Researchers investigating a huge fossil find near Chile's northern coast say they likely know what caused the mass die-off of at least 40 prehistoric whales in the area.

A study team of Chilean and Smithsonian Institution scientists discovered evidence that toxins produced by algae blooms most probably poisoned the animals between 6-9 million years ago.

The findings are published in the journal Proceedings of the Royal Society B.

Experts from the Smithsonian traveled to Chile's Atacama region in 2011 to pick through one of the best-preserved deposits of prehistoric whale fossils ever found, which also included the remains of an extinct aquatic sloth, as well as a whale that bore walrus-like tusks.

Scientists indicate the condition and placement of the whale bones demonstrates the large animals died at sea, before reaching their final resting place on a tidal flat.

Today, toxins from harmful algal blooms, such as red tides, are a main reason for repeated mass strandings of a wide variety of large marine animals, not just whales.

Harmful algal blooms are common along the coasts of continents, where their growth is enhanced by vital nutrients like iron, released during erosion and carried by rivers flowing into the ocean.

Since the Andes of South America are iron-rich, the runoff that has occurred along the west coast of the continent for more than 20 million years, providing ideal conditions for the harmful blooms to flourish.

The scientific team concluded bloom toxins poisoned many ocean-going vertebrates near Cerro Ballena, or, "Whale Hill," in the late Miocene (5-11 million years ago) through inhalation or the ingestion of contaminated prey. Relatively rapid deaths set in after that.

"There are a few compelling modern examples that provide excellent analogs for the patterns we observed at Cerro Ballena -- in particular, one case from the late 1980s when more than a dozen humpback whales washed ashore near Cape Cod, with no signs of trauma, but sickened by mackerel loaded with toxins from red tides," said Nicholas Pyenson, paleontologist at the Smithsonian's National Museum of Natural History and lead author of the study. "Harmful algal blooms in the modern world can strike a variety of marine mammals and large predatory fish. The key for us was its repetitive nature at Cerro Ballena: no other plausible explanation in the modern world would be recurring, except for toxic algae, which can recur if the conditions are right."

The remains were first found during an expansion project of the Pan-American Highway in 2010.

The following year, paleontologists from the Smithsonian and Chile examined the fossils and recorded what was left before the site was paved over.