Trois fois plus de mercure à la surface des océans depuis le début de l’ère industrielle
▻http://www.lemonde.fr/planete/article/2014/08/08/trois-fois-plus-de-mercure-a-la-surface-des-oceans-depuis-le-debut-de-l-ere-
En janvier 2013, à Genève, 130 pays ont signé une convention visant à réduire la pollution au mercure, métal toxique qui s’accumule dans la chaîne alimentaire et qui affecte le système nerveux et le développement. L’objectif est d’en limiter drastiquement les émissions.
Mais l’environnement est déjà durablement contaminé, à commencer par le milieu marin. Une étude internationale (Etats-Unis, France, Pays-Bas), publiée jeudi 7 août dans la revue Nature, montre que la concentration de mercure dans les eaux de surface de quasiment tous les océans a triplé du fait des rejets d’origine humaine.
Pour arriver à cette conclusion, Carl Lamborg de l’Institut océanographique Woods Hole du Massachusetts et ses collègues ont utilisé pour la première fois des mesures directes, dans le but de quantifier le « mercure anthropique », par opposition au mercure naturellement émis par l’activité volcanique terrestre et sous-marine.
Le reste de l’article derrière #paywall.
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Deux recensions (en anglais) de cette étude…
Mercury levels in surface ocean have tripled | Science/AAAS | News
▻http://news.sciencemag.org/environment/2014/08/mercury-levels-surface-ocean-have-tripled
The calculations suggest that the ocean contains about 60,000 to 80,000 tons of mercury from pollution, with almost two-thirds residing in water shallower than a thousand meters, the team reports online today in Nature. Mercury concentration in waters shallower than 100 meters has tripled compared with preindustrial times, they found, whereas mercury levels in intermediate waters have increased by 1.5 times. Higher mercury concentrations in shallower waters could increase the amount of toxin accumulating in food fish, exposing humans to greater risk of mercury poisoning, Lamborg says. Countries ringing the North Atlantic Ocean, where the mercury concentration is among the highest recorded in the study, may be particularly vulnerable.
(…)
The findings contradict previous thought that the Gold Rush of the 1850s was a major source of mercury to the oceans. Lamborg says his results suggest mercury from past gold mining in the United States might have been deposited in nearby soils instead of in the ocean.
La revue Science donne la parole à l’auteur, Carl Lamborg, et lui fait dire qu’en fait on ne sait pas très bien quel est l’impact réel, ainsi qu’à un spécialiste n’ayant pas participé à l’étude. Qui a plutôt l’air de relativiser : il faut poursuivre la recherche…
Even with these data, researchers still can’t ascertain the impact of rising levels of mercury on marine fish, and on the people who consume them, Lamborg notes. That’s because scientists still don’t know precisely how inorganic mercury transforms into toxic methyl mercury.
(…)
[David] Streets points out that more research is needed to paint a better picture of the mercury cycle. “There are still big gaps in our understanding of how mercury moves between air, soil, and water, and between different parts of the world,” he says. “This paper is a good estimate of mercury in oceanic water, but it’s only part of the bigger picture.”
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L’article grand public de Nature, qui publie l’étude originale, est nettement plus alarmiste, fournit des chiffres qui font peur et fait dire au même Lamborg que le mécanisme tampon qui stocke une grande partie du mercure dans des zones où il est moins directement nocif pour le vivant est peut-être en train de disparaitre.
Humans have tripled mercury levels in upper ocean : Nature News & Comment
▻http://www.nature.com/news/humans-have-tripled-mercury-levels-in-upper-ocean-1.15680
But study co-author Carl Lamborg, a marine geochemist at Woods Hole Oceanographic Institution in Massachusetts, says that the deep water’s ability to sequester mercury may soon be exhausted. Humans are on track to emit as much mercury in the next 50 years as they did in the last 150 years, he notes.
“You’re starting to overwhelm the ability of deep water formation to hide some of that mercury from us, with the net result that more and more of our emissions will be found in progressively shallower water,” Lamborg adds. That increases the odds that mercury levels in key food species will rise, increasing humans’ exposure.
Between 5–10% of US women of childbearing age already have blood mercury levels that that increase the risk of neurodevelopmental problems in their children, and an estimated 1.5 million–2 million children are born in the European Union each year with mercury exposure levels associated with IQ deficits. Wildlife and marine life is not spared either. Studies have found that mercury levels compromise the reproductive health and fertility of some fish and birds.
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L’étude, du moins son résumé.
A global ocean inventory of anthropogenic mercury based on water column measurements : Nature : Nature Publishing Group
▻http://www.nature.com/nature/journal/v512/n7512/full/nature13563.html
Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion. Several recent models have estimated that these emissions have increased the oceanic mercury inventory by 36–1,313 million moles since the 1500s. Such predictions have remained largely untested owing to a lack of appropriate historical data and natural archives. Here we report oceanographic measurements of total dissolved mercury and related parameters from several recent expeditions to the Atlantic, Pacific, Southern and Arctic oceans. We find that deep North Atlantic waters and most intermediate waters are anomalously enriched in mercury relative to the deep waters of the South Atlantic, Southern and Pacific oceans, probably as a result of the incorporation of anthropogenic mercury. We estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. Our findings suggest that anthropogenic perturbations to the global mercury cycle have led to an approximately 150 per cent increase in the amount of mercury in thermocline waters and have tripled the mercury content of surface waters compared to pre-anthropogenic conditions. This information may aid our understanding of the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulated in marine food webs.
avec cette illustration
dont je comprends que les plus fortes concentrations se trouvent en profondeur dans le Pacifique (le « confinement » évoqué plus haut).
Et le gag que j’imagine pas tout à fait fortuit du graphique 1a où les points mesurés aux Bermudes sont représentés par des… triangles !
