Researchers identify Anthropocene marker in the South Atlantic

Sediment cores were divided into slices according to the time at which each layer of sediment was deposited.

By Peter Moon  |  Agência FAPESP – Between 1945 and 1963, the United States and the former Soviet Union detonated hundreds of atmospheric, submarine, and underground nuclear devices to test and expand their arsenals of atomic weapons. The radioactive elements generated in the explosions were ejected into the stratosphere and deposited all over the Earth’s surface. In 1963, the two superpowers signed a treaty banning all test detonations of nuclear warheads, except those conducted underground.

Although the Cold War test ban treaty averted future contamination, nothing could be done to eliminate the effects of tests performed before the ban. Those radioactive elements remain.

For more than a decade, researchers at the University of São Paulo’s Oceanographic Institute (IO-USP) in Brazil, in collaboration with colleagues in the Brazilian states of Pará, Pernambuco, and Paraná and in Uruguay, collected samples of sediment from different estuarine systems. One of the researchers at IO-USP was Rubens Cesar Lopes Figueira, who has been supported by FAPESP from the beginning of this research. He has worked on four projects, which started in 2007, 2009, 2011, and 2014.

Samples were collected in Caeté Bay (Pará), the mouth of the Capibaribe River (Pernambuco), the Caravelas Estuary (Bahia), the Santos-São Vicente and Cananeia-Iguape Estuarine Systems (São Paulo), Paranaguá Bay (Paraná), and the Rio de la Plata Estuary (Uruguay).

On analyzing the samples, the researchers detected a common component – radionuclides of the chemical element cesium in the form of the radioactive isotope cesium-137. A radionuclide is an unstable form of a chemical element that radioactively decays, resulting in emission of nuclear radiation.

Cesium-137, which is used in radiotherapy, was involved in Brazil’s worst radioactive contamination accident, in 1987 in Goiânia. However, according to an article written by the researchers and published in the journal Anthropocene, until the 1960s the only source of this artificial radionuclide in the South Atlantic was fallout from US and Soviet nuclear tests.

The presence of radionuclides in environmental matrices, the researchers said, is an important tool for oceanographic research because these chemical elements record processes at spatial and temporal scales according to their levels and distribution in sediments.

“In the case of cesium-137 from estuaries, more than 30 sediment cores were collected with depth profiles of between 1 m and 2 m,” Figueira said.

The cores were divided into slices of approximately 2 cm in thickness corresponding to the time at which each layer of sediment was deposited, with the most recent at the top. In this way, the researchers were able to establish a time scale and to estimate the proportions of cesium-137 deposited along the South American coast after being generated in US and Soviet nuclear detonations, hurled into the stratosphere, and carried southward by air currents.

The researchers were also able to identify precisely when the cesium-137 in the sedimentary samples was deposited in the Southern Hemisphere. The amounts are first perceptible in the layer corresponding to 1954, which is the year H-bomb testing began. Hydrogen bombs are thousands of times more powerful than the atomic bombs that destroyed Hiroshima and Nagasaki.

“The proportions of cesium-137 increased year by year between 1954 and 1963, when they peaked,” Figueira said. “Then they fell sharply owing to the test ban.”

New geological era

According to another author of the study, Paulo Alves de Lima Ferreira, a PhD student who works at IO-USP’s Marine Inorganic Chemistry Laboratory, although cesium from the nuclear tests was detected in all samples, the amounts were far smaller than those found in cores from the Northern Hemisphere, where the tests were actually performed.

“The discovery that cesium-137 can be used as an environmental marker in this way is an example of the work we do,” Ferreira said. “Our work consists of finding all sorts of chemical markers to study the effects of the industrial age over the past 250 years.”

The importance of this detection lies mainly in the possibility of using cesium-137 as a geological marker in the South Atlantic. Indeed, its application to the standard chronostratigraphic model could validate the transition from the Holocene to the Anthropocene, a term coined in 2000 by Dutch chemist and Nobel Laureate Paul Cruitzen to denote the current geological epoch, which is viewed as the period during which human activity has been the dominant influence on climate and the environment.

Since then, this idea has been widely discussed in the scientific community. The International Commission on Stratigraphy (ICS), which aims to standardize geochronology worldwide, has sought markers that define the new geological epoch by occurring globally. Such markers will be incorporated into deposits in the future geological record and will signal the time at which the Anthropocene began.

This is a necessary condition for the ICS to proclaim the end of the Holocene, which began 11,700 years ago at the close of the Paleolithic Ice Age, and officially unveil the Anthropocene.

The article “Using a cesium-137 (¹³⁷Cs) sedimentary fallout record in the South Atlantic Ocean as a supporting tool for defining the Anthropocene” (doi: http://dx.doi.org/doi:10.1016/j.ancene.2016.06.002) by Paulo Alves de Lima Ferreira, Rubens Cesar Lopes Figueira et al. can be retrieved from dx.doi.org/doi:10.1016/j.ancene.2016.06.002.