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Monitoring and assessment of carbon export in the oceans

Phytoplankton assimilate inorganic carbon dissolved in the surrounding seawater and transform it into organic carbon within their cells, leading to a decrease in CO2 concentration, which promotes further transfer of CO2 from the atmosphere to the surface ocean. Phytoplankton lie at the base of the food chain and are grazed by marine zooplankton. These tiny surface-dwelling animals ingest phytoplanktonic carbon for energy, while packaging their carbon waste into faecal material that is excreted and settles into the deep ocean. Thus carbon is pumped from the atmosphere through the surface ocean and into the deep ocean as organic carbon. This transfer is facilitated because ingested or dying plankton also contribute dense silica (diatoms) or calcium carbonate material from their tiny shells, which helps to send particles further into the abyss. Once the organic carbon and calcium carbonate particles reach the ocean interior they remain isolated from the atmosphere for centuries. A small portion even becomes incorporated into marine sediments. This vertical carbon-transport process, termed the “biological pump”, naturally keeps atmospheric CO2 levels lower by concentrating carbon in the deep ocean.

Two complementary approaches have been used to quantify the flux of carbon and sinking particles in the oceans. Firstly, in order to capture particles as they fall from the surface to the deep ocean, large conical or cylindrical traps have been deployed. These sediment traps (Figure 1) are either set adrift to float for a few days at given depths or they are fixed to bothom-moored lines at a variety of depths for longer periods (weeks to years). Beyond simply assessing particle and carbon fluxes, the Agency has further used sediment traps to evaluate associated fluxes of radionuclides, both of anthropogenic and natural origin.

Secondly, information has been gathered based on abundances of natural radionuclides in seawater. Oceanographers have exploited the deficit of the seawater activity of the daughter 234Th, which is particle reactive, relative to that of its parent 238U, which is soluble. In upper waters, where particles are produced, thorium adsorbs onto freshly produced particles and thereby leaves the system as these particles settle. The resulting deficit in 234Th along with its half-life of 24 days, much shorter than its parent, is used to assess its removal from the productive upper ocean. The carbon flux is estimated by converting the 234Th flux in POC flux using the particulate organic carbon to 234Th ratios as measured in large particles collected in the field with either in-situ pumps or sediment traps.

Figure 1 (credit: J.C. Miquel - IAEA-NAML)
Figure 2 (credit: J.C. Miquel - IAEA-NAML)

Sediment trap (Figure 1) used to catch marine particles (Figure 2) that settle from the surface to the deep ocean.