Ice foot, freeze-thaw of sediments, and platform erosion in a subarctic microtidal environment, Manitounuk Strait, northern Quebec, Canada

Abstract

The role of shore ice and freeze-thaw processes in intertidal sediments was studied along the inner shoreline of Manitounuk Strait through a series of drillings through the ice foot and numerous observations at all seasons. Mechanical processes such as drift-ice scouring and clast transport play only a minimal part in the observed changes. The thermal behaviour of the ice foot and the control it exerts on surface and groundwater flow throughout winter are the dominant factors of the shoreline dynamics. As the ice foot freezes to the surface of the tidal flat, a freezing front penetrates to depths of 3 m in the underlying sediments, which consist of postglacial Tyrrell Sea clays. This induces the formation of segregation ice which destroys the structure of the clays and leads to their thaw liquefaction in early summer. As this deep penetration of frost beneath the ice foot also impedes winter groundwater and surface-water flow, forced seepage and springs form icings (naleds), frost blisters, and lens-shaped bodies of intrusive ground ice at the upper margin of the tidal zone. The melting of all this ground ice induces settlement, microcliff collapse, and mud flows. The Manitounuk tidal zone, rather than being a typical tidal flat, is an erosional platform that is cut in soft Quaternary sediments and is being graded to tidal regime. Freeze-thaw processes controlled by the presence and thermal behaviour of the ice foot are the principal agents of platform destruction while waves and tidal currents are the transport agents that remove the sediments from the tidal zone. Platform erosion is presently the main source for sediment deposition in the shallow waters of Manitounuk Strait. Such a series of coastal periglacial processes may play an important role in other subarctic coastal settings. They also play an important ecological role, as they prevent the colonization of the coastal zone by mollusks and restrict the extent of coastal marshes.