How does the formation of exoskeleton by marine organisms help to minimize global warming?

Question

The excessive CO2 being released in the atmosphere through the combustion of fuels is largely absorbed by seas and oceans thus restricting the green house effect and global warming. Choose the appropriate combination of the biological processes that help in minimizing global warming. i. Photosynthesis by phytoplanktonic species ii. Deposition of marl and compaction into limestone iii. Diagenesis of organic sediment into mineral oils iv. Formation of exoskeleton by marine organisms (a) i, ii & iv (b)i, ii & iii (c)Only i & ii (d)Only i & iv The answer given is (d), but how does the formation of exoskeleton actually help?

Answer 1

the exoskeleton is composed of calcium carbonate. Organisms extract co2 from their environment to make this substance. Thus co2 absorbed by the ocean, which is a co2 sink and reservoir, can then be removed from the ocean in part by organisms with exoskeletons. When these organisms die, they often fall to the sea floor and their hard shells form a sediment which eventually is transformed into chalk, marble, limestone etc. Thus, co2 is sequestered in these forms instead of floating around in the atmosphere. (However it is important to note that increased co2 concentration in the ocean also leads to ocean acidification, and when acid reacts with calcium carbonate it dissolves the hard shell and co2 is released. For this reason many shelled organisms are in trouble because of co2 emissions (they die when their shell is dissolved).