Carbon, a fundamental element, cycles continuously through the atmosphere, land, and oceans, influencing Earth’s climate. The ocean serves as a vast reservoir for carbon, holding approximately 50 times more carbon than the atmosphere. This allows the ocean to absorb and redistribute significant amounts of carbon, regulating atmospheric carbon dioxide levels and global temperatures. Understanding the pathways through which carbon enters this marine reservoir is important for comprehending the Earth’s carbon cycle.
Absorption from the Atmosphere
The primary pathway for carbon to enter the ocean is through the direct absorption of atmospheric carbon dioxide (CO2) into surface waters. This process, known as air-sea gas exchange, is driven by the difference in CO2 partial pressure between the atmosphere and the ocean. When atmospheric CO2 concentrations are higher than those in the surface ocean, CO2 molecules diffuse into the seawater.
The ocean acts as a substantial carbon sink, absorbing an estimated 25 to 30 percent of human-generated CO2 emissions over recent decades. This uptake occurs primarily at the air-sea interface. Once dissolved, CO2 undergoes chemical reactions, forming carbonic acid, which then dissociates into bicarbonate and carbonate ions. These chemical forms allow the ocean to store vast quantities of carbon.
Several physical factors influence the rate and extent of CO2 absorption. Ocean temperature is a primary factor, as colder water has a greater capacity to dissolve CO2 than warmer water. This explains why cooler regions, particularly the polar oceans, are significant areas for carbon uptake, while warmer equatorial waters tend to release CO2.
Salinity also affects solubility, with higher salinity reducing the amount of CO2 that can be absorbed. Wind and waves play a role by enhancing gas transfer across the interface through increased surface area and mixing. Ocean currents then transport this dissolved carbon from the surface into deeper ocean layers, sequestering it for extended periods.
Transport from Land by Rivers
Carbon also enters the ocean from terrestrial environments, carried by rivers in various forms. This riverine transport serves as an important link between land-based carbon cycles and the marine system. The carbon arrives primarily as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC).
Dissolved inorganic carbon, largely in the form of bicarbonate ions, originates from the natural weathering of rocks on land. As rainwater, made slightly acidic by atmospheric CO2, interacts with rocks, it dissolves minerals and transports the inorganic carbon into river systems. Global riverine DIC input to the ocean is estimated to be around 0.65 ± 0.3 gigatons of carbon per year.
Dissolved organic carbon, conversely, results from the decomposition of plant and soil organic matter. Rivers carry approximately 0.25 gigatons of DOC to the oceans annually. Particulate organic carbon consists of larger fragments of organic material suspended in the water.
Human activities significantly influence the amount and type of carbon transported by rivers. Agricultural practices, such as fertilizer use, and deforestation can increase carbon runoff into waterways. Climate warming can also accelerate chemical weathering rates, impacting inorganic carbon release. Urbanization and pollution can lead to increased decomposition of organic matter in rivers, potentially altering these ecosystems from carbon sinks to sources that release carbon dioxide back into the atmosphere before it reaches the ocean.
Release from Geological Sources
Beyond atmospheric exchange and riverine input, carbon also enters the ocean through geological processes, though these pathways generally contribute less to the overall carbon flux. These sources involve the release of carbon compounds from Earth’s interior.
Hydrothermal vents, found predominantly along mid-ocean ridges, emit carbon-rich fluids into the deep ocean. These vents release dissolved black carbon (DBC), which forms when hot vent fluids mix with cold seawater. Hydrothermal vents contribute significantly to the deep-sea dissolved organic carbon pool. These vents also release metals like iron and manganese, which can promote phytoplankton growth in surface waters, indirectly influencing the biological carbon pump and atmospheric carbon uptake.
Submarine volcanoes and mid-ocean ridges also contribute carbon through CO2 degassing from magma. Carbon outflux from mid-ocean ridges is estimated to be around 13 ± 4 million tons per year. Most of this degassed CO2 dissolves into the surrounding seawater, becoming part of the marine bicarbonate cycle. These geological inputs are slower and contribute less to the overall carbon flux into the ocean compared to the more dynamic processes of atmospheric absorption and riverine transport.