Continual measurement of the total inorganic carbon in surface seawater

An automated sampling and analysis system was developed to determine the total inorganic carbon (TCO2) in seawater. Measurements are performed on subsamples taken from the shipboard flowing seawater line. The flowing TCO2 system (FTCO2) automatically acidifies and strips the CO2 out of a known volume of sea water and determines the concentration by integrating the infra red (IR) absorbance. Laboratory results have demonstrated an analytical precision of 2–5 μM in TCO2 for this system. Intercomparison of TCO2 measurements with the FTCO2 system and the SOMMA (single operator multimetabolic analyzer) agreed within their respective uncertainties. The TCO2 in surface seawater was determined continually on the flowing seawater line aboard the R/V Thompson in the Arabian Sea. The average difference between the TCO2 determined on surface seawater with the flowing system and the SOMMA was 6.5±8.7 μM. Measurements made with the flowing system on CRMs agreed to ±3 μM with the assigned value. TCO2 in surface waters in the Arabian Sea varied by as much as 40 μM on short spatial and temporal scales. Although the accuracy of the present design of the FTCO2 system is not as good as the SOMMA, the surface TCO2 variability with time and space is captured with the FTCO2 system and is completely missed by discrete measurements made on CTD casts. Continual measurements of TCO2 will be valuable in characterizing the temporal and spatial changes in the carbon dioxide system in surface ocean waters.