Understanding the oceanic carbon pump and gaining a better understanding of the chemical elements in the south-western Indian Ocean: these are the objectives of the 48 scientists ready to board the Marion-Dufresne II for the Swings campaign. The two CNRS researchers heading up this mission, Hélène Planquette, from the Marine Environment Sciences Laboratory (Lemar), and Catherine Jeandel, from the Space Geophysics and Oceanography Laboratory (Legos), explain what is at stake.
Catherine Jeandel, Hélène Planquette, you met in early January in La Réunion. What was your objective?
Hélène Planquette1. On 13 January, we boarded the Marion Dufresne II to lead the Swings (South West Indian Geotraces Section) oceanographic campaign. We are due to disembark on 8 March 2021, after eight weeks at sea in the South West Indian Ocean.
Catherine Jeandel2. A major aim of Swings is to gain a better understanding of the so-called ocean carbon pump: marine biology plays a key role in the natural carbon cycle, sequestering large quantities of CO2 in the waters of the deep ocean.
To do this, you are studying the transport of trace elements and isotopes within bodies of water?
C. J. Absolutely. This mission is part of a very large international project, Geotraces, the aim of which is to describe and quantify the sources of chemical elements in the ocean, their transformation in the ocean once they are there, and finally how they are then subtracted from it. To do this, we are relying on the 'talkative' elements of the periodic table, which serve as tracers... and which are in fact very scarce in the ocean.
H. P. Throughout our mission, we will be taking samples from the surface to the bottom of the ocean to determine the concentrations of these elements, which are present in very low concentrations. We call them 'trace elements', hence the name of this international, collaborative project, which reflects our discipline. Faced with the sheer volume of the world's oceans, scientists from all over the world have realised that they need to pool their efforts.
We're talking about specific chemical elements, each playing a particular role...
H. P. Indeed. Some of these elements are essential for the development of life. They are the vitamins of the ocean and are said to be nutrients, such as iron, which is essential for photosynthesis at the surface. Other elements come into play, such as copper, zinc and cadmium.
C. J. Others are tracers of processes and play the role of 'dyes', because they make it possible to identify the source of the material, or chronometers of the processes studied because they are naturally radioactive. For example, thorium helps us to determine the speed at which matter falls from the surface to the bottom sediments, like marine snow in the water column. As we are unable to land with a stopwatch at a depth of 3,000 m, thorium comes in very handy!
Each of you has your own favourite subjects, right?
H. P. For example, I'm focusing on iron, a trace element essential for photosynthesis and the first link in the biological carbon pump. The Southern Ocean plays an important role in the sequestration of atmospheric CO2 and the players involved in this sequestration are complex, involving biological activity (photosynthesis at the surface, trophic chain, export of carbonaceous matter to the deep ocean, sequestration in sediments) and ocean circulation (marine currents, transport of water masses). It is therefore important to identify where nutrients come from and how they are transformed. To determine these sources and sinks, we use the distributions of other elements such as manganese or aluminium, or isotopes.
C. J. Not all the chemical elements present are produced within the ocean machine, as products of biological activity: others are derived from continental dust, from sediments deposited by rivers on continental margins, etc. We are also studying these sources and transports. For example, isotopes of neodymium, a rare earth element on which I have done a lot of work, can be used to identify the origin of the material at a given point, for example if it comes from South Africa. We also measure barium, a tracer of bacterial activity at medium depth, and radium, a chronometer that can be used to establish how long ago water left the coast. All play their specific role as tracers in this very special toolbox!
Measuring these concentrations in seawater must be a real challenge!
H. P. Absolutely! For traces of iron, it's like looking for the material in a paper clip that's been dissolved in 30 Olympic swimming pools! In the history of our discipline, we first had to agree on the protocols, to make sure we were measuring the same thing. This was known as intercalibration at the end of the 2000s.
C. J. It certainly implies a certain way of working. These trace element measurements are very recent; the first ones date back to the 1980s and 1990s, when we were able to carry them out reliably, i.e. without contamination from outside the sample.
What role does the swings campaign play in Geotraces?
C. J. Marine geochemists (our discipline in oceanography) are aiming to create a global atlas of open-access data on these tracers. What's more, we still have a lot to understand about the mechanisms at work in the ocean, which is not much more homogeneous in terms of ecosystems than the Earth, with its deserts, Amazonian rainforests, Norman meadows and so on.
The task is therefore immense, and working in isolation makes no sense given the stakes involved. The Geotraces programme was conceived in the 2000s by the researchers themselves, who thought about the best places to explore, based on the oceanographic fleets and the scientific interests of each nation. In marine research, collaboration is more important than competition. The swings mission will be exploring the south-western Indian Ocean and sailing in the world's largest current.
H. P. Access to this sector of the Southern Ocean is easy for the French scientific community thanks to the Marion Dufresne, which makes regular rotations around the Crozet and Kerguelen islands, for example. The swings mission is a major contribution to Geotraces as it is the only oceanographic campaign in this sector. In addition to the nutrients mentioned above, we will also be studying the circulation and physical transport of water masses in this sector of the Southern Ocean. Our research is just one building block, but if we can better understand these transports and the mechanisms of the ocean's biological carbon pump, which traps CO2 from the atmosphere, dissolves it and deposits it at the bottom, we will have a better understanding of its role in climate.
What samples and experiments will be carried out on board?
H. P. We have several instruments at our disposal: two rosettes equipped with 24 12-litre bottles, one of which is dedicated to trace metals, which are then collected in a dedicated clean room, specially fitted out in a 20-foot container. A 'fish' will draw water continuously from the stern of the boat, which will be pumped into this clean room. This invaluable equipment is managed by the technical division of the Institut national des sciences de l'Univers (Insu) in Plouzané, near Brest (Finistère). Our foreign colleagues are also providing aerosol and rain collectors.
C. J. We will also be using around fifteen pumps, capable of filtering 1,000 litres of seawater for around three hours, at depths that have been targeted for their interest. We'll also be taking sediment cores at very specific points.
How is the itinerary defined from La Réunion?
C. J. Downstream of the islands, there may be a significant input of matter through the erosion of these islands: we're going to be on the lookout for any planktonic stimulation and richer biodiversity. We will also be comparing the warmer waters of the northern zones with the colder waters of the south. Between South Africa and the first island we encounter, Marion Island, we'll be spending three days exploring hydrothermalism near an underwater mountain known as the mid-oceanic wrinkle. To the south of Kerguelen and Heard, we pass through a deep trench where a circumpolar current, a horizontal circulation this time, transports chemical elements from west to east.
H. P. We have drawn up the route of the campaign to intercept the zones that Catherine has just mentioned, but on board, a small navigation team, known as the anticipation team, of which we are a part, will refine the itinerary a few days in advance. A member of the team has designed a piece of software, a sort of routine that will enable us to adapt our route and our measurements in real time to unforeseen logistical and weather conditions. It's an aid to sampling, so we don't have to 'type blindly', because we'll have access to chlorophyll concentrations, in real time, obtained by satellite, as well as altimetric products that give us the precise position of surface currents.
Measurements are taken 24 hours a day?
C. J. And seven days a week! There are 105 of us on board, including 48 researchers and scientists for the Swings mission alone. In addition to studying the origin of trace elements, their physical, chemical and biological transformations in the water column and the ways in which they leave the ocean, there are also plans to compare the CO2 concentration in surface waters with that in the atmosphere: this is why the Oiso national observation service, which has been carrying out this annual monitoring for 30 years, is on board.
H. P. We should also mention a second temporal data monitoring programme, Themisto: aboard Swings, it uses acoustics to study pelagic ecosystems by establishing the high-resolution 3D distribution of zooplankton and micronekton over a wide gradient of latitudes, in order to understand how these organisms are distributed in relation to ocean physics, biogeochemistry and climate, and also in relation to top predators that concentrate in certain marine areas to feed.
C. J. A final project, Map-Io, uses the ship's platform to establish physical measurements of aerosol distribution (optical thickness, granulometry) in the atmosphere of the Indian Ocean.
The mission also includes a scientific outreach component...
C. J. Absolutely! We have put in place a special programme for teachers in schools, collèges and lycées. Educational documents will be prepared by two associate professors, Yan Serra in SVT-geology and Renaud Blyweert in physics-chemistry, and made available to them at the educational service of the Midi-Pyrénées Observatory in Toulouse. Every Tuesday morning, the Université Fédérale de Toulouse's online newspaper Exploreur, edited by Anne-Claire Jolivet, will also feature an in-depth article on the research themes addressed during the campaign, content shared with the CNRS le journal blog.
H. P. And thirdly, the website https://swings.geotraces.org will be kept up to date throughout the campaign. Featuring a logbook, ship positions and captioned photos, it will be fed by participants who have the desire (and the time!) to share their experiences on board. And a video maker, Sibylle d'Orgeval, will be filming a documentary, accompanied by her assistant Laurent Godard, who is also a painter and dental surgeon!
How is your scientific collaboration organised? How do you share the work?
H. P. Given the number of operations, the multitude of teams and the associated sampling, we felt it was important to share our time on board: one of us will always be on watch, awake. This sharing also applied to the type of sampling: for example, Catherine was responsible for coordinating the standard rosette and I for the clean rosette. Finally, to build up this mission, each of us contacted several collaborators, both in France and abroad, according to their skills. We then brought them together to better identify their needs and refine the campaign strategy.
C. J. The first drafts date back to 2010! Setting up a project like this takes time and perseverance, as well as a team of motivated collaborators. Hélène and I also shared the mountain of administrative documents, visas, authorisations and so on. Each of us is constantly aware of what the other is doing, so that we can deal with any eventuality.
H. P. Like, for example, during the festive season: we took it in turns to keep an eye on things to deal with any last-minute problems. ♦
Read on the Insu website: Heading for the Southern Ocean with the Swings campaign
Notes
1. Hélène Planquette is a CNRS researcher at Lemar (CNRS/UBO/IRD/Ifremer unit), in Plouzané near Brest (Finistère).
2. Catherine Jeandel is CNRS Research Director at Legos (CNRS/Cnes/IRD/UPS Unit), in Toulouse (Haute-Garonne).