The global carbon cycle describes carbon’s continuous movement among Earth’s major reservoirs: atmosphere, oceans, land, and rocks. While carbon rapidly exchanges between the atmosphere, oceans, and living organisms, a significant portion is sequestered over vast timescales within Earth’s crust. Sedimentation is a key component of this cycle, storing carbon for millions of years and helping regulate Earth’s climate by removing carbon from active atmospheric and oceanic systems.
Carbon’s Journey to Sediments
Carbon transitions from the active cycle into geological reservoirs through several distinct sedimentation pathways. One primary mechanism involves the burial of dead organic matter, derived from plants and animals. When these organisms die, their remains can settle in environments with low oxygen levels, such as deep ocean basins, swamps, or lakebeds. The lack of oxygen prevents complete decomposition, allowing the organic material to accumulate over time.
Over millions of years, as layers of sediment build up, increasing pressure and temperature transform this buried organic matter into fossil fuels, including coal, oil, and natural gas. Coal forms from ancient plant material in swampy environments, while oil and natural gas often originate from marine microorganisms. This process effectively locks away atmospheric carbon that was once incorporated into living biomass.
Inorganic carbon also forms geological deposits, primarily through the creation of carbonate rocks like limestone. Many marine organisms, such as corals, shellfish, and plankton, extract dissolved calcium and carbonate ions from seawater to build their shells and skeletons of calcium carbonate (CaCO3). Upon their death, these calcium carbonate remains accumulate on the ocean floor, forming vast layers of sediment. Over geological timescales, these sediments undergo compaction and cementation, solidifying into carbonate rocks.
Long-Term Carbon Storage
Sedimentary rocks and fossil fuel deposits collectively represent Earth’s largest carbon reservoir, holding significantly more carbon than the atmosphere, oceans, and terrestrial biosphere combined. Carbon can remain stored in these forms for millions to hundreds of millions of years, classifying this as the “slow” component of the global carbon cycle.
The amount of carbon stored in sedimentary rocks, particularly limestones, is estimated to be over 100 million gigatons of carbon. This contrasts sharply with the atmosphere, which holds approximately 800 gigatons, and the oceans, which contain around 38,000 gigatons. This vast geological sequestration of carbon has historically played a significant role in regulating Earth’s long-term climate. By continuously removing carbon dioxide from the atmosphere over geological epochs, sedimentation has helped maintain atmospheric CO2 concentrations within ranges conducive to life, preventing runaway greenhouse effects.
Returning Carbon to the Active Cycle
While sedimentation sequesters carbon for prolonged periods, natural geological processes eventually return some of this stored carbon to the active atmosphere and oceans. One significant pathway is the chemical weathering of carbonate rocks. When rainwater, which is slightly acidic due to dissolved atmospheric carbon dioxide, falls on limestone, it slowly dissolves the calcium carbonate. This process releases bicarbonate ions into rivers and groundwater, which eventually flow into the oceans.
Some of this dissolved carbon can then be released as carbon dioxide back into the atmosphere or remain dissolved in ocean water. Volcanic activity also acts as a natural carbon release mechanism. As tectonic plates move, carbon-rich sedimentary rocks can be carried deep into Earth’s mantle through subduction zones. The intense heat and pressure within the mantle melt these rocks, and the trapped carbon dioxide is subsequently released into the atmosphere during volcanic eruptions. Tectonic uplift can also expose deeply buried carbon-rich rocks to the surface, making them more susceptible to weathering and erosion, which releases their stored carbon.
Human Impact on the Sedimentation Carbon Cycle
Human activities have significantly altered the natural balance of the sedimentation carbon cycle, primarily by accelerating the release of carbon that has been stored for millions of years. The extraction and combustion of fossil fuels—coal, oil, and natural gas—represent the most significant human influence. These fuels are highly concentrated forms of organic carbon that were buried and transformed over geological timescales. When burned for energy, they rapidly release vast quantities of carbon dioxide (CO2) into the atmosphere.
This rapid release bypasses the slow, natural geological processes that would otherwise return this carbon to the atmosphere over millions of years. The burning of fossil fuels releases approximately 9 to 10 gigatons of carbon into the atmosphere annually. This rate is far greater than the natural geological release, leading to a significant increase in atmospheric CO2 concentrations.
Changes in land use, such as widespread deforestation and certain agricultural practices, also indirectly impact the sedimentation carbon cycle. While these activities primarily affect the terrestrial carbon cycle by reducing carbon stored in biomass and soils, they can influence the amount of organic carbon available for eventual sedimentation. Ultimately, human actions are accelerating the natural geological release of carbon, contributing to increased atmospheric CO2 levels and climate change.