Marine productivity estimates from continuous O2/Ar ratio measurements by membrane inlet mass spectrometry

Dissolved oxygen/argon (O₂/Ar) ratios in the oceanic mixed layer are indicative of net community production (NCP) because O₂ and Ar share similar physical solubility properties, but only O₂ is biologically produced and consumed. We describe a membrane inlet mass spectrometer (MIMS) that allows continuous high-precision shipboard analysis of O₂/Ar ratios and eventually other gases, calibrated with discrete samples analyzed in the laboratory. We also present O₂/Ar data from the eastern equatorial Pacific. Short-term reproducibilities of 0.05% were achieved. Meridional gradients and small-scale phenomena were clearly resolved. O₂/Ar undersaturations around the equator reflect the interaction of biological and physical forcings. Mixed-layer NCP estimated from wind speed-gas exchange parameterizations was near zero north of 2.75°N, and about 12 mmol m^-2 d^-1 south of 6.75°S. Ar supersaturations, calculated from MIMS O₂/Ar measurements and accompanying O₂ concentration measurements, ranged from -0.8 to +3.0%.