Genomics:GTL

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Carbon Cycling in the Oceans: Solubility and Biological Pumps

Ocean processes regulate the uptake, storage, and release of CO₂ to the atmosphere. The total exchange of carbon between atmosphere and oceans is controlled by two principal processes: The solubility (or physical) pump and the biological pump.

The solubility pump is driven by physical processes. The solubility of CO₂ in water increases with lower water temperature, and the colder water sinks. This gradient, from lower CO₂ concentration near the surface to higher concentrations below about 500 m, helps draw CO₂ from the atmosphere into the oceans. The solubility pump, in combination with ocean circulation, results in net CO₂ emissions at the equator and net CO₂ drawdown at high latitudes. Changes to ocean circulation or stratification due to increased global warming from increased greenhouse gases are predicted to result in decreased ocean uptake of CO₂ by the ocean solubility pump.

The biological pump, whose activities are just being revealed, refers to the composite of biological processes occurring in ocean surface layers. These begin with the microbial photosynthesis of CO₂ into organic matter (much like land plants) and end with either the conversion of organic matter to CO₂ at different depths or with the deposition of a small fraction of organic material into sediments on the ocean floor. The biological pump’s efficiency is a function not only of carbon fixation but also of the depth at which the organic carbon is remineralized to CO₂. Current models, which rely on incomplete carbon-cycle models having little biological input, suggest that if the biological pump were turned off today, atmospheric levels of CO₂ would rise to 680 ppm (~400 ppm higher than preindustrial levels and about 300 ppm higher than current levels). The ocean’s future activity as a carbon sink is uncertain, however, because of potential (and currently uncharacterized) feedbacks among global climate change, ocean circulation, and microbial communities that actively cycle carbon. These natural ocean carbon-sequestration processes extend beyond carbon to affect organic and inorganic pools of nitrogen, phosphorus, oxygen, and many other chemical species.

A key feature of ocean environments is the extremely slow recycling of mineral nutrients. Dead organisms from the photosynthetically active top of the water column sink into its depths and, ultimately, the ocean floor. They carry with them essential nutrients, mainly nitrogen and phosphorus, that are liberated in the darkness of the deep ocean. From there, upwelling currents take several thousand years to return the nutrients to warm surface waters. Consequently, primary production in the top of the water column is limited severely by the lack of mineral nutrients, whereas the nutrient-rich deep waters lack light energy for primary photosynthetic production.

Text adapted from Genomics:GTL Roadmap: Systems Biology for Energy and Environment, U.S. Department of Energy Office of Science, August 2005. DOE/SC-0090.