Tethys Ocean: a vanished realm now lying in the mountains.

08/06/2020

The Tethys Ocean was a vast marine realm that no longer exists. Is it all gone? Not quite. There are still remnants of this ocean on Earth today. The Mediterranean Sea, for example, is the last vestige of the Tethys Ocean. Other remains can be found on land and high up in the mountains.

June 8th, 2020: World Ocean Day. Text: Kathelijne Bonne.

In honor of #WorldOceansDay, we take a closer look at an ocean. There are five oceans: the Atlantic, Pacific, Indian, Southern and Arctic oceans. In the geological past, the continents lay in different locations, and oceans were different as well. Tethys is the most famous vanished ocean. 

In this article, we briefly explain what an ocean is, when the Tethys ocean existed, which creatures it boasted, and how the existence of the Tethys was first suspected. 

What is an ocean?

There are oceans because the outer 'shell' of the earth is broken into pieces. Between those pieces (the continents), ocean basins slowly form, and widen. These large basins are low-lying depressions and are therefore flooded by seawater. But oceans are more than just submerged lowlands. Beneath the water column, on average 4 km deep, lies the oceanic crust, which consists of basalt. Basalt is solidified lava that flows out of large cracks in the middle of the ocean and becomes part of the 'new' ocean crust when it solidifies. The continental crust, contrarily, consists mainly of granite, a completely different rock. 

Let's return to the Tethys Ocean. At the start of the Mesozoic, the time span from 250 to 65 million years ago, the southern continents, including Africa, lay further south as part of a large supercontinent, Pangaea. Africa and its neighbors were separated from the Eurasian continent by a very large ocean, the Tethys. 

Strictly speaking, the Tethys Sea was not a single, vast, uninterrupted expanse of water all the time. It was dotted by smaller continents, volcanoes, plateaus and other landforms. Pieces of Italy, Iberia, Corsica and Sardinia, Anatolia, and a few others, lay for some time as islands in the Tethys Ocean. 

Marine reptiles in warm subtropical waters

The Equator ran across the Tethys Ocean, and there was an overall warm, pleasant climate globally, especially in the Cretaceous Period (last part of the Mesozoic). The Tethys Sea teemed with life, which thrived in the warm, subtropical waters. There were many ammonites, sea lilies, bivalves, sea cucumbers and corals. They were part of a food pyramid and were preyed upon by marine reptiles with big, scary beaks, such as the ichthyosaurus and the plesiosaurus (top picture). They patrolled the coastal areas of the world's oceans. But there were also fish, including sharks and coelacanths. Like today, a myriad of microorganisms was at the bottom of the food chain. Their skeletons (little shells) are made of chalk or silica and have good chances of being preserved in the sedimentary layers. When they die, their skeletons slowly settle on the seafloor. There, they form thick layers. Occasionally, remains of larger animals and plants were also preserved in the seafloor sediments. 

Himalayas and Mount Everest

During the Cretaceous Period, Africa and India steadily drifted northward. The Tethys Ocean became smaller as these continents closed in on Eurasia. Eventually, Arabia and India collided with the great continent and formed the vast transcontinental mountain range stretching from the Pyrenees, over the Alps and the Zagros Mountains all the way to the Himalayas. The sediments deposited on the Tethyan seafloor are now scattered throughout this huge mountain range. 

The summit of the world's highest mountain, Mount Everest, is made up of old Tethyan marine rocks. At more than eight kilometers above sea level, you can find trilobites, sea lilies and shellfish that once crawled on a long-gone seafloor. 

In the late nineteenth century, marine fossils in mountains were a source of great consternation for scientists, including professor Edward Suess. Then, the Earth was considered static and immobile, and the idea of moving pieces of Earth was for crazy dreamers. Suess was the first to suggest that there had been a sea between Europe and Africa. He chose an apt name, Tethys, after the Greek goddess of the sea. However, it would take another 100 years before the evidence for the plate tectonics came. This is the topic of an upcoming feature article. 

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Title picture: Elasmosaurus.

Visit the link of WorldOceansDay and think about how you can take action for the benefit of healthier oceans.