It's going to be biological mercury - from bioaccumulation in top predator tissue which ends up frozen and inert in the permafrost when the top predators die.
The thing that makes this really nasty is that biological mercury is most commonly methylmercury - which makes free mercury look like delicious gravy. It's very readily absorbed by tissues, and doesn't leave.
God help us if it's acquired a second methyl group through tens of thousands of years of incredibly slow decomposition - dimethylmercury is pretty much the most toxic thing we know of.
I think inferring bioaccumulation is the biggest source is a bit of a stretch. Naturally occurring processes can result in organomercury compounds in soil as well.[1]
The thing that makes this really nasty is that biological mercury is most commonly methylmercury - which makes free mercury look like delicious gravy. It's very readily absorbed by tissues, and doesn't leave.
God help us if it's acquired a second methyl group through tens of thousands of years of incredibly slow decomposition - dimethylmercury is pretty much the most toxic thing we know of.