The canyons… of the St. Lawrence

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Les canyons… du Saint-Laurent

In the depths of the St. Lawrence River, far from all eyes, unsuspected avalanches of sediment occur at the bottom of immense canyons. Structures that are just beginning to reveal their secrets.

Map of submarine canyons in the St. Lawrence River off Pointe-des-Monts.

At the top of the Pointe-des-Monts lighthouse, on the North Shore, the view of the St. Lawrence River is spectacular. But if by magic we could remove the water all at once, the surprise would be total. Offshore, we would see huge canyons appear, similar to those we imagine in the United States.

Four kilometers long, with a depth of up to three hundred meters under water, these underwater gorges constitute a veritable highway for the transport of sediments.

Underwater canyons in the St. Lawrence River.

Alexandre Normandeau, marine geoscience researcher at the Geological Survey of Canada, and Patrick Lajeunesse, professor in the Department of Geography at the University Laval, have mapped these canyons up and down since 2019 thanks, among other things, to an underwater robot.

“It's a drone, no more, no less. It records positioning data, but also takes measurements of depth and various characteristics of the seabed. »

— Patrick Lajeunesse is a professor in the Department of Geography at Laval University

Patrick Lajeunesse is a professor in the Department of Geography at Université Laval.

Over the years and surveys, they observed shifts of massive sediment deposits at the bottom of these canyons.

They are associated with sediment avalanches, called turbidity currents. A dynamic similar to that of snow avalanches, but underwater.

By moving along the seabed, these masses of sediment pose a risk to infrastructure, including telecommunications cables.

Sediment avalanches in the submarine canyons of the St. Lawrence.

“If these infrastructures are broken by these sediment avalanches or landslides, it can affect our daily lives . »

— Alexandre Normandeau is a marine geoscience researcher at the Geological Survey of Canada

To better understand the movement and frequency of these sediment avalanches, in the short and long term, scientists have of different tools.

At the exit of these canyons, Jean Carlos Montero Serrano and his team from the Institut des sciences de la mer in Rimouski installed sediment traps in the water column. It is a kind of funnel in which suspended sediments settle over time.

At the same time, sediments are also taken from the seabed, some in the form of cores.

Illustration of a sediment trap placed in the water column to collect suspended sediments.

In the laboratory, the analysis of these cores, taken off Pointe-des -Monts, allows different sedimentary layers to be distinguished, depending on the size of the particles.

“Throughout the long core, we see layers of sand. It testifies to the active dynamics of the canyons at Pointe-des-Monts. »

— Jean Carlos Montero Serrano is a professor of marine geology at the Institute of Marine Sciences at the University of Quebec at Rimouski

< p class="sc-v64krj-0 dlqbmr">Jean Carlos Montero Serrano studies a core of sediment taken from underwater canyons.

These underwater sediment avalanches also occur elsewhere in Canada and around the world. Each system has its particularities and the trigger of these turbidity currents can vary from place to place.

In the canyons off Pointe-des-Monts, they are believed to be associated with strong storms that pound the coast for several hours.

“It happens during storms that increase the height of the waves. It is believed to also take a low tide because the lower the tide, the more wave impact will affect the seabed compared to a high tide. »

— Alexandre Normandeau is a researcher in marine geosciences at the Geological Survey of Canada

Alexandre Normandeau is a marine geoscience researcher at the Geological Survey of Canada.

This deep mixing of sediments can also resuspend potentially toxic organisms that were sleeping at the bottom of the water. Audrey Limoge, professor in the Department of Earth Sciences at the University of New Brunswick, is interested in one species in particular, a dinoflagellate, the Alexandrium catenella.

During its life cycle, this microscopic algae will produce a dormant cyst, a kind of protective envelope, before settling in the sediments at the bottom of the water.

Avalanches, in canyons, hundreds of meters under water. The St. Lawrence River is the scene of a particular phenomenon. When triggered, these avalanches can pose a risk to submarine cables. Report by our colleague from the Découverte program, André Bernard.

When an avalanche occurs in these underwater canyons, the sleeping algae could be propelled to the surface and wake up from new.

“And if the conditions become favorable again, if the cell has access to enough oxygen, it will come out of its envelope and will be able to continue its life cycle, to divide, to proliferate in the water column. »

— Audrey Limoges is a professor in the Department of Earth Sciences at the University of New Brunswick

Audrey Limoges is a professor in the Department of Earth Sciences at the University of New Brunswick.

The proliferation of these algae can in turn cause red tides and the toxins thus released can spread through the food chain. Molluscs, among other things, can accumulate these toxins in large quantities and harm the health of people who consume them, recalls Audrey Limoges.

Underwater canyons are not over to reveal their secrets.

In fact, only 20% of our oceans are mapped. Alexandre Normandeau believes that there is still a lot to be done to understand these underwater avalanches and the risks they pose to the integrity of certain infrastructures at the bottom of the water.

With information from trainee journalist Simone Caron at Découverte

The report by André Bernard, Simone Caron and Jean-François Michaud is broadcast on the show Découverte on Sundays at 6:30 p.m. on ICI Radio-Canada Télé.