Mariana Trench Creatures Have Radioactive Carbon from Nuclear Bomb Tests in Their Muscles

Scientists have discovered that radioactive carbon from nuclear bomb tests carried out in the 1950s and 60s has reached the deepest parts of the world's oceans—including the Mariana Trench, the deepest known point on Earth.

In a study published in the journal Geophysical Research Letters, researchers report that tiny crustaceans found four miles below the ocean surface have radioactive carbon from nuclear bomb tests in their muscle tissues—something they were not expecting.

The team, led by Ning Wang, a geochemist at the Chinese Academy of Sciences in Guangzhou, China, analyzed amphipods—an order of crustacean—recovered from hadal trenches, which are some of the most remote and least explored places on Earth. The amphipods had been recovered from the Mariana, Mussau, and New Britain Trenches in 2017.

Initially, the scientists had been hoping to learn more about the source of organic matter that sustains the creatures at these depths. This is important in the understanding of the ocean carbon cycle and how these animals have adapted to life in the trenches.

When nuclear bombs were tested, they doubled the amount of carbon-14 in the atmosphere. Carbon-14 is radioactive carbon created when nitrogen in the atmosphere interacts with cosmic rays. When the bombs were set off, the neutrons released reacted with the nitrogen to produce "bomb carbon."

This mixed with the ocean surface and was absorbed by organisms living in this environment. In the years that followed, scientists documented elevated levels of Carbon-14 in these marine organisms, which eventually started to fall back down towards pre-test levels.

Researchers were looking for carbon-14 in hadal trench organisms to find out if organic material with elevated levels of bomb carbon from the surface had reached the depths of the oceans. Their findings showed carbon-14 levels in the muscle tissue of the amphipods was far higher than the levels of carbon-14 found in the organic matter there. They also found carbon-14 levels in the gut contents of the amphipods was the same level as samples of organic material from the surface of the ocean.

This indicates that the creatures are purposely feeding on organic matter from the ocean surface that sinks to the ocean floor, rather than the organic material produced by local sources.

"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," Wang said in a statement.

"There's a very strong interaction between the surface and the bottom, in terms of [biological] 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 study co-author Weidong Sun, a geochemist at the Chinese Academy of Sciences. "It's not expected, but it's understandable, because it's controlled by the food chain."

The scientists also discovered the amphipods lived far longer and grew bigger than those in shallower environments—those studied were found to be more than 10 years old, while their counterparts only live for around two years. The team believes their extreme habitat of freezing temperatures and high pressures has led them to develop a very slow metabolism, allowing them to store energy for longer and extending their lifespan in the process.

Rose Cory, from the University of Michigan, who was not involved in the study, said the findings show that evidence of human activity can be found in some of the remotest places on Earth. "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," she said in a statement.