On 7 January 1841, the U.K. iron steamer Nemesis exploded a Chinese ship with a rocket in a battle during the First Opium War—a conflict between China and the United Kingdom over trade. That shot, along with other explosions and gunfire, would have spewed the toxic metal mercury into the air. Now, a new study suggests that corals in the South China Sea may have taken up the metal, keeping a record of this and future wars locked in their skeletons. The finding provides a look at how humans have been polluting the ocean throughout history, and may help us understand how the metal travels in our atmosphere today.

The skeletons of hard corals are made of aragonite, a calcium carbonate mineral. As the organism grows, it pulls additional calcium out of the water to build its skeleton. Corals’ annual growth bands, like the rings of a tree, can be used to track the history of the organism. But because certain metal pollutants—like lead and mercury—can take the place of calcium in the coral’s skeletal structure, the rings can also be an archive of the metals floating in the seawater.

A team of researchers led by Ruoyu Sun, a geochemist now at Trent University, Peterborough, in Canada, wanted to see whether corals could be a good record of mercury pollution, too. So they extracted a 200-year-old core from a Porites lutea coral in the South China Sea, expecting the record to match those gathered from remote ice and peat samples: a gradual increase over time due to mining, coal combustion, and later industrial production.

But the record they found was very different. In the oldest part of the core, dating to between 1800 and 1830, levels of mercury were low and relatively constant. But over the coral’s next 170 years of life, the amount of mercury incorporated into its skeleton spiked repeatedly, sometimes reaching concentrations four to 12 times higher than the baseline. Those spikes line up precisely with a number of violent conflicts that raged in nearby China, including the First Opium War (1839–1842), the Second Opium War (1856–1860), and World War II, according to a study published this month in Environmental Science & Technology . The baseline mercury level also rose throughout the 1900s—but that increase is dwarfed by the wartime spikes.

“We never expected [mercury] would come from the wars,” Sun says—but there’s a good reason why it would. The metal is used in the production of weapons and explosives, and their detonation could also release mercury into the air. When the elemental form of mercury in the atmosphere encounters reactive chemicals like bromine ejected from the ocean via sea spray, it forms what researchers call “reactive gaseous mercury” molecules. These then settle into the ocean, after which corals can take up the dissolved mercury into their skeletons.

For Hannah Horowitz, an atmospheric chemist at Harvard University who wasn’t involved in the study, this local effect of wars fits into the new picture that’s emerging of how mercury behaves in the atmosphere. Once considered a more long-lived chemical that travels vast distances as it drifts through the atmosphere for a year or more, the impacts of mercury are increasingly being thought of as more local. “We’re revising that down to the order of months,” Horowitz says. That would explain why the signal of mining and manufacturing in the West might not make its way to the South China Sea.

Others are more skeptical of the results. Seeing a signal of wars in coral, according to Carl Lamborg, a geochemist at the University of California, Santa Cruz, is “maybe not impossible, but would require enormous inputs of mercury over time.” One large source of uncertainty is that corals may not take up mercury at a constant rate, meaning that a record of mercury from coral skeletons wouldn’t faithfully track the amount of mercury in seawater. Research on how mercury is taken up by corals is still in the early stages, Lamborg stresses. But, he says, “their interpretation is as reasonable as any other.”

And, certainly, this isn’t the last step for Sun and his colleagues. To firm up the connection between coral mercury and wars, Sun next plans to look for specific mercury isotopes in the coral archive. Different sources of mercury in the atmosphere—including volcanoes, coal combustion, and explosives—contain different amounts of the element’s isotopes, Sun says. Liquid mercury derived from the mineral cinnabar—which, among other things, was used as an additive in many explosives in the 19th century—might have its own such fingerprint, he adds. “If that’s the same as the one in the corals then we’ll be sure.”