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.
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.