Fossil fuels largely come from the Carboniferous period. The reason for this is that during this period the trees which covered the planet's surface did not de

Fossil fuels largely come from the Carboniferous period. The reason for this is that during this period the trees which covered the planet's surface did not decompose, and became buried in thick layers, which became coal seams. Whatever the reason, we know that the carbon cycle was not so much of a cycle at this point--the fixed carbon was retained and not recycled back into the atmosphere, resulting in the accumulation of vast quantities of carbon. In the current environment, breakdown is rapid and carbon is rapidly recycled. Accumulation is minimal and while some longer-term storage can occur in places such as peat bogs, these are not stable long-term stores--they can easily burn if they dry out.