Corals keep a record of historical events ranging from the Opium Wars to World War II, a new Canadian study has revealed. This may be because they can absorb the toxic metals leached into their surroundings from such naval battles.

During the First Opium War, in a battle that ensued between China and the United Kingdom because of trade, the UK warship “Nemesis” exploded a Chinese ship through a rocket. The sinking and accompanying gunfire spewed mercury into the environment.

A study from Trent University researchers in Peterborough in Canada found that South China Sea corals took in these metals, recording the event as well as future conflicts in their own skeletons.

Hard corals’ skeletons are composed of aragonite, a mineral consisting of calcium carbonate. Corals absorb excess calcium out of the ocean water over time to build such skeleton. The yearly growth bands of corals can help monitor the organisms’ history, just like what tree rings show, as well as the metal pollutants absorbed and when they first took up residence in the water.

The team used a 200-year-old Porites lutea coral sample and discovered that it was very different from expected results, or only gradual increases of metals over time due to greater industrial production and other phenomena.

They saw that the mercury levels in the core’s oldest sections – dating from 1800 to 1830 – remained quite low and constant. However, the amount of the metal in the skeleton spiked repeatedly from 1830 to the wake of the 20th century, sometimes peaking 12 times higher than starting point.

The spikes in mercury amounts lined up with a couple of violent historical events in neighboring China, particularly the First Opium War, the Second Opium War and World War II. These wartime peaks dwarfed the mercury increases consistently in the 1900s, a period of intense industrialization.

Metal used to produce the explosives and other weapons released mercury into the air, which then formed molecules called “reactive gaseous mercury” because of their interaction with mercury in the atmosphere. The end product fell into the ocean and was then absorbed by corals.

There were a number of doubts on the study conclusion, though – much more staggering amounts of mercury, for instance, would likely be detected if wars are primarily responsible.

The team will next seek more traces of mercury in corals, as there are varying levels of its isotopes found in volcanoes, coal burning and other sources.

“‘If that’s the same as the one in the corals then we’ll be sure,” said geochemist Ruoyu Sun, the study's lead author.

The findings were published in the journal Environmental Science and Technology.