Radioactive fallout in the Mariana Trench

Crabs from the bottom of the deep sea trunks have enriched C-14's nuclear weapons tests

Explosion of a nuclear bomb in the US nuclear weapons test Castle Bravo 1954 in Bikini Atoll. The fallout of these tests has penetrated surprisingly fast into the deepest deep-sea trenches. © US Department of Energy
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Astonishingly fast: The fallout of the nuclear weapons tests has already reached the deepest deep-sea trenches - faster than previously thought possible. Because normally the transport of water from the surface to the deep sea takes centuries. But crabs in the Mariana Trench and other deep-sea trenches have already enriched the radioactive C14 bomb tests, as analyzes show. This also sheds new light on the food strategies of these deep-sea dwellers.

Between 1945 and the mid-1960s, the United States, the Soviet Union and several other countries carried out numerous nuclear weapon tests in the Pacific. The explosions left radioactive fallout on islands such as the Bikini Atoll, but also in the earth's atmosphere. Even today, the stratosphere contains up to 100, 000 times more radioactive plutonium and cesium than at ground level, as recent measurements show.

The atomic bomb explosions also increased the content of the radioactive carbon isotope C-14 in the earth's atmosphere. This C-14 peak peaked in the mid-1960s and has been falling slowly ever since. But even 30 years after the end of the nuclear weapons tests, the C-14 fraction in the atmosphere was still 20 percent higher than before the nuclear test. Today, this C-14 curve helps to time-order objects using radiocarbon dating.

Deep sea - largely isolated from the surface?

But even if the fallout of nuclear tests on the entire earth left its mark, so far was an area considered largely untouched: the extreme deep sea. According to popular wisdom, the water and life of deep-sea trenches such as the Mariana Trench have only limited interaction with the sea surface. For most fabrics, it takes centuries before they reach those depths of more than 6, 000 meters.

However, this slow exchange obviously does not apply to all substances. As early as 2017, researchers in the Mariana Triangle's flea crabs detected unusually high levels of environmental toxins such as PCBs and polybrominated diphenyl ethers (PBDE). These toxins appeared to be in the deep with dead remains of plankton and other upper-layer organisms. display

Bomb C-14 in muscle tissue

As it turns out, this accelerated "elevator" in the depth obviously also applies to the radioactive fallout of nuclear weapons tests. For their study, Ning Wang from the Guanzhou Institute of Geochemistry in China and his colleagues had analyzed the C-14 levels of the Mariana Trench and two further deep-sea trenches in the Western Pacific. For comparative purposes, the researchers also investigated sediment from the bottom of deep-sea beds and water samples.

The deep-sea hare Hirondellea gigas lives among other things at the bottom of the Mariana Trench. Daiju Azuma CC-by-sa 2.5

The surprising result: In the body tissue of the flea crabs the C-14-Werte were noticeably increased. With 10 to 65 per thousand, the C-14 levels in the muscles of the crabs corresponded to those at the sea surface. "This suggests that C-14 is present in the nuclear tests, " the researchers said. Also, the fresh food in the digestive tract of the cancers showed slightly elevated C-14 levels, although these were also significantly below those of the muscle tissue, as the scientists report.

That means: Contrary to expectations, the fallout from nuclear weapons tests has long since arrived in the deep sea. "The C-14 measurements of deep-water flea crabs clearly show a bomb signature in the deep ocean trenches, " say Wang and his team.

How does the fallout get into the crabs?

Interesting, however: in the deep water and in the sediment of the deep-sea trunks, the C-14 values ​​were not increased, as the analyzes showed. But this is to be expected, because the deep-sea flea crabs are usually believed to feed on organic matter and carrion that they find on the seabed - and that over time from higher water layers up to sinks down into the trenches.

But the deviating C-14 values ​​speak in favor of a different strategy. Obviously, the flea crabs in the Mariana Trench and other deep-sea trolls are deliberately picking out the food chunks, which, as it were, fall off the surface in rapid transit. These seem to selectively fish and eat them out of the water. "The data suggests selective seeding of fresh, post-bomb-test organic material from the water surface, " the researchers explain.

Food chain as "express elevator" in the depth

It also sheds new light on surface-to-deepwater interchange: "Although ocean circulation takes hundreds of years to bring water with bomb fallout to the deepest deep-sea trenches, the food chain can do this much faster, " says Wang. Sinking remains of organisms apparently take the express elevator into the depths - and with them also environmental toxins and radioactive fallout.

"The really new thing about it is not only that carbon from the ocean surface can reach the deep ocean in a relatively short time, but also that the 'young' carbon from the surface nourishes life in the deep trenches, " she says Study involved Rose Cory of the University of Michigan. According to study authors, this also means that human activities can even affect biosystems down to almost 11, 000 meters. "We need to think carefully about our future behavior, " the researchers said. (Geophysical Research Letters, 2019; doi: 10.1029 / 2018GL081514)

Source: American Geophysical Union

- Nadja Podbregar