Residues from the nuclear tests in the 1950s and 1960s have been traced in the depths of the Marianas Trench — in the guts and tissues of crustaceans.
The study, titled “Penetration of Bomb 14C Into the Deepest Ocean Trench” and was recently published in American Geophysical Union’s journal Geophysical Research Letters, revealed high levels of radiocarbons that were found in the deep region of the Marianas Trench.
“The food source of hadal endemic fauna provides an insight into the carbon cycle in trenches and a biological adaptation to the impoverished and harsh trench environment,” states the abstract of the study headed by Ning Wang of Chinese Academy of Sciences.
What intrigued the researchers was how this radioactive signature of the thermonuclear tests from decades ago found its way into the guts of amphipods that thrive in the deepest part of the trench, the so-called hadal zone, some 20,000 to 36,000 feet deep.
Amphipods, the shrimp-like scavengers of the ocean, scour the sea bottom for leftovers of organic matter that have come from the surface. “Hadal amphipods are benthic scavengers relying on detritus and larger carrion that can originate from both surface and deep waters,” the research said.
Researchers gathered samples of amphipods from three trenches in the Western Pacific: Marianas Trench, Mussau Trench, and New Britain Trench. Using traps, they went as far down as 11,000 meters.
Gut contents and tissues of these crustaceans show traces of 14C in higher levels compared to the surrounding waters. This meant the radiocarbons have reached the amphipods through other sources: food chain. The report concluded that these creatures have similar radiocarbon-14 values as those organisms in the surface seawater.
Living in a harsh environment, the hadal amphipods’ low metabolic rate allows them to survive for longer periods without food.
“Moreover, the strong connection between the OM in hadal amphipods and the OM in surface water suggests that the supply of OM from other sources is negligible,” the study said. “Therefore, the adaptive strategy of amphipods is critical for living on the limited surface‐derived OM in hadal zones.”
According to the scientists’ hypothesis, hadal amphipods have a low metabolic rate and can store OM for a long period of time. “The high abundance of fatty acids in hadal amphipods indicates the long‐term energy storage for an extended period of starvation, thus supporting this hypothesis,” the report said. “Moreover, the low turnover rate of tissues enables the application of the bomb 14C dating signal in hadal amphipods.”
Despite their environment, hadal crustaceans live longer, about 10 years, compared to amphipods in shallow water that can last only for up to two years. The research also showed that hadal crustaceans have longer bodies. The higher the radiocarbon values in the tissues, the longer the body lengths, the report showed.
Carbon-14 is one of three naturally occurring carbon isotopes but being unstable, it decays over time into a stable product. This rare isotope is present in the atmosphere. When the nuclear tests were conducted until the ban treaty was enforced in 1963, the levels of 14C in the atmosphere more than doubled, but gradually declined after the ban. Remnants of the blasts reach the plants through photosynthesis. Animals and humans feed off of plants that have absorbed this bomb 14C; hence, have also taken in radiocarbons themselves. The same 14C are present in human tissues.
Given that the decay of the radiocarbons has a constant rate, scientists use this method to ascertain the age of organic matter. Living organisms continue to absorb carbon-14 until death. Then whatever remaining carbon-14 in the tissues give the scientists an idea of how old the organism was.
The same method was used with the amphipods in the study. The researchers discovered that the hadal amphipods have longer lifespan and are bigger than their counterparts that live near the surface.
The discovery of radioactive carbons in crustaceans in the hadal region of the Marianas Trench and two other trenches of the Pacific raises a red flag that lead scientists to believe that pollutants have arrived at the nether reaches of the ocean. The rapid transport of these pollutants to the bottom of the ocean was made possible through the food chain.
“We conclude that surface‐derived OM penetrated rapidly into the deepest living biomass via the food chain, and selective feeding on the surface‐derived OM causes amphipods at depths of ~10,900 m to have similar Δ14C values as organisms in the surface seawater.
Accordingly, anthropogenic pollution reaches hadal trenches fast via the food chain,” the report said.
The report was co-authored by Chengde Shen, Weidong Sun, Ping Ding, Sanyuan Zhu, Weixi Yi, Zhiqiang Yu , Zhongli Sha, Mei Mi, Lisheng He, Jiasong Fang, Kexin Liu, Xiaomei Xu and Ellen R.M. Druffel.