How did mercury get down there?

Mercury has been found in the deepest part of the Earth’s oceans, the 11,000-metre-deep Mariana Trench in the northwest Pacific. But how did it get there?

In recent months, two groups of scientists have been surprised to find it but put forward two different explanations.

In July, a team led by Ruoyu Sun from China’s Tianjin University proposed, in a paper in the journal Nature Communications, that it essentially hitches a ride on microscopic particles of sinking organic matter – including faecal material and dead plankton – that constantly rain down from the upper oceans.

However, in a paper just published in PNAS, a team led by Joel Blum from the University of Michigan, US, suggests that a more likely explanation is that sinking carrion from fish that feed in the upper ocean deliver most of the mercury to the trenches.

And most of that mercury began its long journey, Blum and colleagues say, as atmospheric emissions from coal-fired power plants, mining operations, cement factories, incinerators and other human activities.

They draw their conclusions after analysing the isotopic composition of mercury in fish and crustaceans collected at the bottom of the Mariana Trench and the Kermadec Trench in the southwest Pacific.  

“It was widely thought that anthropogenic mercury was mainly restricted to the upper 1000 metres of the oceans, but we found that while some of the mercury in these deep-sea trenches has a natural origin, it is likely that most of it comes from human activity,” says Blum.

Why does it matter whether the mercury was carried by sinking fish carcasses or the steady rain of tiny bits of detritus? Because, Blum says, scientists and policymakers want to know how changing global mercury emissions will affect the levels found in seafood.

To determine how seafood is likely to be impacted, researchers rely on global models, and refining those models requires the clearest possible understanding of how mercury cycles within the oceans and between the oceans and the atmosphere.

“Yes, we eat fish caught in shallower waters, not from deep-sea trenches,” Blum says. “However, we need to understand the cycling of mercury through the entire ocean to be able to model future changes in the near-surface ocean.”

Anthropogenic mercury enters the oceans via rainfall, dry deposition of windblown dust, and runoff from rivers and estuaries. And it is not alone in the trenches.

“Deep-sea trenches have been viewed as pristine ecosystems unsullied by human activities, but recent studies have found traces of anthropogenic lead, carbon-14 from nuclear weapons testing, and persistent organic pollutants such as PCBs in organisms living in even the deepest part of the ocean…,” says co-author Jeffrey Drazen, an oceanographer with the University of Hawaii.

Both teams presented their findings to the online Goldschmidt geochemistry conference in June, but at that stage neither paper had been peer-reviewed.