The carbon cycle in the ocean describes the continuous movement of carbon atoms between the atmosphere, the ocean’s surface, its depths, and the seafloor. This exchange and transformation of carbon, driven by both natural processes and human activities, makes the ocean a significant part of Earth’s global carbon cycle.
Types of Carbon in the Ocean
Carbon exists in the ocean in various forms, broadly categorized as inorganic and organic. Dissolved inorganic carbon (DIC) represents the largest pool of reactive carbon in the ocean, accounting for approximately 95% of the total carbon stored there. This category primarily includes bicarbonate ions (HCO3-), carbonate ions (CO32-), dissolved carbon dioxide (CO2), and carbonic acid (H2CO3). Bicarbonate and carbonate ions are abundant and regulate the ocean’s pH and chemical equilibrium.
Dissolved organic carbon (DOC) consists of organic molecules. Particulate organic carbon (POC) includes larger organic matter such as dead or living organisms, fecal matter, and detritus. Particulate inorganic carbon (PIC), mainly calcium carbonate (CaCO3), makes up the shells and skeletons of many marine organisms.
How Carbon Moves Through the Ocean
Carbon movement within the ocean is governed by physical and biological processes. Gas exchange at the ocean surface allows carbon dioxide (CO2) to dissolve from the atmosphere into seawater and vice versa. The solubility of CO2 in water is influenced by temperature, with colder waters absorbing more gas.
The solubility pump is a physical process where CO2 dissolves into cold, dense surface waters. These cold, carbon-rich waters then sink and move into the deep ocean through thermohaline circulation. This process creates a vertical gradient, where dissolved carbon concentrations are higher at depth.
The biological pump involves marine organisms moving carbon from the surface to the deep ocean. Microscopic marine plants, called phytoplankton, take up dissolved CO2 in surface waters through photosynthesis, converting it into organic matter. Zooplankton consume phytoplankton. As organisms die, their remains, along with fecal pellets, aggregate into “marine snow” that sinks through the water column. While some of this carbon is recycled in shallower waters through respiration, a fraction reaches the deep ocean, where it can be stored if incorporated into sediments.
The Ocean’s Role in Climate
The ocean plays a role in regulating Earth’s climate system. It functions as a carbon sink, absorbing a portion of atmospheric CO2. The ocean has absorbed approximately 25-30% of human-caused CO2 emissions and about 90% of the excess heat generated by these emissions.
This absorption capacity helps moderate global temperatures, acting as a buffer against rising greenhouse gas concentrations. The ocean stores vast quantities of carbon, roughly 38,000 billion tonnes, which is over 28 times more than carbon in land vegetation and the atmosphere combined. This carbon sequestration contributes to habitability.
Human Influence on the Ocean Carbon Cycle
Human activities, particularly the burning of fossil fuels and changes in land use, alter the natural ocean carbon cycle. Since the Industrial Revolution, the ocean has absorbed a substantial amount of anthropogenic CO2, estimated to be around 150 billion tonnes since 1870. This increased absorption of atmospheric CO2 leads to a direct chemical change in seawater, known as ocean acidification.
When CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions and lowers the ocean’s pH. This change in ocean chemistry makes it more difficult for calcifying organisms, such as corals, oysters, crabs, sea urchins, and certain plankton, to build and maintain their shells and skeletons. These impacts can disrupt marine food chains, threaten biodiversity, and affect industries like fisheries and aquaculture. A study in the United States suggested the shellfish industry alone could face annual losses exceeding $400 million by 2100 due to ocean acidification.