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Oxidative weathering of sedimentary rocks plays an important role in the global carbon cycle. Rhenium (Re) has been proposed as a tracer of rock organic carbon (OCpetro) oxidation. However, the sources of Re and its mobilization by hydrological processes remain poorly constrained. Here, we examine dissolved Re as a function of water discharge, using samples collected from three alpine catchments that drain sedimentary rocks in Switzerland (Erlenbach and Vogelbach) and Colorado, USA (East River). The Swiss catchments reveal a higher dissolved Re flux in the catchment with higher erosion rates, but have similar [Re]/[Na+] and [Re]/[SO42−] ratios, which indicate a dominance of Re from OCpetro. Despite differences in rock type and hydro-climatic setting, the three catchments have a positive correlation between river water [Re]/[Na+] and [Re]/[SO42−] and water discharge. We propose that this reflects preferential routing of Re from a near-surface, oxidative weathering zone. The observations support the use of Re as a proxy to trace rock-organic carbon oxidation, and suggest it may be a hydrological tracer of vadose zone processes. We apply the Re proxy and estimate CO2 release by OCpetro oxidation of 5.7 +6.6/−2.0 tC km−2 yr−1 for the Erlenbach. The overall weathering intensity was ∼40%, meaning that the corresponding export of unweathered OCpetro in river sediments is large, and the findings call for more measurements of OCpetro oxidation in mountains and rivers as they cross floodplains.

Humid tropical forests play a dominant role in the functioning of Earth but are under increasing threat from changes in land use and climate. How forest vulnerability varies across space and time and what level of stress forests can tolerate before facing a tipping point are poorly understood. Here, we develop a tropical forest vulnerability index (TFVI) to detect and evaluate the vulnerability of global tropical forests to threats across space and time. We show that climate change together with land-use change have slowed the recovery rate of forest carbon cycling. Temporal autocorrelation, as an indicator of this slow recovery, increases substantially for above-ground biomass, gross primary production, and evapotranspiration when climate stress reaches a critical level. Forests in the Americas exhibit extensive vulnerability to these stressors, while in Africa, forests show relative resilience to climate, and in Asia reveal more vulnerability to land use and fragmentation. TFVI can systematically track the response of tropical forests to multiple stressors and provide early-warning signals for regions undergoing critical transitions.

The transfer of carbon from land to the near-coastal ocean is increasingly being recognized in global carbon budgets. However, a more direct transfer of terrestrial carbon to the deep-sea is comparatively overlooked. Among systems that connect coastal to deep-sea environments, the Congo Submarine Canyon is of particular interest since the canyon head starts 30 km into the Congo River estuary, which delivers ~7 % of the total organic carbon from the world's rivers. However, carbon and sediment transport mechanisms that operate in the Congo Canyon, and submarine canyons more globally, are poorly constrained compared to rivers because monitoring of deep-sea canyons remains challenging. Using a novel array of acoustic instruments, sediment traps and cores, this study seeks to understand the hydrodynamic processes that control delivery of particulate organic carbon via the Congo Submarine Canyon to the deep-sea. We show that particulate organic carbon transport in the canyon-axis is modulated by two processes. First, we observe periods where the canyon dynamics are dominated by tides, which induce a background oscillatory flow (speeds of up to 0.15 m/s) through the water column, keeping muds in suspension, with a net upslope transport direction. Second, fast-moving (up to 8 m/s) turbidity currents occur for 35 % of the time during monitoring periods and transport both muddy and sandy particulate organic carbon at an estimated transit flux that is more than ten times the flux induced by tides. Remarkably, organic carbon transported and deposited in the submarine canyon has a similar isotopic composition to organic carbon in the Congo River, and in the deep-sea fan at 5 km of water depth. Episodic turbidity currents, together with background tidal currents thus promote efficient transfer of river-derived particulate organic carbon in the Congo Submarine Fan, leading to some of the highest terrestrial carbon preservation rates observed in marine sediments globally. [ABSTRACT FROM AUTHOR]

Mountain uplift and erosion have regulated the balance of carbon between Earth’s interior and atmosphere, where prior focus has been placed on the role of silicate mineral weathering in CO2 drawdown and its contribution to the stability of Earth’s climate in a habitable state1–5. However, weathering can also release CO2 as rock organic carbon (OCpetro) is oxidized at the near surface6,7; this important geological CO2 flux has remained poorly constrained3,8. We use the trace element rhenium in combination with a spatial extrapolation model to quantify this flux across global river catchments3,9. We find a CO2 release of 68 − 6 + 18 megatons of carbon annually from weathering of OCpetro in near-surface rocks, rivalling or even exceeding the CO2 drawdown by silicate weathering at the global scale10. Hotspots of CO2 release are found in mountain ranges with high uplift rates exposing fine-grained sedimentary rock, such as the eastern Himalayas, the Rocky Mountains and the Andes. Our results demonstrate that OCpetro is far from inert and causes weathering in regions to be net sources or sinks of CO2. This raises questions, not yet fully studied, as to how erosion and weathering drive the long-term carbon cycle and contribute to the fine balance of carbon fluxes between the atmosphere, biosphere and lithosphere2,11.Silicate weathering of uplifted rock depletes atmospheric CO2, but oxidation of revealed rock organic carbon supplies CO2, offsetting depletion to a degree dependent on regional geological history. [ABSTRACT FROM AUTHOR]

The oxidation of organic carbon contained within sedimentary rocks ("petrogenic" carbon, or hereafter OCpetro) emits nearly as much C2 as is released by volcanism, thereby playing a key role in the long-term global C budget. High erosion rates in mountains have been shown to increase OCpetro oxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OCpetro oxidation in mountains versus floodplains remains difficult to assess as disparate methods have been used in the different environments. Here, we investigate the sources and fluxes of rhenium (Re) in the Rio Madre de Dios to quantify OCpetro oxidation from the Andes to the Amazon floodplain using a common approach. Dissolved rhenium concentrations (n = 131) range from 0.01 to 63 pmol L-1 and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OCpetro oxidation and that this proportion increases downstream. We estimate that in the Andes, OCpetro oxidation releases 11.2+4.5/-2.8 tC km-2 y-1 of C2, which corresponds to ~41% of the total OCpetro denudation (sum of oxidized and solid OCpetro). A Re mass balance across the Rio Madre de Dios shows that 46% of OCpetro oxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OCpetro oxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OCpetro oxidation and C2 release. [ABSTRACT FROM AUTHOR]

The oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge‐top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first‐order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (< 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that > 80% of Re in the rock is associated with OCpetro, based on Re/Na and Re/S ratios. Using estimates of long‐term denudation rates, the observed OCpetro loss and the Re proxy are equivalent to a low OCpetro oxidation yield of < 1.7 × 10−2 tC km−2 yr−1. This is consistent with the low OCpetro concentrations and low denudation rates at this location. In addition, we find the surface cycle of Re is decoupled from that of deep weathering, with an enrichment of Re in surface soils and elevated Re concentrations in stream water, precipitation, and shallow groundwater. A mass balance model shows that this can be explained by a historical anthropogenic contribution of Re through atmospheric deposition. We estimate that the topsoil Re pool could take decades to centuries to deplete and call for a renewed focus on anthropogenic perturbation of the surface Re cycle in low denudation rate settings. [ABSTRACT FROM AUTHOR]

Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere (1-3). Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (C[O.sub.2]) release (4-6). However, some of this soil organic carbon may be eroded and transferred to rivers (7-9). If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological C[O.sub.2] sink (8-10). Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean (11, 12), and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC (10, 13, 14). Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers (13, 14). From the measured biospheric POC content and variability in annual sediment yield (15), we calculate a biospheric POC flux of [2.2.sup.+1.3.sub.-0.9] teragrams of carbon per year from the Mackenzie River, which is three times the C[O.sub.2] draw down by silicate weathering in this basin (16). Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological C[O.sub.2] sink., Photosynthesis and the production of organic carbon by the terrestrial biosphere (O[C.sub.biosphere]) is a major pathway of atmospheric C[O.sub.2] drawdown. Over millennial timescales, some O[C.sub.biosphere] escapes oxidation and contributes to [...]

Much attention has been focused on fine‐grained sediments carried as suspended load in rivers due to their potential to transport, disperse, and preserve organic carbon (OC), while the transfer and fate of OC associated with coarser‐grained sediments in fluvial systems have been less extensively studied. Here, sedimentological, geochemical, and biomolecular characteristics of sediments from river depth profiles reveal distinct hydrodynamic behavior for different pools of OC within the Mackenzie River system. Higher radiocarbon (14C) contents, low N/OC ratios, and elevated plant‐derived biomarker loadings suggest a systematic transport of submerged vascular plant debris above the active riverbed in large channels both upstream of and within the delta. Subzero temperatures hinder OC degradation promoting the accumulation and waterlogging of plant detritus within the watershed. Once entrained into a channel, sustained flow strength and buoyancy prevent plant debris from settling and keep it suspended in the water column above the riverbed. Helical flow motions within meandering river segments concentrate lithogenic and organic debris near the inner river bends forming a sediment‐laden plume. Moving offshore, we observe a lack of discrete, particulate OC in continental shelf sediments, suggesting preferential trapping of coarse debris within deltaic and neritic environments. The delivery of waterlogged plant detritus transport and high sediment loads during the spring flood may reduce oxygen exposure times and microbial decomposition, leading to enhanced sequestration of biospheric OC. Undercurrents enriched in coarse, relatively fresh plant fragments appear to be reoccurring features, highlighting a poorly understood yet significant mechanism operating within the terrestrial carbon cycle. Plain Language Summary: Coarse sediments are generally neglected as pathways and sinks of terrestrial organic carbon (OC) due to their presumed low OC contents and preservation potential. However, sedimentological and geochemical properties of sediments collected from the Mackenzie River reveal the co‐occurrence of coarse‐grained lithogenic particles with substantial quantities of pieces of plant biomass close to the riverbed. We argue that cold conditions in Arctic regions limit organic matter decomposition and favor the accumulation and water saturation of plant debris. These plant fragments will initially sink but are carried along the riverbed due to their shape, which is more elongated and planar than other particles. As rivers meander and navigate bends, water flow can divert coarser particles toward the inner bend, creating sediment‐laden undercurrents. We find that offshore in the Arctic Ocean, discrete plant detritus is apparently less common and we suggest that these particles are trapped within the Mackenzie Delta. The large amount of sediment exported during the spring flood promotes rapid burial of plant debris, likely preventing the decomposition of OC and the release of CO2 to the atmosphere. Our observations reveal a previously hidden mechanism that transports OC‐rich detritus within river systems that needs to be accounted for. [ABSTRACT FROM AUTHOR]

In mountain ranges, earthquakes can trigger widespread landsliding and mobilize large amounts of organic carbon by eroding soil and vegetation from hillslopes. Following a major earthquake, the landslide-mobilized organic carbon can be exported from river catchments by physical sediment transport processes or stored within the landscape where it may be degraded by heterotrophic respiration. The competition between these physical and biogeochemical processes governs a net transfer of carbon between the atmosphere and sedimentary organic matter, yet their relative importance following a large landslide-triggering earthquake remains poorly constrained. Here, we propose a model framework to quantify the post-seismic redistribution of soil-derived organic carbon. The approach combines predictions based on empirical observations of co-seismic sediment mobilization with a description of the physical and biogeochemical processes involved after an earthquake. Earthquake-triggered landslide populations are generated by randomly sampling a landslide area distribution, a proportion of which is initially connected to the fluvial network. Initially disconnected landslide deposits are transported downslope and connected to rivers at a constant velocity in the post-seismic period. Disconnected landslide deposits lose organic carbon by heterotrophic oxidation, while connected deposits lose organic carbon synchronously by both oxidation and river export. The modeling approach is numerically efficient and allows us to explore a large range of parameter values that exert a control on the fate of organic carbon in the upland erosional system. We explore the role of the climatic context (in terms of mean annual runoff and runoff variability) and rates of organic matter degradation using single pool and multi-pool models. Our results highlight the fact that the redistribution of organic carbon is strongly controlled by the annual runoff and the extent of landslide connection, but less so by the choice of organic matter degradation model. In the context of mountain ranges typical of the southwestern Pacific region, we find that model configurations allow more than 90 % of the landslide-mobilized carbon to be exported from mountain catchments. A simulation of earthquake cycles suggests efficient transfer of organic carbon out of a mountain range during the first decade of the post-seismic period. Pulsed erosion of organic matter by earthquake-triggered landslides is therefore an effective process to promote carbon sequestration in sedimentary deposits over thousands of years. [ABSTRACT FROM AUTHOR]

Permafrost thaw in Arctic watersheds threatens to mobilize hitherto sequestered carbon. We examine the radiocarbon activity (F14C) of dissolved organic carbon (DOC) in the northern Mackenzie River basin. From 2003–2017, DOC‐F14C signatures (1.00 ± 0.04; n = 39) tracked atmospheric 14CO2, indicating export of "modern" carbon. This trend was interrupted in June 2018 by the widespread release of aged DOC (0.85 ± 0.16, n = 28) measured across three separate catchment areas. Increased nitrate concentrations in June 2018 lead us to attribute this pulse of 14C‐depleted DOC to mobilization of previously frozen soil organic matter. We propose export through lateral perennial thaw zones that occurred at the base of the active layer weakened by preceding warm summer and winter seasons. Although we are not yet able to ascertain the broader significance of this "anomalous" mobilization event, it highlights the potential for rapid and large‐scale release of aged carbon from permafrost. Plain Language Summary: The thaw of continuously frozen grounds in the Arctic induced by regional warming accelerates the release of carbon to the atmosphere and river systems. Of particular concern is the fate of dissolved organic carbon (DOC) due to its potential for rapid oxidation to carbon dioxide. In order to understand the ramifications of a warming climate, we analyze the radiocarbon age of DOC in the northern Mackenzie River—a major Arctic river basin. DOC in large Arctic rivers has been characterized by young radiocarbon ages, from modern vegetation and surface soils. In June 2018, we recorded a departure from long‐term observations: Older DOC was measured in three large catchments draining into the Mackenzie Delta. This release of aged DOC followed a warm summer and the second warmest winter on record. We infer that the aged DOC derived from thaw of deeper soil horizons and subsequent carbon mobilization and riverine export. This is the first time such an event has been documented; it highlights the potential for abrupt and widespread aged DOC export with important implications for regional and global carbon cycles. Key Points: A widespread pulse of aged dissolved organic carbon (DOC) occurred in the Mackenzie River and its tributaries in June 2018Export of aged DOC is consistent with a prolonged warming period and the formation of supra‐permafrost taliksMobilization of aged DOC and nitrate suggests percolation of supra‐permafrost groundwater through previously frozen soil layers [ABSTRACT FROM AUTHOR]

A large portion of freshwater and sediment is exported to the ocean by a small number of major rivers. Many of these megarivers are subject to substantial anthropogenic pressures, which are having a major impact on water and sediment delivery to deltaic ecosystems. Due to hydrodynamic sorting, sediment grain size and composition vary strongly with depth and across the channel in large rivers, complicating flux quantification. To account for this, we modified a semi‐empirical Rouse model, synoptically predicting sediment concentration, grain‐size distribution, and organic carbon (%OC) concentration with depth and across the river channel. Using suspended sediment depth samples and flow velocity data, we applied this model to calculate sediment fluxes of the Irrawaddy (Ayeyarwady) and the Salween (Thanlwin), the last two free‐flowing megarivers in Southeast Asia. Deriving sediment‐discharge rating curves, we calculated an annual sediment flux of 326−70+91 Mt/year for the Irrawaddy and 159−51+78 Mt/year for the Salween, together exporting 46% as much sediment as the Ganges‐Brahmaputra system. The mean flux‐weighted sediment exported by the Irrawaddy is significantly coarser (D84 = 193 ± 13 μm) and OC‐poorer (0.29 ± 0.08 wt%) compared to the Salween (112 ± 27 μm and 0.59 ± 0.16 wt%, respectively). Both rivers export similar amounts of particulate organic carbon, with a total of 1.9−0.9+1.4 Mt C/year, 53% as much as the Ganges‐Brahmaputra. These results underline the global significance of the Irrawaddy and Salween rivers and warrant continued monitoring of their sediment flux, given the increasing anthropogenic pressures on these river basins. Plain Language Summary: The sediment (clay, silt, and sand) carried by rivers is a crucial but dwindling resource, sustaining agriculture in fertile deltas, while huge amounts of sand particularly are used to produce concrete, glass, and electronics. The amount of sediment that rivers carry globally is, however, not well known. It is especially difficult to measure in large rivers because most sand is carried near the channel bottom, tens of meters beneath the surface. In this study, we present an improved approach to measure the amount of sediment carried by large rivers. It combines sediment samples collected at various depths in the river with measurements of river flow via acoustic sensors. We apply this method to some of the world's largest rivers ‐ the Irrawaddy (Ayeyarwady) and the Salween (Thanlwin) in Myanmar, which have been understudied for decades. Our results show that they both currently discharge immense quantities of sediment to the ocean. However, this is likely to decrease drastically in the coming decades, given the projected industrialization and future damming of these two basins. The results presented in this study thus provide an important baseline against which to measure future changes in sediment discharge by these rivers. Key Points: An updated empirical Rouse modeling framework to calculate sediment flux and composition of large alluvial rivers is presentedModel was applied to compute annual sediment flux of Irrawaddy and Salween rivers as 326−70+91 and 159−51+78 Mt/year, respectivelyFluxes calculated using simple means of depth point samples result in errors of up to 50% relative to Rouse‐based model [ABSTRACT FROM AUTHOR]

Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. In particular, fractionation of its isotopes 187Re (62.6% abundance) and 185Re (37.4%) may be used to improve our understanding of redox reactions during weathering, both in the modern day and in geological archives. Published methods for measurement of Re isotopic composition are limited by the requirements of Re mass to reach a desirable precision, making the analysis of many geological materials unfeasible at present. Here we develop new methods which allow us to measure Re isotope ratios (reported as δ187Re) with improved precision: ±0.10‰ (2σ) for a mass of Re of ∼1 ng to ±0.03‰ (2σ) for a mass of Re of >10 ng. This is possible due to the combination of a modified column chemistry procedure and the use of 1013 Ω amplifiers for measurement via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). For river water samples (with Re concentrations typically ∼10−12 g g−1) we design a field-based pre-concentration of Re that can be used with large volumes of filtered water (5–20 L) shortly after sample collection to provide abundant Re for isotope analysis. As a result of these developments we provide new measurements of δ187Re in standards reference materials (δ187Re values range from −0.06 ± 0.07‰ to +0.19 ± 0.05‰) and a seawater standard (δ187Re = +0.10 ± 0.04‰), providing impetus for further exploration of the Re isotope system. [ABSTRACT FROM AUTHOR]

The article informs that Brantley et al. have studied the temperature dependence of silicate weathering, and its impact on Earth's climate. It mentions that the results showed that silicate weathering increased with temperature, with field rates increasing more than those from laboratory experiments, and highlights the complex interplay between temperature, minerals, and water in determining the rate of silicate weathering and its impact on the carbon cycle and Earth's climate.

Connectivity describes the efficiency of material transfer between geomorphic system components such as hillslopes and rivers or longitudinal segments within a river network. Representations of geomorphic systems as networks should recognize that the compartments, links, and nodes exhibit connectivity at differing scales. The historical underpinnings of connectivity in geomorphology involve management of geomorphic systems and observations linking surface processes to landform dynamics. Current work in geomorphic connectivity emphasizes hydrological, sediment, or landscape connectivity. Signatures of connectivity can be detected using diverse indicators that vary from contemporary processes to stratigraphic records or a spatial metric such as sediment yield that encompasses geomorphic processes operating over diverse time and space scales. One approach to measuring connectivity is to determine the fundamental temporal and spatial scales for the phenomenon of interest and to make measurements at a sufficiently large multiple of the fundamental scales to capture reliably a representative sample. Another approach seeks to characterize how connectivity varies with scale, by applying the same metric over a wide range of scales or using statistical measures that characterize the frequency distributions of connectivity across scales. Identifying and measuring connectivity is useful in basic and applied geomorphic research and we explore the implications of connectivity for river management. Common themes and ideas that merit further research include; increased understanding of the importance of capturing landscape heterogeneity and connectivity patterns; the potential to use graph and network theory metrics in analyzing connectivity; the need to understand which metrics best represent the physical system and its connectivity pathways, and to apply these metrics to the validation of numerical models; and the need to recognize the importance of low levels of connectivity in some situations. We emphasize the value in evaluating boundaries between components of geomorphic systems as transition zones and examining the fluxes across them to understand landscape functioning. © 2018 John Wiley & Sons, Ltd. Geomorphic connectivity emphasizes hydrological, sediment, or landscape connectivity. Common themes and ideas that merit further research include; understanding the importance of capturing landscape heterogeneity and connectivity patterns; using graph and network theory metrics in analyzing connectivity; understanding which metrics best represent the physical system and its connectivity pathways, and applying these metrics to the validation of numerical models; and recognizing the importance of low levels of connectivity in some situations. [ABSTRACT FROM AUTHOR]

Oxidative weathering of sedimentary rocks can release carbon dioxide (CO2) to the atmosphere and is an important natural CO2 emission. Two mechanisms operate -- the oxidation of sedimentary organic matter and the dissolution of carbonate minerals by sulfuric acid. It has proved difficult to directly measure the rates at which CO2 is emitted in response to these weathering processes in the field, with previous work generally using methods which track the dissolved products of these reactions in rivers. Here we design a chamber method to measure CO2 production during the oxidative weathering of shale bedrock, which can be applied in erosive environments where rocks are exposed frequently to the atmosphere. The chamber is drilled directly into the rock face and has a high surface-area-to-volume ratio which benefits measurement of CO2 fluxes. It is a relatively low-cost method and provides a long-lived chamber (several months or more). To partition the measured CO2 fluxes and the source of CO2, we use zeolite molecular sieves to trap CO2 "actively" (over several hours) or "passively" (over a period of months). The approaches produce comparable results, with the trapped CO2 having a radiocarbon activity (fraction modern, Fm) ranging from FmD0.05 to FmD 0.06 and demonstrating relatively little contamination from local atmospheric CO2 (FmD1.01). We use stable carbon isotopes of the trapped CO2 to partition between an organic and inorganic carbon source. The measured fluxes of rockderived organic matter oxidation (171-5 mgCm river chemistry in rivers around the world. The high oxidative weathering fluxes are consistent with the high erosion rate of the study region. We propose that our in situ method has the potential to be more widely deployed to directly measure CO2 fluxes during the oxidative weathering of sedimentary rocks, allowing for the spatial and temporal variability in these fluxes to be determined. [ABSTRACT FROM AUTHOR]

Oxidative weathering of sedimentary rocks can release carbon dioxide (CO2) to the atmosphere and is an important natural CO2 emission. Two mechanisms operate - the oxidation of sedimentary organic matter and the dissolution of carbonate minerals by sulphuric acid. It has proved difficult to directly measure the rates of these weathering processes in the field, with previous work generally using indirect methods which track the dissolved products of these reactions in rivers. Here we design a chamber method to measure CO2 production during the oxidative weathering of shale bedrock, which can be applied in erosive environments where rocks are exposed frequently to the atmosphere. The chamber is drilled directly into the rock face and is a relatively low cost method to provide a long-lived (several months or more), oxygenated environment in contact with a surface area of potential reactant. To partition the measured CO2 fluxes and the source of CO2, we use zeolite molecular sieves to trap CO2 actively (over several hours) or passively (over a period of months). The approaches produce comparable results, with the trapped CO2 having a fraction modern ranging from 0.05 to 0.06 and demonstrating relatively little contamination from local atmospheric CO2 (fraction modern of 1.01). We use stable isotopes of the trapped CO2 to partition between an organic and inorganic carbon source. The measured fluxes of rock-derived organic matter oxidation and carbonate dissolution by sulphuric acid from a single chamber were high, but consistent with the high erosion rate of the study region (of ~ 5 mm yr-1). We propose our in situ method has the potential to be more widely deployed to directly measure CO2 fluxes during the oxidative weathering of sedimentary rocks, allowing for the spatial and temporal variability in these fluxes to be determined. [ABSTRACT FROM AUTHOR]

The article focuses on a study regarding impact of mountain glaciation on rapid oxidation of rock-bound organic carbon. It mentions the exposure of organic matter in rocks to oxidative weathering at the planet Earth's surface releases carbon dioxide. It also mentions th use of rhenium to track the oxidation of rock-bound organic carbon in the mountain watersheds.

Rapid erosion in mountain forests results in high rates of biospheric particulate organic carbon (POC) export by rivers, which can contribute to atmospheric carbon dioxide drawdown. However, coarse POC (CPOC) carried by particles >~1 mm is rarely quantified. In a forested pre-Alpine catchment, we measured CPOC transport rates and found that they increase more rapidly with water discharge than fine POC (<1 mm) and dissolved organic carbon (DOC). As a result, decadal estimates of CPOC yield of 12.3 ± 1.9 t C km^yr"1 are higher than for fine POC and DOC, even when excluding 4 extreme flood events. When including these floods, CPOC dominates organic carbon discharge (~80%). Most CPOC (69%) was water logged and denser than water, suggesting that CPOC has the potential to contribute to long-term sedimentary burial. Global fluxes remain poorly constrained, but if the transport behavior of CPOC shown here is common to other mountain streams and rivers, then neglecting CPOC discharge could lead to a large underestimation of the global transfer of biospheric POC from land to ocean. [ABSTRACT FROM AUTHOR]

[i] Erosion of particulate organic carbon (POC) occurs at very high rates in mountain river cstchnsents, yet the proportion derived recentty from atmospheric CO2 in the terrestrial biosphere (POCnon-fossil) remains poorly constrained. Here we examine the transport of POCnon-fossil in mountain rivers of Taiwan and its climatic and geomorphic controls. In II catchments we have combined previous geochemical quantification of POC source (accounting for fossil POC from bedrock), with measurements of water discharge (Qw) and suspended sediment concentration over 2 yesrs. In these catchments, POC non-fossil concentration (mg L-1) was positively correlated with Qw, with enhanced loads at high flow attributed to rainfall driven supply of POCnon-fossil, from forested hillslopes, This climatic control on POCnon-fossil transport was moderated by catchment geomorphology: the gradient of a linear relation of POCnon-fossil concentration and Qw increased as the proportion of steep hillsloprs (>35°) in the catchment increased. The data suggest enhanced supply of POCnon-fossil, by erosion processes which act most efficiently on the steepest sections of forest. Across Taiwan, POCnon-fossil, yield was correlated with suspended sediment yield, with a mean of2l ± 10 tC km-2 yr-1. Al this rate, export of POCnon-fossil impacts an upper bound on the time available for biospheric growth, of 800 yr. Over longer time periods, POCnon-fossil transferred with large amounts of elastic sediment can contribute to sequestration of atmospheric CO2 if buried in marine sediments. Our results show that this carbon transfer should be enhanced in a wetter and stornsicr climate, and the rales moderated on geological timescales by the regional tectonic setting. [ABSTRACT FROM AUTHOR]

ABSTRACT In humid, forested mountain belts, bedrock landslides can harvest organic carbon from above ground biomass and soil (OCmodern) while acting to refresh the landscape surface and turnover forest ecosystems. Here the impact of landslides on organic carbon cycling in 13 river catchments spanning the length of the western Southern Alps, New Zealand is assessed over four decades. Spatial and temporal landslide maps are combined with the observed distribution and measured variability of hillslope OCmodern stocks. On average, it is estimated that landslides mobilized 7.6 ± 2.9 tC km−2 yr−1 of OCmodern, ~30% of which was delivered to river channels. Comparison with published estimates of OCmodern export in river suspended load suggests additional erosion of OCmodern by small, shallow landslides or overland flow in catchments. The exported OCmodern may contribute to geological carbon sequestration if buried in sedimentary deposits. Landslides may have also contributed to carbon sequestration over shorter timescales (<100 years). 5.4 ± 3.0 tC km−2 yr−1 of the eroded OCmodern was retained on hillslopes, representing a net-carbon sink following re-vegetation of scar surfaces. In addition, it was found that landslides caused rapid turnover of the landscape, with rates of 0.3% of the surface area per decade. High rates of net ecosystem productivity were measured in this forest of 94 ± 11 tC km−2 yr−1, which is consistent with rapid landscape turnover suppressing ecosystem retrogression. Landslide-OCmodern yields and rates of turnover vary between river catchments and appear to be controlled by gradients in climate (precipitation) and geomorphology (rock exhumation rate, topographic slope). Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC () in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean that ranges from − to − (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in from − to − between the major geological formations. Using coupled and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence , resulting in more variable and lower values than elsewhere. In all other catchments, we have found that variability in is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC. In order to quantify the fraction of suspended load POC derived from non-fossil sources () as well as the isotopic composition of fossil POC () carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, reproduces the fraction modern () in our samples, determined from measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean of suspended load POC was low (0.29±0.02, n =459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean in suspended POC varied between − and − from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts. [Copyright &y& Elsevier]

• 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]