The sinking carcasses of fish from near-surface waters deliver toxic mercury pollution to the most remote and inaccessible parts of the world’s oceans, including the deepest spot of all: the 36,000-foot-deep Mariana Trench in the northwest Pacific Ocean. And most of that mercury began its long journey to the deep-sea trenches as atmospheric emissions from coal-fired power plants, mining operations, cement factories, incinerators and other human activities. Those are two of the main conclusions of a study published in the Proceedings of the National Academy of Sciences, co-authored by University of Hawaiʻi at Mānoa researchers Jeffrey Drazen and Brian Popp.
Mercury is a naturally occurring element, but more than 2,000 metric tons of it are emitted into the atmosphere each year from human activities. This anthropogenic mercury enters the oceans via rainfall, dry deposition of windblown dust, and runoff from rivers and estuaries.
Once there, marine microorganisms convert some of it to methylmercury, a highly toxic organic form that can accumulate in fish to levels that are harmful to humans and wildlife.
The research team, led by Joel Blum of the University of Michigan, collected snailfish and crustaceans called amphipods from depths of up to 33,630 feet in the Mariana Trench near Guam and from depths of up to 32,800 feet in the Kermadec Trench near New Zealand.
“These samples were challenging to acquire, given the trenches’ great depths and high pressures,” said Drazen, an oceanographer in UH Mānoa’s School of Ocean and Earth Science and Technology (SOEST). “The trenches are some of the least studied ecosystems on Earth, and the Mariana snailfish was only just discovered in 2014.”
Mercury fingerprinting techniques
Mercury has seven different non-radioactive isotopes which provide a unique chemical signature, or fingerprint, that can be used as a diagnostic tool to compare environmental samples from various locations. The researchers used these fingerprinting techniques to determine that the mercury from deep-sea-trench amphipods and snailfish had a chemical signature that matched the mercury from a wide range of fish species in the central Pacific that feed at depths of around 1,600 feet.
They concluded that most of the mercury in the trench organisms was transported there in the carcasses of fish that feed in near-surface waters, where most of the mercury comes from anthropogenic sources.
While mercury emissions have declined in recent years in North America and Europe, China and India continue to expand their use of coal, and global-scale mercury emissions are rising.
“Our new study provides critical information to understand how changing global mercury emissions will affect the levels found in seafood,” said Popp, Earth scientist in SOEST.
–By Marcie Grabowski