This article says that the arctic permafrost contains insanely high levels of mercury. On the order of two-thirds the total free mercury on the earth’s surface.
This could be bad. Currently, all that mercury is locked up in the ice. If the permafrost thaws too much, a lot of that will enter the global mercury stream. It is not clear how much of an increase our global ecosystems can tolerate.
WaPo says that a lot of it is atmospheric uptake by plants during the brief growing season. Since it is in the soil, I am guessing it is cast off by metabolic processes, so it is probably more elemental than in compounds. The article suggests that the source is pollution, i.e., caused by humans. ISTR burning coal puts a lot of mercury into the air.