One interesting thing about the carbon cycle (or any natural cycle) is its relationships to other natural cycles, such as the water cycle or the rock cycle. For example, limestone is an important part of both the rock cycle AND carbon cycle. The Madison limestone shown in the photo (Bridger Range) formed about 340 million years ago as molecules of CO2 from the atmosphere dissolved into seawater and then were absorbed and converted to CaCO3 by corals and various shelled critters. When these organisms died their shells became sediment and eventually limestone, locking up carbon in the Earth's crust. Limestone formations are an important reservoir for carbon. To learn more about the role of limestone in the carbon cycle, CLICK HERE.
This NOAA page is another good source: CARBON CYCLE BASICS
Here is an article about research related to the Geologic Carbon Cycle.