Alkalinity and total carbonate in the Arabian Sea. Carbonate depletion in the Red Sea and Persian Gulf

An estimation of the annual decrease in alkalinity (At) in the Persian Gulf was calculated to be 0.326 × 1012 mol/yr and in the Red Sea 1.65 × 1012 mol/yr. Together this amounts to a loss of carbon as calcium carbonate of 12 Mton/yr which is about 5% of the annual river input of carbon as hydrogen carbonate. The perturbation of the global CO2 system is small, but the effect on the alkalinity of the northwest Indian Ocean should not be neglected. The depth profile of the specific alkalinity (At/S) in the Arabian Sea shows an increase below 600 m which is partly ascribed to the dissolution of calcium carbonate and partly due to the sinking of cold water with high values of At/S. The depth profile of At - Ct shows low values below the euphotic zone due to decomposition of organic carbon. Station 2359 at 17° 52′N and 60°46′E from the Atlantis II Cruise 93-17 in February 1977 was selected to show the relationships between the total inorganic carbon (Ct) and the concentrations of phosphate and nitrate+nitrite. The slope of 15 fits the phosphate data except in deep water where dissolution of carbonate may produce high values of Ct. The slope of 105:15 for C:N does not fit the data and shows a loss of nitrogen due to denitrification. In the depth range 215–285 m the nitrogen loss was 45.5%.