Biological Pump and Carbon Export

By fixing carbon at the surface and sending a fraction of it sinking into the deep, marine life runs a planetary-scale pump that keeps atmospheric carbon dioxide far lower than it would otherwise be.

Definition

The biological pump is the set of biological processes that transfer organic carbon from the sunlit surface ocean to depth, where it is stored away from the atmosphere; carbon export is the downward flux of this organic matter out of the surface layer.

Scope

This topic covers the production and sinking of organic particles, the processes of export and remineralization through the water column, the measurement of carbon export flux, the role of zooplankton migration and aggregation, and the controls and efficiency of the biological pump and its sensitivity to a changing ocean.

Core questions

• What fraction of surface primary production is exported to the deep ocean?
• How does sinking organic matter attenuate with depth through remineralization?
• What roles do particle aggregation and zooplankton migration play in export?
• How efficient is the biological pump, and how might it change with warming and acidification?

Key theories

Soft-tissue and carbonate pumps

Organic (soft-tissue) sinking lowers surface carbon and atmospheric carbon dioxide, while the carbonate counter-pump from shell formation partly offsets it, and their balance sets the net biological control on atmospheric carbon.

Export and remineralization depth

Most exported organic matter is consumed and respired in the upper water column, so the depth at which it is remineralized determines how long the carbon is sequestered from the atmosphere.

Mechanisms

Phytoplankton fix carbon at the surface; a portion sinks as aggregates, fecal pellets, and dead cells, and is further transported downward by migrating zooplankton. As particles fall, bacteria and animals respire most of the carbon back to dissolved form at shallow depths, while the small surviving fraction reaches the deep ocean and sediments, where it can be stored for centuries to millennia.

Clinical relevance

The biological pump is a major control on atmospheric carbon dioxide and thus climate; understanding and quantifying it is central to projecting the ocean carbon sink and to evaluating proposed marine carbon-dioxide-removal strategies such as ocean fertilization.

History

The concept crystallized in the 1980s when Volk and Hoffert distinguished the ocean's solubility and biological pumps and quantified their roles in glacial-interglacial carbon dioxide change, motivating decades of flux measurements and the iron-fertilization debate sparked by Martin.

Key figures

• Tyler Volk
• Wallace Broecker
• John Martin

Seminal works

• sarmientoGruber2006
• volkHoffert1985

Frequently asked questions

Why is it called a pump?

Like a mechanical pump moving fluid against a gradient, marine life continually moves carbon from the surface to the deep ocean, maintaining a difference in carbon concentration between the two that lowers atmospheric carbon dioxide.

Could enhancing the biological pump fight climate change?

Proposals such as fertilizing the ocean with iron aim to boost carbon export, but the amount and permanence of sequestration are uncertain and there are ecological risks, so such approaches remain scientifically contested.