Sea ice covers the polar oceans, existing in various types with distinct behaviors and characteristics. Fast ice is a specific and important category of sea ice found in high-latitude environments. This phenomenon sets the stage for unique physical processes and ecological interactions within the polar landscape.
Defining Fast Ice and Its Formation
Fast ice is sea ice that is physically fixed, or “fastened,” to a coastline, an ice shelf, or the ocean floor in shallow areas. This attachment is the defining feature that prevents it from drifting with winds and currents. It forms a continuous, stationary belt extending outward from the landmass. The formation process typically begins in the autumn as coastal seawater cools to its freezing point. Initial ice crystals form and then consolidate, growing progressively outward from the shore. The ice sheet can also anchor to grounded icebergs or shoals, extending the fixed boundary further into the ocean. The width of this stationary ice can range from a few meters to hundreds of kilometers, depending on the local seafloor topography and the presence of islands.
Physical Characteristics and Stability
The stability of fast ice is its most distinguishing physical trait, allowing it to move only vertically in response to tidal cycles. Its fixed nature results in a surface that is often smoother and more uniform than other types of sea ice, as it is shielded from the constant movement and collision that causes ridging. Fast ice can reach thicknesses of several meters, especially if it persists over multiple years, although first-year ice is typically between one and two meters thick.
If the ice survives one or more melting seasons, it is classified as multi-year ice, which is thicker and less salty than first-year ice. The ice remains fixed to the substrate below, whether it be the shore or the seabed, and this anchoring effect helps regulate the seaward flow of land ice. In the Arctic, fast ice often extends into waters up to 20 meters deep, while in the Antarctic, it can be anchored in water deeper than that.
Distinguishing Fast Ice from Pack Ice
Fast ice is commonly confused with pack ice, but the two are separated by a fundamental difference in mobility. Pack ice is any extensive area of sea ice that is not attached to the shoreline and is free to drift. This mobile ice is constantly driven by wind and ocean currents, resulting in a highly dynamic environment.
The broken and constantly moving nature of pack ice leads to the formation of ridges, rubble fields, and open water channels called leads, making its surface irregular. In contrast, fast ice is stationary, forming a stable platform that acts as a physical barrier to the mobile pack ice further offshore. The boundary between the two is known as the fast ice boundary. Pack ice is composed of individual pieces of ice called floes. When these floes are densely packed together, the term “pack ice” is used synonymously with “drift ice”. This difference in attachment and movement dictates their respective roles in the polar environment.
Ecological and Human Relevance
Fast ice provides a unique and stable habitat that is integral to the polar ecosystem. Ecologically, it serves as an essential platform for a variety of marine animals that rely on its fixed nature for critical life stages. For instance, Weddell seals use the stable surface of fast ice as a site for giving birth, and emperor penguins use it as a breeding ground.
The ice itself supports an early-season food web, as algae grow within the bottom layer of the ice sheet, fueling zooplankton when the ice begins to melt. For human communities, particularly Indigenous groups in the Arctic, fast ice is a traditional and dependable surface for travel, hunting, and transportation. Furthermore, its presence acts as a protective shield for coastal areas, dampening wave energy and reducing erosion from storm surges.