Yep, that’s a can of luncheon meat from 2001, at 5 km down on the
slopes of the Mariana Trench. A recent submarine expedition to the
deepest reaches of the ocean uncovered such sublime traces of human
influence as flame retardant chemicals and other persistent organic
pollutants (POPs), like PCBs, Budweiser, and low-sodium Spam. The
team published their
results in the most recent issue of Nature Ecology and
Evolution.
Congrats, guys. We did it. There’s nowhere on our planet that
humans haven’t left their mark. We have now literally spammed the
bottom of the ocean.
It seems obvious that the cans of beer and Spam probably fell off
someone’s boat. But how did the runoff and smokestack chemicals get
the whole way out there into the middle of the ocean? For one, they
can disperse via wind and water, which accounts for how some of the
POPs got into the sediment. These chemicals are so long-lived that
they can make it the whole way out into the middle of the Pacific
without being materially changed or broken down by water treatment
plants or the environment at large. The study calls them
“invulnerable to natural degradation.”
Nonreactivity is the same quality that made PCBs valuable as
dielectric fluid, used for “quenching” electrical discharge, and
also what made PBDEs and chemicals like PFOS and PFOA useful as
flame retardants. Those ridiculously inert molecules just will
not burn, conduct electricity or otherwise react. To get rid of
them, you practically have to fire them into the sun. We used them
like they were going out of style until we found out they’re
carcinogenic with chronic exposure, and in high enough acute doses,
they’re ugly bad for the liver. As a consequence, production of both
these POPs stopped decades ago. But they’re still in the deep ocean
food chain, and the sediment on the seafloor.
One of the “voracious” invertebrate seafloor beasties that
Jamieson and colleagues tested for POP contamination. Enlarged
to show texture. Actual size: about 2cm.
We tend to regard the depths of the ocean as a sparsely populated
wasteland, occupied by anglerfish and other horrors of the deep. An
area, in other words, generally apart from humanity’s existence.
“[But] the distance from the surface to full ocean depth is actually
only equal to the widest point of the Mississippi River, and half
the length of Manhattan Island,” the study authors said. Isotope
analysis from Fukushima showed it only takes a couple months, tops,
to sink from the surface to the bottom — and it’s probably faster
for carrion falls, which are also a major source of bioaccumulating
POPs in the food web.
POPs got into the food web because they’re inert molecules: they
can bioaccumulate precisely because they’re so nonreactive and
difficult to break down. When an animal eats something contaminated
with POPs, they just sit there in its tissue, vaguely annoying its
immune system and causing oxidative stress. When it dies, those
molecules of POPs are spatially concentrated, ready to be taken up
by some other organism and absorbed into the food chain.
Instead of just remaining up here on land and in surface waters
or near the shores, the POPs reached
the ocean bottom by way of ocean currents and “marine snow,” the
detritus that filters down to the bottom from the surface. When a
POP-contaminated animal dies in the open ocean, the carrion slowly
drifts down to the bottom, nibbled at the whole way by decomposers
until a few contaminated flakes finally reach the seafloor, where
they more or less stay. “When it gets down into the trenches, there
is nowhere else for it to go,” said expedition lead Alan Jamieson,
of Newcastle University. That’s how the PCBs got into the
crustaceans and amphipods living at depths heretofore considered
beyond the reach of human influence.
“The very bottom of the deep trenches like the Mariana are
inhabited by incredibly efficient scavenging animals, like the
2cm-long amphipods we sampled, so any little bit of organic material
that falls down, these guys turn up in huge numbers and devour it,”
added Jamieson. “The surprise was just how high the levels were –
the contamination in the animals was sky high.” (Respectfully, Prof.
Jamieson, you might still be amazed at the variety of things humans
have left on the bottom of the ocean, including the
world’s first analog computer and so much
ordnance from WWII.)
A Budweiser can seen at 3,780 meters depth at Enigma Seamount.
Budweiser: the official beer of seafloor invertebrates.
In some crustaceans on the slopes of the Mariana trench, the
highest levels of PCBs were 50 times more contaminated than crabs
from paddy fields fed by the Liaohe River, a highly polluted rivers
in China, the authors wrote: “The only Northwest Pacific location
with values comparable to the Mariana Trench is Suruga Bay (Japan),
a highly industrialized area with historically heavy usage of
organochlorine chemicals.”
The researchers also found severe contamination in amphipods
collected in the Kermadec trench, which is in the North Pacific,
7,000 km from the Mariana trench. The pollution was everywhere:
found “in all samples across all species at all depths in both
trenches.”
Like the
ubiquitous anthropogenic lead contamination Clair Patterson
discovered trying to find the age of the earth, PCBs and other POPs
are candidates for a specific chemical marker of the hotly debated
Anthropocene epoch. The Anthropocene is a
proposed division in the geological record of our planet, a
literal and figurative line in the sand marking the emergence of
humans as a species of global influence and worldwide dominance. The
fine but distinct layer of industrial pollution forms just as
distinctive a signal that something has happened as the
powdering of iridium that covered the entire planet after the K-T
impact that killed the dinosaurs. Just as researchers today can
trace the K-T extinction event to that iridium layer, scientists ten
thousand years from now will be able to track the appearance of
pollutants like PCBs in the fossil record. Hopefully we’ll be able
to track their disappearance as well.
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