A journey across the Atlantic seafloor

A journey across the Atlantic seafloor

More than two thirds of our planet are covered by water. What lies underneath is a mystery – only 20 percent of the sea floor has been explored in detail. But hasn’t it always appealed to you, this underwater world with all its mysteries? Then join us for a hike across the seabed.

Imagine you could breathe underwater. You could counteract the tremendous pressure of the deep and descend to the bottom of the sea. Imagine crossing the entire Atlantic, from Portugal to Mexico, 8,200 kilometers. A journey of superlatives, over mountains, valleys, hot springs and asphalt.


37.96320°N, -8.88048°E: The start

The deep sea is teeming with life: schools of shrimp, deep-sea jellyfish, crabs. However, we do not know many deep-sea animals. Photos: MARUM − Center for Marine Environmental Sciences, University of Bremen.

You start in Sines, Portugal. Your feet dig into the wet, cold sand beneath you, and a salty breeze makes your hair stick together. You go towards the Atlantic, leaving the blue and white houses behind you, the city wall, the fishermen. You’re still barefoot, but hiking boots are dangling from your backpack. You have tied ropes to the outside, snap hooks jingle together. You are well equipped, have packed your camera, compass, hiking poles and a flashlight. It gets rough and tiring. But it will also fascinate you.

The water is up to your chin now. You take another breath, then you dive. You feel the coolness on your body. You’ll quickly get used to breathing underwater as you walk across the sandy seabed. If you look up, you can still see the sea surface. You walk a bit over the continental shelf, which forms the last edges of the continental plate. The rocks underneath the shelf are up to many hundreds of millions of years old and are covered here with fine remains of shells. But also pebbles, sand and clay, carried away by the wind and rain on land, collect on the seafloor. You trudge past red, purple, pink corals. You will encounter schools of striped sea bream, mackerel with tiny fins, a sea bass sparkling as if covered with sequins, finally huge groups of anchovies.

37.93155°N, -9.17676°E: The sediment

Maps: The University of Sydney. Copyright © 2015-2016.

You pass the 12-mile zone, here the territorial waters of Portugal end. You are now a little over 22 kilometers behind you and you are about 200 meters below the surface of the water. Now it goes downhill, the continental slope begins. You can think of it like standing on the top of a huge mountain and you have a long, long descent ahead of you. The floor is uneven. Canyons and gullies, which the water has carved into the ground over the centuries, dissect it it again and again.

Everything that is more than 200 meters below the water surface is considered deep sea. It’s full of life that we don’t know. In February 2022, a study found that almost two-thirds of species in the deep sea cannot be assigned to any previously known group. Other estimates assume that up to 91 percent of deep-sea creatures with a cell nucleus are still unknown.
You’re glad you brought your flashlight because it’s pitch black. The only problem is that because light, like that of your flashlight, is swallowed up more by water than by air, you can only see around 50 meters underwater. After that everything is black again. Even if there is a canyon or a colorful coral reef waiting for you – you could not see it from your vantage point.

You remember a mudflat hike that you did as a child. You think of the worms that looked at you from the mudflats back then.

As you think about it, the slope gradually flattens out. You are now at a depth of about two thousand meters and are walking over the so-called continental foot. Underneath the layers of sediment you walk on, the continental crust meets the oceanic crust.

Whenever you put one foot in front of the other, you throw up a small cloud of mud. Your feet sink slightly, a few inches. You remember a mudflat hike that you did as a child. You think of the worms that looked at you from the mudflats back then. The diversity of the deep sea is mainly in its sediment.

Roundworms, isopods and protozoa cavort in the mud below you. You only see most of them when you get on your knees and take some mud in your hands, some you don’t even recognize. Many of them feed on animals and plants that live on the water surface. They convert sunlight into energy and sink to the bottom at the end of their lives. The organisms of the deep sea therefore depend on the sun without ever having seen its light.

To survive in the dark, over 70 percent of deep-sea animals generate light – either themselves or through a symbiosis with bacteria. Researchers call this process »bioluminescence«. One of the most famous bioluminescent fish is the frogfish, with a lantern dangling before its eyes. Finding a live, glowing frogfish is very rare. In 2018, researchers filmed a pair of fan-finned monkfish for the first time. The tentacles of this species of frogfish stick out from the body like individual hairs – and together with the fins and rod they glow to attract prey. If you’re lucky, you’ll meet one.

The further you get away from the continent, the more calcareous the soil becomes, and the sandy erosion material from the land decreases. More than 30 percent of the soil here consists of the remains of tiny sea creatures in the water or on the bottom. These include, for example, foraminifera, small protozoa with a shell whose calcareous shell remains collect on the ground. Or so-called coccolithophores. These are unicellular algae, they form microscopic calcareous skeletons that gradually sink to the bottom.

36.36102°N, -13.57411°E: The Sea Mountain

You’re heading southwest. It is quiet. Every now and then a fish swims past your nose. Then it goes uphill, in front of you is a sea mountain. Seamounts are extinct underwater volcanoes at least 1,000 meters high. They have evolved through volcanic activity over the course of millions of years. Nobody knows exactly how many seamounts lie dormant under the water surface. It is estimated that there are at least 33,000 worldwide.

You have the current of the deep sea behind you, which rises on the mountain slopes. It carries away the sediment, leaving the slopes of the seamounts free of mud. It gets steeper and steeper, the area is rocky and rough. You are now 1,500 meters below the sea surface. This is where the oxygen minimum zone begins.

It derives its name from the fact that a lot of oxygen is consumed by algae and bacteria, in this zone, but – unlike on the surface – there is hardly any replenishment from the air. You feel a current on your neck: cold deep water that contains a lot of oxygen. In large eddies it mixes with the water of the oxygen minimum zone from above.

The elevations you cling to and trip over are millions of years old. They only grow five millimeters in a million years.

You scramble to the top, your hands clinging to black bumps that are firmly attached to the rock. The structures remind you of coal, some are a few millimeters thick, others several centimeters. They are cobalt crusts, formed from dissolved metals that drift through the sea. When these minerals come into contact with the oxygen-rich deep-sea waters that swirl around the mountain, they collect on the slope and form crusts. Stuck in those crusts

Manganese and iron, but also other raw materials: cobalt, nickel, rare earths – all minerals that are essential for the production of smartphones, for example. The elevations you cling to and trip over are millions of years old. They only grow five millimeters in a million years.

Only now do you realize that life is teeming around you. A red soft coral dangles its arms in the current. A fiery red shrimp scurries across the ground at your feet. In the background you discover a sponge that you almost mistake for a white chrysanthemum. 1,000 meters below the water surface you hike through a paradise.

At a depth of 600 meters, a sperm whale swims around a diving robot on the expedition ship EV Nautilus. Encounters between sperm whales and diving robots are extremely rare. Video: EV Nautilus/ Ocean Exploration Trust (OEC).

As you look up while descending, you discover a sperm whale above you. You suppress the impulse to photograph the huge animal with the square head. You know how special this encounter is: Sperm whales are mammals and lung breathers, but they can dive around 2,000 meters deep and hold their breath for up to two hours. But you also know that sperm whales can make one of the loudest sounds on earth. The click of the whales reaches up to 230 decibels, which they use to explore their surroundings like with an echo sounder. Above 198 decibels, the pressure of the noise can burst the air sacs in your lungs, which would kill you. You cover your ears and let the sperm whale pass by. He stays calm, you stay calm. Then you take a look at the compass – west, down.

36.95727°N, -25.06702°E: The Azores

Mussels and crabs abound at the Menez Gwen hydrothermal vent, southwest of the Azores. Video: MARUM − Center for Marine Environmental Sciences, University of Bremen.

For a long time you have hiked on the flat seabed up to 5,000 meters below the water surface, past isolated cold-water corals, sea cucumbers, crabs. You have now arrived at the foot of Santa Maria, the oldest island in the Azores. It is said to have been born 140 to 160 million years ago. Formed from magma, piled up above the sea surface by recurring volcanic eruptions. You bypass the mountain peak, but hike meter by meter towards the sea surface.

Sponges are a kind of ecosystem engineers – and have been for millions of years, as they are evolutionarily one of the oldest groups of animals in the world.

At a depth of 1,000 meters, the huge reefs on glass sponges catch your eye. Like little hairy lanterns, open at the top, they cavort on the flat surfaces of the mountain range you are walking on. Researchers thought these sponges resembled birds’ nests, so they dubbed them “bird’s nest sponges.” Many bacteria live in the sponges. In this way, they can convert substances in the water into tiny particles, which in turn provide food for other creatures in the sea. Sponges are a kind of ecosystem engineers – and have been for millions of years, as they are evolutionarily one of the oldest groups of animals in the world.

You try not to step on the bird’s nest sponges. Their skeleton consists of tiny needles and is delicate, but at the same time robust. Like a framework with pore-like cavities and channels. Water is filtered through there every second.

For an area of ​​the North Atlantic slightly larger than Greece, it has been calculated that a deep-sea sponge bottom filters around 56 million cubic meters of seawater per day – an unimaginable number. To put this in context: the main sewage treatment plant in Stuttgart-Mühlhausen, one of the largest sewage treatment plants in Germany, pumps 64 million cubic meters through the plant every year.

32.90812°N, -38.87008°E: The Mid-Atlantic Ridge

The seabed you are now coming to is the youngest on your journey: the Mid-Atlantic Ridge, a kilometer-long mountain range. It curves from north to south through the Atlantic. In the middle of the ridge is a valley about six miles wide and a mile deep. It contains active volcanoes. Here, in the neovolcanic zone, new ocean floors are constantly being created. About two to four centimeters of new oceanic crust grows every year.

The reason: The continental plates that were once close together are drifting apart. Rising magma swells through the resulting crack. Eventually, it slowly cools down in the form of pillows or tubes. The still warm rock forms hills and ridges, almost like terraces. As it cools, it condenses and sinks. But new rock is constantly being formed and pushing the old one aside over millions of years. The seafloor is therefore older the further away it is from the Mid-Atlantic Ridge.

29.76010°N, -43.23060°E: The massive sulphides

Liquid full of metal compounds at a temperature of 400 degrees flows out of so-called “black smokers” on the Mid-Atlantic Ridge. Video: MARUM − Center for Marine Environmental Sciences, University of Bremen.

A little further southwest you suddenly see shadows. A colony of meter-high monsters towers in front of you. Each one pulls a face and seems to want to block your way. If you could smell underwater, the smell of rotten eggs would fill your nostrils. You pass an area about half the size of the Munich Oktoberfest. There, around 100 rust-brown, black and white-coloured structures rise up to 20 meters in height. Some of them are thin and smooth as pipes, some thick but jagged and uneven, as if rubble had been piled up and glued together. Black smoke pours out of them in huge clouds. You are on a hydrothermal field full of “black smokers”, stony formations made of metal-sulphur compounds.

A colony of meter-high monsters towers in front of you. Each one pulls a face and seems to want to block your way.

You don’t approach. The liquid that shoots up from the vents would be unbearable for you: an acidic, 400-degree hot fluid full of metal compounds that have dissolved from the rocks below the sea floor. Where the liquid comes into contact with the seawater, copper, iron and sulfur minerals are deposited and form a chimney. The fluid continues to rise in black clouds – until it has cooled to the temperature of sea water. Inside, the metal compounds flow into the sea as if in an inverted funnel: first straight up, then they are distributed with the current up to thousands of kilometers across the ocean.

The smokers are teeming with clams, crab, shrimp and colonies of beard worms shaped like Medusa’s head. The organisms here are chemotrophs, which means they use chemicals to get energy for their metabolism, just like plants use light to survive. The long white beard worms with the red fringes, for example, form a community with bacteria that convert substances such as hydrogen sulfide and methane and thus feed the worms.

You march quite a bit further southwest over the Mid-Atlantic Ridge, past hydrothermal vents, side by side with migrating crabs, crawling sea cucumbers, drifting plankton.

11.40299°N, -48.00071°E: The manganese nodule field

Most of the manganese nodule fields are found in the Pacific, like here in the Clarion-Clipperton Zone. Anemones, glass sponges and starfish make themselves comfortable between the tubers. Photos: ROV-Team/GEOMAR (CC BY 4.0) & ROV KIEL 6000, GEOMAR (CC BY 4.0).

Not only on smokers and seamounts, but also at a depth of 4,995 meters, on the flat bottom of the sea, there are nickel, copper, cobalt, manganese and rare earths – elements that are needed for the manufacture of electric cars, for example. You came across a manganese nodule field.

In front of, next to, below and behind you are black balls in the form of marbles, potatoes or cannonballs. You bend down and pick one up. You have to use some force to pull them out of the mud. But once you’ve done it, the manganese nodule lies lightly in your hand.

The potato-sized tuber you picked up from the ground is between two and 20 million years old.

The manganese nodule contains a wide variety of metals. They have accumulated over millions of years, for example on a small rock, a shark tooth or the remains of a shell. Year after year, shift after shift.

The potato-sized tuber you picked up from the ground is between two and 20 million years old. So you have a geological archive in your hands. If you were to cut it open, you could – like a tree – discover rings that provide information about certain climatic conditions. The composition of the various metal compounds in the rings tells you something about environmental fluctuations such as the formation of huge areas of ice or changes in ocean currents. You put the tuber in your backpack.

You tiptoe through the manganese nodule field, jump over sea urchins and soft corals, make your way. You are now steering towards Mexico, at a depth of over 5,000 meters. Again and again you come across parts of quark cups, garbage bags or other small plastic particles on the seabed. Plastic can stay in the sea for decades without being broken down. In addition, there are microplastics, i.e. particles with small diameters, sometimes thinner than a hair. Researchers estimate that at least 14 million tons of microplastics lie at the bottom of the oceans.

25.32810°N, -68.68501°E: The whale carcass

Whale carcasses are energy bombs for deep-sea creatures. Researchers assume that around 700,000 whale carcasses lie on the seabed in various stages of decomposition. Video: EV Nautilus/ Ocean Exploration Trust (OEC).

You almost collided with a giant skull. You look around and see more bones, gnawed flesh, an oily trail of grease on the floor. Worms, crabs, fish. In front of you is the carcass of a humpback whale. Humpback whales migrate to the Puerto Rico area during the winter to mate and raise their calves. In summer they migrate north again to their feeding grounds. This one must have died and sunk on its journey. His carcass now becomes a deep-sea energy bomb.

This whale carcass is a feast for the deep-sea creatures, which are otherwise dependent on the plankton material trickling down from above.

First, sharks or mucous membranes tear the meat from the whale’s bones – the animals destroy up to 60 kilograms of it a day. Then crabs gnaw off the bones, mussels and snails settle, countless octopuses and predatory fish come to eat the small animals that have gathered. Finally, worms and bacteria decompose what is left. This whale carcass is a feast for the deep-sea creatures, which are otherwise dependent on the plankton material trickling down from above. Researchers assume that around 700,000 whale carcasses lie on the seabed in various stages of decomposition. It can take several decades for the bones of a cadaver to decompose. You continue to steer east. It’s not far to the Gulf of Mexico.

27.43240°N, -91.16860°E: The Gulf of Mexico

Salt lakes have formed on the sea floor in the Gulf of Mexico over time. Because of the high salt content, their water is denser and heavier than sea water. Video: EV Nautilus/ Ocean Exploration Trust (OEC).

Around 150 million years ago, the Gulf of Mexico was just a basin into which seawater kept seeping and evaporating. This is how massive salt deposits are said to have formed, over which sediments have accumulated over the years. You are now 600 meters below the surface of the water and you can see how the salt below the seabed affects its surroundings. Here, at the bottom of the sea, a salt lake with a diameter of 30 meters has formed. You approach, passing colonies of mussels and beard worms. The water of the salt lake is slightly bluer than that of your area. Angular formations seem to collect it like in a trough, but there is another reason why it doesn’t mix with the seawater: Because of the high salt content, it is denser and heavier than seawater. There is a dead crab in the middle of the lake. Many creatures not adapted to this environment go into toxic shock when exposed to salt water. But some live from the warmth of the salt water and the high concentration of salt and methane – like the mussels that you have already discovered on the black smokers. You move further south, across the gulf. For this you descend one last time to a depth of 3,000 meters, across the gulf. 

21.91701°N, -93.3301°E: The Asphalt Mountains

Oil has accumulated under the sea floor in the Gulf of Mexico. If oil escapes, it reacts with the cold seawater and forms mountains of asphalt. Video: MARUM − Center for Marine Environmental Sciences, University of Bremen.

Shortly before the continental shelf, which will lead you back to land, you suddenly stumble over black elevations. Some are reminiscent of piles of rubble, others look like a chaos of hoses and pillows. You can hardly believe your eyes, but there are piles of asphalt in front of you. Oil, which can accumulate in the bottom thanks to the geological structure of the salt domes, emerges from the seabed here. When it reacts with the cold water, it turns into asphalt. You break off a piece. The asphalt is tough but not solid. On the structures, the beard worms let their bodies float in the current, seemingly indestructible. You imagine that these worm colonies could also live on the asphalt streets of your hometown. What would become of them up there between people and cars? Suddenly you’re glad that down here, in the dark,

With all the other deep-sea dwellers, they form their own cosmos, their own world full of diversity and life.

As you climb ashore, there’s one question you can’t get out of your head.

For how much longer?

Nelly Ritz

Photo media:
GEOMAR Helmholtz Center for Ocean Research Kiel, MARUM – Center for Marine Environmental Sciences at the University of Bremen,   EV Nautilus /Ocean Exploration Trust (OEC)

This text describes an idea, a thought experiment, a pipe dream. Nelly Ritz researched and created this fictional journey using maps from the University of Sydney GPlates Portal, funded by AuScope NCRIS. She conducted interviews with marine geologists and deep-sea biologists and read expedition logs and numerous research papers.

The original article in German can be found here.