When did the Anthropocene begin, the modern age in which we humans massively influence the earth? A prerequisite for determining the beginning of a new geological era is verifiable geological changes. Researchers have now identified precisely such man-made changes in the marine sediments of Japan’s Beppu Bay: nuclear tests in the 1950s and 1960s left their radioactive traces there. Comparisons with corals from a nearby island enabled an exact dating. The project is part of international research efforts to establish a globally valid reference point for the beginning of the Anthropocene.
According to the currently valid classification of the earth ages, we live in the Holocene, which began around 12,000 years ago with the end of the last ice age. Scientists traditionally determine the beginning of a new geological era based on features in rock strata that indicate serious environmental changes. For several years, research has been concerned with the question of whether our modern age, in which we humans have a massive influence on the shape of the earth, should be defined as a new geological age. The term “Anthropocene” has been in use for this since the year 2000 – the age of man.
Definition of a new age
But how can the beginning of this age be defined? In order to clarify this question, an international working group was formed to determine the beginning of the Anthropocene and a reliable geological marker for it. “The majority of the working group supports the use of plutonium isotope signals in geological samples as markers, since the levels rose sharply in the 1950s and were widespread due to surface thermonuclear bomb tests,” explains a team led by Yusuke Yokoyama from the University of Tokyo in Japan.
Using a combination of methods, Yokoyama and his colleagues have now shown that traces of the atomic bomb tests can be reliably detected in Beppu Bay in southwestern Japan. “Beppu Bay is one of several areas in the Pacific where there are well-preserved traces of human impact on the environment in the sediments deposited on the calm seabed,” says Yokoyama. “Our task was to find clear evidence of plutonium fallout from the 1950s through 1963, when testing largely ceased.”
Traces of nuclear testing in coral and marine sediment
In fact, he and his team found clear evidence of plutonium from the nuclear tests in the marine sediments of Beppu Bay. One problem with sediment samples, however, is that dating is not possible with sufficient precision. “So we also collected coral skeletons from the island of Ishigaki, southwest of Okinawa, that contained plutonium fallout,” explains Yokoyama. Similar to trees, corals form annual rings that allow for great historical precision – but contain less information than sediments. Therefore, the researchers decided to combine both methods. “By comparing sediments and corals, we can more accurately date the signatures we see in the sediments,” Yokoyama said.
In this way, his team was able to identify the fallout from the American atomic bomb tests in the Pacific in 1954 in both coral and sediment. Using the coral as a reference point, they were also able to precisely match the corresponding sediment layer and also link other chemical signatures found only in the sediment and not in the coral to the same year. “This work is not only important to solidify the definition of the Anthropocene,” says Yokoyama. “The successful application of our method also means that it could also be used to improve ocean and climate models or even to study past tsunamis and other geological hazards.”
Source: Yusuke Yokoyama (University of Tokyo, Japan) et al., Scientific Reports, doi: 10.1038/s41598-022-14179-w