Origin of Fossil Periglacial Wedges in Northern and Eastern Hokkaido, Japan

Abstract

Some fossil periglacial wedges discovered in Hokkaido have been recognized as ice-wedge casts. We newly investigated dimensions, character and stratigraphy of 34 fossil periglacial wedges in northern and eastern Hokkaido, for the purpose of examining their origin (permafrost wedge or seasonally frozen ground soil-wedge). The origin of these fossil wedges is discussed on the basis of the relation between the depth and spacing of wedges and an activelayer thickness in the present permafrost region.
Tephrostratigraphy and radiocarbon ages indicate that most of wedges have been formed during the latter half of the Last Glacial Age (ca. 42 ka-12 ka). The occurrence of fossil pollen and fossil woods indicating the boreal forest (especially Larix, Picea and Abies) and the opening of the thermal contraction cracks during this period suggest that the past annual variation in air temperature was characterized by warm summer and cold winter in northern and eastern Hokkaido. Then we calculated the annual air temperature amplitudes to be 15.8°C (northern Hokkaido) and 16.5°C (eastern Hokkaido) with the mean annual air temperature of-3°C corresponding to the limit of discontinuous permafrost zone, and to be 19.9°C (northern Hokkaido) and 20.9°C (eastern Hokkaido) with the mean annual air temperature of -7°C corresponding to the limit of continuous permafrost zone.
On the basis of the annual air temperature amplitude and the mean annual air temperature, the active-layer thickness for different material was calculated by Aldrich's equation. The relation between the depth of the fossil wedges and the calculated active-layer thickness indicates that 9 fossil wedges among 34 have likely been permafrost wedges with the mean annual air temperature of -3 °C, 12 fossil wedges have likely been permafrost wedges with the mean annual air temperature of -7 °C others have been seasonally frozen ground soil-wedges. Our judgement on the origin of the fossil periglacial wedges suggests that northern Hokkaido (Okhotsuk Coastal Plain) and eastern Hokkaido (Syari Plain and Konsen-genya Plain) was located in the southern margin of the continuous permafrost zone or the northern margin of the discontinuous zone during the maximum cold stage of the Last Glacial Age.