Internal consistency of the inorganic carbon system in the Arctic Ocean

Highly accurate and precise measurements of the inorganic carbon system are crucial for monitoring and assessing the uptake of anthropogenic CO2 by the ocean. The Arctic Ocean is an area of particular interest due to Arctic amplification of climate change. Internally consistent constants are essential for making high quality CO2 system measurements and for calculating the full suite of carbon parameters. Temperatures and salinities in the Arctic are near or below the valid ranges of most carbon system constants, but the applicability has never been fully assessed in the Arctic. Using measurements of all four carbon parameters (total alkalinity, total CO2, pH, and pCO2) made on the Arctic GEOTRACES/GO-SHIP cruise in summer 2015, we evaluate the internal consistency of six different sets of constants for surface waters at in situ temperature. Five of the six sets of constants are generally internally consistent, but each is statistically significantly different from each other. Although differences among constants are generally within the uncertainty of the calculations, these results indicate caution should be used when comparing data using different constants as biases may result from the choice of constants. We also find significant uncertainty in converting measured pH to pH at in situ temperature. The uncertainty in the pH temperature conversion limits our ability to determine the accuracy of pH measurements, particularly in polar regions. Measurements in marginal ice zones indicate that dissolution of semi-stable ikaite (CaCO3) may bias total alkalinity measurements, and should therefore be analyzed immediately upon collection or filtered before storage.