Marine soUndscape in the Southern Indian oCean – MUSIC

The objective of the MUSIC is to analyze 3 years of continuous and low-frequency acoustic data (0-120 Hz) recorded by a widespread network of autonomous hydrophones in the southern Indian Ocean (24S-56S; 55E-81E). The network was deployed in 2020, maintained in 2022 and recovered in 2023 during cruises of RV Marion Dufresne from the TGIR French Oceanographic Fleet, for monitoring:
• The dynamics of the three Indian spreading ridges with contrasted spreading rates (14 to 70 mm/yr), from the low-level seismic activity they generate;
• The presence and migration pattern of 6 species of baleen whales from their vocal activity (fin, Antarctic blue, 3 populations of pygmy blue and Minke whales);
• The ambient soundscape, which includes cryogenic sounds from icebergs and anthropogenic noises.
This dataset will be analyzed in light of previous works in the same area to assess the long-term evolution of all these components of the Southern Indian Ocean soundscape.
The project is divided in 4 Tasks:
Task 1 is devoted to bioacoustic analyses to exhaustively identify all whale species present in this area, and to establish their migratory routes and seasonality. Their geographical and temporal distribution, linked to the environment and presence of prey, will be examined against the evolution of environmental variables near the hydrophones (bathymetry, SST, Chlorophyll-A). Catalogs of calls, established with detection tools suited to stereotyped (blue whales) or pulse-like (fin and Minke) calls, will be compared with a set of metrics between the different species and sites. These observations will be placed in their environment to try to link them.
Task 2 will focus on the low-level seismic and volcanic activity generated by the three Indian spreading ridges to study their dynamics. By trilateration of arrival times of earthquake-generated acoustic waves, sources can be located within few km and their level measured. As in Task 1, catalogs of events will be established, yielding the temporal and geographical distribution of the seismo-volcanic activity. Event clusters will be analyzed to determine their tectonic or volcanic origin, which reflects the thermal and mechanical state of the ridge. Is this state stable in time and space and only dependent on spreading rates? A new type of events, short and highly energetic, resulting from hot lava-seawater interactions on the seafloor, would be indicative of active magmatic eruptions. How are they distributed?
Task 3 will investigate the long-term evolution and components of the Indian Ocean soundscape. In 2012, the main seasonal variations in the soundscape were due to natural sources such as baleen whale calls and cryogenic events from drifting icebergs, with a limited anthropogenic contribution. Have these components evolved since then, with global warming, or increase in ship traffic as in other oceans?
Task 4 is methodological. Such large amount of data to process and variety of sounds to detect require new deep-learning techniques to automatically detect and classify acoustic events. Cross-feeding between Tasks 1, 2 and 4 will be crucial, since algorithms need to be trained on annotated catalogs. Source mechanisms of impulsive events (bubble implosion or cracks due to lava quenching) and the role of earthquake moment tensor in the radiation pattern of acoustic waves are still debated and require further modelling.
Partners of MUSIC are Geo-Ocean, providing the data and expertise in geosciences, and Lab-STICC with a solid experience in signal-processing, computer science and bioacoustics. This multidisciplinarity for analyzing this large acoustic database is a strength of the project.
All participants are supportive of Open Science and will make their results available through data repository and collaborative platforms. The general public being very receptive to the sounds of sea, we will continue to participate to public manifestations and outreach.