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 [CO3=] rather than calcification temperature. Down core shell weight variations of Core AAS9/21 show an excellent correspondence with CO2 concentrations in an Antarctic Ice Core, which reveals that planktic forminifera shells can trace atmospheric CO2 due to the resultant change in surface water [CO3=]. Hence, shell weights of G. sacculifer can be used as a proxy to reconstruct atmospheric CO2 concentrations in the past. Here, based on the shell weights, surface water [CO3=] change in the Arabian Sea is quantified and found that a [CO3=] variation of 
8 μmol/kg occurred during the Holocene and a 36 μmol/kg variation occurred during the last glacial period.
Naik, S; S., Naidu, P. D., Govil, P., & Godad, S., 2010. Relationship between weights of planktonic foraminifer shell and surface water CO3= concentration during the Holocene and Last Glacial Period. Marine Geology 275(1-4):278-282. Article (subscription required).
