Toxic man-made mercury pollution has been discovered in the deepest part of the ocean, in the Marianas Trench — more than six miles below the surface.
Researchers from China and the US used submarine robots to identify mercury in the fish and crustaceans living in the deepest part of the western Pacific Ocean.
Mercury enters the atmosphere through the burning of fossil fuels, mining and manufacturing. It can then be transported into the oceans via rainfall.
The liquid metal — which was once used in thermometers before being banned — is highly toxic and can be ingested via polluted seafood.
One of the research teams, from the US, sampled fish and crustaceans from both the Marianas trench off of the Philippines and the Kermadec trench near New Zealand, which reaches around 6.2 miles below the water’s surface. Pictured, snailfish in the Kermadec trench
‘This is a surprise. Previous research had concluded methyl-mercury was mostly produced in the top few hundred metres of the ocean,’ said paper author and environmental scientist Ruoyu Sun of China’s Tianjin University.
‘This would have limited mercury bio-accumulation by ensuring fish which forage deeper than this would have had limited opportunity to ingest the methyl-mercury. With this work, we now believe that isn’t true.’
Mercury in the ocean ends up becoming methyl-mercury — its organic form — which is more dangerous as it can easily concentrate during its journey up the food chain in a process which experts refer to as ‘bio-accumulation’.
This process creates toxic food for fish, which in turn are eaten by larger fish that then grow increasingly toxic, and so on — until they end up on our dinner plates.
Mercury has been implicated in environmental disasters and is particularly hazardous to developing fetuses. At Minamata in Japan in the 1950’s, for example, it led to birth defects and severe neurological symptoms.
Now this dangerous pollutant has been discovered in the Marianas Trench and the Yap Trench — beyond New Zealand — at 6.84 and 5.3 miles deep, respectively.
‘During 2016-2017, we deployed sophisticated deep-sea lander vehicles on the seafloor of Mariana and Yap trenches, amongst the most remote and inaccessible locations on Earth,’ Dr Sun explained.
The landers, he added, ‘captured the endemic fauna at 7,000–11,000 metres [4.3–6.8 miles deep] and collected sediments at 5,500–9,200 metres [3.4–5.7 miles deep].’
‘We are able to present unequivocal mercury isotope evidence the mercury in the trench fauna originates exclusively from methyl-mercury from the upper ocean,’ Dr Sun continued.
‘We can tell this because of the distinctive isotopic fingerprint which stamps it as coming from the upper ocean.’
‘Our findings reveal very little methyl-mercury is produced in the deep oceans and imply anthropogenic [man-made] mercury release at the Earth’s surface is much more widespread across deep oceans than was previously thought.’
During 2016-2017, we deployed sophisticated deep-sea lander vehicles on the seafloor of Mariana and Yap trenches, amongst the most remote and inaccessible locations on Earth,’ Dr Sun explained. ‘We are able to present unequivocal mercury isotope evidence the mercury in the trench fauna originates exclusively from methyl-mercury from the upper ocean’
Another team, from the US, also sampled fish and crustaceans from both the Marianas trench off of the Philippines and the Kermadec trench near New Zealand, which reaches around 6.2 miles below the water’s surface.
Chemical analyses showed that mercury found in the deep-sea species the live in these trenches was largely derived from the atmosphere — and entered the ocean by means of rainfall.
‘We know 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,’ said expedition leader Joel Blum.
‘We identified this by measuring the mercury isotopic composition, which showed that the ocean floor mercury matched that from fish found at around 400-600 metres [1,312–1969 feet] depth in the Central Pacific.’
‘Some of this mercury is naturally-produced — but it’s likely much of it comes from human activity,’ the University of Michigan environmental scientist added.
Chinese researchers have detected mercury pollution in the Marianas Trench and the Yap Trench — beyond New Zealand — at 6.84 and 5.3 miles deep, respectively. Another team, from the US, also found the toxic metal fish and crustaceans from both the Marianas trench and the Kermadec trench, which reaches around 6.2 miles below the water’s surface
‘We know mercury is introduced into the environment from a variety of natural sources such as volcanic eruptions and forest fires,’ said University of Hawaiʻi at Mānoa earth scientist Ken Rubin, who was not involved in the present study.
‘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 the effects of this deposition have spread throughout the ocean into the deep sea and the animals that live there.
This he added, ‘is yet another indicator of the profound impact of modern human activities on the planet.’
The full findings of the study were presented at the 2020 Goldschmidt geochemistry conference, which is being held virtually from June 21–26.
Last weekend Kelly Walsh — son of ocean explorer Don Walsh — descended to the bottom of the Marianas Trench.
He made the dive on Saturday, 60 years after his father made history doing the same— making Kelly Walsh only the twelfth person to visit the trench floor.