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Large volumes of fluid flow through aged oceanic crust. Given the scale of this water flux, the exchange of organic and inorganic carbon that it mediates between the crust and deep ocean can be significant. However, off-axis carbon fluxes in older oceanic crust are still poorly constrained because access to low-temperature fluids from this environment is limited. At North Pond, a sedimented depression located on 8-million-year-old crust on the flank of the Mid-Atlantic Ridge, circulating crustal fluids are accessible through drilled borehole observatories. Here, fluids are cool (≤ 20°C), oxygenated and bear strong geochemical similarities to bottom seawater. In this study, we report concentrations and isotopic composition of dissolved organic and inorganic carbon from crustal fluids that were sampled six years after the installation of borehole observatories, which better represent the fluid geochemistry prior to drilling and perturbation. Radiocarbon-based signatures within carbon reservoirs support divergent shallow and deep fluid pathways within the crust. We also report a net loss of both dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from the fluid during isolation in the crust. The removal of DOC is isotopically selective and consistent with microbe-mediated DOC oxidation. The loss of DIC is consistent with carbonate precipitation, although geochemical signatures of DIC addition to the fluids from DOC oxidation and basalt weathering are also evident. Extrapolated to global fluxes, systems like North Pond could be responsible for a net loss of ~10^11 mol C/yr of DIC and ~10^11 mol C/yr of DOC during the circulation of fluids through oceanic crust at low temperatures. 

Here, we explicitly define a half-cell reaction approach for pH calculation using the electrode couple comprised of the solid-state chloride ion-selective electrode (Cl-ISE) as the reference electrode and the hydrogen ionselective ion-sensitive field effect transistor (ISFET) of the Honeywell Durafet as the hydrogen ion (H+)-sensitive measuring or working electrode. This new approach splits and isolates the independent responses of the Cl-ISE to the chloride ion (Cl−) (and salinity) and the ISFET to H+ (and pH), and calculates pH directly on the total scale (pHEXT total) in molinity (mol (kg-soln)−1) concentration units. We further apply and compare pHEXT total calculated using the half-cell and the existing complete cell reaction (defined by Martz et al. (2010)) approaches using measurements from two SeapHOx sensors deployed in a test tank. Salinity (on the Practical Salinity Scale) and pH oscillated between 1 and 31 and 6.9 and 8.1, respectively, over a six-day period. In contrast to established Sensor Best Practices, we employ a new calibration method where the calibration of raw pH sensor timeseries are split out as needed according to salinity. When doing this, pHEXT total had root-mean squared errors ranging between ±0.0026 and ±0.0168 pH calculated using both reaction approaches relative to pHtotal of the discrete bottle samples (pHdisc total). Our results further demonstrate the rapid response of the Cl-ISE reference to variable salinity with changes up to ±12 (30 min)−1. Final calculated pHEXT total were ≤±0.012 pH when compared to pHdisc total following salinity dilution or concentration. These results are notably in contrast to those of the few in situ field deployments over similar environmental conditions that demonstrated pHEXT total calculated using the Cl-ISE as the reference electrode had larger uncertainty in nearshore waters. Therefore, additional work beyond the correction of variable temperature and salinity conditions in pH calculation using the Cl-ISE is needed to examine the effects of other external stimuli on in situ electrode response. Furthermore, whereas past work has focused on in situ reference electrode response, greater scrutiny of the ISFET as the H+-sensitive measuring electrode for pH measurement in natural waters is also needed.