In the arid coastal region of South America
known as the Atacama Desert, between the Andes Mountains and the Pacific Ocean, there lie
the remains of ancient behemoths of the sea. At a site known as Cerro Ballena or Whale
Hill, there are more than 40 skeletons of marine mammals -- a graveyard of ocean life
dating back 6.5 million to 9 million years ago, in the Late Miocene Epoch. Most of these marine mammals were big baleen
whales of all ages, but there are also skeletons of two different species of seals, an extinct
sperm whale, an aquatic sloth, and a weird toothed whale that looked like a walrus. And the skeletons were deposited there in
four distinct layers, over a period of about 10,000 to 16,000 years. So whatever killed all these animals happened
more than once. Dense bone-beds of marine mammals like this
are pretty rare in the fossil record, which makes this site an incredible find. But, what happened? How did these sea creatures end up buried
in the desert? Well, back in the Miocene, Cerro Bellena was
a beach, a tidal flat where dead and dying animals washed ashore over time. As for what killed all of those creatures,
researchers have considered a couple of different hypotheses - from a natural disaster like
a tsunami, to a fast-acting disease that somehow struck four different times. But the identity of the killer that they finally
settled on might surprise you. The hypothesis that best fits the strange
evidence found at Cerro Ballena is that these marine mammals might've been ultimately killed
by a quirk of South America’s geography. Today, we’d probably call what happened
at Cerro Ballena a mass stranding. These happen when whales beach themselves
on land and can’t get back into the water, and the whales often die as a result. In modern times, mass strandings usually involve
toothed whales - like pilot whales and sperm whales - not baleen whales, like most of the
ones found at Cerro Ballena. But the few mass strandings of baleen whales
that have happened in the last century or so can give us important clues about what
happened millions of years ago. For example, over a five-week period from
1987 to 1988, 14 humpback whales washed up on the coast of Massachusetts. Their bodies showed no signs of attack or
trauma. They included both males and females and one
calf. And they died over a short period of time,
in a small geographic area. But the killer at Cerro Ballena was even less
discriminating, because it affected at least five other species of marine mammals along
with the baleen whales. And, like those unfortunate humpbacks, the
marine mammals of the Miocene also probably died and floated ashore pretty quickly, maybe
in as little as hours or days, washed there by ocean currents and lifted onto the tidal
flat by storm surges. The paleontologists that worked on this site
think that’s what happened, because the whale skeletons were still pretty much intact. That means marine predators didn’t have
time to feast on the dead or dying whales at sea, and land predators didn’t scavenge
the remains, either. Most of the animals were also found with their
ventral side facing up - literally, belly-up - which suggests that they were dead when
they washed ashore. But knowing how the skeletons ended up there
doesn’t explain what killed all the marine mammals in the first place. Even today, it can be hard to know the cause
of a mass stranding. Many explanations have been suggested, like
changes in climate patterns, or the difficulty that whale sonar can have in picking up the
shape of a sloping coastline. The human use of sonar might be a factor too. So, the researchers working on Cerro Ballena
explored four hypotheses to explain the fossil evidence they found there. One idea was: could it have been something
about the behavior of the animals themselves? In general, it does look like there might
be a connection between whale behavior and mass strandings. Species that are more social and live in deeper
water seem to strand more often than solitary species or ones that live in coastal waters. But even today, researchers aren’t sure
why that is. And some populations of orcas and bottlenose
dolphins will beach themselves on purpose, to capture prey that’s tried to escape them
by leaving the water. But this isn’t something baleen whales do. And while most of the skeletons preserved
at Cerro Ballena are whales, there are still other species, like seals, that should’ve
been able to get back off the tidal flat and into the ocean. This means the Miocene graveyard couldn’t
have been caused by the behavior of a single species. So, next hypothesis: could the animals have
been killed by -- and I’m sorry to have to use this word now -- a pandemic? Well, again, there are several different species
at the site, so the disease would’ve had to have been able to infect and kill them
all. It also would’ve had to do it various times
over a period of 10,000 to 16,000 years, which the researchers think is pretty unlikely. Then, could it have been some other kind of
natural disaster? Like, what about a tsunami? Well It turns out, the sediment that the animals
were buried in looks like it was deposited continuously, at a regular, slow pace -- not
all at once. So the researchers were left with one more
hypothesis: What if they were poisoned? This actually happens all the time, in events
known as harmful algal blooms. These blooms happen when colonies of certain
kinds of algae suddenly explode in size. And these algae naturally produce toxic compounds,
like neurotoxins, that infiltrate marine food webs. Marine mammals might consume these toxins
directly or they might eat prey that have consumed them. And the toxins can also be inhaled. And we know that harmful algal blooms can
be deadly. In fact, they were the culprit behind the
mass strandings of humpback whales in Massachusetts in the late 1980s. They’ve also been the cause of strandings
and deaths for many other species, like sea lions, bottlenose dolphins, birds, and sea
turtles, so their effects aren’t specific to a particular species the way a disease might
be. So at Cerro Ballena, the scientists think
that the animals found there probably swam through an algal bloom, and breathed in the toxins
or ate prey that had been poisoned by them. Then they likely died pretty quickly, and
were washed onto the tidal flat where they were preserved. This would explain why so many different kinds
of mammals were found at the site. The killer didn’t have to affect a single
species; all of the victims were just unlucky enough to have been in the wrong place at
the wrong time. But, how could this have happened repeatedly
within a period of 10,000 to 16,000 years? That is where geography comes in. Along the coastline of the Atacama Region,
there’s what’s called an upwelling system in the Pacific Ocean. Upwelling happens when wind pushes the surface
water of the ocean away from the edge of a continent and cold, nutrient-rich water from
the deep ocean rises up to take its place. This can make the area really biologically
productive, with plankton thriving on the nutrients, and forming the basis for marine
food webs. However, the upwelling water doesn’t always
have all the nutrients these organisms need. And one that’s usually in pretty short supply
is iron. But! When a bunch of iron is dumped into this ocean
system, it can wreak havoc on marine life, by creating a harmful algal bloom. This is what researchers think happened in
the Miocene. And it all began with runoff flowing into
the ocean from the Andes Mountains. A study from 2004 was able to piece together
at least one of the mechanisms behind this iron-rich runoff, by studying cores of marine
sediment spanning the last 100,000 years. It confirmed that the cores had alternating
layers of iron-rich and iron-poor sediment. And this pattern roughly coincided with the
astronomical cycle known as precession, the change in the tilt of our planet’s axis
over timescales of about 20,000 years. This cycle can cause lots of changes to climate
all over the world. In this case it probably shifted the location
of climate zones in Chile and influenced the amount of precipitation that fell on parts
of the Andes Mountains. Today, most of the precipitation there happens
in the winter as snow. And in the summer, that snow melts into rivers
that run into the ocean. But in the past, when Earth was tilted at
a slightly different angle, this region probably got less direct sunlight in the winter, and
received even more snow. If this cold winter was followed by an extra
hot summer, it would lead to more run off. And more runoff meant more sediment being
eroded off of the iron-rich rocks of the Andes and carried into the ocean. And those times when there was a lot of iron
washing into the ocean provided a big boost of nutrients for plankton: both the good ones,
like phytoplankton, but also the bad ones that produce toxins, like harmful algae. While that study of sediment cores only looked
at this cycle over the last 100,000 years, it was probably happening back in the Miocene
Epoch, too. And we know from studying climate in the present
that it’s a complicated system. So other factors, like that pattern of warming
ocean currents known as El Niño, probably also played a role in why the strandings happened
four times. So that’s the process the paleontologists
think created the strange evidence we find at Cerro Ballena: The effects of precession and other variables
changed the local climate, which created spikes of iron-rich runoff, which in turn promoted
the growth of harmful algal blooms at least four times off the South American coast. And groups of unlucky marine mammals swam
through the toxic blooms, died, and then floated toward the tidal flat, where storm surges
lifted them onto the sand. They eventually were buried and fossilized,
preserving four tragic moments in deep time. Now, we’ll probably never know for sure
exactly what happened at Cerro Ballena. But the hypothesis that the researchers constructed
does a good job of explaining the observed evidence, by using the present as a tool for
understanding the past. And it stands today as a stark reminder of
how tightly our planet’s systems are connected: from big baleen whales, to tiny toxic algae,
to the oceans they all live in, and even the far-off mountains that feed the seas. Ok let's see what kind of terrible puns Kallie has lined up for me this week Mountains of thanks to this month’s Eontologists:
Patrick Seifert, Jake Hart, Jon Davison Ng, Sean Dennis, and Steve! You can join them by pledging your support
at patreon.com/eons. Also thanks for joining me today in the Konstantin
Haase studio. If we peaked your interest by mentioning aquatic
sloths, be sure to check out our episode, How Sloths Went From the Seas to the Trees. And if you want to join us for more adventures
in deep time, just go to youtube.com/eons and subscribe.
Eons is one of my favourite YouTube channels 🙂