The ocean holds a significantly larger amount of carbon than the atmosphere or terrestrial ecosystems. A substantial portion of this carbon originates directly from the atmosphere, entering the ocean through various interconnected processes. The ocean’s ability to absorb and store atmospheric carbon dioxide plays a considerable role in regulating Earth’s climate.
The Initial Journey: From Air to Water
Carbon dioxide from the atmosphere dissolves into the surface waters of the ocean through a physical process. The concentration of CO2 in the atmosphere, known as its partial pressure, directly influences how much gas can enter the ocean. When the partial pressure of CO2 in the air is higher than in the surface water, CO2 molecules move across the air-sea interface and dissolve into the ocean.
Once dissolved, this CO2 can be transported to deeper parts of the ocean through a mechanism called the solubility pump. This physical process occurs predominantly in colder, high-latitude regions where surface waters become dense. These cold, CO2-rich waters then sink, carrying the dissolved gas with them to the deep ocean. The amount of CO2 that seawater can hold is influenced by factors like temperature and salinity; colder water dissolves more gas, making polar regions effective at absorption.
Carbon’s Chemical Forms in the Ocean
Upon dissolving in seawater, carbon dioxide undergoes a series of chemical reactions, forming various inorganic carbon compounds. Dissolved CO2 first reacts with water to form carbonic acid (H2CO3). This carbonic acid is unstable and quickly dissociates, losing a hydrogen ion to become bicarbonate (HCO3-). Bicarbonate can further dissociate, losing another hydrogen ion to become carbonate (CO3^2-) ions.
These chemical transformations establish the ocean’s carbonate system, a complex equilibrium of these different carbon forms. Bicarbonate ions are by far the most abundant form of inorganic carbon in the ocean, accounting for approximately 90% of the total dissolved inorganic carbon. The presence of these different carbon forms also provides the ocean with a natural buffering capacity, which helps to stabilize its pH by absorbing changes in acidity.
Life’s Role: The Biological Carbon Pump
Marine organisms actively participate in the uptake and transport of carbon from the surface waters to the deep ocean, a process known as the biological carbon pump. Microscopic marine plants, called phytoplankton, are primary producers that absorb dissolved carbon dioxide and bicarbonate ions from surface waters through photosynthesis. They convert this inorganic carbon into organic matter, forming the base of the marine food web.
As phytoplankton are consumed by zooplankton and other marine life, carbon moves up through the food web. When these organisms die, or produce waste products, the organic carbon they contain can sink. Particles such as dead organisms, fecal pellets, and shells made of calcium carbonate (CaCO3) descend through the water column. This sinking material transports carbon from the sunlit surface layers to the deep ocean and eventually to the seafloor sediments, contributing to long-term carbon storage.
The Ocean’s Capacity as a Carbon Reservoir
The ocean absorbs and stores a substantial portion of atmospheric carbon. This vast capacity results from the combined physical and biological processes that transport carbon from the surface to the deep ocean. The deep ocean and seafloor sediments can store carbon for hundreds to thousands of years, effectively removing it from active exchange with the atmosphere for extended periods.
This continuous absorption and storage of carbon helps mitigate the increase of CO2 in the atmosphere, which is produced by both natural processes and human activities. The ocean’s role as a carbon reservoir influences the planet’s overall carbon balance.