The global carbon cycle describes the continuous movement of carbon atoms between Earth’s atmosphere, oceans, land, and sediments. Deposition refers to the mechanisms by which carbon is removed from the atmosphere and stored for varying durations in different reservoirs, often called carbon sinks. These processes regulate Earth’s climate by controlling atmospheric carbon dioxide levels.
Biological Carbon Deposition
Living organisms capture atmospheric carbon dioxide through photosynthesis. Land plants, such as trees and grasses, absorb carbon dioxide from the air, combining it with water and sunlight to create sugars and build their biomass, including wood, leaves, and roots. This carbon is stored within their tissues.
Terrestrial ecosystems, like forests and grasslands, accumulate significant amounts of carbon. Carbon also transfers to the soil as plants shed leaves, roots decay, or when organisms decompose, forming soil organic matter. This biological storage is a dynamic part of the carbon cycle, with carbon held for years to centuries before being released back through respiration or decomposition.
Oceanic Carbon Deposition
The ocean functions as the planet’s largest active carbon sink, absorbing a substantial portion of atmospheric carbon dioxide through distinct mechanisms. One process is the “solubility pump,” where carbon dioxide dissolves directly from the atmosphere into colder, denser surface waters, particularly at high latitudes. These carbon-rich waters then sink and are transported to the deep ocean through large-scale ocean currents, like the thermohaline circulation, sequestering carbon for thousands of years.
The “biological pump” is another pathway, driven by marine life. Microscopic plant-like organisms called phytoplankton at the ocean surface absorb dissolved carbon dioxide for photosynthesis, converting it into organic matter. When these organisms die, or are consumed by zooplankton and their waste products are released, the carbon-rich organic particles, often called “marine snow,” sink through the water column. This sinking transports carbon to the deep ocean and seafloor sediments for hundreds to thousands of years.
Geological Carbon Deposition
Geological processes account for the slowest and most long-term storage of carbon, locking it away for millions of years. A primary mechanism is the accumulation of calcium carbonate, from the shells and skeletons of marine organisms like plankton and corals. After these organisms die, their remains settle on the seafloor, forming layers of sediment.
Over geological time, these sediments undergo compaction and cementation, transforming into sedimentary rocks such as limestone. Limestone is a major reservoir, holding a substantial portion of Earth’s carbon. Similarly, under specific conditions of heat and pressure, buried organic matter from ancient plants and marine organisms can be chemically altered over millions of years to form fossil fuels like coal, oil, and natural gas, representing another significant form of long-term carbon storage within the Earth’s crust.
Human Influence on Carbon Deposition
Human activities have disrupted the natural processes of carbon deposition, diminishing the capacity of these carbon sinks. Deforestation, for instance, directly reduces carbon absorption by plants, meaning less carbon is stored in terrestrial biomass. Converting forested areas or other natural vegetation to agricultural lands or urban developments can also release carbon previously stored in soils back into the atmosphere.
The oceans’ ability to absorb carbon is also impacted by human emissions. Increased atmospheric carbon dioxide leads to greater absorption by seawater, which reacts to form carbonic acid, causing ocean acidification. This decrease in ocean pH hinders marine organisms, especially those building shells and skeletons from calcium carbonate, from forming and maintaining their structures. Such impairment can weaken the biological pump and reduce the long-term geological deposition of carbonates, compromising the ocean’s capacity as a carbon sink.