A Single Freeze Can Redirect How Iron Minerals Form

A brief encounter with ice may permanently change the geological future of a common iron mineral, according to research from Umeå University published in Science and reported by Phys.org. The study focused on ferrihydrite, a poorly ordered iron oxide only a few nanometers across. It is widespread in glacial sediments, icebergs and cold soils, where its large surface area allows it to bind nutrients, pollutants and organic carbon. Its transformation can therefore influence how much reactive iron reaches polar oceans and how much carbon remains stored in frozen ground. The researchers found that freezing does more than temporarily preserve the mineral. As ice crystals grow, compounds excluded from the ice become concentrated in microscopic pockets of liquid. The resulting pressure pushes ferrihydrite particles together, removes water and hydroxyl groups from their surfaces, and bonds the particles into larger structures. A single freezing cycle at minus 20 degrees Celsius increased the particles’ size by about 30 times, producing micrometer-scale flakes shaped by the boundaries of the ice grains. Additional freezing caused little further change because most of the transformation took place during the first cycle. The bonded structures also remained intact when exposed to ultrasound and prolonged acid treatment, according to the report. The mineral’s later development changed as well. Over a year in water, untreated ferrihydrite transformed into goethite, a yellow-brown iron mineral commonly associated with cool, damp soils. Samples that had been frozen once did not form goethite. Under accelerated aging conditions, they instead transformed into hematite, the red iron mineral associated with warmer, drier soils. That difference could complicate some methods used to reconstruct past climates. The proportions of goethite and hematite in ancient soils, along with oxygen isotopes in those minerals, are used to estimate earlier environmental conditions. Those approaches generally assume that the transformations happened in liquid water. The researchers also reported that hematite absorbs a wider range of sunlight than goethite. A freeze-driven increase in hematite formation could therefore speed light-driven iron cycling in polar soils after they thaw. The findings suggest that ice can actively drive geochemical change rather than simply act as a freezer. Phys.org reported that increasing freeze-thaw cycles in a warming climate could make this process increasingly important for iron cycling and ecosystems.