Have you ever seen a heavy, solid rock that’s been seamlessly broken into thin plates by some invisible force? Or have you observed those eerily perfect circular patterned rock formations along mountain slopes? Maybe you’ve noticed mysterious, repetitive mounds scattered through the countryside in the middle of fields. The movement of both rocks and soil are controlled by freeze and thaw, creating mounds, organizing rock circles, and cracking the rocks themselves. These phenomena are macro views of the freezing and thawing that naturally occurs in soil.
But, there are microscopic changes occurring as well. My research at Oregon State University is looking at the impact that increasing frequency of freeze-thaw cycles may have on Alaskan soils. These soils have stored an estimated forty percent of Earth’s terrestrial organic carbon for centuries. The soil’s ability to continue storing carbon belowground will depend on soil resilience to changes in the climate. These changes include increasing variability in winter air temperature and the resulting increase in freeze-thaw cycles.
Water is one of the few substances that expands as it freezes. In addition, frozen water crystals – ice – attract more water along their edges. This creates beautiful and diverse frozen structures within the soil. Water, ice, and freeze-thaw processes have been crucial to shaping landscapes across the globe, from the chilly Arctic circle, to the snowy mountain tops in the distance, to the frost-covered yard behind your home. How? Well, we know that some soils are formed by the breakdown of rocks into smaller mineral components. This process is called weathering, and it can be caused by a multitude of agents including (but not limited to) trees and plants, lichen, microorganisms, flowing water, and…ice. Alternating freezing and thawing of rock, soil, and water is crucial to the weathering process.
It takes a while for the ground to freeze even after winter sets in. At an air temperature of 32oF, the ground isn’t necessarily frozen. But by mid-winter, soil can be frozen to large depths – especially in areas that have permanently frozen ground within one to two meters of the soil’s surface. This permanently frozen ground is called permafrost. Frozen rock, soil, water, or a mixture of all three can be classified as permafrost as long as the material remains frozen for two or more consecutive years. Permanently frozen ground has a huge impact on soil temperature, cooling soil from the bottom up in areas with permafrost present. Alaska contains many of these permafrost-affected landscapes.
As winter temperatures penetrate the ground from above, water held within soil pores begins to shape the soil. Cooling water first contracts and then expands during freezing as liquid water crystallizes into ice. Soil is not just one homogenous material. Rather, soil is a beautiful, messy mixture of mineral particles, organic matter, and interconnected pore spaces filled with air and water. So, as temperature acts on soil, from either the atmosphere or the underlying permafrost, the soil doesn’t just solidify like an ice cube. It doesn’t thaw evenly like an ice cube, either.
Very tiny pores will keep water in a liquid state during freezing. Ice crystals attract liquid water along a thermal gradient, resulting in the growth of ice lenses. Movement of soil water toward the ice lenses causes soil dehydration, resulting in soil contraction. Simultaneously, the growth of ice lenses results in localized expansion. Cracks appear during this process, cracks that grow in both girth and depth during the thaw season. This process is followed by additional freezing and expansion the next winter. The raised polygons gradually become more pronounced and the wedges deeper. These ice wedges, over time, become incorporated into the underlying permafrost.
Freeze-thaw cycles can alter the ground’s surface using only mineral soil and water. Yet, these cryogenic features become even more distinguished and fascinating when organic matter is added to the mix. And, despite low precipitation along Alaska’s North Slope, the shallow water table controlled by underlying permafrost results in dense vegetation and thus organic matter.
Freezing deforms the soil. Frost heaving allows mineral subsurface layers – or horizons – to be squeezed up through the soil. This moves horizons from lower to higher. This feature is known as a “mud boil” or “frost boil.” During thaw, organic rich surface horizons may be swirled further below the surface as they thaw earlier in the season than the subsurface. This may fill the cracks left behind by soil contraction due to dehydration during freezing. This process of soil horizons being moved vertically throughout the profile is called cryoturbation.
So far, these features are macroscopic. What happens when we get a little closer? The cracks formed in soil by freezing and expansion of water can occur on a tiny scale. Cracks forming in rocks and in minerals within rocks control a process called mineral weathering. Through this mechanism, freeze-thaw can influence the soil and pore water as minerals that serve as vital nutrients to living organisms are broken off from rocks.
Freeze-thaw cycles aren’t only seasonal. They can occur monthly, weekly, daily, or as night changes to day. When freeze-thaw cycles are occurring at higher rates, the soil can be greatly impacted on a microscale. Rates of mineral weathering and other soil attributes may be affected by the mechanical stress exerted by freeze-thaw cycles.
Members of other groups, along with soil scientists from Oregon State, are working together to analyze soil core samples from Alaska. We are comparing soil aggregate stability, carbon quality, microbial activity, and mineral composition in samples that were frozen versus samples that underwent freeze-thaw. Our team hopes to gain insight into how freeze-thaw cycles increasing in both frequency and intensity on soil will impact the soil ecosystem. This information will have implications for the larger, global ecosystem that will become immediate as the climate warms.
Answered by Erin Rooney, Oregon State University
Although you might think that frozen soil has nothing going on, soil is a dynamic substance year-round. Learn more here.