An ancient graveyard of marine mammals in Chile’s Atacama Desert presents a scientific mystery. This site, dating back to the Late Miocene epoch, contains an extraordinary concentration of fossilized remains, prompting questions about what caused the demise of so many creatures in one location. Unraveling this enigma offers insights into past marine ecosystems and environmental events.
The Ancient Graveyard
The fossil site, Cerro Ballena, or “Whale Hill,” is located along the Pan-American Highway in Chile’s Atacama Desert, a few kilometers north of Caldera. This locality is part of the BahÃa Inglesa Formation and dates to the Late Miocene epoch, approximately 6 to 9 million years ago. The area was once a coastal tidal flat where dead animals washed ashore and were subsequently buried by sediments.
Cerro Ballena is renowned for its exceptional abundance and preservation of marine vertebrate fossils. Researchers have documented over 40 individual skeletons, many of them nearly complete, including various types of cetaceans like baleen whales and an extinct species of sperm whale. The site also contains remains of pinnipeds, such as seals, an unusual walrus-like whale, billfishes, predatory fish, and even aquatic sloths, specifically Thalassocnus natans.
Solving the Miocene Mystery
Paleontologists and geologists investigated the Cerro Ballena site to understand the mass mortality event, examining various hypotheses. Catastrophic events, such as a single massive tsunami, were considered, but the evidence did not align with such a scenario.
Scientists also evaluated the possibility of a fast-acting disease. However, the presence of multiple species across different trophic levels, from large whales to aquatic sloths and fish, made a single disease outbreak less likely. The recurring depositions, found in four distinct layers over a period spanning 10,000 to 16,000 years, further pointed away from a singular, isolated disaster.
The Toxic Explanation
The leading hypothesis for the mass deaths at Cerro Ballena points to harmful algal blooms (HABs), often referred to as “red tides.” These blooms occur when microscopic algae multiply rapidly, producing potent neurotoxins. Marine mammals can be exposed to these toxins by consuming contaminated prey or by inhaling aerosolized toxins directly from the water.
These neurotoxins can cause rapid organ failure and disorientation in marine animals, leading to their death at sea. Their carcasses then floated towards the coastline, where ocean currents and storm surges carried them onto the ancient tidal flat. The Andes Mountains, rich in iron, likely contributed to these blooms by providing nutrient-rich runoff into the coastal waters, creating conditions conducive to algal growth.
Uncovering the Clues
Evidence supports the harmful algal bloom hypothesis at Cerro Ballena. Geological analysis revealed four distinct bone-bearing layers, indicating that these mass mortality events occurred repeatedly over time rather than as a single catastrophe. This repetitive pattern, spanning an estimated 10,000 to 16,000 years, is a hallmark of recurring natural phenomena like harmful algal blooms.
Paleontological evidence from the fossilized remains provided further insights. The skeletons, including over 40 individuals of various species, were largely complete and articulated, many found belly-up and facing the same direction. This suggests that the animals died suddenly at sea and floated to the surface before being washed ashore and rapidly buried.
The multispecies nature of the graveyard, encompassing large whales, seals, and even aquatic sloths, points to a widespread environmental cause that affected the entire marine food web, aligning with the impact of a broad-spectrum toxin. Taphonomic studies, which examine the processes of fossilization, further supported this conclusion. The lack of extensive scavenging or disarticulation of the skeletons indicates quick burial by sand, preserving the remains in remarkable condition on the protected tidal flat.