Is atmospheric carbon a carbon sink?

A natural or man-made reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period..

rivers

Trees absorb carbon dioxide (when talking about a "carbon sink", it means the carbon as any form).

Formation of carbonates in the ocean

The atmospheric source of carbon that is part of the carbon cycle is carbon dioxide. Carbon dioxide moves into the atmosphere from the oceans, the land, rocks and soil, from vegetation and from...