A novel design for continuous measurement of CO ₂ from deep remineralization of petrogenic organic carbon

Abstract The oxidation of petrogenic organic carbon contributes a large CO 2 flux to the atmosphere, regulating the geological carbon cycle. While few in situ fluxes have been measured in near‐surface sedimentary rocks, deep fracture‐controlled metamorphic terranes remain unexplored. Here, we present a novel design for continuous in situ measurements of subsurface CO 2 flux feasible for metamorphic rocks. The accumulative CO 2 measurements were performed by either the passive mode or active recirculation on an 80‐m deep borehole in eastern Taiwan. Measurements in the passive mode yielded stepwise CO 2 releases in the fractured zone and steady accumulation in the pulverized zone at rates (18.9–132 mgC m −2 day −1 ) resembling those for other active orogens. By contrast, the active recirculation produced an overestimation of CO 2 flux. The high flux combined with variation patterns highlights heterogeneous metamorphic terranes as an important CO 2 source and the necessities of extended monitoring using the passive mode.