The ocean carbon cycle

Carbon, a building block of life, is constantly moving through different environmental compartments such as biota, the atmosphere, the ocean, soil and sediment, as part of what is called ‘the global carbon cycle.’ A change in any of these fluxes could have wide-ranging impacts on ecosystems and our climate. The IAEA Environment Laboratories apply nuclear and isotopic techniques to better understand the carbon cycle, to evaluate the ocean’s capacity to store carbon and to study how this might be impacted in future climate scenarios.

Carbon exists in many forms in the global carbon cycle, including carbon dioxide (CO₂) and methane (CH₄), two prominent greenhouse gases. CO₂ exists naturally but is also increasingly being emitted into the atmosphere through human activities like the burning of fossil fuels. One quarter of this anthropogenic CO₂ released into the atmosphere is taken up by the ocean. Some of this CO₂ returns to the atmosphere, and some is exported to the deep ocean, where the reservoir of carbon is 50 times larger than that stored in the atmosphere. The ocean provides a vital service to our planet through this capacity to regulate atmospheric CO₂ levels and thereby limits climate change and its impacts. However, a small change in the fluxes to the ocean carbon pool could impact the ocean’s storage capacity and in turn impact atmospheric CO₂ levels. The IAEA works with Member States to gain a better understanding of the carbon cycle processes and stocks of carbon which Member States can then use to construct the climate model to predict the impacts of climate change.

The biological pump

Through one of the primary carbon storage mechanisms, the biological carbon pump, phytoplankton (microscopic marine plants at the bottom of the oceanic food chain) take up CO₂ in the surface ocean and as part of photosynthesis convert it to particulate and dissolved organic carbon - carbon-containing molecules typically produced by living things. A fraction of this carbon makes its way to the deep ocean, where it is recycled back into inorganic carbon and stored, isolated from the atmosphere.

The flux of carbon to the deep ocean can be measured directly by collecting sinking particles (living and dead microscopic organisms, faecal matter) in sediment traps, and indirectly using naturally-occurring isotopes of thorium and polonium. These radioisotopes decay at known rates and are used as ‘clocks’ to determine how quickly carbon-containing particles are sinking.

The IAEA Environment Laboratories examine the carbon’s fate by using isotopes to analyse microbial processes in the deep ocean. Microbes are responsible for the transformation of organic material from sinking particles into inorganic carbon. Both naturally-occurring radiocarbon and radio-labelled tracers can be used to measure these microbial processes in terms of the cycling of carbon in the deep ocean.

The application of these tools in a variety of ocean settings helps to determine the extent of sinking carbon flux across different ecosystems and evaluate its sensitivity to climate change.

Working with Member States across the globe

The IAEA Environment Laboratories work with Member States and participate in research missions around the world to collect samples to measure particle flux, including in the Arctic Ocean, a region which is particularly sensitive to ocean warming, and in oxygen-minimum zones, such as those off the coasts of Peru and Mauritania. Such deoxygenation zones are predicted to expand under future climate change scenarios.

An understanding of rates of carbon recycling and the conditions that affect it is important for evaluating the role of the ocean in regulating our climate and how that might change in the future.