The biological pump, also called the marine biological carbon pump, is a process that helps the ocean store carbon from the atmosphere and land runoff deep inside the ocean and in seafloor sediments. This process uses living things to move carbon away from the atmosphere and land into the deep ocean. The biological pump is part of the larger "marine carbon pump," which includes both physical and biological processes. It is responsible for moving organic matter created by phytoplankton during photosynthesis (called the soft-tissue pump) and calcium carbonate (CaCO₃) from shells made by organisms like plankton and mollusks (called the carbonate pump).

Scientists calculate how much carbon the biological pump moves by comparing how much carbon settles to the ocean floor (sedimentation) with how much carbon is released back into the atmosphere (remineralization).

The biological pump is not one single process but the total of many smaller processes that all work together. Overall, it moves about 10.2 billion tonnes of carbon each year into the deep ocean and a total of 1,300 billion tonnes of carbon over an average of 127 years. This keeps carbon away from the atmosphere for thousands of years or more. If the ocean did not have a biological pump, carbon dioxide levels in the atmosphere would be about 400 parts per million (ppm) higher than they are today.

Overview

Carbon is important for Earth's climate and life. It moves between different parts of Earth, such as the geosphere, cryosphere, atmosphere, biosphere, and hydrosphere. This movement of carbon is called the Earth's carbon cycle. It is also connected to the movement of other elements and compounds. The ocean plays a key role in the carbon cycle by helping to control the amount of carbon dioxide (CO₂) in the atmosphere. The biological pump is a process that moves organic carbon from the ocean's surface to the deep ocean, and it is central to the ocean's carbon cycle.

The biological pump works by moving organic carbon that survives breakdown in the sunlit surface region of the ocean. This carbon is carried to the deep ocean through physical mixing, the movement of organisms like zooplankton and fish, and the sinking of organic material. The biological pump has three main stages. The first stage is when planktonic phototrophs in the sunlit surface region use carbon dioxide, nitrogen, phosphorus, and other elements during photosynthesis to create carbohydrates, lipids, and proteins. Some plankton, such as coccolithophores and foraminifera, use calcium and dissolved carbonates to form a protective shell made of calcium carbonate.

Once carbon is fixed into an organism's body, the organism may remain in the sunlit surface region to be reused in the nutrient cycle, or it may sink to the ocean floor after dying. As these sinking particles move downward, they often join together to form larger groups, which helps them avoid being broken down by predators or bacteria. These groups sink faster and have a better chance of reaching the ocean floor.

When carbon is broken down by bacteria during its descent or after reaching the ocean floor, it is changed back into inorganic carbon, which can be used again in the production of organic matter. Some particles avoid being broken down entirely and are stored in ocean sediments, where they may stay for millions of years. This stored carbon helps reduce the amount of CO₂ in the atmosphere.

The biological pump involves the movement of organic carbon into the deep ocean through processes like photosynthesis, the transfer of carbon through food webs, physical mixing, and the sinking of particles due to gravity. During photosynthesis, phytoplankton use inorganic carbon to create organic matter, releasing dissolved organic matter (DOM) and being eaten by zooplankton. Larger zooplankton, such as copepods, produce fecal pellets that may sink or join other organic material to form larger, faster-sinking groups. Some DOM is used by bacteria and released as carbon dioxide, while the remaining hard-to-break-down DOM is carried into the deep ocean. Particles and DOM that reach the deep ocean are eventually broken down, returning organic carbon to the deep ocean's inorganic carbon reservoir. About 1% of the particles that leave the ocean's surface reach the seafloor, where they are consumed, released as carbon dioxide, or buried in sediments. These processes remove organic carbon from the ocean's surface and return it to inorganic carbon at greater depths, maintaining the difference in inorganic carbon levels between the surface and deep ocean. Over long timescales, deep-ocean carbon is returned to the atmosphere through thermohaline circulation.

Primary production

The first step in the biological pump is the creation of both organic and inorganic carbon compounds by phytoplankton in the upper, sunlit parts of the ocean. During photosynthesis, phytoplankton make organic compounds like sugars, carbohydrates, lipids, and proteins. This process can be shown with the following equation:

Carbon dioxide + Water + Light → Organic compounds + Oxygen

In addition to carbon, the organic matter in phytoplankton also contains nitrogen, phosphorus, and small amounts of trace metals such as magnesium, cadmium, iron, calcium, barium, and copper. The amounts of carbon, nitrogen, and phosphorus in phytoplankton are usually close to a ratio of 106 parts carbon to 16 parts nitrogen to 1 part phosphorus, known as the Redfield ratio. While trace metals are much less common in phytoplankton, they are still needed for some biological processes and can limit photosynthesis if their levels in the ocean are too low.

Oceanic primary production is responsible for about half of all carbon fixation on Earth. Each year, marine phytoplankton fix approximately 50 to 60 billion metric tons of carbon, even though they make up less than 1% of Earth's total photosynthetic biomass. Most of this carbon fixation (about 80%) happens in the open ocean, while the remaining 20% occurs in highly productive upwelling regions. Although upwelling areas produce 2 to 3 times more fixed carbon per unit area, the open ocean covers more than 90% of Earth's ocean surface, making it the larger contributor overall.

Forms of carbon

Phytoplankton is essential for ocean life because it changes inorganic compounds into organic materials. This process creates the base of the marine food web. In the diagram below, arrows show how dissolved organic matter (DOM) is produced and removed. Arrows pointing toward the DOM pool indicate production, while arrows pointing away show removal. Dashed arrows represent biological processes that help transfer DOM. As DOM moves deeper into the ocean, the part that is easily broken down (labile DOM) decreases quickly, while the part that is hard to break down (refractory DOM) increases. DOM stands for dissolved organic matter.

The marine biological pump relies on several key pools, parts, and processes that affect its function. There are four main carbon pools in the ocean.

Dissolved inorganic carbon (DIC) is the largest pool, containing about 38,000 Pg C. It includes dissolved carbon dioxide (CO₂), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and carbonic acid (H₂CO₃). The balance between carbonic acid and carbonate affects the pH of seawater. Carbon dioxide dissolves easily in water, and its solubility decreases as temperature increases. During photosynthesis, dissolved CO₂ is used, which can lower CO₂ levels in seawater and draw more CO₂ from the atmosphere. The opposite process, respiration, releases CO₂ back into water, increasing CO₂ levels and allowing more CO₂ to return to the atmosphere. Organisms like coccolithophores create calcium carbonate, which releases CO₂ into water.

Dissolved organic carbon (DOC) is the second-largest pool, containing about 662 Pg C. DOC can be divided into refractory (hard to break down), semi-labile (moderately breakable), or labile (easily breakable) types. The labile part is about 0.2 Pg C, is available for life, and is produced quickly (about 15–25 Pg C per year). The refractory part is the largest (about 642 Pg C) but turns over very slowly (about 0.043 Pg C per year). Refractory DOC may remain in the ocean for over 1,000 years.

Particulate organic carbon (POC) contains about 2.3 Pg C and is smaller compared to DIC and DOC. Despite its size, POC is highly active, with the fastest turnover rate of any organic carbon pool. It is produced globally at about 50 Pg C per year through primary production. POC can be split into living parts (like phytoplankton, zooplankton, and bacteria) and non-living parts (like detritus). Phytoplankton is especially important because it supports marine primary production and serves as food for larger organisms in the open ocean.

Particulate inorganic carbon (PIC) is the smallest pool, containing about 0.03 Pg C. It exists as calcium carbonate (CaCO₃) in solid form and affects the ocean’s carbonate system and pH. PIC is produced at about 0.8–1.4 Pg C per year, with most dissolving in the upper ocean and the rest sinking to deep sediments. Coccolithophores and foraminifera are major sources of PIC in the open ocean. PIC is important because it helps move carbon to the deep ocean through the carbonate pump, where it sinks from the sunlit zone to the ocean floor.

Particulate inorganic carbon (PIC) is usually calcium carbonate (CaCO₃) and plays a key role in the ocean’s carbon cycle. This carbon is used by many planktonic species (like coccolithophores and foraminifera) and larger marine animals (like mollusk shells) as protection. Fish also release calcium carbonate during osmoregulation, and it can form in events called whiting. While PIC is not directly taken from the atmosphere, it forms from dissolved carbonate, which is in balance with CO₂. This process removes carbon from the ocean by burying it in sediments.

The chemical reactions involved are:
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
Ca + 2HCO₃⁻ → CaCO₃ + CO₂ + H₂O

Although this process fixes a large amount of carbon, it also removes two units of alkalinity for every unit of carbon stored. This creates a difference in alkalinity between the surface and deep ocean, which raises the pH of surface water. This change increases the amount of dissolved CO₂ in surface water, which can raise atmospheric CO₂ levels. When calcium carbonate is buried in sediments, it lowers overall ocean alkalinity, which can raise pH and atmospheric CO₂ levels unless new alkalinity from weathering balances it. Some carbon is permanently stored in ocean sediments and becomes part of Earth’s geological record. Calcium carbonate can form large deposits, like the White Cliffs of Dover in England, which are mostly made of ancient coccolithophore plates.

Oceanic carbon cycle

Three main processes, called pumps, help move atmospheric carbon dioxide (CO₂) into the ocean and spread it throughout the oceans. These pumps are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. The total amount of carbon active on Earth’s surface for less than 10,000 years is about 40,000 gigatons of carbon (Gt C). A gigaton is one billion tons, roughly the weight of 6 million blue whales. About 95% of this carbon (around 38,000 Gt C) is stored in the ocean, mostly as dissolved inorganic carbon. The way dissolved inorganic carbon is arranged in the ocean plays a key role in controlling the ocean’s acid-base balance.

The biological pump works alongside the solubility pump, which moves large amounts of dissolved inorganic carbon (DIC) from the ocean’s surface to its interior. This process uses only physical and chemical changes, not biological ones.

The solubility pump operates because of two ocean processes:
1. CO₂ dissolves more easily in colder water than in warmer water.
2. Deep water forms in high-latitude regions where water is cooler and denser, driving the thermohaline circulation.

Because deep water forms in areas where CO₂ dissolves well, it contains more dissolved inorganic carbon than average surface levels. These two processes work together to move carbon from the atmosphere into the ocean’s interior. When this deep water rises to the surface in warmer, equatorial regions, it releases CO₂ into the atmosphere because the gas dissolves less easily in warm water.

The carbonate pump is sometimes called the "hard tissue" part of the biological pump. Some ocean organisms, like coccolithophores, create hard structures made of calcium carbonate, a form of particulate inorganic carbon, by using bicarbonate. This process is an important part of the ocean’s carbon cycle.

The chemical reaction for this process is:
Ca + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O

The biological carbon pump changes inorganic carbon (CO₂) into organic carbon in the form of sugar (C₆H₁₂O₆). The carbonate pump, however, changes inorganic bicarbonate into calcium carbonate and releases CO₂. This means the carbonate pump works in the opposite way to the biological pump, reducing the amount of CO₂ moved into the biological pump.

The continental shelf pump operates in shallow waters near the edges of continents, moving carbon (dissolved or as particles) from coastal areas to the deep ocean. This process happens where the solubility pump interacts with cooler, denser water from the continental shelf. The shallow depth of continental shelves limits how much water can cool, so shelf waters often cool more than open ocean water. This cooling supports the solubility pump, increasing the storage of dissolved inorganic carbon. The extra carbon storage is also helped by higher biological activity on shelves. These dense, carbon-rich waters then sink to the ocean floor and mix with the deep ocean. As global sea levels rise due to warming, the area of continental shelves will increase, making the shelf pump stronger.

Processes in the biological pump

In the diagram on the right, phytoplankton take in carbon dioxide (CO₂) from the atmosphere, which has dissolved into the top layer of the ocean (90 gigatons per year), and turn it into tiny organic particles (particulate organic carbon, or POC) during a process called primary production (about 50 gigatons of carbon per year). These tiny organisms are then eaten by copepods, krill, and other small zooplankton, which are in turn eaten by larger animals. Any phytoplankton not eaten form clumps, and along with waste from zooplankton, these clumps sink quickly and move out of the top layer of the ocean (less than 12 gigatons of carbon per year). Krill, copepods, zooplankton, and microbes in the surface ocean and at deeper levels consume and break down POC into CO₂ (dissolved inorganic carbon, or DIC), so only a small amount of carbon from the surface reaches the deep ocean (below 1,000 meters). When krill and small zooplankton eat, they also break particles into smaller pieces that sink more slowly, slowing the movement of POC to the deep ocean. This process releases dissolved organic carbon (DOC), either directly from cells or indirectly through bacteria (shown as a yellow circle around DOC). Bacteria then break down DOC into DIC (CO₂), a process called microbial gardening.

The biological carbon pump is a major factor in how carbon is distributed vertically in the ocean and affects the level of CO₂ in the ocean’s surface, which influences how CO₂ moves between the ocean and atmosphere. This process involves phytoplankton, the animals that eat them, and bacteria that break down their waste. It plays a key role in the global carbon cycle by moving carbon from the atmosphere to the deep ocean, where it is stored for centuries. Phytoplankton use sunlight to produce POC in the upper part of the ocean (euphotic zone, 0–200 meters deep). This POC is processed by microbes, zooplankton, and their predators into fecal pellets, organic clumps ("marine snow"), and other forms, which then sink or are carried deeper by zooplankton and fish. While both dissolved organic carbon (DOC) and POC are produced during primary production, only POC leads to efficient movement of carbon to the deep ocean. Most DOC in surface waters is broken down by bacteria, but some DOC produced in the euphotic zone (about 15–20% of total production) is not quickly broken down and accumulates in the ocean’s surface as semi-labile DOC. This semi-labile DOC eventually moves to the deep ocean, contributing to the biological carbon pump. The efficiency of this process varies by region, being more active in subtropical oceans with less nutrients. The overall success of the biological carbon pump depends mostly on how much POC is exported to the deep ocean.

Most carbon in living and non-living ocean matter is created at the ocean’s surface, where it begins to sink toward the ocean floor. The deep ocean receives most of its nutrients from the upper layers as they sink in the form of "marine snow," which includes dead or dying animals, microbes, fecal matter, sand, and other materials. A single phytoplankton cell sinks about one meter per day. Since the average ocean depth is about four kilometers, it may take over ten years for these cells to reach the ocean floor. However, when phytoplankton clump together or are packed into fecal pellets by predators, they form larger, faster-sinking aggregates called marine snow. These aggregates sink much faster than individual cells and reach the deep ocean in days.

In the diagram on the right, phytoplankton use sunlight in the euphotic zone to capture CO₂ and produce POC. This POC is processed by microbes, zooplankton, and their predators into organic aggregates (marine snow), which sink or are carried deeper by zooplankton and fish to the mid-depth (200–1,000 meters) and deep ocean (bathypelagic) zones. Export flux refers to the movement of material out of the surface layer (about 100 meters deep), and sequestration flux refers to the movement of material out of the mid-depth layer (about 1,000 meters deep). Some POC is broken down back into CO₂ in the deep ocean by microbes and zooplankton, creating a difference in CO₂ levels between the surface and deep ocean. This deep-ocean CO₂ returns to the atmosphere over thousands of years through ocean currents. Between 1% and 40% of the carbon produced by phytoplankton moves out of the euphotic zone, decreasing rapidly as it travels deeper, with only about 1% reaching the ocean floor.

Of the 50–60 petagrams of carbon produced each year, about 10% moves out of the ocean’s surface layer, and less than 0.5% eventually reaches the ocean floor. Most of the carbon is reused in the upper ocean, and a large portion is broken down during the sinking of particles. The carbon that leaves the surface layer is sometimes considered "sequestered" because it is removed from contact with the atmosphere for centuries. However, in areas like the Southern Ocean, much of this carbon can return to the atmosphere within decades.

The biological carbon pump is studied by comparing the movement of carbon (export) to its breakdown (remineralization). It is estimated that up to 25% of the carbon captured by phytoplankton in the surface ocean is moved to deeper layers. About 20% of this (5% of surface levels) is buried in ocean sediments due to minerals that help particles sink. These organic particles are centers of microbial activity and play a key role in breaking down organic matter and redistributing nutrients in the ocean.

Observations show that the movement of minerals (calcium carbonate, opal, and rock material) and organic carbon are closely linked in the deep ocean. Much of the organic matter in the ocean is found in marine snow aggregates (larger than 0.5 millimeters) made of phytoplankton, dead matter, minerals, and fecal pellets. The formation and sinking of these aggregates move organic matter from the surface to the deep ocean and sediments. The amount of organic matter that leaves the surface layer depends on how fast these aggregates sink and how quickly microbes break them down. Recent studies show that the movement of these minerals and organic matter is closely connected in the deep ocean, leading to the idea that the presence of these minerals helps move organic carbon to the deep ocean.

Minerals help about 60% of the organic carbon in the North Atlantic and 40% in the Southern Ocean to sink. Strong links also exist in the deep ocean between these minerals and organic carbon movement. This suggests that minerals help organic carbon move to deeper layers by attaching to sinking particles.

Quantification

The geological part of the carbon cycle works slowly compared to other parts of the global carbon cycle. It is a major factor in how much carbon is in the atmosphere, which affects Earth's temperature.

The biological pump is an important part of Earth's carbon cycle, so scientists work hard to measure how strong it is. However, the processes that form the biological pump are hard to study because they happen deep in the ocean and involve complex interactions between living things. One way scientists estimate the strength of the biological pump is by measuring how much life grows using nutrients like nitrate and ammonium. These nutrients come from different sources linked to the breakdown of sinking material. From this, scientists calculate the f-ratio, which helps show how strong the biological pump is in a certain area. Using findings from local studies to understand the entire ocean is difficult because ocean currents vary in different regions.

Effects of climate change

Changes in how land is used, burning fossil fuels, and making cement have caused more carbon dioxide (CO₂) to build up in the atmosphere. Today, about one-third (around 2 billion tons of carbon each year) of CO₂ from human activities may be entering the ocean, but this is not certain. Some studies suggest that higher CO₂ levels might affect the growth of plants in the ocean.

Climate change could change how the biological pump works in the future. Warming and layering of the ocean’s surface might reduce the flow of nutrients to the upper ocean layer where plants grow, which could lower the amount of plant life there. Also, ocean acidification, which happens when CO₂ dissolves in seawater, might change the success of certain ocean organisms that build hard shells. This could affect how well the biological pump moves carbon from the surface to deeper parts of the ocean. These changes might also influence other processes in the ocean that help move carbon downward.

A diagram on the right shows possible effects of melting sea ice and thawing permafrost on carbon movement in the Arctic. On land, plants take in carbon, while soil microbes release methane and CO₂. Lakes release methane into the air, and carbon moves from land to the ocean through rivers and streams. In the ocean, methane can be released from frozen soil under the sea, and CO₂ is absorbed because the water has less CO₂ than the air. Many processes are linked to sea ice. Scientists estimate carbon movement in the atmosphere in units of teragrams of carbon per year (Tg C/year) where possible. The lake data covers areas north of about 50º N, which is a larger area than the Arctic tundra used for other land-based estimates. Uncertainty ranges are shown in brackets when available. The arrows in the diagram do not show the size of each process.

The biological pump has likely influenced changes in atmospheric CO₂ levels during past ice ages and warmer periods. However, it is unclear how the biological pump will respond to future climate changes. To make accurate predictions, it is important to understand how phytoplankton, a key part of the biological pump, will react to rising CO₂ levels. Different types of phytoplankton may respond differently to climate changes. For example, diatoms, which are good at moving carbon to the deep ocean by forming large, sinking groups, may become less common because the ocean could become more layered in the future. Fewer diatoms might reduce how much carbon is moved to the deep ocean.

Lower ocean pH, caused by ocean acidification, might make it harder for coccolithophores to create their hard shells, which could affect the biological pump. However, some species are more affected than others. Changes in the numbers of these or other phytoplankton types might greatly impact ocean productivity and how much carbon is stored in the ocean.

A 2015 study found that coccolithophore levels in the North Atlantic have grown ten times since the 1960s. Scientists think this increase is likely due to more CO₂ being absorbed and rising temperatures.

In a 2017 study, scientists used models to predict how two important phytoplankton species—diatom Chaetoceros diadema and coccolithophore Emiliania huxleyi—might spread in the future. They used data from the IPCC’s scenario 8.5, which predicts how much the Earth’s climate might change by 2100. Their models suggested that the areas covered by these two species could decrease by 8% and 16%, respectively, under this scenario. These changes might reduce how much carbon is moved to the deep ocean through the biological pump.

A 2019 study suggested that if ocean acidification continues at current rates, Antarctic phytoplankton might become smaller and less effective at storing carbon by the end of the century.

Monitoring

Tracking the biological pump is very important for learning how the Earth's carbon cycle is changing. Scientists use many methods to track the biological pump, which can be used from different places like ships, autonomous vehicles, and satellites. Right now, satellite remote sensing is the only way to see the whole surface of the ocean over large areas and over time.

Needed research

To fully understand the biological pump in the deep ocean, scientists need to study the deep water column using multiple fields of science:

• Physics: The layering of water affects how quickly particles sink. To learn where particles come from and how long dissolved inorganic carbon (DIC) stays in the deep ocean after particles break down, scientists must measure how water moves and mixes.
• Biogeochemistry: Organic matter, both as particles and dissolved substances, moves from the ocean’s surface to deeper layers. This process determines how much easily broken-down organic matter reaches the seafloor, where it is either used for energy by seafloor organisms or stored in the sediment for longer periods.
• Biology and ecosystems: Tiny animals called zooplankton and microorganisms break down sinking particles in the water column. Organic matter that reaches the deep ocean provides food for all living things in the water column and on the seafloor, including zooplankton, seafloor invertebrates, and microbes. This process helps maintain the number, density, and variety of species in these environments.