With forest recovery, wetlands are becoming even more abundant. Mining discharges (smelter emissions, tailing releases, mine shaft seepage) appear superimposed upon high wetland methylation. We compare historical deposition rates with a combination of modeling and sediment core studies. For the first time, we reconstruct 140 years of historical mercury emissions from copper and iron mining, showing how mercury was broadcast broadly around regional environments up to the present. Environmental significance The manuscript addresses the enigma of low atmospheric mercury deposition and falling emissions in modern-day Upper Peninsula environments, yet elevated THg and MeHg in rivers, lakes, and fish. We argue that mercury loading from mining is historically superimposed upon strong regional wetland effects, producing a combined elevation of both THg and MeHg in the Western Upper Peninsula. DOC can mobilize metals and elevate MeHg concentrations. High MeHg concentrations are geographically correlated with low pH and dissolved organic carbon (DOC), a consequence of biogeochemical cycling in wetlands, characteristic of the Upper Peninsula. Sediment profiles from the Keweenaw Waterway show that THg accumulation increased 50- to 400-fold above modern-day atmospheric deposition levels during active mining and smelting operations, with lingering MeHg effects. Moreover, lake sediments in the vicinity of mining operations have higher THg concentrations. We show that mercury emissions from copper and iron operations were substantial (60вЂ“650 kg per year) and dispersed over relatively large areas. Copper smelters and iron furnace-taconite operations broadcast mercury almost continuously for 140 years, whereas mills discharged tailings and old mine shafts leaked contaminated water. Many regional ores (silver, copper, zinc, massive sulfides) contain mercury in part per million concentrations. A contributing explanation for elevated THg loading is that a century ago the Upper Peninsula was a major industrial region, centered on mining.

Moreover, Western Upper Peninsula (WUP) fish possess higher methyl mercury (MeHg) levels than Northern Lower Peninsula (NLP) fish. However, state (MDEQ) surveys of rivers and lakes revealed elevated total mercury (THg) in Upper Peninsula waters and sediment relative to downstate. A geographic enigma is that present-day atmospheric deposition of mercury in the Upper Peninsula of Michigan is low (48%) and that regional industrial emissions have declined substantially ( ca.

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