Flexi Says: If the organism is rapidly buried it may be transformed over millions of years into coal, oil, or natural gas. The carbon may be stored for millions of years. Many ocean creatures use calcium carbonate (CaCO3) to make their shells. When these organisms die, their organic material becomes part of the ocean sediments, which may stay at the bottom of the ocean for thousands or millions of years. Eventually, these sediments may be subducted into the mantle. The carbon could cycle back up into the atmosphere: The ocean sediments melt and form magma, and the CO2 is released when volcanoes erupt.