Arctic rivers are turning rusty orange as warming triggers freeze-thaw cycles that dissolve iron minerals in permafrost, releasing metals into waterways and threatening fragile ecosystems.
Ice isn’t just frozen water – it’s a hidden chemical engine. A new study reveals that as the Arctic warms and freeze-thaw cycles become more frequent, rivers are taking on a rusty orange tint due to iron being released from the ground.
Ice That Acts Like a Reactor
Scientists from Umeå University have discovered that ice can actually speed up certain chemical reactions, making it more effective than liquid water at dissolving iron minerals. Their findings, published in PNAS, challenge the long-held belief that cold conditions slow chemical processes.
“Ice isn’t just a passive frozen block,” explains Professor Jean-François Boily, co-author of the study. “When water freezes, it forms tiny liquid pockets between ice crystals. These micro-zones become highly acidic, turning into powerful chemical reactors that can dissolve iron minerals—even at temperatures as low as minus 30°C.”
Why Rivers Are Turning Orange
As permafrost thaws across the Arctic, minerals locked in frozen soils are exposed. Combined with the chemical power of freeze-thaw cycles, iron is released into streams and rivers. The result: many waterways, like Alaska’s Kutuk River, are now stained a rusty orange. Along with iron, other metals such as zinc and copper may also leach into the water, raising concerns for aquatic ecosystems.
Experiments That Uncover the Process
The research team tested a common iron oxide mineral called goethite together with naturally occurring organic acids. Advanced microscopy revealed that repeated freezing and thawing made the minerals break down faster. Freshwater and brackish water enhanced this effect, while saltier water tended to slow it down.
Doctoral researcher Angelo Pio Sebaaly, first author of the study, notes: “With climate change, freeze-thaw cycles are becoming more common. Each cycle releases more iron into soils and rivers, with potentially wide-ranging effects on water quality and ecosystems.”
Beyond the Arctic
The findings also apply to other cold, acidic environments such as mining areas, acid sulfate soils along coastlines, or even frozen dust in the atmosphere. The Umeå team is now investigating whether the same reactions occur in all forms of iron-bearing ice.
A Warning for the Future
The study underscores that ice is not just a storage medium but an active force in shaping ecosystems. As the planet warms, the hidden chemistry of freezing and thawing may accelerate environmental changes—turning clear rivers into rust-colored streams and altering the delicate balance of polar and mountain ecosystems.