Radioactive carbon originating from nuclear tests has reached the deepest points of the ocean, further proof that human impacts reach every point on Earth. But does this also have positive aspects?
Sewage and runoff, industrial wastewater, agricultural fertilizers, plastic waste left on beaches after visits to the sea, and countless other pollutants find their way into the oceans, where they harm the marine ecosystem.
Most solid waste drifts toward the shore or gathers into large floating patches concentrated in the centers of oceans.
Plastic waste has even been documented at a depth of about 11 kilometers in the deepest point of the ocean, the Mariana Trench, located in the western Pacific Ocean.
In the last century, between the 1950s and 1990s, another significant pollutant was added to the oceans: low level radioactive nuclear waste, dumped at least 50 sites in the North Atlantic and Pacific oceans in metal barrels or concrete cylinders.
Additionally, during numerous nuclear tests conducted in the Pacific Ocean for the development and improvement of nuclear weapons, radioactive carbon C14, also called “bomb carbon,” was created.
During atomic explosions, a rapid and strong flux of neutrons reacts with nitrogen atoms in the atmosphere, creating carbon with radioactive properties carbon 14.
During the nuclear tests conducted between 1955 and 1963, the natural amount of carbon 14 in the atmosphere doubled. Later, the bomb carbon combined with oxygen atoms and formed radioactive carbon dioxide, which ultimately reached ocean waters through biological and chemical processes.
Today, thanks to this significant increase in carbon 14 in the atmosphere and oceans, bomb carbon helps identify and date various natural processes.
Unlike naturally occurring radioactive carbon in the atmosphere, which decays slowly with a half-life of 5,730 years and is used to date organic processes that occurred thousands or tens of thousands of years ago, bomb carbon can inform researchers about processes on much shorter timescales from a few years to several decades.
“The nuclear tests allowed us to use the ‘disturbance’ humans created on Earth as an indicator,” says Dr. Galen Antler, a researcher at an inter-university institute in the United States and at a major American university.
“Because we know the source of the new carbon 14 and the time it was created, we can track it in various reservoirs and estimate how quickly carbon moves between different reservoirs on Earth, such as the atmosphere, shallow ocean, and deep ocean.”
A new study published this year in Geophysical Research Letters by a team of Chinese and American researchers used bomb carbon to show how far the remnants of our nuclear activity can reach in the ocean.
The study showed that bomb carbon reached the bodies of crabs living in the deepest points of the oceans the sub-oceanic trenches in the western Pacific, with depths reaching approximately 11 kilometers.
Researchers collected crabs from these trenches (at depths between 6 and 10.9 kilometers) and found that the radioactive carbon in their bodies matched the expected levels from nuclear tests in the ocean.
The presence of bomb carbon in the crabs’ bodies suggests that their food originates from the surface and sinks to the trenches where the crabs live.
The carbon consumed by surface organisms sinks as the organism decomposes or is eaten by other animals, eventually reaching the ocean floor and consumed by crabs.
Based on the work with bomb carbon, scientists discovered that deep-sea crabs feed on carbon originating from the surface, and likely due to this diet, their lifespan is five times longer than similar organisms in shallow waters.
This leads to two interesting conclusions: first, it demonstrates the wide-reaching influence of human activity.
Nuclear tests conducted decades ago created radioactive atoms that still exist in crab bodies today and reached deep ocean regions. Practically no area on Earth on land, in the air, or in the sea is unaffected by long-term human activity.
Second, this discovery provides new insights into the carbon cycle, describing the movement of carbon atoms, combined with other atoms, between the atmosphere and oceans, and back to the atmosphere through various paths.
The carbon cycle begins when some atmospheric carbon (carbon dioxide released from biological activity, fossil fuel combustion, or forest fires) dissolves at the ocean surface. From there, carbon moves through multiple pathways.
One example is when carbon is consumed by photosynthetic organisms in the ocean.
When these organisms die, or the organisms that consumed them die, carbon sinks to the ocean floor. Some carbon returns to water through bacterial decomposition on the way down, and some sinks with tissue and bones, creating sediments that over millions of years can form sedimentary rocks.
Regarding climate change, this process has a positive implication: carbon that sinks to the seabed becomes buried and effectively removed from the carbon cycle, meaning it cannot return to the atmosphere.
This seabed carbon burial process, also called the biological pump, involves biological processes that pull carbon from surface waters, which are in equilibrium with the atmosphere and bury it in sediments for millions or even hundreds of millions of years.
Bomb carbon teaches researchers much about oceanographic processes and the consequences of human impacts.
“While until now the understanding was that thermohaline circulation ocean circulation beginning in the North Atlantic, moving south, and rising in the North Indian and Pacific Oceans takes several thousand years, researchers show that even in distant and deep ocean regions, human influence is still detectable,” adds Antler.
“These environments may not be so remote from us and are probably much more vulnerable to human impacts than previously thought.”