The whale cemetery: In Chile, the mystery of the mass death of prehistoric cetaceans is solved.
In the distant past, when the Atacama Desert in Chile was a seabed, enormous mass die-offs of cetaceans and other marine animals occurred there, with deaths occurring almost simultaneously. The mystery has just been revealed. The culprit was a red tide-type toxin that still proliferates in our oceans today.
By: Oasis Team. Photos: Smithsonian Institution.
The site now known as Cerro Ballena is located in northern Chile, in the Atacama Desert, near the port city of Caldera. In 2010, the expansion of the Pan-American Highway in that region led to the discovery of a large paleontological site. There, a group of scientists discovered more than 40 skeletons of prehistoric marine mammals. These fossils represent the remains of at least four episodes of mass whale and other marine mammal deaths on ancient beaches during the Late Miocene, about 7 million years ago, suggesting that these disasters were similar in nature and repetitive. Among the most spectacular finds is an entire family of whales, with a male, female, and calf, all together.
Three years of investigation culminated in the identification of the most likely cause for these enigmatic events: In the oceans, the only possible culprit for such mass deaths are single-celled algae, some of which are highly toxic. On some occasions, when there is an excessive proliferation of these algae, these tiny organisms produce an extremely toxic molecule, capable of killing in a short time any animal that ingests a large quantity of them. Exactly as whales and other marine animals do. Indeed, tiny spheres of similar dimensions to those of the algae were found in the fossils.
In scientific terms, the phenomenon is called "toxin-producing dinoflagellate blooms." It is common and relatively frequent even today, and one of its manifestations is the so-called "red tide." The work resulted from a collaboration between the Smithsonian Institution in the USA and scientists from the University of Chile, and was recently published in the journal Proceedings of the Royal Society B.
Among the fossils discovered are dozens of complete skeletons of extinct species of humpback whales and sperm whales, as well as traces of other marine mammals, including an extremely rare prehistoric dolphin belonging to the genus Odobenocetops, and an ancient marine sloth of the species Thalassocnus natans. Researchers also identified teeth of Carcharodon hastalis, a gigantic shark very common during the Miocene and Pliocene epochs, and fossilized remains of penguins, swordfish, and other predatory bony fish. “[At least 10 different types of marine animals were identified, recurring in four different layers,” stated Nicholas Pyenson, a Smithsonian researcher and first author of this work. “This demanded an explanation.”
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Intriguing position
What most intrigued the researchers was the way the skeletons were positioned. “The humpback whales were mostly belly up, and whales only end up belly up if they arrive somewhere already dead,” said Pyenson. “This is a graveyard, not the crime scene – the crime happened elsewhere.” The skeletons also showed a similar orientation, with their heads pointing away from the sea, which lends more credence to the hypothesis that the animals died in the ocean before being washed ashore. The distribution of the fossils in four distinct layers suggests that the deaths resulted not from an isolated catastrophe, but from four mass die-offs that occurred over a period of 5 to 16 years.
The team investigated several phenomena that could explain such a high mortality rate, from tsunamis to virus outbreaks. However, none of these hypotheses fit the observations. The researchers found no geological evidence of tsunamis in the sediments analyzed, and the fossils were in excellent condition. Viruses and other pathogens tend to be more specific, so they were not suitable to explain the diversity of animals found in Cerro Ballena.
“I realized there was only one good explanation: harmful algal blooms,” said Pyenson. When conditions are favorable, microalgae proliferate in large masses, making large quantities of nutrients available to organisms located at the top of the food chain. Dinoflagellates are among the most common groups of microalgae in the oceans and can produce very exuberant blooms known as red tides. Some species of dinoflagellates produce extremely potent toxins, which are concentrated along the food chain and can cause the death of fish and other animals, including humans.
These blooms can arise as a consequence of human activity, such as discharges of effluents rich in inorganic nutrients. However, a large part is triggered by the seasonal enrichment of the waters with nutrients such as iron, minerals brought by ocean currents or by the leaching of natural deposits present on the continents. Interestingly, the productivity of the waters along the Chilean coast, near Cerro Ballena, is strongly shaped not only by coastal upwelling currents (nutrient-rich currents), but also by ferruginous effluents from slopes rich in iron deposits located in the Andes.
Analyses of sediments from Cerro Ballena show the presence of fine deposits stained with iron oxides, as well as apatite spherules with dimensions similar to those of dinoflagellate cysts. The team plans to return to the site in search of even more convincing evidence to support this hypothesis. Meanwhile, at Cerro Ballena, the fossils remain on the surface, at the edge of the Pan-American Highway...