Scientists argue that banking carbon dioxide in the oceans depends as much on withdrawal as deposition

Both the biological and physical elements of the marine biological carbon pump must be considered, climate researchers argue.

Viewing BCP under human-induced climate change
Under human-induced climate change, export flux declines, and the interior ocean stores more dissolved inorganic carbon. Credit: Global Change Biology (2024). DOI: 10.1111/gcb.17124

A climate researcher from the GEOMAR Helmholtz Center for Ocean Research in Kiel, Dr. Ivy Frenger, and Dr. Angela Landolfi from the National Research Council, Rome, Italy, published an opinion paper arguing that the deposition of organic matter — produced from carbon dioxide (CO2) at the ocean surface — is not directly linked to the storage of CO2 in the deep ocean.

An overview of the marine biological carbon pump

Marine phytoplankton and zooplankton play a key role in the marine biological carbon pump (BCP). Phytoplankton are photosynthetic organisms, harnessing the sun’s energy and converting carbon dioxide into organic biomass to grow and survive. These microscopic organisms are predated upon by larger predatory zooplankton. Some carbon is released as carbon dioxide while some is converted into body mass, which sinks to the deep ocean when the zooplankton die.

The process of sinking organic matter or “export flux” is the key area of contention here. Organic matter trickling down into the interior ocean is broken down by bacteria, which produces inorganic matter. This inorganic matter becomes sequestered in the deep ocean, which in turn, captures CO2.

Deep ocean benthic zone
The deep ocean's benthic zone harbours an array of life which depends on the deposition of decaying organic matter from the mid and upper ocean for survival.

In their opinion paper, Dr. Frenger and her colleagues implore the scientific community to not only factor in the deposition and storage of CO2, but also “withdrawal”—like cash from a bank account—from this reservoir through ocean circulation.

Dr. Frenger proposes a more scientifically precise approach to gauge the CO2 reservoir resulting from biological processes in the deep ocean. This method involves measuring oxygen content and physical characteristics (such as the temperature of the ocean's interior).

"You have to consider the ocean circulation because it determines how much of the biologically produced CO2 can actually accumulate in the interior ocean in the long term, isolated from exchange with the atmosphere.” Dr. Ivy Frenger, climate researcher from the GEOMAR Helmholtz Center for Ocean Research.

The changes in these variables, influenced by climate change, clarify the seemingly contradictory response of the BCP. Despite a decrease in carbon export, the BCP contributes to increased carbon storage in the deep ocean due to shifts in ocean circulation, delaying the feedback of biologically stored carbon to the surface. This leads to a greater accumulation of CO2 in the ocean's interior under climate change compared to a scenario without the BCP.