Abstract
Abstract. The exchange of both anthropogenic and natural inorganic carbon between the Atlantic Ocean and the Mediterranean Sea through Strait of Gibraltar was studied for a period of two years under the frame of the CARBOOCEAN project. A comprehensive sampling program was conducted, which was design to collect samples at eight fixed stations located in the Strait in successive cruises periodically distributed through the year in order to ensure a good spatial and temporal coverage. As a result of this monitoring, a time series namely GIFT (GIbraltar Fixed Time series) has been established, allowing the generation of an extensive data set of the carbon system parameters in the area. Data acquired during the development of nine campaigns were analyzed in this work. Total inorganic carbon concentration (CT) was calculated from alkalinity-pHT pairs and appropriate thermodynamic relationships, with the concentration of anthropogenic carbon (CANT) being also computed using two methods, the ΔC* and the TrOCA approach. Applying a two-layer model of water mass exchange through the Strait and using a value of −0.85 Sv for the average transport of the outflowing Mediterranean water recorded in situ during the considered period, a net export of inorganic carbon from the Mediterranean Sea to the Atlantic was obtained, which amounted to 25±0.6 Tg C yr−1. A net alkalinity output of 16±0.6 Tg C yr−1 was also observed to occur through the Strait. In contrast, the Atlantic water was found to contain a higher concentration of anthropogenic carbon than the Mediterranean water, resulting in a net flux of CANT towards the Mediterranean basin of 4.20±0.04 Tg C yr−1 by using the ΔC* method, which constituted the most adequate approach for this environment. A carbon balance in the Mediterranean was assessed and fluxes through the Strait are discussed in relation to the highly diverse estimates available in the literature for the area and the different approaches considered for CANT estimation. This work unequivocally confirms the relevant role of the Strait of Gibraltar as a controlling point for the biogeochemical exchanges occurring between the Mediterranean Sea and the Atlantic Ocean and emphasizes the influence of the Mediterranean basin in the carbon inventories of the North Atlantic.
Highlights
1.1 The oceanographic setting there still remain large uncertainties in relation to the final fate of the carbon dioxide that is released to the atmosphere as a result of human activities, solid evidence has arisen indicating that the oceans represent the major sink for anthropogenic CO2 (Sabine et al, 2004; Waugh et al, 2006)
The selection criterion lies on the hydrodynamics of the area since at this particular place located of the Atlantic side of the Strait, tidal currents in the Mediterranean outflowing water (MOW) layer are not strong enough to reverse the subinertial flow that is due to tidal effect and the fraction of MOW that returns to the Mediterranean basin can be considered negligible (GarcıaLafuente et al, 2007)
The spatial distribution of the variables measured along the GIFT leg during the monitoring period is shown in Fig. 3, where values have been interpolated through the entire water column from the average of all data obtained at the five sampling depths in each station during the nine cruises conducted in the area (Table 1)
Summary
There still remain large uncertainties in relation to the final fate of the carbon dioxide that is released to the atmosphere as a result of human activities, solid evidence has arisen indicating that the oceans represent the major sink for anthropogenic CO2 (Sabine et al, 2004; Waugh et al, 2006). The ocean absorption of CO2 contributes to moderate future climate change, but this behavior as a carbon reservoir will result in deleterious consequences for the marine environment. Quantifying the concentration and distribution of anthropogenic carbon (CANT) in the oceans has become thereby a crucial issue in the understanding of the global carbon cycle and its future evolution in a high CO2 world. Over the past decade, considerable efforts have been devoted to assess the capacity of the ocean to sequester. Huertas et al.: CO2 exchange through the Strait of Gibraltar
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