As glaciers melt, they’re spilling sulfate-rich minerals into mountain streams and wetlands, sparking a hidden danger: toxic methylmercury. A new study from CU Boulder, published on May 18, 2025, reveals how these wetlands, especially in subalpine peatlands, are becoming hotspots for this neurotoxin, threatening fish, birds, and even people who eat contaminated wildlife.

How It Happens

When glaciers thaw, they release sulfate trapped in ice, which flows into watersheds. In wetlands, sulfate fuels bacteria that turn harmless mercury into methylmercury, a potent toxin that builds up in food chains. Hannah Miller, a PhD student and lead author, found that subalpine peatlands—wet, plant-packed areas below the treeline—are prime spots for this process.

“We know little about methylmercury in high-elevation wetlands,” Miller said, “but climate change is making it a bigger issue.”

The study tested soil from Colorado’s Boulder watershed, focusing on peatlands, alpine meadows, and river zones. They found a “Goldilocks zone” for sulfate—0.6 to 1.4 milligrams per gram of soil—where methylmercury production peaks. Too little sulfate, and nothing happens; too much, and it slows down. In lab tests, peatlands with moderate sulfate churned out the most methylmercury, driven by bacteria thriving in soggy, carbon-rich soils.

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Why Peatlands Are Risky

Subalpine peatlands are perfect for trouble. Their dense plants provide carbon for bacteria, and waterlogged soils create low-oxygen zones where sulfate-reducing bacteria go wild, spitting out methylmercury. Above the treeline, sparse vegetation limits this process, but below, it’s a different story. Boulder’s watershed has seen a 200% sulfate spike over 30 years, matching trends in the Andes (up to 2000% increases), Alps, and Himalayas as glaciers shrink.

Methylmercury doesn’t stay put. It seeps into water, where fish soak it up. Small fish get eaten by bigger ones, then birds like eagles or herons gobble those, passing the toxin up the chain. Humans eating contaminated fish face risks like neurological damage, especially in kids or pregnant women.

“Mountain ecosystems seem low-risk, but peatlands are a real concern,” Miller noted.

Other Factors at Play

Sulfate’s the star, but iron, carbon, and water matter too. Iron can tweak methylmercury production, though the study focused on sulfate’s lead role. Wetter conditions could expand peatlands, boosting toxin output, while droughts might shrink them. Wildfires, more common with climate change, could also stir things up by altering soil chemistry.

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Why It Matters

Glaciers are melting fast—global ice loss hit 1.3 trillion tonnes per year in 2020, per NASA. This floods wetlands with sulfate, ramping up methylmercury risks worldwide, from Peru’s Andes to Europe’s Alps. In Colorado, where 80% of water comes from mountains, contaminated wetlands could hit local wildlife and communities. With 83% of people worried about clean water (per a 2024 Gallup poll), this issue’s hitting home.

What’s Next?

Stopping methylmercury is tough, but Miller’s team suggests monitoring sulfate levels, restoring wetlands, and curbing upstream mining to cut runoff.

Co-author Eve-Lyn Hinckley, a CIRES Fellow, said, “With warming, wildfires, and changing elements, this research is urgent.”

The team’s digging deeper into how carbon and iron interact with sulfate, hoping to predict where hotspots will pop up next. For now, as glaciers keep melting, keeping an eye on these wetlands is key to protecting ecosystems and human health.

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