The desert is typically no place for a whale, but a portion the Atacama Desert in Chile is a mass grave for large marine mammals that died in large numbers millions of years ago. Three years after the fossils were announced, a cause of death has likely been determined. The researchers were able to discover that these deaths were caused by four different mass strandings, and toxic algae appears to be the culprit. The research was performed by a cooperation between the Smithsonian and scientists from Chile, led by Nicholas Pyenson, and the results were published with an open access license in the Proceedings of the Royal Society B.
A project to expand the Pan-American highway in Northern Chile in 2010 unearthed a huge bed of fossils dating back to the late Miocene, 5-11 million years ago. The site has been dubbed Cerro Ballena, which translates to “whale hill,” because of there were 40 near-perfect whale skeletons discovered among the specimens.
In addition to normal marine creatures like the whales, seals, and fish, the team also found some fairly exotic fossils from extinct species. These include Thalassocnus, a marine sloth, and Odobenocetops, a dolphin with a face like a walrus—tusks and all.
Researchers noticed something odd about the fossils: they were all oriented the same way: belly up, head pointing inland. This was a clear indication that something affected all of these animals at the same time. They most likely all died at sea and were washed ashore. Layering of the fossils also showed that mass death was not an isolated event: it happened four different times over a time span of 10,000-16,000 years.
Scientists were initially unsure of what would have caused so many marine creatures to die at once, and why it happened repeatedly. A tsunami had been hypothesized, but analysis of the sediment from that time span had ruled it out. Finally, they came to the only plausible explanation: algae.
Algae feeds on the nutrients (especially iron) brought into coastal waters from erosion and runoff. If too many nutrients enter the water quickly, the algae flourish in what is known as a bloom, which can be toxic to marine life. Small marine creatures feed on algae, and large marine creatures feed on the smaller ones. If the food source is contaminated at the most basal level, it has devastating effects on the entire food chain. In some instances, the animals don’t even have to eat the algae; inhalation is enough to have fatal consequences.
A whale stranding was attributed to algal blooms about 25 years ago near Cape Cod, resulting in 14 humpback whales washing ashore dead. Their digestive tissues had high levels of toxins, due to eating mackerel which fed on the algal bloom. In the case of Cerro Ballena, increased runoff from the iron-rich Andes mountains could very well have caused multiple instances of algal blooms. The team found large quantities of iron oxide in the sediment, as well as fossilized dinoflagellates. Though that alone is not enough to say for sure what happened, it seems to fit.
The fossils themselves are in remarkable condition. After the animals were beached, they were out of reach from marine creatures who would have eaten the carcasses. Once on land, there were a limited number of animals in the desert who would have scavenged the animals, scattering the bones. The only damage to the fossils appears to have been inflicted by crabs.
The study of Cerro Ballena was under the time crunch of the highway construction project, so much of the analysis couldn’t happen on site. This turned out to be a pretty good problem to have. The researchers documented the site as much as they could with 3D scans and photographs—including 360˚ panoramas of the dig sites—which have been made available to the public. Using Google Maps, the team developed an interactive map of the origin of the specimens. Selected specimens have been put into the Smithsonian’s 3D display and will be available to view and download for 3D printing purposes.