In 2010 deep in the barren Atacama Desert of northern Chile, road builders came across a curious site during a project to expand the Pan-American Highway: Dozens of fossil skeletons of whales and other marine mammals lay buried in the rock. Before the road workers paved over the site, researchers managed to excavate some of the fossils, and Smithsonian paleontologist Nicholas Pyenson and a team of imaging experts created 3-D scans of many of them.
Those scans, which preserved the orientation of the bones, have allowed Pyenson and his colleagues to come up with a hypothesis for how the marine creatures died: they were victims of a harmful algal bloom, the researchers report February 25 in the Proceedings of the Royal Society B.
The Atacama site, known as Cerro Ballena (Spanish for “whale hill”), probably has hundreds of skeletons still buried in the rock. But scientists managed to unearth several dozen before road construction continued — 31 large baleen whales of various ages, at least two types of seals, an extinct species of sperm whale, a walrus-like toothed whale and an aquatic sloth. The orientation of the bodies indicated that the animals had all died at sea and washed onto a tidal flat where they were buried rapidly in the sand some 6 million to 9 million years ago. This happened four times over a period of 10,000 to 16,000 years.
This collection of evidence “evokes a modern stranding event,” the researchers write, similar to a mass stranding of 14 humpback whales over five weeks in 1987 to 1988 on a stretch of coastline on Cape Cod, Massachusetts. The whales had died after eating Atlantic mackerel that were contaminated from a harmful algal bloom, the name for a boom in algal species that produce neurotoxins. Those toxins can harm or kill creatures that eat the algae or animals further up the food web. In modern times, these blooms often occur in areas where agricultural runoff dumps large amounts of nutrients, such as phosphates, into fresh or salt waters. But the blooms can also be fed naturally.
On the west coast of South America, iron-rich runoff from the Andes Mountains feeds a naturally productive region of ocean upwelling, resulting in harmful algal blooms. That process may have been going on in this area for millions of years, and it could have killed all those different species of marine mammals found in the Atacama, Pyenson and his colleagues write. The animals then washed onto shore, where their carcasses were largely protected from being broken apart by waves or by being picked apart by large predators (there were none on land as the region was desert then, as it is now).
The skeletons can’t be the result of tsunamis, the researchers write, because there would be smaller animals included among the dead. And diseases wouldn’t have killed off such a variety of animals or killed them multiple times over a period of thousands of years. “In modern settings, [harmful algal blooms] are the only known natural cause for such repeated, multispecies accumulations,” the researchers write. So it’s likely that a similar even created the whale graveyard in Chile.