The global carbon cycle describes the movement of carbon atoms between the atmosphere, the oceans, the land biosphere, and rocks. This constant circulation regulates the planet’s climate over vast timescales. While much of the cycle involves rapid exchange, a small but continuous process of sedimentation acts as the planet’s long-term carbon removal mechanism. This slow, geological process effectively locks carbon away from the active surface cycle, storing it for millions of years within the Earth’s crust. It is the deep ocean and subsequent rock formation that ultimately prevents carbon from perpetually accumulating in the atmosphere.
Defining the Long-Term Carbon Cycle
The movement of carbon is often divided into two primary systems based on their speed and reservoir size. The “fast” carbon cycle involves the exchange of carbon between the atmosphere, the surface ocean, and the land biosphere, largely driven by photosynthesis and respiration. This cycle operates relatively quickly, with carbon turning over in decades to a few millennia.
In contrast, the “slow” carbon cycle operates over immense geological periods, often taking 100 to 200 million years to complete a full circuit. This system involves the lithosphere, or Earth’s rock layer, as its main reservoir, holding carbon in the form of sedimentary rocks. Sedimentation is the physical gateway that transfers carbon from the fast, surface cycle into this vastly larger, slow reservoir. This geological sequestration regulates the planet’s climate over deep time.
The Process of Carbon Burial
The initial step in long-term carbon removal begins in the ocean through the biological pump. Microscopic marine plants called phytoplankton absorb dissolved carbon dioxide from the surface water during photosynthesis. This biological uptake reduces the concentration of carbon dioxide in the surface layer, allowing the ocean to absorb more from the atmosphere.
When these organisms die, or are consumed and excreted by zooplankton, the carbon-rich organic matter sinks toward the ocean floor as “marine snow.” A small fraction of this sinking material is inorganic carbon in the form of calcium carbonate shells, produced by organisms like foraminifera. As this material descends, much of it is consumed and remineralized back into dissolved carbon dioxide by microbes. However, a small percentage survives this journey and settles onto the seafloor, forming the initial layers of carbon-rich sediment.
Geological Sequestration
Once carbon-rich sediments settle on the seafloor, they enter geological sequestration. Over millions of years, the accumulation of subsequent layers exerts tremendous pressure on the buried material, initiating diagenesis. Diagenesis encompasses the physical, chemical, and biological changes that convert soft, unconsolidated sediment into hard rock.
This transformation involves two main mechanisms: compaction and cementation. Compaction forces water out and reduces porosity, while cementation occurs as dissolved minerals precipitate, binding the sediment grains together. The ultimate result is lithification, the formation of solid sedimentary rock. Inorganic carbon from marine shells is chemically transformed into massive layers of carbonate rock, such as limestone and dolomite, which hold the majority of sequestered carbon. Buried organic matter, preserved in anoxic conditions, is subjected to increasing heat and pressure, transforming it into kerogen and generating fossil fuels.
Key Global Sediment Sinks
The effectiveness of carbon burial is concentrated in specific geological environments that serve as major sediment sinks. Deep ocean basins, particularly the abyssal plains, receive a slow, continuous rain of pelagic sediment, primarily the calcium carbonate shells of plankton. Although the accumulation rate is slow, the sheer volume of these vast areas means they store enormous quantities of inorganic carbon.
Continental shelves and slopes have much faster accumulation rates due to their proximity to land runoff, which delivers vast amounts of terrestrial sediment and organic matter to the sea. Marginal seas and coastal ecosystems, often referred to as “blue carbon” environments, are particularly effective sinks, including salt marshes, mangrove forests, and seagrass meadows. These areas possess waterlogged, anoxic soils that dramatically slow the decomposition of plant matter, preserving high concentrations of organic carbon in the sediment. The total carbon stored in these crustal and oceanic reservoirs is estimated to be many orders of magnitude larger than the carbon currently circulating in the atmosphere and biosphere.