Limestone is one form of carbon sink. It is generated as ocean organisms generated micorscopic shells which collect on the bottom. As the ocean get more carbon dioxide mixed in it, rather than oxygen, then it tends to re-dissolve the limestone back into carbon dioxide and dissolved calcium.
Soil humus and peat is another form of carbon sink. Though it has been noted that in some types of warmer climate conditions soil humus is broken down faster than it forms.
