Atomic grains of sand: How the history of humans is written into the fabric of the Earth

Moments in human history are etched into the Earth. Researchers are now gathering evidence of our impact on the planet through the marks we've left on nature.

The microbes living in this French harbor have never recovered from World War Two. Between 2012 and 2017, Raffaele Siano collected sediment cores from the seabed at Brest harbor, curious about what he would find. When he and his colleagues at the French Institute for Ocean Science (Ifremer) analyzed the DNA fragments in those cores, they discovered something remarkable.

The oldest and deepest layers of sediment, dating back to before 1941, contained traces of plankton called dinoflagellates that were very different from the genetic traces found in the shallower, more recent layers. "There was a group, an order of these dinoflagellates, that was very abundant before World War Two, and after the war, it almost disappeared," says Siano. He and his colleagues published a study detailing their findings in 2021.

Siano notes that Brest harbour was bombed during the war. Then, in 1947, a Norwegian cargo ship exploded in the Bay of Brest. The disaster killed 22 people and released ammonium nitrate, a toxic chemical used to make fertilizer and explosives, into the sea. Even younger sediment from the 1980s and 1990s showed further changes in the plankton community in the harbour. "We linked this to another type of pollution from intensive agricultural activities," says Siano.

Nature has a way of remembering. Traces of certain human activities, especially those that cause a lot of pollution, can appear in tree rings, coastal sediments, and ecosystems. These traces might be signs of the Anthropocene, a proposed geological epoch where humanity is believed to have permanently and significantly changed Earth. Human history is embedded in the very fabric of our planet and the life that shares it with us. Siano and his colleagues are mainly ecologists, but they also collaborate with historians. "The land changed due to human impact and historical events," Siano says. When the team analyzed the sediment cores from Brest, they found a gradual increase in heavy metal pollution over time. Younger layers of sediment contained higher amounts of mercury, copper, lead, and zinc, for example.

The report notes that similar levels of some metals, especially lead and chromium, were found in Pearl Harbor, a major U.S. naval base in Hawaii that was heavily bombed by Japanese warplanes in 1941. However, Siano mentions that he can't be certain whether these metals came directly from the bombs themselves. Regardless, there is evidence in both Brest and Pearl Harbor of a catastrophic and polluting moment in human history.

Other researchers have also searched the planet for geological records of human-caused pollution. In China, soil sediments show a sharp increase in metal contamination since 1950, which matches the rise in air pollution there during the second half of the 20th century. A separate study looks at how the emergence of industries like shipbuilding may be linked to higher levels of heavy metal deposits in tree rings from certain parts of China.

Even Roman metallurgy, from many centuries ago, has left its mark. A 2022 study found a noticeable rise in lead contamination in ice, sediment, and peat cores from Europe, linked to the development of Roman industry. However, the authors note that it is sometimes difficult to pinpoint which specific events caused spikes in lead contamination.