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What if the ocean dried up?

By Pedro Marone Tura

Black and white illustration of a seascape. The top third of the image shows a sunny sky. The middle portion shows waves on the ocean surface and marine life along the water column and seafloor. The bottom third is black and has a big question mark

Illustration by Joana Ho.

Children ask the best questions. On my last trip to the beach, a child, upon learning that I was an oceanographer, asked me one of the best questions I have ever heard, "What would happen if the sea dried up?"

"Do you want to know what would happen to the fish and the weather?" I asked.

"No, no. What's down there? If there was no water, what would I see?" I was stunned for a few seconds at both the simplicity and complexity of the question. What could I tell a child about the bottom of the sea that would answer their curiosity and at the same time not bore them to death (the main challenge)? "Much more than shipwrecks and corals," I began.

Although this topic is new among the FAQs (Frequently Asked Questions) at beach gatherings - usually related to surfing - I realized that marine geomorphology is a subject that is little appreciated by the general public. Sitting on the beach, have you ever wondered what the sea bottom is like between Brazil and Africa? Is it uniform and boring? Or, do I find the same beach sand filling the entire ocean floor? Two thirds of our planet is underwater and, except when the subject is oil or the Brazilian pre-salt layer, we care little about what lies beyond the waterline.

Marine geomorphology tells us incredible stories about the evolution of planet Earth itself. Each beach, plain, parcel, and island is the result of years of sedimentary processes - the balance between deposition and erosion of sediments - on a landscape forged by tectonic forces. How many years? All of them. Even though slow, ocean and coastal environments are always changing. Add human interactions to the equation and the result is the current landscape. But if they are so dynamic, how is it possible to know the history and evolution of these places? Perhaps you have heard the expression: 'The present is the key to the past' (or perhaps ‘history always repeats itself’). Besides being poetic, this phrase represents one of the most important concepts in geology - uniformitarianism. Essentially, if I observe a certain outcome for a certain process today, this was also true throughout geological history. In this way it is possible to reconstruct past environments by looking for clues to what happened. Logical or magical?

But of course, no child wants to hear about sedimentary facies, proxies, or reflectors. So I stopped myself. Instead, I started by talking about one of the most fascinating environments for any age group: hydrothermal vents. Worthy of a science fiction setting, amidst the darkness of the deep ocean, there are real chimneys, which constantly spew a variety of chemical elements into the ocean. The sources are concentrated at the boundary between tectonic plates, in regions with no light. In other words, the main process of transforming inorganic matter into organic matter - photosynthesis - does not occur in these regions. Instead of light, the organisms harness energy from chemical reactions, a process known as chemosynthesis. What was at first thought to be a lifeless region has proven to be one of the most amazing from a biogeochemical point of view.

Underwater photograph of a hydrothermal vent. The volcano-like structure is spewing dark smoke against a deep blue background

Hydrothermal vents and their black fumaroles (also called black smokers or chimneys). Chemical elements are often discharged into the ocean from these vents which are used as an energy source for the organisms living near the vents (Ocean Networks Canada/Flickr). (CC BY-NC-SA 2.0)

"Now look back. What do you think of the Serra do Mar?"

The Serra do Mar is a mountain range with an impressive size and extent. But, with all due respect, it doesn't even compare to the structures we find hidden in the ocean. Imagine walking across a plain and encountering mountains that rise up for kilometers in height, without breaking the surface of the ocean. Uninformed sailors cannot imagine the incredible formations that lie beneath their feet. Such structures are not rare and exist all over the ocean. In fact, a mountain range cuts across the entire Atlantic Ocean, at the meeting of the South American and African tectonic plates, the so-called Mid-Atlantic Ridge. Interestingly, the range emerges from the sea near the Arctic Circle, in Iceland. How about visiting an underwater mountain range on your next vacation?

Two photographs, side by side. On the left, a diver swims in crystal-clear waters, surrounded by rocks. On the right, a river flows at the bottom of a deep valley.

Left: Diving in the Meso-Atlantic range in the Silfra fissure, Iceland.

(Source, CC BY-NC-SA 2.0).

Right: Path between the mountain range in Thingvellir National Park, Iceland.

(Source: CC BY-NC-ND 2.0).

Shaped like irregular cones, seamounts exist in all ocean basins. The diameter of these structures varies, but generally does not exceed a few kilometers. But of course, there are exceptions. Imagine a structure rising three kilometers above the ocean floor, with an area larger than the state of Santa Catarina. This structure is the Rio Grande Rise. As if its size were not enough, a canyon about 800 meters high and more than 25 km thick cuts through this entire 'geological building.' A Grand Canyon of its own. Impressive? No doubt about it. Due to these numbers, the Rio Grande Elevation has been affectionately called the 'Brazilian Atlantis' by the media and advertising channels.

And so the conversation took a good few minutes of our afternoon and added other curious people. There is no shortage of examples. New techniques of bottom measurement and exploration have shown that we still know little about our oceans. In fact, we know more about the surface of the Moon and planets like Mars than we do about the ocean floor. If you have ever felt annoyed that you were born at a time when space travel is still premature, cheer up. You were born at the right time to explore the oceans!


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About Pedro Marone Tura:

I am an oceanographer and currently a doctoral student in biological oceanography at the Oceanographic Institute - USP. I work with the vertical flow of particles in the water and the biogeochemical cycling of elements in the marine environment. Always passionate about oceanography and its interdisciplinarity, today I discovered a new passion: education. I see in it the opportunity to bring society closer to the academic world.

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