Shell weights of Globigerinoides sacculifer and the elemental concentration of magnesium and calcium (Mg/Ca) from Globigerinoides ruber measured from an Arabian Sea sediment core, AAS9/21, exhibit an inverse relationship with each other, which reveals that shell weights are mainly controlled by surface water [CO sub(3) sup(=)] rather than calcification temperature. Down core shell weight variations of Core AAS9/21 show an excellent correspondence with CO sub(2) concentrations in an Antarctic Ice Core, which reveals that planktic foraminifera shells can trace atmospheric CO sub(2) due to the resultant change in surface water [CO sub(3) sup(=)]. Hence, shell weights of G. sacculifer can be used as a proxy to reconstruct atmospheric CO sub(2) concentrations in the past. Here, based on the shell weights, surface water [CO sub(3) sup(=)] change in the Arabian Sea is quantified and found that a [CO sub(3) sup(=)] variation of approx. mu mol/kg occurred during the Holocene and a approx. 36 mu mol/kg variation occurred during the last glacial period