Total organic and inorganic carbon exchange through the Strait of Gibraltar in September 1997

The total organic carbon (TOC) and total inorganic carbon (CT) exchange bet ween the Atlantic Ocean and the Mediterranean Sea was studied in the Strait of Gibraltar in September 1997. Samples were taken at eight stations from western and eastern entrances of the Strait and at the middle of the Strait (Tarifa Narrows). TOC was analyzed by a high-temperature catalytic oxidati on method, and CT was calculated from alkalinity-pH(T) pairs and appropriat e thermodynamic relationships. The results are used in a two-layer model of water mass exchange through the Strait, which includes the Atlantic inflow , the Mediterranean outflow and the interface laver in between. Our observa tions show a decrease of TOC and an increase of CT concentrations from the surface to the bottom: 71-132 muM C and 2068-2150 mu mol kg(-1) in the Surf ace Atlantic Water, 74-95 muM C and 2119-2148 mu mol kg(-1) in the North At lantic Central Water, 63-116 muM C and 2123-2312 mu mol kg(-1) in the inter face layer, and 61-78 muM C and 2307-2325 mu mol kg(-1) in the Mediterranea n waters. However, within the Mediterranean outflow, we found that the conc entrations of carbon were higher at the western side of the Strait (75-78 m uM C, 2068-2318 mu mol kg(-1)) than at the eastern side(61-69 muM C, 2082-2 324 mu mol kg(-1)). This difference is due to the mixing between the Atlant ic inflow and the Mediterranean outflow on the west of the Strait. which re sults in a flux of organic carbon from the inflow to the outflow and an opp osite flux of inorganic carbon. We estimate that the TOC input from the Atl antic Ocean to the Mediterranean Sea through the Strait of Gibraltar varies from (0.97 +/- 0.8)10(4) to (1.81 +/- 0.90)10(4) mol Cs-1 (0.3 x 10(12) to 0.56 x 10(12) mol C yr(-1)), while outflow of inorganic carbon ranges from (12.5 +/- 0.4)10(4) to (15.6 +/- 0.4)10(4) mol Cs-1 (3.99-4.90 x 10(12) mo l C yr(-1)). The high variability of carbon exchange within the Strait is d ue to the variability of vertical mixing between inflow and outflow along t he Strait. The prevalence of organic carbon inflow and inorganic carbon out flow shows the Mediterranean Sea to be a basin of active remineralization o f organic material. (C) 2001 Elsevier Science Ltd. All rights reserved.