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In streams, short‐term element‐specific solute fluxes are often not balanced with long‐term chemical weathering fluxes determined in the residual solids from fractional element loss and denudation rate. The ratio of both estimates—the "Dissolved Export Efficiency" (DEE)—is frequently <1, indicating deficits in the stream dissolved load. To explore the cause of the stream deficits, we performed a daily water sampling campaign for one year in a forested headwater watershed in Southern Germany. We sampled surface runoff, above‐canopy and below‐canopy precipitation, subsurface flow from the organic soil layer, upper, and deep mineral soil, and groundwater. Regolith samples were obtained from a drill core and revealed the weathering front to lie between 7 and 15 m depth. We found a DEE < 1 for K, Si, Al, Fe. These elements are characterized by shallow slopes in C‐Q relationships, and the imbalances were found to originate in the deep saprolite. Their export pathway potentially includes "hidden" Critical Zone compartments or fluxes, presumably unsampled colloids that are exported preferentially during rare flushing events with stochastic temporal distribution. The DEE of nutritive elements like Ca, Mg, and P is also <1. These elements are characterized by steeper C‐Q slopes, and their imbalance can be explained by deep nutrient uptake followed by nutrient retainment in re‐growing forest biomass or export in plant debris. The collective evidence for these imbalances, including previous evidence from metal stable isotopes, suggests that the deep Critical Zone represents the location for chemical or biogenic retention and release of solutes. Plain Language Summary: Mineral dissolution by chemical weathering produces solutes in the weathering zone that are eventually exported by streams. Yet, often an imbalance between element‐specific solute export fluxes in stream water and element‐specific chemical weathering fluxes determined from residual solids in the weathering zone is apparent. This imbalance is expressed as a deficit in the stream dissolved load. To explore the origin of this deficit, we performed a daily water sampling campaign for one year in a forested watershed in Southern Germany. We studied concentration‐discharge relationships and quantified solute fluxes in stream‐ and groundwater. A comparison of solute fluxes in stream water with chemical weathering fluxes integrating mineral dissolution over the whole weathering zone revealed indeed deficits in the stream dissolved load. These deficits originate in deep isovolumetrically weathered bedrock (saprolite). The elements K, Si, Al, and Fe, which show invariant concentration changes with increasing discharge, may follow a "hidden" export pathway involving colloids that are only mobilized and exported during rare flushing events that were missed during this study's sampling period. Deficits of nutrients (Ca, Mg, P), which show dilution behavior with increasing discharge, can be explained by deep nutrient uptake and retention in growing forest biomass or plant litter erosion. Key Points: Imbalances between short‐term and long‐term estimates of element‐specific chemical weathering fluxes (DEE < 1) emerge in deep saproliteElements with shallow C‐Q slopes showing chemostatic or enrichment behavior are likely exported in colloidal form during flushing eventsElements with steeper C‐Q slopes showing weak dilution behavior remain in re‐growing biomass or are exported in particulate biogenic form [ABSTRACT FROM AUTHOR]

A negative feedback between silicate weathering rates and climate is hypothesized to play a central role in moderating atmospheric CO 2 concentrations on geologic timescales. However, uncertainty regarding the processes that regulate the operation of the negative feedback limits our ability to interpret past variations in the ocean–atmosphere carbon cycle. In particular, the mechanisms that determine the flux of weathered material for a given climatic state are still poorly understood. Here, we quantify the processes that determine catchment-scale solute fluxes for two lithologic end-members—basalt and granite—by applying a recently developed solute production model that links weathering fluxes to both discharge and the reactivity of the weathering material. We evaluate the model against long-term monitoring of concentration–discharge relationships from basaltic and granitic catchments to determine the parameters associated with solute production in each catchment. Higher weathering rates in basaltic catchments relative to granitic catchments are driven by differing responses to increases in runoff, with basaltic catchments showing less dilution with increasing runoff. In addition, results from the solute production model suggest that thermodynamic constraints on weathering reactions could explain higher concentrations in basaltic catchments at lower runoff compared to granitic catchments. To understand how the response to changing discharge controls weathering fluxes under different climatic states, we define basalt/granite weatherability as the ratio of the basalt catchment flux to the granite catchment flux. This weatherability is runoff-dependent and increases with increasing runoff. For HCO 3 − and SiO 2 (aq) fluxes, for modern global runoff, the derived mean basalt/granite weatherability is 2.2 (1.3–3.7, 2 σ ) and 1.7 (1.6–2.1, 2 σ ), respectively. Although we cannot determine the array of individual processes resulting in differences among catchments, the relative differences in certain model parameters that represent catchment-scale weathering fluxes of granitic and basaltic lithologies are robust. Our approach provides a mechanism that links runoff with the distribution of global sub-aerial silicate lithologies to understand how the basalt/granite weatherability of the Earth’s surface may have varied on geologic timescales. The relationships between basalt/granite weatherability and runoff derived in this study could be used to parameterize the silicate weathering negative feedback to model past changes in p CO 2 . [ABSTRACT FROM AUTHOR]

Mineral waters were monthly sampled in selected springs from Furnas and Fogo trachytic central volcanoes (São Miguel, Azores, Portugal). Water temperatures between 15.1°C and 90.2°C, characterize poorly mineralized waters of Na–HCO3 and Na–HCO3–Cl types. According to the spring location, two watersheds were selected in each volcano to evaluate solute fluxes and chemical weathering rates (Fogo volcano: Ribeira Grande river — RRG; Furnas volcano: Ribeira Quente river — RRQ). Na, Mg, K and Ca fluxes in groundwater represented 43%, 60%, 46% and 57% of the total (subsurface plus surface fluxes) in RRG, and respectively 43%, 53%, 46% and 49% in RRQ. Average HCO3 flux is ten times higher in RRQ (130.1×106 mol/yr) compared to RRG (13.8×106 mol/yr), reflecting the volcano degassing and the lower ratio between groundwater and river water fluxes. Based on these values, total CO2-consumption by weathering ranged from 1.5×106 mol/km2/yr (RRG) to 4×106 mol/km2/yr (RRQ). TDS load varied between 3772t/yr (RRG) and 15388t/yr (RRQ), and the ratio between values in groundwater and in surface water is respectively equal to 0.72 and 2.04. The associated chemical weathering rates in groundwater were 206t/km2/yr (RRG) and 399t/km2/yr (RRQ). Coupled to river water, these values indicate that total chemical weathering rates are respectively equal to 493t/km2/yr and 594t/km2/yr. A similar approach developed for the entire archipelago showed that the chemical weathering rates due to groundwater are in the range from 33 to 321t/km2/yr, being partially controlled by the age of the islands. Results of the present study point out to the need to consider groundwater solute fluxes due to weathering when establishing geochemical budgets. [ABSTRACT FROM AUTHOR]

Abstract: The role of fluid residence time and catchment length scales in controlling the chemical composition of rivers is evaluated by comparing numerical simulations and scaling arguments to concentration–discharge data from small catchments. The analysis suggests that poorly-crystalline aluminosilicates are an important control on the composition of stream waters and therefore chemical equilibrium between the dissolving and precipitating phases determines the maximum concentration and the maximum silicate weathering flux. The modeling results suggest that the residence time of fluid relative to the residence time required to approach chemical equilibrium can be used to assess the controls on solute fluxes in small catchments, and possibly larger rivers. Catchments that show little variability in concentration with discharge (or “chemostatic behavior”) likely have average fluid residence times that exceed the time required to reach chemical equilibrium. Conversely, decreases in concentration with increasing discharge are explained by average residence times shorter than required to approach chemical equilibrium, resulting in dilution. Solute fluxes are also strongly impacted by the distribution of fluid residence times in a basin. The fluid residence time model provides an alternative framework for assessing both the relationship between discharge and concentration observed for individual catchments, and controls on the solute fluxes of rivers. If fluid residence times are a dominant control on weathering fluxes, the chemistry of different rivers could vary entirely as a function of the nature subsurface flow paths and the composition of the system at equilibrium, which is complex to predict and strongly coupled to biological processes, temperature and the composition of the system. As a result of thermodynamic and hydrologic restrictions on the amount of weathering, global solute fluxes may depend more strongly on the geometry, relief, runoff and permeability of basins then on temperature and rates of erosion. [Copyright &y& Elsevier]