Matthieu Lengaigne: Between Iron and the Sea

In 18 months, Matthieu Lengaigne will set sail aboard the “Pourquoi pas?” The name of thisIfremer research vesselpays tribute to the explorer Jean Charcot. TheIRD research directorwill join others in exploring the South Pacific Ocean off the coast of Tonga. This exceptional research expedition was made possible by a 13-million-euro European Research Council (ERC) Synergy grant, which he secured alongside his two CNRS colleagues, Cécile Guieu and Karin Sigloch.

A metal that is scarce in the ocean

The oceanographer is one of the key figures in the award-winning Harvest project, which will study how iron released by underwater volcanoes stimulates phytoplankton growth. Over the course of six years, this work will involve more than a hundred scientists, as the issue is more complex than it seems.“Iron is a limiting factor for ocean life because it is essential for phytoplankton growth yet is scarce in the ocean, as it quickly precipitates and settles to the seafloor ,” explains the researcher. Many questions remain to be answered, particularly regarding the presence of iron in areas too far from continental coasts to receive the precious metal from exposed rocks. Initial research is focusing on the role of undersea volcanoes in this supply.

It was during an initial reconnaissance trip that Cécile Guieu witnessed a chlorophyll bloom in the Tonga volcanic arc. That’s when the idea for the project was born.“Understanding iron availability for phytoplankton requires a wide range of expertise, particularly in physics and biogeochemistry,” explains Guieu, who is pleased to be well-qualified in these areas. At the Marbec laboratory since 2019, he has focused in particular on the impact of climate change on open-ocean marine ecosystems. In other words, how, under the influence of global warming, ocean physics (temperature, currents) alters biochemistry (oxygen, phytoplankton), which in turn influences marine life.

IPCC Literature

A graduate of the École Normale Supérieure who initially focused on quantum physics, Matthieu Lengaigne could have kept his distance from the sea. But he shifted his focus to climate and ocean physics, drawn more to the vastness of the ocean than to abstract concepts. Building on his dissertation at the Locean laboratory on the El Niño climate phenomenon, he has become a specialist in complex modeling of climate and ocean systems. And the models he will help develop from the ground up will contribute to the IPCC’s body of research.

The modeler will combine his numerical simulations with observational campaigns. While Harvest will make it possible to simulate the fate of hydrothermal iron and its effects on primary production, the models will be based on data collected aboard the “Pourquoi pas?” As the discussion progresses, it becomes clear that the ocean expedition will be a major challenge. The team must find a volcanic zone suitable for measurements in an ocean region where the seafloor has never been mapped. But with funding from the ERC—half of which will be allocated to two 30-day ocean expeditions—the team can assemble all the necessary navigation and measurement equipment. A fleet of autonomous vehicles will accompany the ship to map the seafloor in real time and measure currents, temperature, chlorophyll, iron—in short, all the data needed to identify the most active volcanoes.

He will set sail in December 2027 with thirty sailors to operate the ship and an equal number of researchers.“It’s always both a human and scientific adventure. We’re bringing on board specialists from many fields, such as microturbulence, to understand the upwelling and dissolution of iron,” says the researcher

"Humble in our ignorance"

This trip to the South Pacific is not the first for the El Niño specialist—a phenomenon that influences the climate of the entire region. In fact, his work in the Southern Hemisphere had led to a position at the IRD. This was followed by an assignment in India, specifically in Goa, where he arrived in 2009 with his family and belongings. He would remain there for 10 years, primarily to mentor Indian students at the National Institute of Oceanography.“I devoted about 70% of my work time to training young Indians in the analysis of model simulations, a skill that doesn’t exist there,” says the man who speaks of his joy at being surprised every day by his life as an expatriate:“Every morning, I didn’t know what the day would bring. It teaches you to be humble about what you don’t know so you can work effectively without offending people.”

And since he doesn’t mention it, we can’t resist asking him to tell us about the CNRS bronze medal he won in 2012—if only to understand this oddity, given that he isn’t a researcher at that institution. “I discovered on that occasion that it was possible,” the researcher concludes matter-of-factly regarding the award, which recognized his work on the ocean’s influence on cyclonic activity in India.