The ocean is not just a vast body of water. It is also one of Earth’s most powerful regulators of carbon. In the Arabian Sea, a region shaped by dramatic seasonal monsoons, scientists set out to understand how carbon dioxide moves through seawater and how biological life transforms it.
In 1995, research teams from the University of Miami and Woods Hole Oceanographic Institution joined a series of oceanographic cruises under the Joint Global Ocean Flux Study. Their goal was ambitious. They wanted to map the entire carbon dioxide system of the Arabian Sea across seasons, depths, and biological activity.
To accomplish this, researchers measured several key chemical signals in seawater, including total dissolved inorganic carbon, alkalinity, pH, and carbon dioxide pressure. One of the central measurement techniques involved coulometric analysis using a UIC Inc. carbon analyzer, integrated into a Dissolved Inorganic Carbon Extractor system. In this process, seawater samples were acidified to release carbon dioxide, which was then transported by nitrogen gas into the coulometric cell where the carbon content was quantified with exceptional precision.
The data revealed a remarkably stable surface ocean chemistry across most of the year. Surface waters typically showed a pH near 8.1 and total inorganic carbon around 1950 micromoles per kilogram. Yet the ocean is far from static. Seasonal monsoon winds drive powerful upwelling events that bring carbon rich deep water toward the surface. These processes cause major variations in carbon dioxide concentrations, especially along coastal regions.
Deep beneath the surface, the chemistry tells another story. Below roughly 600 meters, seawater becomes undersaturated with respect to aragonite, and deeper than about 3400 meters it becomes undersaturated with calcite. These conditions allow calcium carbonate minerals to dissolve, contributing additional carbon and alkalinity to the deep ocean.
Perhaps most intriguing is what the carbon chemistry reveals about life. By combining carbon measurements with nutrient data, the researchers reconstructed the chemical signature of phytoplankton growth in the Arabian Sea. Their analysis suggested a typical biological composition of
(CH₂O)125(NH₃)14(H₃PO₄)(SiO₂)13.
When this organic matter decomposes, oxygen is consumed and carbon dioxide is produced. Most oxidation occurs using oxygen, but a significant portion is driven by nitrate in low oxygen waters. This process helps explain the strong oxygen minimum zone that defines the Arabian Sea.
Together, these measurements offer a deeper understanding of how carbon cycles through one of the world’s most dynamic marine systems. They also provide a crucial baseline for detecting future changes as atmospheric carbon dioxide continues to rise.
The Arabian Sea, shaped by monsoons and biological productivity, stands as a natural laboratory for understanding the ocean’s role in the global carbon balance.
Reference: Millero, F. J., Degler, E. A., O’Sullivan, D. W., Goyet, C., & Eischeid, G. (1998). The carbon dioxide system in the Arabian Sea. Deep-Sea Research Part II: Topical Studies in Oceanography, 45(10–11), 2225–2252. https://doi.org/10.1016/S0967-0645(98)00069-1
