Researchers from Umeå University have discovered that ice can dissolve iron minerals more effectively than liquid water -- a finding that challenges long-standing assumptions about frozen environments.
The study, published in PNAS, shows that ice at -10°C releases more iron from minerals than water at 4°C. This breakthrough may explain why many Arctic rivers are turning rusty orange as thawing permafrost releases iron into waterways.
"It may sound counterintuitive, but ice is not a passive frozen block," says Jean-François Boily, Professor at Umeå University and co-author of the study. "Freezing creates microscopic pockets of liquid water between ice crystals. These act like chemical reactors, where compounds become concentrated and extremely acidic. This means they can react with iron minerals even at temperatures as low as minus 30 degrees Celsius."
To investigate the process, scientists studied goethite -- an abundant iron oxide mineral -- combined with a naturally occurring organic acid, using advanced microscopy techniques.
They found that repeated freeze-thaw cycles enhance iron dissolution. When ice melts, organic compounds that were previously locked inside are released, fueling further chemical reactions. Salinity also matters: fresh and brackish water speed up the process, while seawater tends to suppress it.
The results primarily apply to acidic environments, including:
Researchers aim to determine whether this phenomenon occurs in all types of iron-bearing ice, with ongoing experiments in the Boily laboratory seeking to answer that question.
"As the climate warms, freeze-thaw cycles become more frequent," says Angelo Pio Sebaaly, doctoral student and first author of the study. "Each cycle releases iron from soils and permafrost into the water. This can affect water quality and aquatic ecosystems across vast areas."
The team emphasizes that ice should no longer be viewed as a passive storage medium, but as an active driver of chemical processes. With more frequent freezing and thawing in polar and mountain regions, the ecological consequences could be profound.
Warming climates and more frequent freeze-thaw cycles could significantly affect water quality and ecosystems.