Buffering means having a repository of the item being buffered. So while the carbon dioxide levels swing wildly in the atmosphere, the plankton absorb excess to keep the active amount in the atmosphere more nearly steady. It's the same idea of a buffer in an acidic solution. Or a buffer in a computer I/O system. (Not the same as a buffer to polish a car.)
Calcium just exists in the crust. (Ultimate it was produced in a nova or supernova.) Calcium is very active and comines with acids such as carbonic acid to produce limestone (and marble.)
the plankton absorb excess to keep the active amount in the atmosphere more nearly steady Do you mean "the plankton absorb excess which keeps the active amount in the atmosphere more nearly steady"?
The sentence as originally constructed suggests that absorbing the CO2 is somehow a function of the plankton, rather than a marvellous coincidence.
Reason I am asking is that some people believe in the Gaia Hypothesis, that various interworkings of things like plankton, seawater and CO2 operate on some kind of feedback system.