Continuous O 2 /Ar measurements in surface sea water by membrane inlet mass spectrometry Jan Kaiser and Michael L. Bender Department of Geosciences, Princeton University, Princeton, New Jersey, USA thermocline mixed layer  100 m O2O2 O2O2 CO 2 phyto- plankton Ar atmosphere References Gruber N, Gloor M, Fan SM, Sarmiento JL (2001): Air-sea flux of oxygen estimated from bulk data: Implications for the marine and atmospheric oxy-gen cycles, Global Biogeochem. Cycles, 15, Kana TM, Darkangelo C, Hunt MD, Oldham JB, Ben-nett GE, Cornwell JC (1994): Membrane inlet mass spectrometer for rapid high-precision determina-tion of N 2, O 2, and Ar in environmental water samples, Analytical Chemistry, 66, rel. std. deviation  1.4 ‰ 1.2 ‰ rel. std. deviation  4.6 ‰ rel. std. deviation  6.8 ‰ Results from MIMS Fig. 7: O 2 /Ar ratios of distilled water (30 ºC). Faraday cup and channeltron have about equal precision (10 -3 ), but the O 2 /Ar ratios are strongly influenced by changing temperature and water levels in the vacuum manifold. Fig. 8: CO 2 measurements with precisions <10 -2 (or averaged over 10 min) are possible. Fig. 9: Good precision on N 2 isotope ratio measurements opens up the possibility of calibrations via isotope dilution. Methods The measurements involve a quadrupole mass spec-trometer (QMS) coupled via a membrane inlet (Fig. 4) to a continuous supply of surface sea water from an underwater sampling system. We plan to install the QMS on the NOAA vessel “Ka’ Imimoana” (Fig. 6) that is regularly servicing the TAO mooring array in the tropical Pacific (Fig. 5) (TAO = Tropical Ocean Atmosphere pro- ject, The O 2 /Ar ratios of the sample will be calibrated regularly to a supply of air equilibrated water which will allow us to correct for discrimination effects in sample inlet and analyzer (cf. Figs. 7-9). In addition to O 2 and Ar (Fig. 7), N 2 and CO 2 measurements are also possible ( Figs. 8+9). We Abstract Simultaneous O 2 and Ar measurements in surface seawater allow one to estimate oceanic O 2 outgassing and to probe oceanic productivity. Previously, the sea-to-air flux of O 2 has been estimated from tempe-rature-based oceanic heat budgets and modeled gas fluxes, often considering only solubility effects due to temperature chan-ges and neglecting variations of biological O 2 production. Whereas measuring the small net annual average air-sea O 2 flux on a global scale might be out of the scope of the present project (Fig. 2), it is pos-sible to determine the individual terms, i.e. the O 2 flux to the atmosphere in sum-mer and the return flux (reoxygenation/ ventilation) in fall and winter. We want to make the first long continuous measurements of biological O 2 supersaturation and sea-air fluxes which would provide constraints on the net productivity of ocean systems over very large scales. Measurements will involve a host of novel techniques, in particular membrane inlet mass spectrometry and O 2 optodes. Introduction Biological O 2 fluxes are due to enhanced photo-synthetic production that causes O 2 super-saturation. In addition to that, O 2 super-saturation is affected by physical processes such as bubble entrainment in breaking waves (Fig. 1). O 2 /Ar ratios allow one to partition the outgassing into physically- and biologi- cally-forced flux components. Ar is used to correct for physical supersaturation, because it is an inert gas, but has similar solubility characteristics as O 2. Fig. 2: Modeling of atmospheric potential oxy-gen (APO ≈ CO 2 + O 2 ) predicts significant O 2 outgassing in the tropical Pacific. Fig. 3: O 2 /Ar ratios in the upper tropical Pacific record on processes such as net production, subduction and diapycnal mixing on regional scales. are currently investigating whether it will be possible to replace conventional pCO 2 instruments (showerhead equilibrators with NDIR detection of CO 2 in the equilibrated headspace) by the mem-brane inlet mass spectrometer. Main issues here are CO 2 and N 2 O production from O 2 and residual carbon in the ion source and from N 2 + O 2 ion molecule reactions, respectively. Another scope of the project is to investigate whether the mass- spectrometric measurements can be used not only to determine O 2 /Ar ratios, but also the gas concentrations themselves. The currently achieved reproducibility suggests that this may be possible. Intercalibrations of the mass spectrometer measurements are going to comprise Winkler titrations, oxygen optode measurements and isotope dilution techniques. Gruber et al. (2001 ) © M.B. Hendricks © NOAA © A. Milligan 4 ºC ([O 2 ]/[Ar]) / ([O 2 ] sat /[Ar] sat )