cover pic by Sofia Alexiou, NOC
Dr Maribel García-Ibáñez & Mr Gareth Lee, University of East Anglia, UK
EPISODE 1: Why we measure carbon dissolved in the ocean
Atmospheric carbon dioxide (CO2) enters the ocean through three “pumps”: the solubility pump (or physical pump), the biological pump, and the carbonate pump. Continuous contact and interaction between the atmosphere and the ocean allows CO2 to be readily absorbed into the surface ocean. This is what is called the solubility pump or physical pump. The biological pump consists of the transformation in the ocean surface of dissolved CO2 into organic matter, whose deposition creates a flow of organic carbon to the deep ocean. Phytoplankton, the base of the oceanic food webs, absorb dissolved CO2 to synthesize organic matter. As it passes through the food web, the organic matter is transported to deeper layers of the oceans, being oxidized and decomposed. Part of this organic material reaches the seafloor, joining seabed sediments. The carbonate pump, on the contrary, releases CO2 in the ocean surface layer through the creation of calcium carbonate external structures by some marine organisms such as coccolithophores and foraminifera. This calcium carbonate precipitates during photosynthesis and sinks.
Understanding how CO2 behaves in the ocean, therefore, gives us information about how the ocean uptakes atmospheric CO2 and how it is the redistributed in the ocean. Human activities have increased atmospheric CO2 concentrations since the industrial revolution. These anthropogenic CO2 emissions occur on top of an active natural carbon cycle that circulates carbon between the atmosphere, ocean and land reservoirs. The ocean dominates the storage of CO2 due to its high solubility in seawater and its sequestration through water sinking away from the surface. In fact, the oceans have absorbed about 30% of the anthropogenic CO2 emitted to the atmosphere since the industrial revolution. But this anthropogenic CO2 is not evenly distributed throughout the oceans. While CO2 concentration in the surface layers of the ocean increases as CO2 increases in the atmosphere, its penetration into the deep ocean depends on the slow vertical mixing of the water column and the circulation of water. About half of the anthropogenic CO2 is found in the first 400 m of the water column. However, in some regions where vertical movements of water are relatively fast, such as the Southern Ocean, the time scale necessary for deep penetration of anthropogenic CO2 is of the order of decades instead of centuries.
During the CUSTARD cruise, we, the UEA CO2 team, are quantifying two variables of the carbon system in the water of the Southern Ocean to help in answering the question of how deep in the ocean the CO2 is stored and, therefore, for how long it is kept out of the atmosphere. Understanding how the ocean uptakes the atmospheric CO2, which processes are responsible for this uptake, and where in the water column the CO2 is stored will allow us to understand how the ocean will continue this task, favourable to us all, in the future.
Please see our next blog post ‘Carbon on CUSTARD- EPISODE 2’ to learn more about how we go about measuring dissolved carbon in the ocean.
Scientists measure Carbon on CUSTARD research expedition, here’s whyTweet