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

Abstract

An estimation of the annual decrease in alkalinity ( A t) in the Persian Gulf was calculated to be 0.326 × 10 12 mol/yr and in the Red Sea 1.65 × 10 12 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 CO 2 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 ( A t/ 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 A t/ S. The depth profile of A t - C t 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 (C t) 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 C t. 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%.