South West Indian Geotraces Section – SWINGS

In the remote Southern Ocean (SO) considered as a nutrient “hub” between the Atlantic, Pacific and Indian Oceans, the sources, sinks and processes controlling vital nutrient distribution remain a black-box to date. In this context, SWINGS is a multidisciplinary 4-year project dedicated to elucidate trace element sources, transformations and sinks along a section crossing key areas of the SO. Major French contribution to the international GEOTRACES program (www.geotraces.org), SWINGS involves 78 scientists (19 international laboratories, 6 countries). A 63-day oceanographic cruise in the South Indian Ocean (47 participants on board), process-oriented field studies and integrative modeling experiments will be carried out early 2021 to tackle the following objectives:
1) establish the relative importance of sedimentary, atmospheric and hydrothermal sources of TEIs in the Indian sector of the SO, 2) investigate the drivers of the internal trace element cycles: biogenic uptake, remineralization, particle fate, and export, and 3) quantify TEI transport by the Antarctic Circumpolar Current and the numerous fronts at the confluence between Indian and Atlantic Oceans.
SWINGS strategy relies on the strong coupling between physical oceanography, biogeochemistry and modeling. A major and original focus will be put on the characterization of the physical and chemical particle speciation in suspended and sinking particles that will be collected during SWINGS. Together with a high resolution sampling of the dissolved phases, the resulting SWINGS harvest of data will allow a major step forward in the understanding and quantification of dissolved-particle exchanges, a major recognized bolt for the element cycle modelling. Dedicated tracers (e.g. Th and Pa isotopes) will help to characterize the particle dynamics. Ra isotopes will support the quantification of land-ocean transfers while Nd ones will trace the origin of the dissolved and particulate matter. Specific attention will be paid to the ocean interfaces: atmospheric and land contacts, and a segment of the South West Indian Ridge suspected to be the home of active hydrothermal sites. State of the art in situ mass spectrometer will be deployed for this exploration. The cruise track –at the Atlantic-Indian boundary- will cross up to 6 currents or fronts. These jets are major pathways of the general circulation, critical for chemical specie transport: they will be thoroughly documented. New model experiments will be designed and take place after first data acquisition, in order to evaluate the sensitivity in TEI distributions to the representation of sources and transports and explore the importance of “the island effect” on the TEI distribution around naturally fertilized islands.
SWINGS is structured in 4 tasks: 1) Management of the project, design and management of the cruise, physical parameter measurements and data management; 2) Thorough sampling of particulate and dissolved phases of TEIs all along the track; 3) Characterization of the biological uptake and remineralization mechanisms; 4) Original modelling development, coupling tracer distributions with refine simulations of the circulation to quantify TEI transport and transformation.
The first year (2020) will be fully dedicated to the cruise preparation. The cruise is planned for early 2021 on the R/V Marion-Dufresne. Analyses, data validation, interpretation will start in boreal spring 2021. Post-cruise meetings, communications and peer-reviewed publications will happen until 2024 and likely beyond. SWINGS being a GEOTRACES section (#GS02) will follow the mandatory sampling resolution, intercalibration procedure and data validation: all the acquired data will be granted open access following a rigorous Data Management Plan.