Abstract. Oxidative weathering of sedimentary rocks can release carbon dioxide (CO 2 ) to the atmosphere and is an important natural CO 2 emission. Two mechanisms operatethe 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 CO 2 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 CO 2 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-tovolume ratio which benefits measurement of CO 2 fluxes. It is a relatively low-cost method and provides a long-lived chamber (several months or more). To partition the measured CO 2 fluxes and the source of CO 2 , we use zeolite molecular sieves to trap CO 2 "actively" (over several hours) or "passively" (over a period of months). The approaches produce comparable results, with the trapped CO 2 having a radiocarbon activity (fraction modern, Fm) ranging from Fm = 0.05 to Fm = 0.06 and demonstrating relatively little contamination from local atmospheric CO 2 (Fm = 1.01). We use stable carbon isotopes of the trapped CO 2 to partition between an organic and inorganic carbon source. The measured fluxes of rock-derived organic matter oxidation (171 ± 5 mgC m −2 day −1 ) and carbonate dissolution by sulfuric acid (534 ± 16 mgC m −2 day −1 ) from a single chamber were high when compared to the annual flux estimates derived from using dissolved 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 CO 2 fluxes during the oxidative weathering of sedimentary rocks, allowing for the spatial and temporal variability in these fluxes to be determined.

Release of carbon from high-latitude soils to the atmosphere may have significant effects on Earth's climate. In this contribution, we evaluate visible-near-infrared spectroscopy (vis-NIRS) as a time-and cost-efficient tool for assessing soil organic carbon (SOC) concentrations in South Greenland. Soil samples were collected at two sites and analyzed with vis-NIRS. We used partial least square regression (PLS-R) modeling to predict SOC from vis-NIRS spectra referenced against in situ dry combustion measurements. The ability of our approach was validated in three setups: (1) calibration and validation data sets from the same location, (2) calibration and validation data sets from different locations, and (3) the same setup as in (2) with the calibration model enlarged with few samples from the opposite target area. Vis-NIRS predictions were successful in setup 1 (R 2 = 0.95, root mean square error of prediction [RMSEP] = 1.80 percent and R 2 = 0.82, RMSEP = 0.64 percent). Predictions in setup 2 had higher errors (R 2 = 0.90, RMSEP = 7.13 percent and R 2 = 0.78, RMSEP = 2.82 percent). In setup 3, the results were again improved (R 2 = 0.95, RMSEP = 2.03 percent and R 2 = 0.77, RMSEP = 2.14 percent). We conclude that vis-NIRS can obtain good results predicting SOC concentrations across two subarctic ecosystems, when the calibration models are augmented with few samples from the target site. Future efforts should be made toward determination of SOC stocks to constrain soil-atmosphere carbon exchange.

Abstract. 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 10 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 15 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 20 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.

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.