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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]

Abstract: Secure storage of anthropogenic carbon dioxide (CO2) in geological reservoirs requires predicting gas–water–rock interactions over millennial timescales. Noble gases and carbon isotope measurements can be used to shed light on the nature of competing dissolution—precipitation processes over different timescales, from the fast dissolution of gaseous CO2 in groundwater to more sluggish reactions involving dissolution and precipitation of newly formed minerals in the reservoir. Here we study a compilation of gas analyses including noble gases and δ13C of CO2 from nine different natural CO2 reservoirs. Amongst these reservoirs, the Bravo Dome CO2 field (New Mexico, USA) shows distinct geochemical trends which are explained by degassing of noble gases from groundwater altering the composition of the gas phase. This groundwater degassing is synchronous with the dissolution of CO2 in groundwater. Progressive creation of alkalinity via CO2-promoted mineral dissolution is required to explain the observed positive correlation between CO2/3He and δ13C of the gas phase, a unique feature of Bravo Dome. The differences between Bravo Dome and other natural CO2 reservoirs are likely explained by the more recent filling of Bravo Dome, reflecting CO2–water–rock interactions over thousands of years rather than over millions of years in older reservoirs. [Copyright &y& Elsevier]

Abstract: Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth''s best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental margin was subducted to between 8 and 13kbar (30–45km) by 34.2±3.6Ma. These data define: (i.) an upper limit on the timing of eclogite facies metamorphism at 26.2±1.8kbar (70–80km) and 553±12°C, (ii.) plate velocity (1–6cm·a−1) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximum duration of 10Ma (28–38Ma) for juxtaposition of Alpine upper-plate and European basement units and subsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30Ma. One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slow to account for Tauern Barrovian conditions (550°C at 9–13kbar) within the maximum 10Ma interval between eclogite exhumation and the thermal peak (28–32Ma). Given that the Tauern Window is a classic locality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this work raises questions of fundamental importance concerning the length scales of the mechanisms responsible for heat transfer within orogenic crust. [Copyright &y& Elsevier]

Abstract: Reactions between CO2-charged brines and reservoir minerals might either enhance the long-term storage of CO2 in geological reservoirs or facilitate leakage by corroding cap rocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute mineral surface reaction rates and the significance of other rate limiting steps in natural systems. Here we use the chemical evolution of groundwater from the Jurassic Navajo Sandstone, part of a leaking natural accumulation of CO2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral–fluid reactions, under elevated CO2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2×10−14 and 4×10−16 mol m−2 s−1, respectively. Fluid ion-activity products are close to equilibrium (e.g. ΔG r for plagioclase between −2 and −10 kJ/mol) and lie in the region in which mineral surface reaction rates show a strong dependence on ΔG r. Local variation in ΔG r is attributed to the injection and disassociation of CO2 which initially depresses silicate mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer feldspar hydrolysis reactions consume H+ and liberate solutes to solution which increase mineral saturation in the fluid and rates slow as a consequence. The measured plagioclase dissolution rates at low ΔG r of 2×10−14 mol m−2 s−1 would be compatible with far-from-equilibrium rates of ~1×10−13 mol m−2 s−1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reaction rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium whereas most laboratory experiments are run far-from-equilibrium. [Copyright &y& Elsevier]

Abstract: Hot springs in the Marsyandi Valley, Nepal, vent CO2 sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earth''s surface. The δ 13C of spring waters ranges up to +13‰, while that of the coexisting free gas phase is close to −4‰. Empirical and thermodynamic modelling of this isotopic fractionation suggests >97±1% CO2 degassing. The calculated minimum total CO2 degassing in the Marsyandi catchment is 5.4×109 mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO2 degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO2 flux of 0.9×1012 mol/yr, or ∼13% of solid Earth CO2 degassing. The calculated flux is a factor of three greater than the estimated CO2 drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO2 flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability. [Copyright &y& Elsevier]

Abstract: An analytical solution to the equations describing the flow of a buoyant fluid released into a porous medium below a horizontal impermeable boundary is used to model the growth of CO2 accumulations beneath thin mudstone beds in the Utsira sand reservoir at Sleipner in the North Sea. Here supercritical CO2 has been injected at a rate of ∼1 MT/yr since 1996 and imaged by time-lapse seismic data in 1999, 2001 and 2002. The CO2 rises as a narrow plume and is partially trapped by a number of thin mudstones before reaching the caprock to the reservoir. The radii of the individual layers of trapped CO2 increase as the square root of time since initiation as predicted by the modelling for constant input flux. However apparent negative initiation times for horizons low in the reservoir suggests that net input fluxes for these layers have decreased with time, most probably as the spreading layers have increased their leakage rates. Accumulation of CO2 in the layers higher in the reservoir was initiated up to 3 yr after injection started. Modelling of the thickness profiles across three of the higher layers suggests that their net input fluxes have increased with time. The observation that the central thicknesses of the deeper layers have remained approximately constant, or have slightly decreased since first imaged in 1999, is consistent with the model predictions that the central thickness is directly proportional to net input flux. However, estimates of the permeability of the reservoir from the rate of increase of the radii of the CO2 accumulations are an order of magnitude less than measured permeabilities on the reservoir sandstone. Permeabilities estimated from the modelling of layer thickness changes scatter in the same range. These discrepancies may arise from, 1) approximations in the model not being valid, 2) the measured permeabilities not being representative of the permeability for two-phase flow on the scale of the reservoir or, considered less likely, 3) that much less CO2 is being stored in the imaged CO2 accumulations than estimated from the seismic reflection profiles. The most probable cause of the discrepancy is that the relative permeability for the CO2 phase is significantly reduced at lower CO2 saturations. [Copyright &y& Elsevier]

Palmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11–26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141–4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ∼1.8×109 mol yr−1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ∼2.3×109 mol yr−1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (∼8.7×109 mol yr−1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ∼8×108 mol yr−1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ∼3.1±0.8×109 mol yr−1 (87Sr/86Sr ∼0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation. [Copyright &y& Elsevier]

The outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics, erosion and climate. Here we use cosmogenic nuclides (10Be and 26Al) analysed in quartz from river sediments from the Upper Ganges catchment to make the first direct measurements of large-scale erosion rates in a rapidly uplifting mountain belt. The erosion rates are highest in the High Himalaya at 2.7±0.3 mm/yr (1σ errors), fall to 1.2±0.1 mm/yr on the southern edge of the Tibetan Plateau and are 0.8±0.3 to <0.6 mm/yr in the foothills to the south of the high mountains. These relative estimates are corroborated by the Nd isotopic mass balance of the river sediment. Analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years. The data presented here, along with data recently published for European river catchments, demonstrate that a log–linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes.The erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the Indian Himalaya. This confirms that much of the exhumation in the Himalayan mountain chain is now balanced by erosion. However, exhumation rates calculated from high blocking temperature systems, such as 40Ar/39Ar in muscovite, imply lower exhumation rates. Rocks presently at the surface must have undergone a three- to six-fold increase in exhumation rate within the last few million years. We show how this could be explained either by climatic forcing of erosion rate changes or by tectonics. Published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates. [Copyright &y& Elsevier]

Abstract: Understanding the controls on chemical weathering, especially of silicate minerals, remains a major challenge, despite its importance in controlling the evolution of the Earth''s surface. In particular, it has proved hard to distinguish the temperature sensitivity of silicate weathering rates from other factors. Here we present a new compilation of chemical and physical erosion rates in small catchments and show that silicate weathering rates are not governed by any single parameter but require consideration in multiple dimensions. The overall variation in silicate weathering rates with physical erosion rates, rainfall, and temperature can be quantitatively described by a parameterization based on considering their limiting relationships. At lower erosion rates mineral supply limits weathering. At higher erosion rates there is abundant material but kinetic and therefore climatic factors limit weathering. A predictive model describing the field data based on transport and weathering (kinetically) limited scenarios yields theoretically sensible values for fitted parameters. In the transport-limited case, the supply of silicate cations from weathering is directly proportional to the supply of material by erosion, consistent with complete leaching of cations from fresh regolith. In the kinetically limited case, weathering scales directly with runoff, as the square root of erosion rate, and with an activation energy of 74±29 kJ/mol, consistent with expected values in the Earth''s surface settings. [Copyright &y& Elsevier]

• Epidosites from ophiolites have lower δ 44 Ca than hydrothermal fluids. • The fractionation of −0.2 to −0.5 is most likely a kinetic isotope effect. • Epidote can form from oversaturated solutions at T > 250 °C. • Hydrothermal silicates may commonly form under non-equilibrium conditions. Calcium isotope ratios in epidote from epidosites in ophiolites of varying Phanerozoic ages have 44Ca/40Ca ratios that are lower by 0.1 to 0.6‰ relative to typical mid-ocean ridge hydrothermal fluids. Epidosites are inferred to form in high-temperature parts of seafloor hydrothermal systems at temperatures above 300 °C and where fluid fluxes are high, so the Ca isotopic composition of the epidote is likely to reflect fractionation during growth of crystals from aqueous solution. Available Ca isotope data from MOR hydrothermal vent fluids and mantle peridotites constrain the δ 44 Ca of likely modern hydrothermal fluids to a narrow range at δ 44 Ca = − 0.05 ± 0.1. A reactive-transport model is used to evaluate whether the δ 44 Ca of hydrothermal fluids might have been higher during the Cretaceous and Late Cambrian, the ages of the Troodos, Oman, and Betts Cove ophiolites from which we have data. For these calculations we use the epidosite 87Sr/86Sr as a guide to the extent of Ca isotopic exchange that affected the ancient hydrothermal fluids, which were derived from seawater with higher Ca and Sr, and lower sulfate concentration, than modern seawater. The calculations suggest that the ancient hydrothermal fluid δ 44 Ca values were not much different from modern values, with the possible exception of the Late Cambrian example. We infer that the epidote-fluid Ca isotope fractionation averaging Δ 44 Ca = − 0.2 to −0.6, is most likely due to kinetic isotope fractionation during mineral precipitation. There is evidence from the literature that hydrothermal epidote may commonly form from oversaturated solutions, which makes the kinetic isotope interpretation plausible. The equilibrium epidote-fluid Ca isotope fractionation is estimated to be Δ 44 Ca eq ≈ 0 based on recently reported DFT calculations. Our results suggest that kinetic calcium isotope fractionation can affect hydrothermal silicate minerals, and may be only slightly smaller in magnitude than the effects observed in Ca-bearing minerals at low temperature. Kinetic isotope effects during mineral growth could provide new insights into the formation mechanisms of hydrothermal silicate minerals. [ABSTRACT FROM AUTHOR]

Abstract: We present sulfur and oxygen isotope data from 41 samples of dissolved riverine sulfate from along 65km of the Marsyandi River in the Northern Himalayas. Coupled sulfur and oxygen isotopic composition of riverine sulfate ( and respectively) yield unique constraints on the source of sulfur to the river system. The headwaters of the Marsyandi River have light and , which requires that the source of sulfate to the river is through the anoxic weathering of pyrite (likely via Fe3+). The and of sulfate in tributaries to the Marsyandi increase downstream, which could result either from inputs from evaporites or bacterial sulfate reduction with subsequent sulfide precipitation in warmer and wetter catchments; either of these processes could result in heavy and of the residual river sulfate. Elemental ratios such as Sr/Ca and Ca/SO4 suggest, as previous studies have also concluded, that evaporite weathering is not important in the Marsyandi River. We conclude that the isotope data is most consistent with the onset of bacterial sulfate reduction and secondary sulfide precipitation in the soils in the warmer and wetter downstream catchments. Our results have implications for understanding the source of sulfate to the ocean as well as the redox and acidity budget within rapidly eroding catchments. [Copyright &y& Elsevier]

Abstract: CO2 sequestration is regarded as an important strategy for reducing anthropogenic CO2 emissions. Both the nature and rate of fluid–mineral reactions in CO2–water–rock systems are crucial, yet poorly constrained, parameters in understanding the fate of CO2 injected in geological formations. This study models reactions and reaction rates in an exhumed CO2-charged aquifer where CO2-rich brines have bleached red sandstones by dissolution of hematite grain coatings. We show that the vertical movement of the reaction front is dominated by diffusion and this allows calculation of reaction rates for the dissolution of hematite grain coatings and K-feldspar. Using mineral surface areas calculated from BET measurements, we estimate K-feldspar dissolution rates of ; and hematite reaction rates of . The rates for K-feldspar are lower than previous, experimentally derived, estimates of K-feldspar dissolution rates by 1–2 orders of magnitude, likely explained by the proximity of the natural system to equilibrium. The inferred hematite reaction rates are 5–6 orders of magnitude slower than laboratory experiments and appear to be controlled by the chemical gradients imposed by the more sluggish K-feldspar dissolution. As the majority of potentially mobile trace metals are hosted in iron-oxide grain coatings, we argue that the rate of contaminant mobilization by CO2-charged brines will be lower than suggested by laboratory experiments. [Copyright &y& Elsevier]

Abstract: We analysed the water chemistry of a 6-yr time series of the Liwu River, Taiwan, as well as spot samples of tributaries and groundwater. In this steep and well-drained catchment impacted by typhoon events, the river carries a high dissolved load (average of 320mg/L) dominated by Ca2+ and Mg2+ (73Eq% and 21Eq% of the cationic charge), and SO4 2− and HCO3 − (48Eq% and 51Eq% of the anionic charge). Hysteresis in Ca/Na-discharge data and absence of hysteresis in matching Ca/Sr-discharge data are interpreted as reflecting a mixing of three components. While river chemistry at low- to medium-flow reveals a binary mixing between a rapid surface runoff characterised by the chemistry of first-order tributaries and a component likely to have a deep groundwater chemistry, samples collected at high-flow show the contribution of a third water source not immediately mobile and therefore called slow surface runoff. The relative contribution of these end-members has been estimated through hydrograph separations, and then validated by modelling of the observed riverine 87Sr/86Sr variations (0.0013) during a typhoon. The 37-yr average estimates that 16±3% of the river discharge comes from the deep groundwater reservoir while slow and rapid surface runoff provide 21±5% and 63±5%, respectively. The slow surface runoff is the least well characterised of the end-members but the modelled evolution of water chemistry along its flow-path suggests that cations (except Na) and sulphate are removed from the water in the slow surface runoff source region by the combination of biological uptake, bacterial reduction and pedogenesis (clays and carbonate precipitation) — processes that might be partly driven by the evapo-transpiration that removes 75% of the water within this reservoir. The silicate cation denudation rate for the Liwu is 18tkm−2 yr−1, which is one of highest measured so far in the world for felsic lithologies. This high value has to be modulated by the facts that 1) deep chemical weathering of silicate accounts for more than 1/3 of this flux, and 2) sulphuric-acid weathering is significant in this catchment. In Taiwan, the input of chemicals in river flux from deep reservoirs is likely to be promoted by rapid uplift of fractured rocks, but the underlying processes (chemical reaction in a porous media) responsible for such significant groundwater contribution are also present in alluvial and sedimentary basins. [Copyright &y& Elsevier]

• 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]