By first thought, one might think that the Grand Canyon in the United States and the Bothnian Bay have nothing in common. But – surprise, surprise!- also at the bottom of the Bothnian Bay, there are canyons or canyon-like seabed features. The Grand Canyon, not to mention the deepest canyon on earth, which stretches to a depth of 3.5 km below the sea level, found in Antarctica, are, of course, huge compared to the canyons in the Bothnian Bay. But the Canyons of the Bothnian Bay aren’t exactly tiny. They are often tens of meters deep, hundreds of meters wide and kilometers, up to tens of kilometers long depressions at the seabed.
Official definition for a canyon says that it is a relatively narrow, deep depression with steep sides, the bottom of which generally has a continuous slope, developed characteristically on some continental slopes. But as we know, canyons occur also in the continental shelf, like here in the Baltic Sea.
The development of canyons, both on land and on the seabed, is accompanied by flowing water. The flow of water can erode deep canyons even on the seabed over time. Canyons may develop (also) during tectonic processes in tectonic lineaments and fracture zones of the bedrock. These zones can be eroded more easily than the surrounding rock. Over time, deep channels can be eroded, as the bottom currents are directed to the depressions and erode them deeper and deeper. Ice ages may also play a role in the birth of these canyons.
Another type of canyon is the submarine extension of the terrestrial river valleys. Some of these submarine “rived beds” may have originally been born on dry land. They are the channels of ancient rivers from a time when the sea level was much lower than at present. The canyons of the Bothnian Bay are most likely developed in this way.
Canyons play a big role in the sea. They control the bottom currents and can transport oxygenated and nutrient-rich water from one sea area to another. For example, canyons that split the Archipelago Sea take care of the exchange of water between the Baltic Proper and the Gulf of Bothnia. Currents in canyons can also transport mineral and organic material from the coasts deeper into the sea.
The SEAmBOTH project studied several canyons in the Bothnian Bay, for example, a canyon north-west of Hailuoto Island. We’ve named it Hailuoto Subway Canyon. That is a relatively large, tens of kilometers in length, a kilometer wide in some places, and more than 20 meters in depth. The canyon extends from the coastal area to almost in the middle of the Botnian Bay deep.
The Hailuoto Subway Canyon was surveyed with acoustic-seismic sounding methods, and a large number of seabed sediment samples were also taken from the area. The results indicate that at least occasionally sediment transport occur at the bottom of the canyon. The currents transport mineral and organic matter both in suspension and along the bottom. Canyons can probably offer the sea dwellers a great place to eat, where plenty of fast food is available.
And yes, Hailuoto Subway Canyon, a submarine “fast food” place, fits somehow in our present situation as well, where fast food restaurants (like Subway) and other places offer takeaway food to citizens exhausted by the corona situation, around the Baltic Sea, and around the world. Fortunately, in those submarine fast-food places they don’t have to keep that one meter distance from each other. At least not yet 😉
Aarno Kotilainen, GTK