The Sciences

Effects of 20th Century Nuke Tests Have Reached the Bottom of Earth's Oceans

Scientists know because they have found isotopes released by the explosions in the bodies of tiny organisms living 6.5 km below the ocean's surface.

In a new problem, scientists have found that pollutants released by human activity can reach the deepest points in the world’s oceans, and pretty quickly too, via the food chain. Marine organisms at these depths seem to be feeding on these pollutants.

The researchers, all from China, had their study published on April 8, 2019. They observed tiny organisms called amphipods, which can survive at 6,500 m below the surface or deeper. This portion of the ocean is called the hadal zone, named after Hades, the god of the underworld in Greek mythology. The zone includes some of the remotest and least-explored places on Earth. Its typical characteristics are low temperature, high pressure, limited food and frequent geological activity, according to the researchers. So understanding where the occupants of this chthonic realm get their food from could be key to understanding biological adaptation at such depths.

Also read: The Deepest-Dwelling Fish in the Sea Is Small, Pink and Scaleless

Amphipods are crustaceans, but they’re not as big as their more famous cousins, the crabs, lobsters, prawns and shrimps. They all have multiple limbs and outer shells. The lesser-known amphipods have been known to dwell on beaches and the oceanic depths, and everywhere in between. They are marine scavengers that mop up the remains of other plants and animals in the water and rid the oceans of the dead. Biologists also see amphipods as model organisms for research on how life has adapted to hadal conditions, and study them as a way to understand the effect of human activities in the deep.

Oceanographers from the Woods Hole Oceanographic Institution (WHOI), Massachusetts, have reported that, apart from feasting on other deep marine life, “amphipods … consume just about anything that falls to the seafloor” and recycle nutrients “from even hard-to-digest material back into the environment.”

The deep ocean is one of the largest ecosystems on Earth but it’s quite far below the surface. However, fragments of pollutants in the shallower parts gradually sink down to the seafloor. On the way, marine animals ingest them, so the pollutants accumulate inside the animals’ bodies. And as the animals eat each other, the pollutants start to build up in larger and larger quantities.

Until recently, scientists reasoned that amphipods feed on organisms in the deep and not on biological matter originating from near the surface. But recent findings of concentrated pollutants even at hadal depths have been causing scientists to reexamine the long-held idea.

For example, scientists led by Alan Jamieson, a senior lecturer in deep-sea biology at Newcastle University, UK, studied six trenches around the Pacific Rim that plunged between 7,000 m and 10,890 m below sea level, and in 2014 reported finding microplastic contaminants “in the very deepest reaches of the oceans”.

Confirming prior work

The Chinese group didn’t look for plastics but for the residues of a horror of the previous century: nuclear bomb tests. Specifically, the researchers looked for the carbon-14 isotope, which is radioactive, in three trenches in the tropical west Pacific Ocean. They were able to find it easily because nuclear tests in the 1950s and 1960s added to the C-14 occurring naturally in the atmosphere, doubling it. C-14 from the tests also mixed quickly into the surface ocean and terrestrial carbon pools.

Based on their data, they were able to conclude that hadal amphipods obtain their food from surface organisms rather than from carbon arising from deep-Earth processes moving up to the seafloor, as Doug Bartlett, a marine biologist at the Scripps Institution of Oceanography, University of California, San Diego, put it. The researchers add in their paper that this indicates “anthropogenic pollution can reach the deepest ocean trench rapidly via the food chain”.

The results also suggests that hadal amphipods have an unexpectedly long lifetime, of over 10 years. This is more than four-times higher than the common longevity of amphipods that live in shallow waters. The researchers write in their paper that “these characteristics seem to be a result of the evolution of amphipods in the extreme environment in hadal zones, and studying the mechanism behind this evolution would expand our understanding of life and evolution”.

While the Chinese group claims it is the first to examine C-14 concentrations in hadal organisms, Bartlett says the study simply confirms older data. “It is not a particularly surprising result,” he said, “given the results from prior studies of hadal amphipods” led by his colleague Lisa Levin, and Jamieson. “But it is nice to see prior work verified using radiocarbon methodology.”

Also read: How a Soviet A-Bomb Test Led the US Into Climate Science

Nuclear fallout is infamous for being extremely persistent. Radioactive substances released in a nuclear test linger in the atmosphere, within sediments and soil, and inside living tissue. It is telling that our instruments can still notice a higher concentration of certain isotopes, including C-14, released in numerous tests in the 1950s. By one estimate, subterranean aquifers exposed to such fallout can be unfit for consumption for many millennia.

It is extra-worrisome, therefore, to find that even creatures as insulated from the surface as amphipods aren’t spared the effects. It is even more worrisome now, when consumerism has replaced Cold-War paranoia as this century’s existential threat of choice, and inundated the world with microplastics.

T.V. Padma is a freelance science journalist.