Mercury pollution has reached ocean’s deepest valley

Man-made mercury pollution has reached the bottom of the deepest part of the ocean, scientists have discovered.

Two groups of scientists from China and the US found toxic mercury in fish and crustaceans living more than 11,000 metres below the surface of the ocean in the Mariana Trench.

“This is a surprise” said Dr Ruoyu Sun, who led a group of researchers from Tianjin University in China. “Previous research had concluded that methlymercury was mostly produced in the top few hundred meters of the ocean.

“This would have limited mercury bioaccumulation by ensuring that fish which forage deeper than this would have had limited opportunity to ingest the methylmercury. With this work, we now believe that isn’t true”.

Mercury can become concentrated in marine organisms as small amounts are ingested by some species which in turn are eaten by larger species, resulting in more harmful concentrations of the liquid metal accumulating in animals higher up the food chain.

It is generally poisonous at high levels to humans and other animals and can be especially dangerous to the developing foetus.

Dr Sun's team used a deep-sea vehicle to capture animals from between 7,000-11,000m and take sediment samples at 5,500-9,200m.

“We are able to present unequivocal mercury isotope evidence that the mercury in the trench fauna originates exclusively from methylmercury from the upper ocean,” Dr Suns said. ”We can tell this because of the distinctive isotopic fingerprint which stamps it as coming from the upper ocean.”

A second research team, led by Dr Joel Blum from the University of Michigan, sampled fish and crustaceans from two of the deepest Pacific Trenches – the Kermadec trench near New Zealand and the Mariana Trench of the Philippines.

They discovered that mercury found in trench species in both locations was largely derived from the atmosphere and had entered the ocean through rainfall.

Dr Blum said: ”We know that this mercury is deposited from the atmosphere to the surface ocean and is then transported to the deep ocean in the sinking carcasses of fish and marine mammals as well as in small particles.

“We identified this by measuring the mercury isotopic composition, which showed that the ocean floor mercury matched that from fish found at around 400-600m depth in the Central Pacific. Some of this mercury is naturally-produced, but it is likely that much of it comes from human activity.

“This work shows that human-released mercury has reached and entered foodwebs in even the most remote marine ecosystems on earth. This better understanding of the origin of mercury in the deepest reaches of the ocean will aid in modeling the fate of mercury in the atmosphere and oceans”.

Professor Ken Rubin, an independent researcher at the Department of Earth Sciences at the University of Hawaii, said: ”We know that mercury is introduced into the environment from a variety of natural sources such as volcanic eruptions and forest fires.

“However, human activities, such as coal and petroleum burning, mining, and manufacturing are mainly responsible for mercury deposition to marine environments. We are now learning from these two studies that the effects of this deposition have spread throughout the ocean into the deep sea and the animals that live there, which is yet another indicator of the profound impact of modern human activities on the planet.”