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From the mouths of manatees

Captive dugong (Dugong dugon) at the Sydney Aquarium.
[Image courtesy of Mark T. Clementz]

The Eocene epoch is the period of time in Earth's history that lasted from about 56 million years ago to 34 million years ago—and researchers say that our planet was a much wetter place then, especially in the tropics.

But, how do they know what the Earth was like so long ago? Mark Clementz and Jacob Sewall looked at the fossil teeth of sea cows, like manatees and dugongs, and studied the oxygen isotopes that were stored in them. These oxygen isotopes can help researchers to track the path of moisture around the globe.

Fossil sirenian, Halitherium schinzi, at the Paris National Museum of Natural History.
[Image courtesy of Mark T. Clementz]

By comparing the levels of oxygen isotopes in the teeth of modern sea cows with the isotopes stored in the ancient teeth of sea cows from the Eocene epoch, these researchers could tell that Earth's tropical water cycle was accelerated at the time.

Scientists already knew that the Eocene epoch was a very warm time in Earth's climate history. But this new discovery means more rain fell during the Eocene and there was much more water vapor in the air as well.

Records of rainfall had already linked greenhouse gases at higher latitudes to an accelerated water cycle during the Eocene epoch. Now, Clementz and Sewall have made it clear that the warmer, tropical parts of the planet were also very active at the time.