While the famous Minoan culture was just beginning, woolly mammoths (Mammuthus primigenius) were disappearing from Saint Paul Island, according to a group of paleontologists who have dated this extinction to 5,650 years ago, thousands of years after mainland populations died off.
This is an artist’s rendition of a woolly mammoth. Image credit: Flying Puffin / CC BY-SA 2.0.
The scientists found evidence that St. Paul Island – the largest of the Pribilof Islands, a group of four Alaskan islands located in the Bering Sea – experienced a phase of dry conditions and declining water quality at about the same time the mammoths vanished.
The findings were published this week in the Proceedings of the National Academy of Sciences.
“Past events on St. Paul Island provided a unique opportunity for research,” explained co-author Dr. Matthew Wooller, from the University of Alaska Fairbanks.
Mammoths were trapped there when rising sea levels submerged the Bering Sea land bridge, and survived several thousands of years longer than isolated mainland populations.
Dr. Wooller and co-authors used a variety of proxies – things in the environment that can be used to independently document the presence of an organism – to investigate the timing, causes, and consequences of mammoth disappearance from the island.
In 2013, they collected a sediment core from the bed of one of the few freshwater lakes on the island.
Three different spores from fungi that grow on large animal dung were extracted from the core and used to determine when the mammoths were no longer on the island.
“We see a reduction in the three species of fungus, all of which are associated with the dung of large animals. These spores are a marker for the presence of large animals like mammoths,” said lead author Prof. Russ Graham, from Pennsylvania State University.
Beside the mammoths, the only animals appearing on the island in ‘prehistoric’ times were arctic foxes, shrews and polar bears, and there is no evidence of polar bears before 4,000 years ago.
Humans did not arrive on the island until 1787 CE. The only large mammals present were mammoths.
Sediment DNA from the lake core showed the presence of mammoth DNA until 5,650 years ago, plus or minus 80 years. After that time, there is no mammoth DNA and so no mammoths on the island.
The team also measured the stable oxygen isotope ratios of the prehistoric remains of aquatic insects preserved in the sediment from before, during and after the extinction of mammoths.
The remains of aquatic organisms living in lakes retain water isotope signatures within their bodies, which allowed the researchers studying their exoskeletons to determine that lake levels had diminished. The remains also changed over time, indicating decreasing lake levels and water quality leading up to the mammoth extinction.
Nitrogen isotope analyses of mammoth bones and teeth also signaled progressively drier conditions leading up to the extinction event.
Pollen from the lake core indicate that the area around the lake was denuded of vegetation by the mammoths.
Like elephants today, when the water became cloudy and turgid, the mammoths probably dug holes nearby to obtain cleaner water. Both of these things increased erosion in the area and helped fill in the lake, decreasing the available water even more.
After the extinction of the mammoths, the core shows that erosion stopped and vegetation returned to the area. In essence, the mammoths contributed to their own demise.
“Five independent indicators of extinction show that mammoths survived on St. Paul until 5,600 years ago,” the scientists said.
“Vegetation composition remained stable during the extinction window, and there is no evidence of human presence on the island before 1787 CE, suggesting that these factors were not extinction drivers.”
“Instead, the extinction coincided with declining freshwater resources and drier climates between 7,850 and 5,600 years ago, as inferred from sedimentary magnetic susceptibility, oxygen isotopes, and diatom and cladoceran assemblages in a sediment core from a freshwater lake on the island, and stable nitrogen isotopes from mammoth remains.”
“Contrary to other extinction models for the St. Paul mammoth population, this evidence indicates that this mammoth population died out because of the synergistic effects of shrinking island area and freshwater scarcity caused by rising sea levels and regional climate change.”
“Degradation of water quality by intensified mammoth activity around the lake likely exacerbated the situation.”
“The St. Paul mammoth demise is now one of the best-dated prehistoric extinctions,” the researchers said.
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Russell W. Graham et al. Timing and causes of mid-Holocene mammoth extinction on St. Paul Island, Alaska. PNAS, published online August 1, 2016; doi: 10.1073/pnas.1604903113
