US researchers have obtained a seawater sample from 20,000 years ago, the age of the Last Glacial Maximum. This water sample was trapped in sediments during a time when mammoths still roamed the Earth and humans just started making pottery.

As reported in Geochimica et Cosmochimica Acta, researchers extracted the water from sedimentary rock collected from the bottom of the ocean near the Maldives. They put the sample into a hydraulic press and squeezed the rock under extremely high pressure, releasing water once trapped in its pores. They then used the presence of certain types of elements to date it, confirming predictions of what ancient seawater was like 20,000 years ago.

"Previously, all we had to go on to reconstruct seawater from the last Ice Age were indirect clues, like fossil corals and chemical signatures from sediments on the seafloor," Clara Blättler, an assistant professor of geophysical sciences at the University of Chicago, said in a statement. "But from all indications, it looks pretty clear we now have an actual piece of this 20,000-year-old ocean."

The samples were extracted using the JOIDES Resolution ship, which is capable of collecting rock cores almost 2 kilometers (over a mile) long from up to 5 kilometers (3 miles) underneath the ocean floor. The team was studying sediments to understand how they formed and are shaped by the Asian monsoon cycle. However, the water they extracted revealed a different story: the seawater from the rock was saltier than expected.

"That was the first indication we had something unusual on our hands," Blättler said. "Since so much fresh water was pulled into glaciers, the oceans would have been significantly saltier – which is what we saw. The properties of the seawater we found in the Maldives suggests that salinity in the Southern Ocean may have been more important in driving circulation than it is today."

For the team to uncover such a sample, the ancient water must have penetrated the porous rock and was then covered by sediments, allowing the sample to sit undisturbed until the researchers drilled and pressed for it. This is the first discovery of such an ancient water sample. 

Ocean currents play an enormous role in the world’s climate. Current models are tested against present-day conditions, but the ability to gather direct data about past seawater allows scientists to refine their models and form more precise predictions of our planet's future.