Mixing in the region of the midrift islands of the Gulf of California: effect on surface pCO2

Abstract

Surface seawater carbon dioxide partial pressure (pCO₂) and the CO₂ flux through the air-sea interface in the region of the midrift islands of the Gulf of California were estimated. We used surface temperature (ºC), salinity and pH data collected during summer 1990, and we assumed a constant specific alkalinity value. Mixing by tide-related phenomena is particularly strong in this region of the gulf, and our objective was to compare the effect of post-spring tides with that of post-neap tides. Carbon dioxide was either near equilibrium or flowing from the water to the atmosphere, even with post-neap tides. Surface cooling, and larger pCO₂ and CO₂ flux were detected with post-spring tides. relative to post-neap tides (<4 mM m^–2 d^–1 with post-neap tides. up to 23 mM m^–2 d^–1 with post-spring tides). There is a fortnightly modulation of the CO₂ system in the surface waters of this region due to the spring-neap tide sequence. Tidal mixing is a “physical pump” that carries CO₂ efficiently from intermediate depths (500 m) to the atmosphere in this region of the gulf.