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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]

Oxidation of rock-derived, petrogenic, organic carbon (OC petro ) during weathering of sedimentary rocks is a major source of carbon dioxide (CO 2 ) to the atmosphere. This geological respiration is thought to be enhanced by physical erosion, suggesting that mountain belts could release large amounts of CO 2 to counter the CO 2 sequestration achieved by the erosion, riverine transfer and oceanic burial of organic carbon from the terrestrial biosphere. However, OC petro oxidation rates in mountain belts have not been quantified. Here we use rhenium (Re) as a proxy to track OC petro oxidation in mountain river catchments of Taiwan, where existing measurements of physical erosion rate allow the controls on OC petro oxidation to be assessed. Re has been shown to be closely associated with OC petro in rocks and following oxidation during chemical weathering forms a soluble oxyanion ( ReO 4 − ) which contributes to the dissolved load of rivers. Soils on meta-sedimentary rocks in Taiwan show that Re loss is coupled to OC petro loss during weathering, confirming previous observations from soil profiles on sedimentary rocks elsewhere. In Taiwan rivers, dissolved Re flux increases with the catchment-average sediment yield, suggesting that physical erosion rate is a major control on OC petro oxidation. Based on our current understanding of Re mobility during weathering, the dissolved Re flux can be used to quantify an upper bound on the OC petro oxidation rate and the associated CO 2 transfer. The estimated CO 2 release from this mountain belt by OC petro oxidation does not negate estimates of CO 2 sequestration by burial of biospheric OC offshore. The findings are compared to OC transfers estimated for the Himalaya, where OC petro oxidation in the mountain belt remains unconstrained. Together, these cases suggest that mountain building in the tropics can result in a net sink of OC which sequesters atmospheric CO 2 . [ABSTRACT FROM AUTHOR]

Uranium (U) isotope signatures (δ238U) recorded in sedimentary archives provide insight into the paleo redox state of the ocean. But the robust interpretation of these sedimentary U isotope records requires characterisation of the U isotope signature of rivers, the main source of U to the ocean. The main controlling factors on riverine δ238U remain poorly constrained. Here, we investigate the elemental and isotope signatures of uranium in the dissolved and solid loads of a well-characterised river, the Mackenzie Basin (Canada). In the Mackenzie Basin, the solid load δ238U shows a positive relationship with U and vanadium (V) contents, consistent with the suggestion that particulate δ238U are explained by variable contributions via erosion of silicate and black shale. The δ238U of the dissolved and solid loads are correlated which, at first sight, suggests no U isotope fractionation during chemical weathering, and a purely lithological control on both the river dissolved and solid δ238U. Moreover, relationships between dissolved U and δ238U and major elements such as calcium and sulfate, also support the idea of a lithological control. However, the δ238U of end members inferred from mixing relationships are not consistent with binary mixing of two sources, suggesting some potential U isotope fractionation during weathering. In fact, the abundance of U in the river dissolved load is always lower than that predicted by silicate rock weathering. This suggests that 1) the weathering of silicate only can explain the abundance of U in the river dissolved load and 2) secondary weathering processes scavenge a proportion of the U released by primary mineral breakdown. The broad negative relationship between δ238U and the depletion of dissolved U is also consistent with the control of dissolved δ238U by secondary weathering processes following silicate mineral breakdown. The relationships observed between dissolved U, δ238U and the large-scale environmental controls on weathering processes (such as weathering intensity or runoff) support this hypothesis. Overall, our interpretations of the variation in the river dissolved δ238U challenge the common assumption of the control of dissolved U by black shale and carbonate weathering. In addition, we suggest that the extent of secondary weathering processes can imprint on the U isotope signature of rivers, now and in the past. [ABSTRACT FROM AUTHOR]

The erosion of major mountain ranges is thought to be largely cannibalistic, recycling sediments that were deposited in the ocean or on the continents prior to mountain uplift. Despite this recognition, it has not yet been possible to quantify the amount of recycled material that is presently transported by rivers to the ocean. Here, we have analyzed the Li content and isotope composition (δ7Li) of suspended sediments sampled along river depth profiles and bed sands in three of the largest Earth's river systems (Amazon, Mackenzie and Ganga-Brahmaputra rivers). The δ7Li values of river-sediments transported by these rivers range from +5.3 to -3.6‰ and decrease with sediment grain size. We interpret these variations as reflecting a mixture of unweathered rock fragments (preferentially transported at depth in the coarse fraction) and present-day weathering products (preferentially transported at the surface in the finest fraction). Only the finest surface sediments contain the complementary reservoir of Li solubilized by water-rock interactions within the watersheds. Li isotopes also show that river bed sands can be interpreted as a mixture between unweathered fragments of igneous and sedimentary rocks. A mass budget approach, based on Li isotopes, Li/Al and Na/Al ratios, solved by an inverse method allows us to estimate that, for the large rivers analyzed here, the part of solid weathering products formed by present-day weathering reactions and transported to the ocean do not exceed 35%. Li isotopes also show that the sediments transported by the Amazon, Mackenzie and Ganga-Brahmaputra river systems are mostly sourced from sedimentary rocks (>60%) rather than igneous rocks. This study shows that Li isotopes in the river particulate load are a good proxy for quantifying both the erosional rock sources and the fingerprint of present-day weathering processes. Overall, Li isotopes in river sediments confirm the cannibalistic nature of erosion and weathering. [ABSTRACT FROM AUTHOR]