| Summary: | Recent studies have debated the potential importance of alkalinity production in shallow coastal sediments to the global ocean alkalinity budget, suggesting that the benthic alkalinity source could act as negative feedback to rising atmospheric CO2 conditions and ocean acidification. At present, the limited quantitative understanding of benthic alkalinity fluxes from shallow coastal areas contrasts with the potential significance of coastal sediments to the global carbon cycle. This study was conducted in the Bay of Cádiz (30°31’ N, 6°15’ E), a shallow coastal ecosystem covering an area of 152 km2 (max. depth of 7 m) connected to the North East Atlantic. Cores of surface sediment and near-bottom waters were collected in Summer 2016 at two locations (RSP and SPC) within the Bay of Cádiz. Concentrations of carbonate and non-carbonate (dissolved ammonia, phosphate, silicate, borate and sulfate) constituents of total alkalinty (AT) were analyzed in near-bottom and pore water and their sediment-water diffusive fluxes (J) were estimated using Fick's Law. Pore water AT increased with depth, as did the concentrations of other parameters (excluding sulphate), and ranged from 3240 and 21,240 μmol/kg and 3545 and 6130 μmol/kg at RSP and SPC, respectively. Total concentration of non-carbonate alkalinity (N-CIBs) in pore water followed the same trend, varying between 7.1 – 172 and 7.2 – 45 μmol/kg, respectively, with N-CIBs accounting for 0.2 – 1.1 % of AT at RSP, and 0.3 - 0.7% of AT at SPC. The benthic alkalinity generation in the Bay of Cádiz varied from 280 to 2900 μmol/m2/day, and was in the range of alkalinity effluxes measured in the coastal sediments of the Baltic Sea and the North Sea; AT efflux was dominated by carbonate alkalinity with non-carbonate inorganic bases (N-CIBs) contributing as low as 1 -3 %. The alkalinity release from the sediment in the Bay of Cadiz could have a significant impact on the carbonate system, and thus on the pCO2 regulating the exchange of CO2 between the atmosphere and the sea in ...
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