MAY 8, 2019
Radioactive carbon released
into the atmosphere from 20th-century nuclear bomb tests has reached the
deepest parts of the ocean, new research finds.
A new study in AGU's
journal Geophysical Research Letters finds the first evidence
of radioactive carbon from
nuclear bomb tests in muscle tissues of crustaceans that inhabit Earth's ocean trenches, including the
Mariana Trench, home to the deepest spot in the ocean.
Organisms at the ocean surface have
incorporated this "bomb carbon"
into the molecules that make up their bodies since the late 1950s. The new
study finds crustaceans in deep ocean trenches are feeding on organic matter
from these organisms when it falls to the ocean floor. The results show human
pollution can quickly enter the food web and make its way to the deep ocean,
according to the study's authors.
"Although the oceanic
circulation takes hundreds of years to bring water containing bomb [carbon] to
the deepest trench, the food chain achieves this much faster," said Ning
Wang, a geochemist at the Chinese Academy of Sciences in Guangzhou, China, and
lead author of the new study.
"There's a very strong
interaction between the surface and the bottom, in terms of biologic systems,
and human activities can affect the biosystems even down to 11,000 meters, so
we need to be careful about our future behaviors," said Weidong Sun, a
geochemist at the Chinese Academy of Sciences in Qingdao, China, and co-author
of the new study. "It's not expected, but it's understandable, because
it's controlled by the food chain."
The results also help
scientists better understand how creatures have adapted to living in the
nutrient-poor environment of the deep ocean, according to the authors. The
crustaceans they studied live for an unexpectedly long time by having extremely
slow metabolisms, which the authors suspect may be an adaptation to living in
this impoverished and harsh environment.
Creating radioactive particles
Carbon-14 is radioactive
carbon that is created naturally when cosmic rays interact with nitrogen in the
atmosphere. Carbon-14 is much less abundant than non-radioactive carbon, but
scientists can detect it in nearly all living organisms and use it to determine
the ages of archeological and geological samples.
Thermonuclear weapons tests
conducted during the 1950s and 1960s doubled the amount of carbon-14 in the
atmosphere when neutrons released from the bombs reacted with nitrogen in the
air. Levels of this "bomb carbon" peaked in the mid-1960s and then
dropped when atmospheric nuclear tests stopped. By the 1990s, carbon-14 levels
in the atmosphere had dropped to about 20 percent above their pre-test levels.
This bomb carbon quickly fell
out of the atmosphere and mixed into the ocean surface. Marine organisms that
have lived in the decades since this time have used bomb carbon to build
molecules within their cells, and scientists have seen elevated levels of
carbon-14 in marine organisms since shortly after the bomb tests began.
Life at the bottom of the sea
The deepest parts of the ocean
are the hadal trenches, those areas where the ocean floor is more than 6
kilometers (4 miles) below the surface. These areas form when one tectonic
plate subducts beneath another. Creatures that inhabit these trenches have had
to adapt to the intense pressures, extreme cold, and lack of light and
nutrients.
In the new study, researchers
wanted to use bomb carbon as a tracer for organic material in hadal trenches to
better understand the organisms that live there. Wang and her colleagues
analyzed amphipods collected in 2017 from the Mariana, Mussau, and New Britain
Trenches in the tropical West Pacific Ocean, as far down as 11 kilometers (7
miles) below the surface. Amphipods are a type of small crustacean that live in
the ocean and get food from scavenging dead organisms or consuming marine
detritus.
Surprisingly, the researchers
found carbon-14 levels in the amphipods' muscle tissues were much greater than
levels of carbon-14 in organic matter found in deep ocean water. They then
analyzed the amphipods' gut contents and found those levels matched estimated
carbon-14 levels from samples of organic material taken from the surface of the
Pacific Ocean. This suggests the amphipods are selectively feeding on detritus
from the ocean surface that falls to the ocean floor.
Adapting to the deep ocean
environment
The new findings allow
researchers to better understand the longevity of organisms that inhabit hadal
trenches and how they have adapted to this unique environment.
Interestingly, the researchers
found the amphipods living in these trenches grow larger and live longer than
their counterparts in shallower waters. Amphipods that live in shallow water
typically live for less than two years and grow to an average length of 20
millimeters (0.8 inches). But the researchers found amphipods in the deep
trenches that were more than 10 years old and had grown to 91 millimeters (3.6
inches) long.
The study authors suspect the
amphipods' large size and long life are likely the byproducts of their
evolution to living in the environment of low temperatures, high pressure and a
limited food supply. They suspect the animals have slow metabolisms and low
cell turnover, which allows them to store energy for long periods of time. The
long life time also suggests pollutants can bioaccumulate in these unusual
organisms.
"Besides the fact that
material mostly comes from the surface, the age-related bioaccumulation also
increases these pollutant concentrations, bringing more threat to these most
remote ecosystems," Wang said.
The new study shows deep ocean
trenches are not isolated from human activities, Rose Cory, an associate
professor of earth and environmental sciences at the University of Michigan who
was not involved in the new research, said in an email. The research shows that
by using "bomb" carbon, scientists can detect the fingerprint of
human activity in the most remote, deepest depths of the ocean, she added.
The authors also use "bomb" carbon to show that the main
source of food for these organisms is carbon produced in the surface ocean,
rather than more local sources of carbon deposited from nearby sediments, Cory
said. The new study also suggests that the amphipods in the deep trenches have
adapted to the harsh conditions in deep trenches, she added.
"What is really novel
here is not just that carbon from the surface ocean can reach the deep ocean on
relatively short timescales, but that the 'young' carbon produced in the surface ocean is fueling, or sustaining,
life in the deepest trenches," Cory said.
