The internal consistency of the marine carbon dioxide system for high latitude shipboard and in situ monitoring

Deep convection in the Labrador Sea supplies large amounts of anthropogenic carbon to the ocean's interior. We use measurements of all four measurable CO2 system parameters made along AR7W (across Labrador Sea) between 2013 and 2015 to assess the internal consistency of the carbonate system, including, as appropriate, conversion to in situ temperature (T) and pressure (P). The best agreement between measured and calculated values was obtained through combination of T,P-dependent (pH or pCO(2)) and non-dependent (TA or DIC) parameters. Use of the dissociation constants of Mehrbach et al. (1973) as refit by Dickson and Millero (1987) and Lueker et al. (2000) yielded the best internal consistency irrespective of the input parameters used. A Monte Carlo simulation demonstrated that the propagated uncertainty (Le combined standard uncertainty) of calculated parameters of the carbonate system is (a) always larger than the analytical precision of the measurements themselves; (b) strongly dependent on the choice of input parameters and uncertainties; (c) less dependent on choice of the specific set of constants. For calculation of other parameters of the carbonate system from TA and DIC measurements made throughout the Labrador Sea time-series, the estimated combined standard uncertainty of calculated pCO(2) and pH based on the Monte Carlo simulation is similar to 13 mu atm and similar to 0.012 pH units respectively, with accuracy relative to laboratory-based measurement estimated to be between - 3 and - 13 mu atm and 0.002 and 0.007 pH units. Internal consistency especially at in situ temperature and pressure conditions is important for rapidly developing sensor-based monitoring programs in the region, including measurement of pH and/or pCO(2) from gliders, profiling floats and moorings. We highlight uncertainty associated with the large pressure effect on pH and pCO(2), and recommend a study of carbonate system internal consistency under deep ocean conditions that addresses pressure effects on calculations.