The ocean carbon cycle is the continuous movement of carbon atoms between the ocean, the atmosphere, and the marine biosphere. This circulation system is fundamental to regulating Earth’s climate and sustaining marine ecosystems. The ocean is the planet’s largest active carbon reservoir, holding approximately 50 times more carbon than the atmosphere.
Ocean-Atmosphere Carbon Exchange and Circulation
Carbon dioxide enters the ocean at the sea surface, where it dissolves from the atmosphere. This process is governed by water temperature. Colder, denser waters, especially in polar regions, can hold significantly more dissolved CO2 than warmer tropical waters. This solubility pump is the physical mechanism driving carbon from the air into the sea.
Once dissolved, this carbon is transported by large-scale ocean currents. The thermohaline circulation, a current system driven by differences in water temperature and salinity, is a large part of this process. It moves carbon-rich surface waters from high-latitude regions into the ocean’s depths.
This circulation includes downwelling and upwelling. In downwelling zones, surface water with its dissolved carbon is transported into deeper layers. Conversely, upwelling brings deep, carbon-rich water back to the surface. This physical mixing distributes carbon throughout the world’s oceans.
The Marine Biota’s Role in Carbon Cycling
Life in the ocean drives the biological carbon pump, which moves carbon from the surface to the depths. This system is initiated by phytoplankton, microscopic marine algae in the sunlit upper ocean. Through photosynthesis, these producers convert dissolved CO2 into organic carbon, forming the base of the marine food web.
This newly created organic carbon is then transferred up the food chain as phytoplankton are consumed by small animal-like organisms called zooplankton. These, in turn, are eaten by larger animals, including fish, marine mammals, and seabirds. Throughout this cycle, carbon is incorporated into the biomass of these organisms.
A fraction of this organic carbon is exported to the deep ocean. This occurs when organisms die and their remains sink, or through the sinking of fecal pellets. This constant shower of organic material, often called “marine snow,” is a direct pathway for transporting carbon from the surface into deeper waters.
As this organic material descends, it is decomposed by microbes. This process, known as remineralization, converts the organic carbon back into dissolved CO2 and releases nutrients. This recycling of carbon at depth prevents it from immediately returning to the atmosphere.
Deep Ocean Carbon Sequestration
The deep ocean serves as a long-term reservoir for carbon, storing it for centuries to millennia away from the atmosphere. This sequestration is a result of the physical and biological pumps.
The sinking of organic matter is a primary pathway for delivering carbon to the deep sea. An additional mechanism, the carbonate pump, also contributes to long-term storage. Some marine organisms, including coccolithophores, foraminifera, and corals, construct hard shells from calcium carbonate (CaCO3).
When these calcifying organisms die, their calcium carbonate structures sink. A portion reaches the seafloor and accumulates in sediments. Over geological timescales, these layers can become rock, such as limestone, locking carbon away for millions of years. This process is one of the most permanent forms of carbon removal in the ocean system.
Anthropogenic Carbon and Ocean Uptake
Since the Industrial Revolution, human activities have altered the global carbon cycle. The burning of fossil fuels, cement production, and land use changes have released large amounts of carbon dioxide into the atmosphere. The concentration of atmospheric CO2 has risen rapidly, reaching levels not seen in at least the last 800,000 years.
The ocean has provided a buffer against a more rapid accumulation of atmospheric CO2, absorbing an estimated 25-30% of all anthropogenic emissions to date. This uptake occurs through the same physical exchange processes that govern the natural cycle.
This oceanic absorption has slowed the pace of global warming by reducing the amount of heat-trapping gas in the atmosphere. Without this service, atmospheric CO2 levels would be higher, and the resulting climate impacts would be more pronounced.
Consequences of Increased Ocean Carbon
The absorption of excess CO2 is causing a shift in seawater chemistry known as ocean acidification. When CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions. This increases the water’s acidity, measured as a decrease in pH, and reduces the availability of carbonate ions.
This reduction in carbonate ions poses a threat to calcifying marine organisms that rely on these ions to build their shells and skeletons. As carbonate becomes less available, it is more difficult for them to grow, and in acidic conditions, their existing shells can begin to dissolve.
The effects extend beyond calcifying organisms, impacting entire marine ecosystems. Changing water chemistry can impair the sensory abilities and behavior of fish, disrupting their ability to detect predators or find suitable habitats. These shifts at the base of the food web can cascade upwards, altering biodiversity and the functioning of marine ecosystems.
Ocean acidification does not occur in isolation. It acts in concert with other stressors, such as rising ocean temperatures. The combined effect of warming and acidification can create challenging conditions for marine life, potentially altering the ocean’s future capacity to absorb atmospheric CO2.