A new study led by the Chinese Academy of Sciences in Guangzhou indicates that radioactive carbon-14 produced by above-ground nuclear weapons tests in the mid-20th century has been found in the muscle tissues of crustaceans in the deepest parts of the ocean. The presence of the nuclear isotopes in animals living up to seven mi (11 km) down shows that pollutants may be reaching the deep ocean much faster than previously thought.

Carbon 14 is a rare isotope of carbon that every living creature on Earth absorbs. It's produced in the Earth's atmosphere when cosmic ray neutrons strike nitrogen atoms in the upper atmosphere. The naturally radioactive form of carbon is absorbed by every organism in the form of food or respiration until it dies.

The rate at which carbon-14 breaks down has been used by archaeologists for decades to date ancient organic artifacts because it has a half-life of 5,568 years ± 30 years, which means that every 5,568 years there's half as much carbon-14 as there was to begin with, then half as much again in 5,568 years, and so on.

One reason why this dating system works is that scientists can, by taking into account the effects of solar activity on cosmic rays, calculate how much carbon-14 is being produced, therefore what the ratio of C-14 to non-radioactive C-13 is, and how old the artifact is. Unfortunately, future archaeologists are going to have a much harder time because atmospheric nuclear tests from 1945 to 1980 doubled the amount of C-14 in the air.

This peaked in the 1960s, when the United States, the Soviet Union, and Britain abandoned atmospheric tests and by the 1990s the excess had dropped to only 20 percent more than the pre-atomic levels. However, manmade C-14 has since been absorbed by plants and animals, making it an excellent trace isotope to learn more about how chemical compounds move through the environment.

But what has this to do with the deep ocean? If you look at how slowly water circulates in the deep hadal trenches over four mi (six km) beneath the surface, it looks like a C-14 peak should be irrelevant because none of it should get down there for hundreds or even thousands of years.

According to the Chinese Academy team, the answer to this question lies in the life that lives at these tremendous depths. It's a very odd place to find any kind of life because, outside of a few volcanic vents, there's no source of energy to sustain life. Sunlight is unable to penetrate such depths, so it's cold, and it's dark.

But there is life, such as a small crustacean called an amphipod that the team, led by geochemist Ning Wang Wang, collected in 2017 from the Mariana, Mussau, and New Britain Trenches in the tropical West Pacific Ocean. In shallow waters, the amphipods live only about two years and grow to a size of about 20 mm (0.8 in), but in the deep trenches, they live for 10 years and grow to 91 mm (3.6 in) long.

According to the team, this is because in the low-temperature and high-pressure environment, the crustaceans develop a very slow metabolism and cell turnover. It's also because the amphipods share a similar lifestyle with other deep-living creatures. There's very little food, so they have become incredible opportunists. Instead of actively swimming about and hunting for prey, the amphipods sit quietly and conserve energy until some tasty morsel comes along.

This poses another question, where does the food come from? Ultimately, it comes from the one big source that's available – the surface. What happens is that, over time, the fish and plankton in the shallower, sunlit waters send food drifting down into the deepest areas in the form of scraps, dead plants and animals, and wastes that the bottom dwellers feed on.

What this means for the Chinese Academy study is that this rain of food is a shortcut to transport the C-14 to the bottom. The organisms at the top ingest it, and then it rains down for the amphipods and others in turn. By analyzing the crustaceans, they not only found excess C-14 in the muscle tissues, but also in their gut contents, which matches samples taken in the shallows.

This, in itself, doesn't pose an environmental hazard, but it does show how pollutants can reach the deepest corners of the abyss in a very short time.

"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," says Wang.

The findings were published in Geophysical Research Letters.