Years ago a 1000 year old shark was found frozen in ice somewhere.
The 1000 year old shark had mercury levels around current levels, sugessting that sharks (fish) accumulate mercury from naturally occurring sources.
I did some google searches, and it appears volcanic activity is a concern for some -- for putting Mercury into the atmosphere. I don't know how much of a factor it is...
It appears that volcanic eruptions can sprew quite a bit of Mercury into the atmosphere. I imagine Mount St. Helen's recently, and Krakatoa (1800's) could have put more Mercury in the atmosphere than all the coal fired power plants in the 20th Century and 21st Century (not counting smelters). Plus there are a number of active volcanoes such as Hawaii and other places that may be naturally adding to the Mercury in the atmosphere.