By Richard Black
BBC News environment correspondent
Sea birds are transporting industrial and agricultural pollutants to the Arctic, according to new research.
It is not clear how pollutants end up inside sea birds
Canadian scientists found that birds carry pollutants like DDT and mercury, and deposit them in sites where other animals feed.
They say this process may be contributing to the high levels of industrial chemicals found in some Arctic peoples.
The research is published in the journal Science.
It has been a rallying cry of environmental groups campaigning against industry's use of toxic chemicals: how can it be right to use them when they end up in the planet's most pristine regions?
According to a recent study, the levels of one important class of industrial chemicals, polychlorinated biphenyls (PCBs), are 30 times higher in Canadian Inuits living north of the Arctic circle than in residents of temperate Quebec.
PCBs are one example of a Persistent Organic Pollutant or POP - chemicals which are broken down very slowly by natural degradation, and which accumulate in living organisms, including humans; other examples are pesticides, including DDT, and multi-purpose substances such as hexachlorobenzene (HCB).
How these substances end up in the Arctic and inside Arctic organisms has not been entirely clear - the presumption has been that they are transported by several routes, as particles in the air, in the sea and perhaps in the bodies of migratory species.
Now a group led by Jules Blais, from the University of Ottawa, has confirmed the involvement of one migratory bird species, the northern fulmar (Fulmarus glacialis).
"We were studying a population of northern fulmars on Devon Island in the Canadian Arctic, specifically at Cape Vera which is one of the most northern and isolated fulmar colonies in North America," he told the BBC News website.
"There's a large breeding colony, about 10,000 pairs at this location; and what we've been doing is looking at the distribution of some of these chemicals like PCBs and mercury and DDT in areas near the breeding colony."
Sites very close to breeding colonies are enriched in nitrogen, which comes from guano - bird excrement.
Blais found that sites high in nitrogen also contained elevated levels of DDT. Locations frequented by the fulmars showed levels of HCB 10 times higher, mercury 25 times higher and DDT 60 times higher than in the surrounding area - strongly suggesting that the chemicals were also deposited in guano.
These fulmars feed from the North Atlantic during the breeding season, scooping plankton, squid, fish and carrion from the sea; it appears that these foodstuffs are also the birds' source of DDT, mercury and HCB.
If the pollutants then stayed at the breeding sites, there might not be a significant problem; but Dr Blais believes that is not the case.
"These nesting sites are oases of biological life in a region which is largely barren," he said.
Persistent pollutants can build up in wildlife and humans
"We're seeing higher rates of insect emergence in these places, for example, which supports a local population of a bird called the snow bunting; and there are carnivores in the area such as foxes which may also be there because of the life supported by nutrients imported by fulmars.
"And people in the north eat seabird eggs; so there might be a link between what people are exposed to and this biological transport of contaminants from the ocean, but that is something we are still exploring."
The production and use of some POPs is now prohibited by a United Nations treaty, the Stockholm Convention; and for Gwynne Lyons, a toxics officer with the environmental group WWF, this study proves that more substances must be banned.
"This is interesting research which underlines the need for regulatory authorities worldwide to prevent the release of persistent pollutants that can build up in wildlife and humans," she told the BBC News website, "but the Stockholm Convention only covers 12 substances, and it's going to be incredibly slow to get agreement on further chemicals."
The European Union wants another nine chemicals added. Norway, too, has proposed extending the ban; and at its meeting in May, parties to the Stockholm Convention set up a committee which will consider these proposals.