The carbon dioxide system in the Arabian Sea

In 1995 the WHOI (C. Goyet) and MIAMI (F.J. Millero) groups participated on a number of research cruises in the Arabian Sea as part of the U.S. Joint Global Ocean Flux Study (JGOFS) sponsored by the National Science Foundation (NSF). This paper gives the results of our total inorganic carbon dioxide (TCO₂), total alkalinity (TA) and potentiometric pH measurements made on Arabian Sea water samples during these cruises. Measurements made on Certified Reference Material (CRM) indicate that the reproducibility of the measurements was ±0.007 in pH, ±3.2 μmolkg^-1 in TA, and ±1.2 μmolkg^-1 in TCO₂ (N = 180). The surface measurements (0-30 m) of pH and normalized TCO₂ and TA were quite uniform throughout the year (pH = 8.1 ± 0.05, NTCO₂ = 1950 ± 20 μmolkg^-1 and NTA = 2290 ± 5 μmolkg^-1). The larger variations in NTCO₂ in the surface waters are related to changes in primary production and upwelling in the coastal waters. The depth profiles of pH, pCO₂, TA, and TCO₂ were similar to those in the Equatorial Pacific Ocean. The components of the carbonate system (CO₂, HCO₃/^-, CO₃/^2-) and the saturation state (Ω) for calcite and aragonite were determined from the measurements of TA and TCO₂. The waters below 600 and 3400 m in the Arabian Sea were undersaturated (Ω < 1.0) for aragonite and calcite, respectively. The CO₂ measurements have been combined with the nutrient data to examine the stoichiometric ratios of C/N, C/P, C/O₂, and C/SiO₂ of the waters. Marked differences were found for the waters above and below the oxygen minimum zone. The surface water results have been used to develop the following stoichiometry for phytoplankton in the Arabian Sea (CH₂O)₁₂₅(NH₃)₁₄(H₃PO₄)(SiO₂)₁₃. The oxidation of this material is due to reactions with O₂ (77%) and NO₃ (23%) with the resultant formation of N₂ and N₂O. The maximum amount of organic carbon oxidized has been estimated to be 3.1 μmol kg^-1 in the deep waters with as much as 0.9 μmol kg^-1 in the oxygen minimum zone with NO₃. The maximum amount of CaCO₃ dissolved in the deep waters is 116 μmol kg^-1. These results, together with the organic material collected from the sediment traps, should be useful in characterizing the formation and degradation of plant material in the Arabian Sea.