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.