Reservoir - (n.)

In geochemistry, a reservoir is the mass of an element (such as carbon) or a compound (such as water) within a defined “container” (such as the ocean or the atmosphere or the biosphere). The definition is arbitrary, in principle, but may coincide well with actually existing bodies on Earth (such as the ocean). Reservoir properties are “size” and “reactivity,” with the latter given as the ratio of input to size.