Calcium: chlorinity ratio and carbonate dissolution in the northwestern Indian Ocean

Abstract

Analysis of 128 water samples from the northwestern Indian Ocean gave an average calcium: chlorinity ratio of 0.02164 ± 0.000009. An identical ratio (0.02165) was obtained using the data from selected Meteor stations in the region. The ratio in the northern Indian Ocean is significantly higher than the oceanic averages. The northern Indian Ocean, occupying only 3% of the global oceanic are, receives about 9% of the total global river runoff annually. It is suggested that the high calcium flux from the rivers in a relatively small area increases the Ca:Cl ratio in the upper layers of the region. The ratio in the deep waters in the region is also higher than in other oceanic areas, presumably due to a high rate of calcium removal at the surface by biological activity and its subsequent input to the bottom waters. A more rapid upward movement of deep waters in the Indian Ocean than in the Pacific and Atlantic oceans appears to result in a faster upward flux of calcium in the region. The ratio increases below the lysocline. The calcite saturation depth lies between 2500 and 3000 m. Carbonate content of the sediments, however, shows no appreciable decrease above 4000 m, which supports kinetic models for CaCO₃ dissolution. The data support the Broecker-Takahashi model (Deep-Sea Research, 25, 65-91, 1978), indicating that variations in the lysocline depth can be accounted for largely by variations in carbonate ion concentration. The "critical carbonate ion concentration" for the region is 90 ± 5 μM kg^-1. It is concluded that calcite dissolution does not occur above 4000 m.