SWINGS: an oceanographic expedition to unlock the mysteries of the Southern Ocean
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The famous ship Marion Dufresne, a veteran of oceanographic campaigns, set sail on Wednesday 13 January 2021 from Reunion Island to South Africa for the SWINGS (South West Indians Geotraces Section) expedition of the international GEOTRACES programme.
For 8 weeks, 48 scientists and crew members crossed part of the Indian Ocean, passing through the Marion, Crozet, Heard and Kerguelen islands, before returning to Reunion Island 57 days later. This was an exceptional scientific journey, but one that was necessary to take samples of water and particles. The analyses of these samples will lead to a better understanding of the role of the Southern Ocean in CO2 sequestration.
The Southern Ocean: a gigantic ocean at the heart of the climate machine
Of the five ocean basins on the planet Earth, the Southern Ocean is the least known. Located far from the coasts, geographically between 40° south and the Antarctic continent, it is the most inaccessible and the most feared by all sailors who have to face sometimes extreme weather conditions.
If we were to draw it, we would choose the shape of a huge ring of water circling the globe and encircling the Antarctic. This is one of its characteristics: the Southern Ocean is the only ocean that provides a connection between the three major ocean basins (Pacific, Atlantic and Indian) of our planet!
It is this feature that makes it a key element in the balance of the climate machine. The Southern Ocean interacts continuously with an atmosphere marked by extreme weather conditions, particularly strong winds and exceptional temperature variations. The world’s largest and most intense ocean current – the Antarctic Circumpolar Current – ensures exchanges between the Southern Ocean and the three basins mentioned. It carries more than 140 million cubic metres of water per second from west to east – equivalent to 140 times the amount of water in all the rivers on Earth every second!
The ocean circulation within the Southern Basin is an extremely turbulent circulation. The vortex structures in the Southern Basin interact very vigorously with each other and thus extend their influence to depths that can exceed a thousand metres.
The horizontal and vertical movements of these eddies allow the transport, deep injection and dispersion of the heat absorbed by the sea water, its salt content and dissolved gases, especially oxygen and CO2. The role of the Southern Ocean is therefore essential in the sequestration of atmospheric CO2, and therefore in the balance of the climate machine.
Two categories of actors are involved in this carbon sequestration process: biological activity, which ensures photosynthesis at the surface and then, via the food chain, the export of carbonaceous matter to the abyss and sequestration in the sediments; and the physical pump: the dissolution of CO2 and then oceanic circulation, which transports the CO2 into the water masses via marine currents.
Water samples to analyse carbon sequestration
Understanding these two CO2 sequestration processes – biological or physical – therefore requires being able to quantify them, which is partly possible thanks to measurements of so-called “geochemical tracers”. Geochemical tracers are elements present in very low concentrations (i.e. “trace”) in seawater. Some of them play a crucial role in photosynthetic activity and are valuable for marine life.
During the SWINGS campaign, massive water samples were taken. Once analysed, they will make it possible to quantify the origin (atmospheric, sediments, hydrothermalism, etc.) of the geochemical tracers found there. They will also make it possible to study the transformations of state (physical, chemical and biological) that they undergo in the water column, as well as the way in which they are exported to the ocean depths.
To find out more about the SWINGS expedition, click on the link below:
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