Inorganic carbon and biological oceanography above a shallow oxygen minimum in the entrance to the Gulf of California in the Mexican Pacific

The eastern Pacific intermediate oxygen minimum layer (OML) is particularly well-developed and shoals close to the Mexican coast. We obtained hydrographic profiles including oxygen concentration [O], measured dissolved inorganic carbon (DIC), continuous surface pCO2 in water and air and took biological data south of the Gulf of California in March 2005. The core of the OML, with close to zero [O] was centered around 475 m with a thickness of the core varying between 80 m and 700 m and the upper limit of the core ranging from 500 m to 80 m. At the surface [O] was close to air-saturation and showed no relation with the depth of the OML contrary to DIC and pCO2 concentrations. Below 50-m depth the changes in DIC and Apparent Oxygen Utilization (AOU) yielding a molar ratio of dDIC 5 0.79 3 dAOU. When the OML shoaled, surface temperature, chlorophyll concentration [CHL], and depth-integrated zooplankton increased, resulting in positive correlations among plankton biomass, pCO2, and DIC in surface waters. When [CHL] is recalculated as particulate organic carbon (POC), a linear relationship with the sum of DIC and POC is observed as expected for concomitant transport of DIC and inorganic nutrients supporting POC formation. Neglecting ventilation of carbon into the atmosphere and the production of dissolved organic carbon, the results suggested that , 16% of the carbon transported up from the OML was present in surface waters in the form of POC and the rest in inorganic form.