The biological carbon pump is a natural ocean system that moves carbon from the atmosphere and surface waters into the deep ocean and seafloor sediments. This process relies on marine organisms to transform dissolved inorganic carbon into organic matter. It acts as a mechanism for sequestering carbon, removing it from active circulation in the upper ocean and atmosphere. This pump functions as a fundamental component of Earth’s broader carbon cycle, operating across the global ocean.
Journey of Carbon in the Ocean
The journey of carbon within the ocean’s biological pump begins in the sunlit surface waters, known as the euphotic zone, where microscopic marine plants called phytoplankton play a foundational role. These single-celled organisms absorb dissolved carbon dioxide from the seawater during photosynthesis. This biological uptake reduces the carbon dioxide concentration in surface waters, enabling the ocean to absorb more carbon dioxide from the atmosphere.
As phytoplankton grow and reproduce, they incorporate carbon into their biomass. This carbon then moves up the marine food web through grazing by small marine animals, primarily zooplankton. Larger marine organisms, including fish, then feed on these zooplankton.
A significant portion of the carbon taken up by phytoplankton is recycled within the upper ocean layers through respiration by marine organisms and bacteria. However, a fraction of this organic matter avoids immediate recycling and begins to sink. This sinking material often aggregates into “marine snow,” which consists of dead organisms, discarded feeding webs, fecal pellets from zooplankton, and other organic detritus.
Marine snow particles, along with larger dead organisms like fish and marine mammals, descend through the water column due to gravity. The rate at which these carbon-rich particles sink varies based on their size and density; smaller particles might take weeks or months to reach the seafloor, while denser materials can sink in days. This gravitational settling is a primary pathway for transporting carbon from the surface to the ocean’s interior.
Once these carbon-rich particles reach depths typically below 500 meters, the carbon is considered sequestered. At these depths, it is unlikely to return to the atmosphere for hundreds to thousands of years. The process of sequestration removes carbon from atmospheric exchange, storing it in the deep ocean or within seafloor sediments for extended periods.
Global Climate Regulation
The biological carbon pump plays a role in regulating Earth’s climate by influencing atmospheric carbon dioxide levels. By transferring carbon from the surface to the deep ocean, it acts as a natural carbon sink, mitigating the greenhouse effect. Without this biological mechanism, atmospheric carbon dioxide concentrations would be higher.
Approximately 2.8 billion tons of carbon, equivalent to about 10 billion tons of carbon dioxide, are sequestered annually by the biological carbon pump. This amount is a notable fraction compared to global fossil fuel emissions, which were around 36.6 billion tons of carbon dioxide in 2023.
The effectiveness of the biological carbon pump directly impacts the ocean’s capacity to absorb atmospheric carbon dioxide. When carbon is removed from surface waters and transported downwards, it creates a gradient that allows more atmospheric carbon dioxide to dissolve into the ocean. This continuous removal helps to maintain the ocean’s role as a carbon reservoir, which holds roughly 50 times more carbon than the atmosphere.
Variations in the efficiency of this pump and the strength of the deep ocean carbon sink regulate Earth’s climate and have been linked to past glacial-interglacial cycles. The carbon sequestered at depth by the pump remains out of contact with the atmosphere for potentially several thousand years. This long-term storage is a natural mechanism that helps to balance the global carbon budget over geological timescales.
Environmental Influences on the Pump
The efficiency and strength of the biological carbon pump are sensitive to various environmental factors. Changes in ocean temperature affect the pump’s function. As surface waters warm, the ocean experiences increased stratification, meaning layers of water become more distinct and mix less readily.
This increased stratification limits the upward mixing of nutrient-rich deep waters into the sunlit surface layer. A reduction in nutrient supply impacts phytoplankton growth and overall primary production. Less organic matter produced at the surface translates to less carbon available for transport to the deep ocean, thereby weakening the pump’s effectiveness.
Nutrient availability influences the productivity of phytoplankton. Certain phytoplankton are larger and more effective at carbon export but require ample nutrients. If nutrient-depleted conditions cause shifts in phytoplankton communities towards smaller species, the overall carbon export to the deep ocean can decrease.
Ocean acidification, a consequence of increasing atmospheric carbon dioxide dissolving into seawater, also poses a threat to the biological carbon pump. A more acidic ocean makes it harder for calcifying organisms to produce their calcium carbonate shells or structures. These structures serve as ballast, helping organic matter sink more quickly, so their reduced formation can slow down the transport of carbon to depth.
The interplay of these factors creates a complex picture for the future of the biological carbon pump. While some changes might partially compensate by allowing particles to sink faster, the predicted overall trend points to a potential decline in the ocean’s carbon sink capacity due to global warming and associated environmental shifts.