Studies of ground ice in Pre-Yedoma deposits

Six similar small epigenetic ice wedges IW 1, IW 2, IW 3, IW 5, IW 6 and IW 7 were sampled in Krest Yuryakh deposits. These ice wedges were located at the coastal cliff of a thermo-terrace of about 2-2.5 m height with distances of 5 to 50 m from each to other. The enclosing deposits were presented by grey, blue-grey (dark blue) clay loam. This clay loam had a characteristic nut-like structure with brown iron oxide films and included shells (molluscs) and detritus. The cryostructure of clay loam was bended and reticulate (net-like) with an ice schlieren thickness of about 1-1.5 cm. The ice wedges width was from 5-10 cm to 30-40 cm. The visible thickness was about 0.7-0.9 m. The bottom parts of the ice wedges were cut by wave-cut notch. The ice wedge ice was yellowish grey, brownish grey and showed clearly observed vertical oriented ice veins (widths about 2-3 mm). There was brown iron oxide film in the contact of ice wedges and sediments.

Each ice wedge was sampled by axe in the height from 0.8 to1.2 m a.s.l.

Sample points of the ice wedges IW 1, IW 2, IW 3 and IW 5 are displayed in Figure 4.6.5-1. IW 3 was also sampled for ³⁶Cl and ¹⁰Be dating (sample Oy7-01-301).

Figure 4.6.5-1: The similar ice wedges IW 1, IW 2, IW 3 and IW 5 in the section Oy7-01.

Ice wedges Oy7-03 IW 1, IW 2, IW 3, IW 5

The ice wedges IW 1, IW 2, IW 3 and IW 5 were studied in the section Oy7-03, which exposes Pre-Krest Yuryakh deposits. The ice wedges were located at the coastal cliff in a height of about 5 m. The enclosing deposits consisted of brownish-grey silty fine (laminated) sand. In the upper part sand sediments were covered by red-brown, dark-brown peat and peaty silt layers.

The section had a very complicated cryogenic construction, which is still subject of discussion (Figure 4.6.5-2). In the bottom part (about 1-2 m a.s.l.) composite sand ice wedges (IW 1 and IW 5) were located. The composite sand ice wedges were comparable to Oy7-03 IW 4 attributing to the oldest stratigraphical unit Kuchchugui Suite. The width of the composite wedges was about 0.3-0.4 m. The visible thickness was about 0.5-0.7 m. The Composite sand ice wedges passed into an ice wedge (IW 1) in the middle part of the section (about 2.0-2.4 m a.s.l.).

Ice wedge IW 1 crossed ice wedge IW 2. Apparently, we observed the corner of a polygon relic. Ice wedges IW 1 and IW 2 can be considered as syngenetic ice wedges because of their characteristic cryogenic construction. The enclosing sediments were characterized by a belt like cryostructure. The width of ice belts was about 2-3 cm. The distance between ice belts was about 10-12 cm. The width of ice wedges was about 0.4-1 m. The ice wedge ice was clean and

transparent and had big crystals (size ca. 0.3-0.5 mm). The ice was characterized by a lot of air bubbles (1-5 mm in diameter) which showed a vertical orientation. The width of the well pronounced elementary ice veins was about 3-6 mm.

A small epigenetic ice wedge (IW 3) penetrated from peat layers into silty sand in the upper part of the section (ca. 3.5-4.5 m a.s.l.). The width of the ice wedge was about 5-8 cm. The ice was clean, transparent and had no visible air bubbles. The width of elementary ice veins was about 2-8 mm.

Figure 4.6.5-2: Section Oy7-03 ice wedges IW 1, IW 2, IW 3 and IW 5.

Ice wedge Oy7-03 IW 4

The ice wedge IW 4 (composite sand-ice wedge, sand-ice wedge) in the section Oy7-03 can be attributed to the oldest stratigraphical unit Kuchchugui Suite.

This IW was located in the 2 m high cliff of the coastal outcrop. The surrounding sediments consisted of brownish-grey covered by yellowish-grey silty fine laminated sand. In the upper part sand sediments included vertically oriented grass roots in situ and peat inclusions. This section was covered by a dark-brown peat layer. The cryogenic structure of the silty sand was belt-like near the ice wedge, and massive and bended (with a thickness of the ice layers of about 1 cm) in the covered yellowish-grey silty fine laminated sand.

The composite sand-ice wedge consisted of vertically and sub-vertically oriented ice layers and sediment (sand) rich layers. The thickness of ice and ice-sand veins varied from 1-2 to 8-10 mm. The width of the composite sand-ice wedge was about 0.4 m, the visible height about 0.8 m. The composite sand-ice wedge had a rounded thawed head and was buried under the yellowish-grey silty sand with a well pronounced bended cryostructure. Clear detectable was the rounded character of the bended cryogenic structure above the ice wedge head. Four samples of the sand-ice wedge and two samples of texture ice near and above composite sand-ice wedge were taken (Figure 4.6.5-3). A similar ice wedge from the oldest unit was described and sampled in 2002 (Schirrmeister et al., 2003).

Figure 4.6.5-3: The composite sand-ice wedge Oy7-03 IW 4 in sediments of the Kuchchugui Suite.

Ice wedge Oy7-07 IW 1

The ice wedge IW 1 in the section Oy7-07 (Figure 4.6.5-4) was located in a 6 m high cliff of the coastal outcrop. The section exposed Eemian and Pre-Eemian deposits. The geological profile consisted of yellowish-grey silty fine laminated sand with vertically oriented grass roots (this horizon can be associated with the Kuchchugui Suite). The silty sand was characterized by a massive cryostructure and was covered by well-bedded grey loam with peat lenses and showed bended and belt-like cryostructures. The thickness of ice lenses was about 1-3

cm. In the upper part of geological profile there was dark grey loam with peat inclusions and lenses and plant remains (this horizon can be associated with the Krest Yuryakh Suite). The cryogenic structure of this horizon was massive and basal. The width of the ice wedge was about 1.7 m and the visible height was about 2 m. The ice wedge had a thawed head and was buried under the dark grey loam with peat inclusions.

The colour of ice wedge IW 1 was dirty yellowish-grey with numerous mineral patterns and air bubbles (size ca. 1-5 mm in diameter). The width of vertical oriented elementary ice veins was about 3-5 mm. Seven samples were taken.

Small epigenetic ice wedges (vertical oriented cracks) penetrated from covering dark grey loam with peat inclusions (ca. 4-5 m a.s.l.) into silty sand in the upper part of the section. The width of these ice wedges was about 3-8 cm. The ice was clean, transparent and had no visible air bubbles. Sample numbers Oy7-07-111, -302 and -303 were taken from these small epigenetic ice wedges (not shown here).

Figure 4.6.5-4: Outcrop Oy7-07 exposed the ice wedge IW 1 with a thawed head.