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Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes CO2 from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net CO2 consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth CO2 degassing fluxes need to be further investigated. [ABSTRACT FROM AUTHOR]

The mass-independent minor oxygen isotope compositions (△' 17O) of atmospheric O2 and CO2 are primarily regulated by their relative partial pressures, pO2/pCO2. Pyrite oxidation during chemical weathering on land consumes O2 and generates sulfate that is carried to the ocean by rivers. The △'17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct O2 incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation—including oxygen sources—in weathering environments remains elusive. To address this issue, we present sulfate source estimates and △'17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting △'17O values imply no direct tropospheric O2 incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17Oenriched sources such as reactive oxygen species. Sulfate △'17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived △'17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate △'17O as a proxy for past pO2/pCO2 should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling. [ABSTRACT FROM AUTHOR]

Highlights • Li and Li-isotopic ratios in river water may be described by two dimensionless numbers. • Multiple hydrological, climatic and tectonic parameters make up the dimensionless numbers. • Interpretation of Li-isotope records must account for these multiple controls. • The reactive transport model relates catchments in terms of reaction stoichiometry and hydrology. • Li-isotope systematics in Himalayan catchments illustrate these dependencies. Abstract Analytical solutions to reactive-transport equations describing the evolution of Li concentrations and isotopic ratios are presented for one-dimensional flow paths where reaction stoichiometry is constant along the flow path. These solutions are considered appropriate for chemical weathering in rapidly eroding catchments. The solutions may be described by two dimensionless numbers; 1) a Damköhler number describing the product of reaction rate and fluid residence time, and 2) a net partition coefficient which describes the fraction of Li re-precipitated in secondary minerals as the product of a fluid-secondary mineral partition coefficient and the mass fraction of secondary mineral precipitates. In settings where water entering flow paths is dilute, Li concentrations will increase along the flow path until they reach a limiting value determined by the net partition coefficient. Simultaneously, 7Li/6Li isotopic ratios will increase to a limiting value of the source rock ratio minus the secondary mineral–fluid Li-isotopic fractionation factor. Waters with Li-isotopic ratios in excess of this limiting value must have evolved with a change of reaction stoichiometry and/or partition coefficient along the flow path such that at some point net removal of Li to secondary minerals exceeds that supplied by dissolution of primary minerals. The modelling shows that the multiple controls on chemical weathering rates (temperature, rainfall, erosion rate, hydrology) cannot be inferred from Li concentration and isotopic ratio data alone, which only provide two independent constraints. Caution should be exercised in interpretation of oceanic Li records in terms of potential climatic variables. The model is illustrated by a set of Li concentration and isotopic ratio measurements on river waters and bed sands in the Alaknanda river basin which forms the headwaters of the Ganges. This illustrates how values of the Damköhler number and net partition coefficient can be used to trace weathering processes. Water samples from catchments with similar lithologies and climates scatter along contours of approximately constant net partition coefficient, reflecting similar reaction stoichiometries, but with more variable Damköhler numbers reflecting variations in flow path length, fluid flux and/or reaction rate. Samples from the lower, warmer and less rapidly eroding catchments have high 7Li/6Li isotopic ratios with lower Li concentrations and must reflect at least a two-stage weathering process where reaction stoichiometry and/or Li fluid–mineral partition coefficients change along the flow path so that net Li is removed in the later stages. [ABSTRACT FROM AUTHOR]

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties. [ABSTRACT FROM AUTHOR]

• Oxygen isotopes in river water sulfate enable partitioning of sulfate source. • Isotope data demonstrates 56% of sulfate in the Mekong River is derived from pyrite. • Pyrite oxidation releases CO 2 through weathering of carbonates with sulfuric acid. • Carbon consumption from silicate weathering offset in large rivers. The weathering of carbonate rocks with sulfuric acid releases carbon dioxide (CO 2) to the atmosphere, offsetting the CO 2 drawdown from carbonic acid weathering of silicates thought to regulate global climate. Quantifying CO 2 release from sulfuric acid weathering requires the partitioning of riverine sulfate between its two main sources: sedimentary sulfate and sulfide. Although the sulfur (δ 34 S S O 4 ) and oxygen (δ 18 O S O 4 ) isotope ratios of sedimentary sulfates (gypsum and anhydrite) of different ages are well constrained, the δ 34 S of sulfide minerals is highly variable, restricting the utility of δ 34 S for partitioning sulfur sources. Here, we use oxygen isotope ratios in the river water (δ 18 O H 2 O) and sulfate molecules (δ 18 O S O 4 ) to partition the fraction of sulfate and associated uncertainty delivered by the oxidative weathering of pyrite (f pyr). The partitioning is illustrated using the Mekong River, one of the world's largest river basins, presenting new δ 18 O S O 4 , δ 18 O H 2 O and δ 34 S S O 4 data collected on 18 tributaries and 6 mainstem sites over two field seasons at peak flux. The geological, geomorphological and climatic diversity of the Mekong River basin make it an ideal field site to quantify the role of sulfuric acid weathering and its implications for the carbon cycle. There is a 12‰ range in both the difference between δ 18 O S O 4 and δ 18 O H 2 O (Δ 18 O S O 4 − H 2 O) and δ 34 S in the river waters of the basin. In the Mekong tributaries, sources of sulfate are highly variable with the fraction of sulfate derived from pyrite oxidation (f pyr) ranging from 0.19 to 0.84. In the mainstem, f pyr reflects the flux-weighted mean of these tributary inputs, with 56 ± 7 % (1 σ) of the sulfate delivered to the ocean at the Mekong mouth being derived from the oxidative weathering of pyrite. As a result, we estimate that ∼70% of CO 2 consumed through silicate weathering in the Mekong basin is offset by the release of CO 2 via the dissolution of carbonates by sulfuric acid. [ABSTRACT FROM AUTHOR]