Search

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]

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. Plain Language Summary: When rocks undergo chemical weathering, breakdown of organic matter that has been stored in the rocks over time can release carbon dioxide to the atmosphere. It has remained a challenge to track and quantify this process, but the element rhenium provides a tool to do this. When organic matter in rocks undergoes oxidative weathering, rhenium is oxidized and enters river water. We can use measurements of dissolved rhenium fluxes as a proxy to estimate rock organic carbon oxidation. However, we still lack information on the sources of rhenium in river catchments and need a better understanding of the pathway that rhenium takes from rocks, through soils into streams and rivers. Here, we measured dissolved rhenium alongside other weathering products in three catchments that drain alpine landscapes made up of sedimentary rocks. The catchments behave in a remarkably similar way, where the relative amount of rhenium in river water increases with water flow. Based on our ideas of how water moves through the landscape, our results suggest that rhenium is moved from a near‐surface zone in all three catchments, where oxygen and rock organic carbon react. By applying the proxy to the catchments, we confirm that oxidative weathering rates and their carbon dioxide release increase with physical erosion rates. Key Points: We explore the mobility of rhenium in three alpine catchments, in the context of its use as a proxy for rock organic carbon oxidationThe relative abundance of dissolved Re increases with discharge, explained by a model where Re is mobilized from near surface zoneThe findings support rhenium as a proxy for oxidative weathering, and we find rock organic carbon oxidation increases with erosion rate [ABSTRACT FROM AUTHOR]

Abstract Erosion of organic carbon from the terrestrial biosphere and sedimentary rocks plays an important role in the global carbon cycle across a range of timescales. Over geological timescales (>104 years), erosion and burial of particulate organic carbon (POC) from the terrestrial biosphere (POC biosphere) is an important CO 2 sink, while oxidation of organic carbon derived from sedimentary rocks (petrogenic, POC petro) releases CO 2 to the atmosphere. Over decadal to millennial timescales, the balance between POC biosphere production and degradation affects atmospheric CO 2 concentrations. To better constrain the controls on erosional carbon transfers, here we quantify POC biosphere and POC petro fluxes in a mountain range with relatively low runoff, the Longmen Shan, which drains the eastern margin of the Tibetan Plateau. We measure total organic carbon content ([OC total ]) and the carbon isotopic compositions (13C/12C expressed as δ13C; 14C/12C expressed as fraction modern or F mod) of organic matter in suspended sediments collected from six gauging stations on the Min Jiang, a tributary of the Yangtze River, from 2005 to 2012. We find that POC petro has a large range of δ13C, from −26.2‰ to −13.2‰. This POC petro mixes with POC biosphere to set the δ13C of POC in river sediments. Binary mixing models reveal the possibility of aged POC biosphere at two gauging stations which drain the high elevations of the eastern Tibetan Plateau, with modelled biospheric F mod values of 0.82 ± 0.09 and 0.84 ± 0.08. This is consistent with prior suggestions of aged biospheric carbon being eroded from the plateau. The annual POC petro yields range from 0.04 ± 0.02 tC km−2 yr−1 to 1.69 ± 0.56 tC km−2 yr−1 across the five study catchments, with basin average yield that appears to be linked to catchment average slope as a likely proxy for erosion rate. Here, the variability in the petrogenic organic carbon content of rocks masks the signal of the weathering and oxidation of this rock-derived organic carbon. The annual POC biosphere yields range from 0.21 ± 0.04 tC km−2 yr−1 to 3.33 ± 0.57 tC km−2 yr−1. These values are towards the lower end of those measured in mountain ranges around the world, which we suggest not only reflects the relatively low erosion rates of the Longman Shan, but also the low annual runoff (<1 m yr−1). Across this region, the river POC biosphere discharge is related to the intensity of runoff events. Our data suggest that a wetter (and/or stormier) climate could increase the erosional export of POC biosphere in this tectonically-active mountain range. Depending on the fate of POC biosphere downstream in larger river systems, this could act as carbon-cycle climate feedback over geological timescales. [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]

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]

Mineral weathering plays a primary role in the geologic carbon cycle. Silicate weathering by carbonic acid consumes CO² and stabilizes Earth's climate system. However, when sulfuric acid drives weathering, CO² can be released to the atmosphere. Recent work has established that sulfuric acid weathering resulting from sulfide mineral oxidation is globally significant and particularly important in rapidly eroding environments. In contrast, if SO4 ²-produced by sulfide oxidation is reduced during continental transit, then CO² release may be negated. Yet, little is known about how much SO4 ²-reduction takes place in terrestrial environments. We report oxygen and sulfur stable isotope ratios of SO4 ²-in river waters and mass budget calculations, which together suggest that SO4 ²-released from pyrite oxidation in the Peruvian Andes mountains is conservatively exported across ~300 km of the Amazon floodplain. In this system, floodplain SO4 ²-reduction does not counteract the large SO4 ²-flux from Andean pyrite weathering or measurably affect the stable isotope composition of riverine SO4 ²-. These findings support the hypothesis that uplift and erosion of sedimentary rocks drive release of CO² from the rock reservoir to the atmosphere. [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]