Saturday 17 July 2021

The rise and fall of the world’s largest lake

 When continental plates smashed together about 12 million years ago, they didn’t just raise new mountains in central Europe—they created the largest lake the world has ever known. This vast body of water—the Paratethys Sea—came to host species found nowhere else, including the world’s smallest whales. Two new studies reveal how the ancient body of water took shape and how surrounding changes helped give rise to elephants, giraffes, and other large mammals that wander the planet today.

To build that timeline, paleo-oceanographer Dan Palcu of the University of São Paulo and his colleagues at the main campus assembled clues from geological and fossil records. At its largest, the body of water—which some scientists consider to have been an inland sea—stretched from the eastern Alps into what is now Kazakhstan, covering more than 2.8 million square kilometers. That’s an area larger than today’s Mediterranean Sea, they write this week in Scientific Reports. Their analyses further estimate the lake once contained more than 1.77 million cubic kilometers of water, more than 10 times the volume found in all of today’s fresh- and saltwater lakes combined.

At its largest, the megalake Paratethys (shown superimposed on modern geography) stretched from the eastern Alps to today’s Kazakhstan. DAN PALCU; NATURAL EARTH

But climate shifts caused the lake to shrink dramatically at least four times in its 5-million-year lifetime, with water levels falling by as much as 250 meters between 7.65 million and 7.9 million years ago. During that largest episode of contraction, the lake lost as much as one-third of its water and more than two-thirds of its surface area. That sent water salinity in the lake’s central basin—which closely matches the outlines of today’s Black Sea—skyrocketing, from about one-third as salty as today’s oceans to a level on par with seawater.

Those shifts wiped out many aquatic species, including numerous species of single-celled algae and other small free-floating organisms, the researchers report. Creatures that could survive the brackish water, including some mollusks, survived to repopulate the lake when it expanded during wetter times, Palcu says.

The Paratethys soon became home to a wide variety of mollusks, crustaceans, and marine mammals found nowhere else on Earth. Many of the whales, dolphins, and seals living there were miniature versions of those found in open seas, says evolutionary biologist Pavel Gol’din of the National Academy of Sciences of Ukraine’s I. I. Schmalhausen Institute of Zoology, who was not involved with the work. One species, the 3-meter-long Cetotherium riabinini—1 meter shorter than today’s bottlenose dolphin—is the smallest whale ever found in the fossil record. Such dwarfism might have helped these animals adapt to a shrinking Paratethys, Gol’din says.

The changes to the climate that triggered lake shrinkage also influenced the evolution of land animals, says evolutionary biologist Madelaine Böhme of the University of Tübingen. As water levels dropped, the newly exposed shorelines became grasslands—and hot spots for evolution, she notes.

The Paratethys Sea was home to many species found nowhere else, including Cetotherium riabinini (depicted with human for scale), the smallest known whale in the fossil record. PAVEL GOL’DIN; LENA GODLEVSKA/WIKIMEDIA COMMONS

Recently, Böhme and her colleagues focused on the geological record in western Iran, where sediments chronicle repeated long-term changes in climate. The fossil record shows that in areas north of the Paratethys, the ancestors of modern-day sheep and goats roamed side by side with primitive antelope. And in what is now western Iran, south of the lake, the progenitors of today’s giraffes and elephants thrived.

Four lengthy dry periods that occurred between 6.25 million and 8.75 million years ago likely drove those creatures to migrate southwestward into Africa, Böhme and her colleagues reported last month in Communications Earth & Environment. Here, they evolved to produce the diversity of creatures for which today’s African savanna is famous.

The Paratethys was destined for a sadder fate. It ceased to exist sometime between 6.7 million and 6.9 million years ago, when erosion created an outlet at the lake’s southwestern edge. This outlet—which is likely now submerged beneath the Aegean Sea—birthed a short river that eventually found its way to the Mediterranean. But the massive lake had one last hurrah, Palcu says: The water draining from it likely carved “an impressive waterfall” as it flowed down to the sea.

(Source: Science Mag)

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