Clues as to the fate of animals on St Paul Island, one of the last-known groups of mammoths, revealed by sediment cores from lake

One of the last known populations of woolly mammoths was wiped out 5,600 years ago by a lack of freshwater, research suggests.

Woolly mammoths became extinct in North America and mainland Asia around 10,000 years ago, but a small number of survived on islands lying between Siberia and Alaska which, before sea-level rises, were part of the Bering Land Bridge.

While those on Wrangel Island breathed their last around 4,000 years ago, the story of the mammoths on St Paul Island has remained something of a mystery. Dating of remains previously found on the island suggested that the beasts were alive around 6,500 years ago, but it wasn’t clear whether the remains were of the last animals, or what caused their extinction.

Now a team of researchers from Canada and the US say they have an answer.

Writing in the Proceedings of the National Academy of Sciences, the scientists reveal how they turned to sediment cores taken from a lake on St Paul Island to determine what finished off the mammoths, and when.

To pinpoint when the mammoths died out on the island, the team examined traces of ancient DNA found within sediment cores that had been collected and radiocarbon dated. “When the mammoths lived along the lake they probably waded into it, they probably defecated in it, urinated it in, bathed in it, so their DNA got into the lake water and settled to the bottom ,” said Russell Graham, the co-author of the study and professor of geosciences at Pennsylvania State University.

By extracting this sedimentary ancient DNA, and looking at where in the core it was present, the researchers discovered that mammoths seem to have to disappeared around 5,600 years ago. The conclusion was backed up by the analysis of fungal spores from within the core, which are thought to have grown in the mammoths’ dung. A new set of mammoth remains added further weight to the theory, with radiocarbon dating putting the youngest of the remains at 5,530 years ago.

To unravel the cause of the mammoths’ demise, researchers studied other markers within the sediment core, including the type of microorganisms and creatures that once lived in the water and which provide clues about the conditions within the lake. The team also looked at the ratio of different forms of oxygen over time - an indicator of the rate of evaporation of the water. Their findings reveal that between 7,850 and 5,600 years ago the lake became saltier, shallower and more turbid.

With rising sea levels shrinking the size of the island, and lakes becoming shallower, the mammoths would have been forced to rely on fewer water supplies, said Graham, adding that the animals themselves could have made a bad situation worse. “They started congregating around the two probably remaining water holes and when modern elephants do that they destroy the vegetation and it causes increased erosion and infilling of lakes - and that was happening as well, you can see that through the sediment,” he said.

The upshot, he adds, is that the increasingly harsh conditions proved too much for the mammoths.



“Everything just pointed in the same direction that it was a shortage of [freshwater] that did them in,” said Graham.