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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]

Weathering of organic carbon contained in sedimentary rocks (petrogenic OC, OC petro) is an important control on the concentrations of carbon dioxide (CO 2) and oxygen in the atmosphere. Of particular significance are steep mountainous catchments, where high rates of physical erosion introduce OC petro to the surface, where oxygen in air and water can help drive oxidative weathering reactions, yet measurements of CO 2 emissions from OC petro oxidation are still scarce. Here, we use in situ gas accumulation chambers and show that CO 2 fluxes, and their environmental controls, can be determined during a stand-alone, short-term (8 days) field campaign, applied to a remote setting. In the rapidly eroding Waiapu River catchment, New Zealand, dominated by mudstones, we measured high rates of CO 2 release (222–1590 mgC m−2 d−1) in five accumulation chambers in the near-surface of naturally fractured and bedded rock outcrops. The corresponding CO 2 concentrations are very high (p CO 2 ~4700–27,100 ppmv), and such values could influence acid-hydrolysis reactions during chemical weathering of co-occurring silicate minerals. The CO 2 is radiocarbon depleted (fraction modern, F 14C = 0.0122–0.0547), confirming it is petrogenic in origin. Stable carbon isotopes suggest a source from OC petro , but δ13C values of the CO 2 are lower by ~3.5–3.7 ± 0.1 ‰ from those of OC petro (−25.9 ± 0.1 ‰), consistent with isotope fractionation associated with microbial respiration of OC petro. Over 6 days of measurement, we find that CO 2 fluxes respond quickly to changes in temperature and humidity, indicating an environmental regulation that is captured by our short-term installation. The approaches applied here mean that future research can now seek to constrain the climatic, lithological and biological controls on OC petro oxidation across regional to global scales. • CO 2 fluxes from oxidative weathering can be measured during short field campaigns. • Their size, regulated by temperature and humidity, reaches that of soil respiration. • Carbon isotopes reveal that measured CO 2 is sourced from petrogenic organic matter. • Microbial respiration is potentially associated with carbon isotope fractionation. • Partial pressures of CO 2 in rock can be high and induce silicate weathering. [ABSTRACT FROM AUTHOR]