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

Abstract

Dissolved oxygen/argon (O 2 /Ar) ratios in the oceanic mixed layer are indicative of net community production (NCP) because O 2 and Ar share similar physical solubility properties, but only O 2 is biologically produced and consumed. We describe a membrane inlet mass spectrometer (MIMS) that allows continuous high‐precision shipboard analysis of O 2 /Ar ratios and eventually other gases, calibrated with discrete samples analyzed in the laboratory. We also present O 2 /Ar data from the eastern equatorial Pacific. Short‐term reproducibilities of 0.05% were achieved. Meridional gradients and small‐scale phenomena were clearly resolved. O 2 /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 O 2 /Ar measurements and accompanying O 2 concentration measurements, ranged from −0.8 to +3.0%.