A landscape cracked by cold
Across the vast, flat tundra of Arctic Siberia, the ground is fractured into immense geometric patterns. This landscape, known as polygonal ground, is the result of an endlessly repeating cycle of freezing and thawing that has shaped the region for millennia. The process begins in the depths of winter when intense cold, with average January air temperatures plummeting to between –27 and –23°C, causes the permafrost to contract and crack.
In the spring, meltwater trickles into these fissures and freezes, forming a thin vein of ice. Year after year, century after century, the process repeats. The cracks reopen along the same lines of weakness, and new layers of ice are added. Over thousands of years, these thin veins grow into massive wedges of solid ice that can be several meters wide at the top and penetrate deep into the ground. In northern Siberia, the permafrost itself can be over 600 meters deep. The relentless pressure of the growing ice wedges pushes up the surrounding soil, creating raised rims that form the distinctive polygonal shapes, which can range from a few meters to over 90 meters in diameter.
Micro-worlds and ancient time capsules
Each polygon is a distinct microhabitat. Younger, low-centered polygons have raised rims and wet, boggy centers, creating a haven for water-loving sedges and sphagnum moss. Over time, as the ice wedges grow and push the earth upward, they can form high-centered polygons. These are drier and host a different community of plants, such as lichens and dwarf shrubs. This mosaic of wet and dry patches creates a surprisingly varied environment in the otherwise uniform tundra.
The ice wedges are geological features; they are records of ancient ecosystems. Trapped within the ice are air bubbles from past atmospheres, pollen from long-gone plants, and even viable microorganisms. Some of these microbes have been dormant for tens of thousands of years. Radiocarbon dating of organic material found within the ice shows that some Siberian ice wedges began forming during the Late Pleistocene, over 18,000 years ago.
As the Arctic warms, this ancient permafrost is beginning to thaw. The melting of the ice wedges causes the ground to collapse in a process called thermokarst, which dramatically reshapes the landscape, forming ponds, lakes, and depressions called alasses. This thawing also releases vast quantities of stored carbon and methane, as well as releasing ancient bacteria and viruses from their long slumber.