Distribution and transport of natural and anthropogenic CO2 in the Gulf of Cádiz

During the 2002 SEMANE cruise (16–22 July), we sampled seawater for total alkalinity (TA) and total inorganic carbon (TCO2) measurements. In this area six water masses are clearly identified by their different chemical properties: (1) Spanish Coastal Water (SCW), (2) North Atlantic Surface Water (NASW), (3) North Atlantic Central Water (NACW), (4) North Atlantic Deep Water (NADW), (5) Lower Deep Water (LDW), and (6) Mediterranean Water (MW). TA and TCO2 concentrations were higher in the Mediterranean Sea than in the Atlantic Ocean. At this time period, the surface water of the Gulf of Cádiz was acting as a slight source of CO2 for the atmosphere with a net CO2 flux of 18.6±4 mmol m−2 day−1.The Mediterranean Sea is a significant CO2 source for the Atlantic Ocean. The net export of inorganic carbon from the Mediterranean Sea to the Atlantic Ocean, ranges from 0.02 to 0.07 pg C yr−1 and strongly depends upon the water masses transport. As expected, the Mediterranean Sea contains higher concentrations of anthropogenic carbon than the Atlantic Ocean waters. The estimated input of anthropogenic carbon from the outflowing Mediterranean Sea waters to the Gulf of Cádiz ranges from 0.032 to 0.066 pg C yr−1. This anthropogenic CO2 transport into the Atlantic Ocean represents up to 4.8% of the TCO2 outflowing from the Mediterranean Sea through the Gibraltar Strait.This study shows that the Mediterranean Sea is a significant source of anthropogenic carbon to the Atlantic Ocean and that the complex circulation in the Gulf of Cádiz facilitates the sequestration of this anthropogenic carbon below the mixed-layer depth, thus illustrating the importance of the contribution of the Mediterranean Sea in the sequestration and storage of anthropogenic carbon into the ocean.