New research flips a long-held belief that frozen environments slow down chemical reactions and helps explain why Arctic rivers are turning orange. It turns out that ice is actually better than liquid water at releasing iron from common minerals.
For the last few years, rivers in the Arctic have been turning orange due to widespread thawing of permafrost from warming temperatures. The rivers get their color from a high concentration of iron released from local mineral deposits. Affected rivers have less dissolved oxygen and are more acidic than healthy nearby streams, making it harder for wildlife to survive.
It was once believed that when iron-rich mineral deposits were locked in ice, the iron would stay put, but a new study from Sweden’s Umeå University shows that the ice itself is actually working better than permafrost melt to release the iron. The study showed that ice at -10 °C (14 °F) releases more iron from mineral deposits than liquid water at 4 °C (39.2 °F).
“It may sound counterintuitive, but ice is not a passive frozen block,” says study co-author Jean-François Boily. “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.”
In carrying out the study, the researchers examined goethite – a mineral in which iron oxide is mixed with soils and sediments – in the presence of an organic acid. They not only found that frozen water was better at releasing the iron than liquid water, but they also discovered that repeated freezing and thawing cycles were the most efficient means by which the iron is released. Additionally, they found that fresh and brackish water increased the release of iron, while seawater suppressed it.
“As the climate warms, freeze-thaw cycles become more frequent,” says first study author Angelo Pio Sebaaly. “Each cycle releases iron from soils and permafrost into the water. This can affect water quality and aquatic ecosystems across vast areas.”
The researchers only looked at iron being released in acidic environments such as those found near mines and say they will next try to discover ice’s effect on iron dissolution in other environments. The research could have an impact on how environmental scientists understand and possibly combat the effect the orange rivers are having on wildlife.
The study has been published in the journal PNAS.
Source: Umeå University
