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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 organicmatter and the dissolution of carbonate minerals by sulfuric acid. It hasproved difficult to directly measure the rates at which CO2 isemitted in response to these weathering processes in the field, with previouswork generally using methods which track the dissolved products of thesereactions in rivers. Here we design a chamber method to measure CO2 production during the oxidative weathering of shale bedrock, which can beapplied in erosive environments where rocks are exposed frequently to theatmosphere. The chamber is drilled directly into the rock face and has a highsurface-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 andthe source of CO2 , we use zeolite molecular sieves to trap CO2 “actively” (over several hours) or “passively” (over aperiod of months). The approaches produce comparable results, with thetrapped CO2 having a radiocarbon activity (fraction modern, Fm)ranging from Fm  =  0.05 to Fm  =  0.06 and demonstrating relativelylittle contamination from local atmospheric CO2 (Fm  =  1.01). Weuse stable carbon isotopes of the trapped CO2 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 bysulfuric acid ( 534±16  mgC m −2  day −1) from a single chamberwere high when compared to the annual flux estimates derived from usingdissolved river chemistry in rivers around the world. The high oxidativeweathering fluxes are consistent with the high erosion rate of the studyregion. We propose that our in situmethod 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 bedetermined.

biogeosciences

Technical note: In situ measurement of flux and isotopic composition of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; released during oxidative weathering of sedimentary rocks