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."
"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."