Plants convert carbon dioxide from the atmosphere into organic carbon via photosynthesis. Most of this carbon eventually returns to the atmosphere when plant material (or animals that eat plants) decompose. A small fraction of this material, however, ends up in rivers. They carry it out to sea, where some settles to the seafloor and is buried and disconnected from the atmosphere for millions of years and eventually makes its way back to the surface in the form of rocks. At the same time, rivers also erode carbon-containing rocks into particles carried downstream. The process exposes carbon to air, oxidizing the previously locked-up carbon into carbon dioxide that can leak back out to the atmosphere. (Illustration by Eric Taylor, Woods Hole Oceanographic Institution)

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Plants convert carbon dioxide from the atmosphere into organic carbon via photosynthesis. Most of this carbon eventually returns to the atmosphere when plant material (or animals that eat plants) decompose. A small fraction of this material, however, ends up in rivers. They carry it out to sea, where some settles to the seafloor and is buried and disconnected from the atmosphere for millions of years and eventually makes its way back to the surface in the form of rocks. At the same time, rivers also erode carbon-containing rocks into particles carried downstream. The process exposes carbon to air, oxidizing the previously locked-up carbon into carbon dioxide that can leak back out to the atmosphere. (Illustration by Eric Taylor, Woods Hole Oceanographic Institution)