Prof. Sambuddha Misra, Dept. of Centre for Earth Science. IISc.

CO2 from the Abyss: How Oceans Control Climate

As a reactive gas, CO2 readily dissolves in seawater to form carbonic acid; thus, the CO2 in atmosphere and surface ocean are in equilibrium. Whereas, the abyssal deep-ocean, due to long-term accumulation of organic matter respired CO2, contains approximately nine times more CO2 than the surface-ocean and atmosphere combined. The exchange of CO2 between the deep-ocean and the atmosphere, caused by density driven ocean overturning, is the likely modulator of Glacial-Interglacial CO2 fluctuation. Since carbonic acid is a weak acid, the seawater pH and CO2 concentration are tightly coupled. Thus, a measure of seawater pH can be utilised to calculate atmospheric CO2 concentration. Microscopic carbonate-secreting marine organisms, like coccoliths and foraminifera, incorporate trace quantities of dissolved elements from seawater into their calcium carbonate shells. The boron content and isotopic composition of these shells are a function of their growth habitat’s pH. Utilising this relationship, I will demonstrate how reconstruction of CO2 concentrations of atmosphere and the deep ocean is possible by utilising boron isotope ratios (d11B) of foraminifera.