Method Development: Continuous Underway DIC, CO2 and O2/Ar Measurements by Membrane Equilibrator Dual Inlet Mass Spectrometry (MEDIMS)

The goal of the project is to develop a continuous, underway technique for measuring dissolved inorganic carbon (DIC) in surface seawater. The innovation is the use of isotope dilution mass spectrometry, which will be added to an existing instrument package that uses Equilibrator Inlet Mass Spectrometry (EIMS) to measure oxygen:argon ratios. The new system will mix seawater with a known amount of 13C-containing sodium bicarbonate, followed by acidification to convert all DIC to CO2, which will be transferred to a mass spectrometer by equilibration through a membrane inlet with CO2-free carrier gas. Seawater DIC concentration will be quantified by comparison of the measured ratio of the isotope tracer with the known mixing ratio of the added label. (The proposal includes a plan to monitor pCO2 from the EIMS gas stream-the panel notes that N2 is also possible and may be of some interest to this project). Measurements of surface seawater DIC concentrations have two essential applications. The first is quantification of net community production (NCP), the uptake of CO2 and release of O2 by marine ecosystems. Knowledge of NCP is fundamental to a basic understanding of the marine carbon cycle. Measurements of DIC should help constrain estimates of NCP over seasonal time scales. Measurement of oxygen:argon ratios are also used to quantify NCP, but reflect shorter timescale since oxygen re-equilibrates with the atmosphere in about 10 days. Combining DIC with oxygen:argon ration measurements should enable a better estimate of NCP and the impact of episodic events on this quantity. The second application of DIC measurements is improved quantification of the oceans’ uptake of anthropogenic CO2, particularly if measurements of source waters for deep and intermediate water mass formation are possible. Both applications (NCP and uptake of anthropogenic CO2) were deemed by the panel to be highly relevant to NOAA’s Global Carbon Cycle program.