The oceanic carbon cycle involves the exchange and storage of carbon within the marine environment, moving carbon between the atmosphere, the water column, and the seafloor. The ocean is the largest active carbon reservoir on Earth, holding approximately 38,000 billion tonnes of carbon. This immense capacity means the oceans contain about 50 times more carbon than the atmosphere.
How the Ocean Absorbs Atmospheric Carbon
The ocean absorbs atmospheric carbon dioxide (\(text{CO}_2\)) through a physical and chemical process known as the solubility pump. This mechanism relies on the principle that gases dissolve more readily in colder liquids. Consequently, \(text{CO}_2\) gas is more soluble in the frigid surface waters of the high latitudes than in warmer tropical regions.
Once the \(text{CO}_2\) dissolves into the seawater, it undergoes a rapid chemical reaction to form dissolved inorganic carbon (DIC) species. The initial reaction is the formation of carbonic acid (\(text{H}_2text{CO}_3\)), which quickly dissociates into bicarbonate (\(text{HCO}_3^-\)) and carbonate (\(text{CO}_3^{2-}\)) ions. Bicarbonate ions make up nearly 88% of the ocean’s total inorganic carbon pool.
This carbon-rich surface water, particularly in regions like the North Atlantic and Southern Ocean, then cools further and becomes denser. The increased density causes the water to sink into the deep ocean as part of the global thermohaline circulation, often called the ocean conveyor belt. This sinking motion physically transports the elevated concentrations of DIC away from the surface and sequesters it in the abyssal depths.
The Role of Marine Life in Carbon Transport
Marine organisms drive a separate process for carbon transfer called the biological pump. This pump begins in the sunlit surface layer where microscopic plants called phytoplankton perform photosynthesis. Through this process, phytoplankton convert dissolved \(text{CO}_2\) into organic matter, fixing inorganic carbon into particulate organic carbon (POC).
This organic carbon moves up the marine food web as phytoplankton are consumed by grazers, such as zooplankton. When these organisms die or excrete waste, the resulting detritus aggregates into larger, heavier clumps. This continuous shower of sinking organic material is colloquially termed “marine snow.”
Marine snow is composed of dead or dying organisms, fecal pellets, and other organic debris. The gravitational settling of this marine snow is the active export mechanism that transports carbon to the deep ocean floor. These larger aggregates can descend at speeds of \(100 text{ meters}\) per day or more, in contrast to smaller particles that sink much slower.
While sinking, much of the organic carbon is remineralized back into inorganic carbon by bacteria and other microbes in the mid-water depths. The fraction that survives this degradation and reaches the deep sea is isolated from the atmosphere for long periods. This biological transport mechanism works in parallel with the solubility pump, providing a second major pathway for carbon sequestration.
Major Carbon Storage Compartments
The ocean’s carbon inventory is distributed across three distinct reservoirs, characterized by their size and residence time. The surface ocean is the smallest active reservoir and features rapid exchange with the atmosphere. Carbon in the surface layer cycles quickly, often on timescales of years or less, constantly balancing the atmospheric \(text{CO}_2\) concentration.
The deep ocean, which comprises the vast majority of the water column, represents the largest active carbon reservoir. Carbon transported to this depth, primarily as dissolved inorganic carbon (DIC) via the solubility pump, is locked away for centuries. The carbon sequestered through the biological pump has an estimated residence time ranging from 200 to 800 years.
The third compartment, marine sediments and rock, holds the largest amount of carbon in the entire Earth system, though it is not part of the short-term active cycle. This carbon, which includes carbonates and fossil fuels, is stored on geological timescales. The burial of this material represents the ultimate long-term sequestration of carbon from the atmosphere-ocean system.
Oceanic Carbon and Global Climate Regulation
The oceanic carbon cycle regulates the Earth’s climate by acting as a carbon sink. The ocean absorbs a fraction of the carbon emissions generated by human activity, taking up approximately 25 to 30% of the excess atmospheric \(text{CO}_2\). This absorption capacity has slowed the rate at which atmospheric \(text{CO}_2\) levels have risen.
This service comes at the expense of changes to the ocean’s chemistry, a phenomenon known as ocean acidification. When the ocean absorbs excess \(text{CO}_2\), the subsequent formation of carbonic acid leads to a decrease in the water’s \(text{pH}\). This shift in chemistry reduces the concentration of carbonate ions (\(text{CO}_3^{2-}\)), which are the building blocks for calcifying organisms.
Marine life, including corals, mollusks, and certain plankton, rely on these carbonate ions to build their shells and skeletons from calcium carbonate. The increasing acidity and reduced availability of carbonate ions make it difficult for these organisms to survive and grow. The ocean’s function as a buffer simultaneously places stress on marine ecosystems, demonstrating the interconnected nature of global climate and ocean health.