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The balance between the burial of biospheric organic carbon (OC bio) in the ocean and the oxidation of rock-derived organic carbon across landscapes (OC petro) helps to regulate atmospheric CO 2 and O 2 over geological time. However, we lack reconstructions of these processes over the timescales necessary to properly understand the drivers and feedbacks operating. Here we use a sediment core from the Zhuoshui River delta in Taiwan, which receives sediments from a rapidly uplifting and eroding catchment, to reconstruct the variations in OC content and radiocarbon composition since the Last Glacial Maximum (LGM). We find that the export of OC bio and the oxidation of OC petro are both modulated by climate-driven changes in physical erosion and temperature. During cold and dry periods, such as the LGM and the Younger Dryas (YD), shallow erosion and low temperature enhanced the preservation of OC bio in the biosphere in catchment, while deep erosion and high temperature during warm and wet periods, such as the Holocene, favored the dilution and degradation of OC bio. The weathering intensity of OC petro (ω) was inversely related to physical erosion, suggesting a lower intensity of oxidation during the Holocene period while the overall oxidation flux was enhanced. This suggests that the degree of weathering was primarily controlled by physical erosion. We propose a proxy to estimate the ratio of fluxes of OC bio export and OC petro oxidation, and show that the net balance between these two processes shifted from a carbon sink during the late deglacial period to a carbon source during the mid-late Holocene. Our study reveals that with CO 2 rise, a warmer and wetter climate would promote the exposure and oxidation of OC petro by erosion in this mountain range, leading the organic carbon balance towards a CO 2 source to the atmosphere. • Erosion and temperature control carbon dynamic in erosive setting. • Cold, dry periods boost carbon preservation. • Warm, wet climate shifts mountains to carbon sources. [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]

• 0.3‰ of δ 187 Re variation across rivers from the Mackenzie River basin. • For each river, the δ 187 Re of the dissolved load is higher than that of the corresponding river sediment • The δ 187 Re of river water and sediments is controlled by both provenance and modern oxidative weathering processes • The average δ 187 Re of Mackenzie bedrock (∼ − 0.05 ‰) and dissolved load (∼ − 0.01 ‰) are lower than Atlantic seawater δ 187 Re. Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. Here, we report the first rhenium isotopic measurements (δ 187 Re) from river-borne materials to assess the influence of chemical weathering on Re isotopes at continental scale. The δ 187 Re was measured in water, suspended sediments and bedloads from the Mackenzie River and its main Arctic tributaries in Northwestern Canada. We find that the δ 187 Re (relative to NIST SRM 989) of river waters ranges from −0.05‰ to +0.07‰, which is generally higher than the corresponding river sediment (−0.25‰ to +0.01‰). We show that the range of δ 187 Re in river sediments (∼0.30‰) is controlled by a combination of source bedrock isotopic variability (provenance) and modern oxidative weathering processes. After correcting for bedrock variability, the δ 187 Re of solids appear to be positively correlated with the amount of Re depletion related to oxidative weathering. This correlation, and the offset in δ 187 Re between river water and sediment, can be explained by preferential oxidation of reactive phases with high δ 187 Re (i.e. rock organic carbon, sulfide minerals), but could also result from fractionation during oxidation or the influence of secondary weathering processes. Overall, we find that both basin-average bedrock δ 187 Re (∼−0.05‰) and dissolved δ 187 Re (∼−0.01‰) in the Mackenzie River are lower than the δ 187 Re of Atlantic seawater (+0.12‰). These observations provide impetus for future work to constrain the Re isotope mass balance of seawater, and assess the potential for secular shifts in its δ 187 Re values over time, which could provide an additional isotopic proxy to trace current and past redox processes at Earth's Surface. [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]

• Assessment of reactive phases in suspended particulates in a major Arctic river. • Neodymium isotopes show equilibration between Fe-Mn oxides and dissolved loads. • Reactive iron flux from the Mackenzie River is large; 0.21(+0.06,−0.05) Tg/yr. • Sedimentary rock weathering provides an important source of labile trace metals. Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present. [ABSTRACT FROM AUTHOR]