Dead predators (seals, cats, etc.), which fed on other predators - probably fish - which accumulate mercury through eating other aquatic organisms. This is why tuna has relatively high levels of mercury, and humans moreso - once it's in you, it tends not to leave. The higher up the food chain you are, the more mercury you consume and retain.
The reason there's such an accumulation in the permafrost is that predators which die up there don't decompose, they just freeze. Sure, they might get snacked on by scavengers before they freeze, but those scavengers then get eaten by predators, resulting in the vast majority of organic tissue and therefore mercury in the arctic ending up as frozen organic matter. The permafrost comprises incredibly vast quantities of organic material, animal and plant, and not much else. Elsewhere on earth, decomposition happens, flies eat the rotting meat, the mercury gets scattered to the four winds, and goes back into the food chain.
It's also the same reason for the vast quantities of methane that are beginning to be released - huge quantities of organic matter that's been in the deep freeze, decomposing.
The arctic is going to be the biggest charnel pit the earth has ever seen.
Just speculating: maybe atmospheric dust (pollen, etc.), and/or migrating animals contain it and it gets deposited wherever the dust comes down, or the animal dies.
But in permafrost, it is quickly fixed by humidity and ice, and it isn't returned to the oceans by a constant flow of water?
My guess would be that first it was there everywhere else too, just that in warmer climate it would not remain frozen, and got released from the soil. And then got transported by various ways:
"If the mercury is transported across waterways, it could be taken up by microorganisms and transformed into methylmercury, he said."
"Mercury released into the atmosphere can travel large distances and could affect communities and ecosystems thousands of miles away from the release site, he said."
It wouldn't be trapped in ice if it had been there for that long... I think he meant: how did it get trapped in permafrost? The only hint the article gave was this:
> Mercury accumulates in aquatic and terrestrial food chains [...]
and:
> [...] trapped in permafrost [...] since the last Ice Age.
I read before (this study) that mercury accumulates at the poles more than anywhere else. Context was affect on wildlife there, some birds turning gay [0][1] (that effect is location independent, but I read it first in a northern location context). A possible reason for it to "go north" is mentioned here: [2].
> Environmental scientists at Harvard have discovered that the Arctic accumulation of mercury ... is caused by both atmospheric forces and the flow of circumpolar rivers that carry the element north into the Arctic Ocean.
[1] also is a pretty good article on the effects of mercury on wildlife.
There also is mentioning of possible pathways to the north here: [3] - unlike [2] they claim it's mostly through the atmosphere though.
> From our observation site north of Alaska’s Brooks Range, we determined that gaseous elemental mercury in the atmosphere is the dominant source of Arctic mercury. We calculated that it accounted for 70 percent of the mercury that finds its way into tundra soil.
The reason there's such an accumulation in the permafrost is that predators which die up there don't decompose, they just freeze. Sure, they might get snacked on by scavengers before they freeze, but those scavengers then get eaten by predators, resulting in the vast majority of organic tissue and therefore mercury in the arctic ending up as frozen organic matter. The permafrost comprises incredibly vast quantities of organic material, animal and plant, and not much else. Elsewhere on earth, decomposition happens, flies eat the rotting meat, the mercury gets scattered to the four winds, and goes back into the food chain.
It's also the same reason for the vast quantities of methane that are beginning to be released - huge quantities of organic matter that's been in the deep freeze, decomposing.
The arctic is going to be the biggest charnel pit the earth has ever seen.