Ice at 10 degrees Celsius releases iron from more abundant minerals compared to liquid water at 4 degrees Celsius, according to researchers from Umeå University, Chimiques de Rennes, and CNRS. This discovery sheds light on why many Arctic rivers are taking on a rusty orange hue as permafrost begins to thaw in warmer climates.
Schematic diagram of the iron mineral dissolution reaction of ice. Image credit: Sebaaly et al. , doi: 10.1073/pnas.2507588122.
“It may seem counterintuitive, but ice is not merely a static frozen mass,” stated Professor Jean François Boyley from Umeå University.
“Frozen states create microscopic pockets of liquid water between ice crystals.”
“These pockets function like chemical reactors, where compounds become concentrated and highly acidic.”
“This implies that even at temperatures as low as 30 degrees Celsius, they can engage with iron minerals.”
To investigate this phenomenon, Professor Boyley and his team examined goethite, a diverse array of iron oxide minerals, along with naturally occurring organic acids.
Through advanced microscopy and a series of experiments, they found that repeated freeze-thaw cycles enhance iron dissolution significantly.
When ice undergoes freezing and thawing, it releases organic compounds that were previously trapped, fostering additional chemical reactions.
Salt concentration also plays a critical role; fresh brackish waters promote iron dissolution, whereas seawater inhibits it.
The outcomes of this research are particularly relevant in acidic environments like mine drainage sites, frozen atmospheric dust, acid sulfate soils along the Baltic coast, or acidic freezing locales where iron minerals interact with organic matter.
“As global temperatures rise, the freeze-thaw cycles are becoming more frequent,” remarked Angelo Pio Severly, a doctoral candidate at Umeå University.
“Each cycle liberates iron from the soil and permafrost into the water, potentially impacting water quality and aquatic ecosystems over vast areas.”
“These findings emphasize that ice is an active participant, rather than a passive medium for storage.”
“It is crucial to recognize the growing impact of freeze and thaw processes in polar and mountainous regions on ecosystems and elemental cycling.”
The research team’s paper was published on August 26, 2025, in the Proceedings of the National Academy of Sciences.
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Angelo P. Severly et al. 2025. Ice as a kinetic and mechanical driver for iron oxide dissolution of oxalate oxide. Proceedings of the National Academy of Sciences 122 (35): E2507588122; doi: 10.1073/pnas.2507588122
Source: www.sci.news
