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Oxygen minimum zones (OMZs) are important sources of CO 2 to the atmosphere when physical forces bring subsurface water with high dissolved inorganic carbon (DIC) to the surface. This study examines, for the first time, the influence of the OMZ of the coastal North Eastern Tropical Pacific off Mexico on surface CO 2 fluxes. We use variations in the oxycline depth and subsurface water masses to discern physical oceanographic influences. During two cruises, in November 2009 and August 2010, DIC and total alkalinity (TA) measurements were used to estimate pCO 2 and air-sea CO 2 fluxes. At the OMZ layer, Subtropical Subsurface Water (StSsW) was found to have high pCO 2 values (1290 6 70 latm). Due to strong vertical stratification, however, the relationship between DpCO 2 at the air-sea interface and the oxycline/StSsW upper limit depth was weak. During November, the region was a weak source of CO 2 to the atmosphere (up to 2.5 mmol C m ) prevented subsurface mixing of water from the OMZ to the upper layer; particularly in November 2009 which was during an El Niño event. Results suggest that advection of surface water masses, reinforced by strong vertical stratification, controlled surface pCO 2 , and air-sea CO 2 fluxes.

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