Permafrost

Significance of Microtopography and Cryoturbation in Mature Larch
Forest near Yakutsk, Eastern Siberia, Russia
Yuki Sawada 1 , Go Iwahana 2 , Tetsuoh Shirota 1 , Ryusuke Hatano 3
Masami Fukuda 1 , Alexander N. Fedorov 4
(1. Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan; 2. Graduate School of
Engineering, Hokkaido University, Sapporo, Japan; 3. Graduate School of Agriculture, Hokkaido
University, Sapporo, Japan; 4. Permafrost Institute, Russian Academy of Science, Yakutsk, Russia.)
Abstract: The microtopography (earth hummocks) and cryoturbation structures are measured
and described by digital photogrammetry and sketch on pit sections. A study plot was
established in mature larch forest near Yakutsk, eastern Siberia. The 1cm DEM and contour
map of 2cm interval of a 4m 2 quadrat revealed the existence of a number of small mounds and
surrounding troughs on the mineral soil surface, which was covered with an organic soil layer.
Mean diameter of the mounds and mean intervals are approximately 30-50cm, and relative
height is 10-20cm. Sidewalls of the troughs have an apparent straight shape. The dead root and
funnels were found within the troughs, indicating the root expansion into the troughs, and it
forced trough edges being straight.
Cryoturbation structures were appeared in the sidewalls on the quadrat pit. Upper and
lower boundaries of the mixed layer have a wave-like form. The concave, bowl-shaped
depressions in the lower boundary are located under the convex mounds on the mineral soil
surface. The wave length of the lower boundary is approximately 1m. Black mineral soils are
deposited in the troughs, and they had an extension down to the large pools of the black mineral
soil. These structures indicate that the surficial mineral soil darkened with organic matter
submerged into the active layer, and these finally stored in the pools of black mineral soil
within the active layer. Large amount of organic-affected black soils can be stored within the
active layer.
The soil movements can be explained by the circulation model, which is proposed for
earth hummock growth by Mackay (1980). He demonstrated that the cell-like soil circulation
occurs if there are bowl-shaped frozen table, because of the upward and downward movement
of melting soil with the driving force of the gravity. Bowl-shaped lower boundaries appeared in
the sections were probably associated with the frost table when soil circulation occurred.
Key words: Siberia, Continuous permafrost, microtopography, cryoturbation, soil carbon pool,
digital photogrammetry
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