐形成。     

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.     

Silicate and carbonate mineral weathering in soil profiles developed on Pleistocene glacial drift (Michigan, USA): Mass balances based on soil water geochemistry

Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO₂. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg²⁺/Ca²⁺ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca²⁺ and Mg²⁺ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg²⁺ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry.     

Modelling the long-term carbon cycle,atmospheric CO2, and Earth surface temperature from late Neoproterozoic to present day

Over geological timescales, CO₂ levels are determined by the operation of the long term carbon cycle, and it is generally thought that changes in atmospheric CO₂ concentration have controlled variations in Earth's surface temperature over the Phanerozoic Eon. Here we compile independent estimates for global average surface temperature and atmospheric CO₂ concentration, and compare these to the predictions of box models of the long term carbon cycle COPSE and GEOCARBSULF.We find a strong relationship between CO₂ forcing and temperature from the proxy data, for times where data is available, and we find that current published models reproduce many aspects of CO₂ change, but compare poorly to temperature estimates. Models are then modified in line with recent advances in understanding the tectonic controls on carbon cycle source and sink processes, with these changes constrained by modelling ⁸⁷Sr/⁸⁶Sr ratios. We estimate CO₂ degassing rates from the lengths of subduction zones and rifts, add differential effects of erosion rates on the weathering of silicates and carbonates, and revise the relationship between global average temperature changes and the temperature change in key weathering zones.Under these modifications, models produce combined records of CO₂ and temperature change that are reasonably in line with geological and geochemical proxies (e.g. central model predictions are within the proxy windows for >~75% of the time covered by data). However, whilst broad long-term changes are reconstructed, the models still do not adequately predict the timing of glacial periods. We show that the ⁸⁷Sr/⁸⁶Sr record is largely influenced by the weathering contributions of different lithologies, and is strongly controlled by erosion rates, rather than being a good indicator of overall silicate chemical weathering rates. We also confirm that a combination of increasing erosion rates and decreasing degassing rates over the Neogene can cause the observed cooling and Sr isotope changes without requiring an overall increase in silicate weathering rates.On the question of a source or sink dominated carbon cycle, we find that neither alone can adequately reconstruct the combination of CO₂, temperature and strontium isotope dynamics over Phanerozoic time, necessitating a combination of changes to sources and sinks. Further progress in this field relies on >10⁸ year dynamic spatial reconstructions of ancient tectonics, paleogeography and hydrology. Whilst this is a significant challenge, the latest reconstruction techniques, proxy records and modelling advances make this an achievable target.     

Fluxes of high- versus low-temperature water-rock interactions in aerial volcanic areas: Example from the Kamchatka Peninsula, Russia

Volcanic areas play a key role in the input of elements into the ocean and in the regulation of the geological carbon cycle. The aim of this study is to investigate the budget of silicate weathering in an active volcanic area. We compared the fluxes of the two major weathering regimes occurring at low temperature in soils and at high temperature in the active volcanic arc of Kamchatka, respectively. The volcanic activity, by inducing geothermal circulation and releasing gases to the surface, produces extreme conditions in which intense water-rock interactions occur and may have a strong impact on the weathering budgets. Our results show that the chemical composition of the Kamchatka river water is controlled by surface low-temperature weathering, atmospheric input and, in some limited cases, strongly imprinted by high-temperature water-rock reactions. We have determined the contribution of each source and calculated the rates of CO₂ consumption and chemical weathering resulting from low and high-temperature water/rock interactions. The weathering rates (between 7 and 13.7 t/km²/yr for cations only) and atmospheric CO₂ consumption rates (∼0.33-0.46 × 10⁶ mol/km²/yr for Kamchatka River) due to rock weathering in soils (low-temperature) are entirely consistent with the previously published global weathering laws relating weathering rates of basalts with runoff and temperature. In the Kamchatka River, CO₂ consumption derived from hydrothermal activity represents about 11% of the total HCO₃ flux exported by the river. The high-temperature weathering process explains 25% of the total cationic weathering rate in the Kamchatka River. Although in the rivers non-affected by hydrothermal activity, the main weathering agent is carbonic acid (reflected in the abundance of in rivers), in the region most impacted by hydrothermalism, the protons responsible for minerals dissolution are provided not only by carbonic acid, but also by sulphuric and hydrochloric acid. A clear increase of weathering rates in rivers impacted by sulphuric acid can be observed. In the Kamchatka River, 19% of cations are released by hydrothermal acids or the oxidative weathering of sulphur minerals.Our results emphasise the important impact of both low and high-temperature weathering of volcanic rocks on global weathering fluxes to the ocean. Our results also show that besides carbonic acid derived from atmospheric CO₂, hydrochloric acid and especially sulphuric acid are important weathering agents. Clearly, sulphuric acid, with hydrothermal activity, are key parameters that cause first-order increases of the chemical weathering rates in volcanic areas. In these areas, accurate determination of weathering budgets in volcanic area will require to better quantify sulphuric acid impact.     

Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO₂ consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ³⁴S values of 16 to 20‰_CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS₂. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO₂ fluxes from silicate weathering range from ~ 0.012 × 10⁶ to 0.039 × 10⁶ mol/km²/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO₂ consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO₂ consumption rates in particular, based on major ion abundances, should be treated with skepticism.     

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.     

The control of Phanerozoic atmosphere and seawater composition by basalt–seawater exchange reactions

We present results from a long term geochemical cycling model, with a focus on the sensitivity of atmospheric carbon dioxide, oxygen, and the major element composition of seawater to seafloor spreading rates. This model incorporates rock weathering, basalt–seawater exchange reactions, and the formation and destruction of chemical sediments and organic matter. Hydrothermal reactions between seafloor and seawater involving calcium, magnesium, sodium, potassium, sulfate and carbon are the high temperature counterparts to low temperature redox, weathering, precipitation and diagenetic reactions. A major source of uncertainty is the extent to which these exchange fluxes are controlled by seafloor spreading rate. In addition, the return fluxes of these components to the atmospheric and primary silicate reservoirs reflect not only the overall rates of subduction and metamorphism, but the distribution of the overlying sedimentary burden and authigenic minerals formed during basalt alteration as well. In particular, we show how the stoichiometry of exchange fluxes (Mg/Ca and SO₄/Ca) may buffer atmospheric CO₂ and O₂ concentrations.     

Atmospheric CO2 consumption by chemical weathering in North America

CO₂ consumption by chemical weathering is an integral part of the boundless carbon cycle, whose spatial patterns and controlling factors on continental scale are still not fully understood. A dataset of 338 river catchments throughout North America was used to empirically identify predictors of bicarbonate fluxes by chemical weathering and interpret the underlying controlling factors. Detailed analysis of major ion ratios enables distinction of the contributions of silicate and carbonate weathering and thus quantifying CO₂ consumption. Extrapolation of the identified empirical model equations to North America allows the analysis of the spatial patterns of the CO₂ consumption by chemical weathering.Runoff, lithology and land cover were identified as the major predictors of the riverine bicarbonate fluxes and the associated CO₂ consumption. Other influence factors, e.g. temperature, could not be established in the models. Of the distinguished land cover classes, artificial surfaces, dominated by urban areas, increase bicarbonate fluxes most, followed by shrubs, grasslands, managed lands, and forests. The extrapolation results in an average specific bicarbonate flux of 0.3 Mmol km⁻² a⁻¹ by chemical weathering in North America, of which 64% originates from atmospheric CO₂, and 36% from carbonate mineral dissolution. Chemical weathering in North America thus consumes 50 Mt atmospheric CO₂-C per year. About half of that originates from 10% of the area of North America.The estimated strength of individual predictors differs from previous studies. This highlights the need for a globally representative set of regionally calibrated models of CO₂ consumption by chemical weathering, which apply very detailed spatial data to resolve the heterogeneity of earth surface processes.     

Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH,CO2, and oxalate

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO₂ (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m⁻² h⁻¹ at pH 2.78 in the batch experiments to 0.009 μmol Si m⁻² h⁻¹ at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H⁺ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO₂ demonstrated that dissolution rates were independent of CO₂ within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO₂ have been elevated over the last decade. This release of magmatic CO₂ has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO₂ may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO₂ can be excluded.     

CO<Subscript>2</Subscript> Air–Sea Exchange due to Calcium Carbonate and Organic Matter Storage,and its Implications for the Global Carbon Cycle

Release of CO₂ from surface ocean water owing to precipitation of CaCO₃ and the imbalance between biological production of organic matter and its respiration, and their net removal from surface water to sedimentary storage was studied by means of a quotient θ = (CO₂ flux to the atmosphere)/(CaCO₃ precipitated). θ depends not only on water temperature and atmospheric CO₂ concentration but also on the CaCO₃ and organic carbon masses formed. In CO₂ generation by CaCO₃ precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric CO₂ concentration (195–375 ppmv), and increasing CaCO₃ precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic carbon counteracts the CO₂ production by carbonate precipitation and it results in lower CO₂ emissions from the surface layer. When atmospheric CO₂ increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO₂ transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric CO₂. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon from land, the calculated CO₂ flux to the atmosphere is a function of the CaCO₃ and C_org net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO₂ flux near the Last Glacial Maximum is 17 to 7×10¹² mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time it is 38–42×10¹² mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic emissions of CO₂ to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total alkalinity owing to the CaCO₃ storage and consequent emission of CO₂. These results are in agreement with the conclusions of a number of other investigators. As the CO₂ uptake in mineral weathering is a major flux in the global carbon cycle, the CO₂ weathering pathway that originates in the CO₂ produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large imbalances of the atmosphere and land organic reservoir at 10²–10⁴-year time scales.     

The dissolution of olivine added to soil: Implications for enhanced weathering

Chemical weathering of silicate minerals consumes atmospheric CO₂ and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (“enhanced weathering”) has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg²⁺ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10^−16.4 and 10^−15.5 moles(Mg) cm⁻² s⁻¹. Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg²⁺ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements.     

Chemical weathering processes in the Yalong River draining the eastern Tibetan Plateau,China

To better understand chemical weathering and controlling processes in the Yalong River of the eastern Tibetan Plateau, this study presents major ion concentrations and stable isotopes of the dissolved loads. The isotopic compositions (δ¹³C-DIC, δ³⁴S and δ¹⁸O-SO₄) of the dissolved loads are very useful to quantify solute sources and define the carbon budget related with chemical weathering in riverine systems. The isotopic composition of sulphate demonstrates that most of the sulphate is derived from sulphide oxidation, particularly in the upper reach of the Yalong River. The correlations between δ¹³C-DIC, water chemistry and isotopes of sulphate, suggest that the carbon dynamics are mainly affected by carbonate weathering by sulphuric acid and equilibration processes. Approximately 13% of the dissolved inorganic carbon in the Yalong River originates from carbonate weathering by strong acid. The CO₂ consumption rates are estimated to be 2.8 × 10⁵ mol/km²/yr and 0.9 × 10⁵ mol/km²/yr via carbonate and silicate weathering in the Yalong River, respectively. In this study, the influence of sulphide oxidation and metamorphic CO₂ on the carbon budget is estimated for the Yalong River draining the eastern Tibetan Plateau.     

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 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.     

CO2 and sulfuric acid controls of weathering and river water composition

Reactions of CO₂ with carbonate and silicate minerals in continental sediments and upper part of the crystalline crust produce HCO₃⁻ in river and ground waters. H₂SO₄ formed by the oxidation of pyrite and reacting with carbonates may produce CO₂ or HCO₃⁻. The ratio, ψ, of atmospheric or soil CO₂ consumed in weathering to HCO₃⁻ produced depends on the mix of CO₂ and H₂SO₄, and the proportions of the carbonates and silicates in the source rock. An average sediment has a CO₂ uptake potential of ψ = 0.61. The potential increases by inclusion of the crystalline crust in the weathering source rock. A mineral dissolution model for an average river gives ψ = 0.68 to 0.72 that is within the range of ψ = 0.63 to 0.75, reported by other investigators using other methods. These results translate into the CO₂ weathering flux of 20 to 24 × 10¹²mol/yr.     

Geochemical alterations in surface waters of Govind Ballabh Pant Sagar,Northern Coalfield,India

Major ions showed high concentrations, ionic strength and chemical activity in the surface waters of Govind Ballabh Pant Sagar reservoir. Various geochemical ratios showed the dominance of silicate over carbonate weathering and major ions such as Na⁺ + K⁺ account for about 52 % of the cation budget. The high Na⁺ and K⁺ showed sedimentation of rock/coal particles consisting of highly weathered silicate minerals contributed by the discharge of mine water, fly ash mixing during transportation, etc. Further, Ca²⁺ + Mg²⁺/Na⁺ + K⁺ ratio was <1 (0.92) indicating the occurrence of silicate weathering in the reservoir catchment. The comparative assessment showed that the proportion of Ca²⁺ + Mg²⁺/Na⁺ + K⁺ tends to be lower along the coal mining belts compared to non-coal mining regions in the world. The Ca²⁺/SO₄ ^2? ratio <1 revealed not only H₂CO₃ but H₂SO₄ also acting as a source of protons for rock weathering. The cause underlying these differences can be related directly to geological substrate and anthropogenic activities.     

Oxygen and carbon isotope composition of structural carbonate in weathering apatites from laterites, southern Brazil and western Senegal

The oxygen (δ¹⁸O_c) and carbon (δ¹³C_c) isotope compositions of the structural carbonate group (CO₃) in apatites from lateritic profiles were investigated. The weathering profiles, located in southern Brazil and in western Senegal, are developed on three different types of apatite-rich parent rock: carbonatite, metamorphosed marine phosphorite and sedimentary marine phosphorite. The parent rock apatites are of magmatic, hydrothermal, metamorphic and sedimentary origins. The in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically.The overall range of measured δ¹⁸O_c and δ¹³C_c values of apatites of weathering origin (22 to 27 SMOW for δ¹⁸O_c and −15 to −10 PDB for δ¹³C_c) is much smaller than the range of measured and/or published isotope compositions of parent rock apatites (4–35 for δ¹⁸O_c and −11 to +1 for δ¹³C_c). In any profile, the apatites of weathering origin can exhibit lower, similar or higher δ¹⁸O_c values than parent rock apatites. In contrast, their δ¹³C_c values are systematically and significantly lower than those of the parent rock apatites. Apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition.Assuming that apatite CO₃ fractionates O in a way similar to calcite CO₃, the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions. The very low δ¹³C_c values exhibited by these apatites indicate that the dominant sources of dissolved CO₂ in the soil water are organic. The isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies.