Buried Snow in the Lena-Amga Plain V.B
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V.B. SPEKTOR ET AL. Kriosfera Zemli, 2011, vol. XV, No. 4, pp. 16–21 http://www.izdatgeo.ru BURIED SNOW IN THE LENA-AMGA PLAIN V.B. Spektor, V.V. Spektor, N.T. Bakulina* Melnikov Permafrost Institute, Siberian Branch of the Russian Academy of Sciences, 36, Merzlotnaya str., Yakutsk, 677010, Russia; [email protected] * State Committee of the Sakha Republic (Yakutia) on Geology and Mineral Resources, State Unitary Enterprise “Centrgeoanalitika”, 13, Kirova str., Yakutsk, 677000, Russia Some new forms of buried stratified ice have been discovered in the Late Pleistocene section of the Lena-Amga Plain (Central Yakutia, Russia) in the course of core drilling at several watershed sites elevated to 220–250 m asl. The buried ice, which belongs to the traditionally distinguished ice complex, makes three separate layers of firn (snow recrystallized to difference degrees), at the depths 12.0 to 17.0, 23.3 to 24.5, and 33.6 to 39.0 m below the ground surface lying under wedge ice found in the uppermost section between 2.5 and 5.0 m. The firn layers are separated by syngenetically frozen sandy silt and silt. INTRODUCTION FIELD DATA Buried ice is widespread in plainland Central The most complete section with different forms Yakutia, especially in the Lena–Amga–Aldan inter- of buried ice was found in the borehole drilled in fluve (Lena-Amga Plain) where it belongs to the ice 2004 at 62°08′ N, 131°18′ E, 82 km east of Yakutsk complex [Soloviev, 1959; Katasonov, 1975, 1979; Iva- city. The experiment of 2004 consisted in core drilling nov, 1984]. The ice complex of the area consists of up with successive bit diameters of 147, 127, 108, 89, and to 70–80 m thick fine-grained sediments that bear 76 mm, and core recovery using an air-flushing sys- frozen water in the form of ice wedges and structure- tem. Thus obtained undisturbed core material select- forming ice. The total permafrost thickness reaches ed for further studies was then transported, in the hundreds of meters, the upper layer of at least 100 m frozen state, to an underground laboratory of the In- being syngenetically frozen ground. stitute of Permafrost (Yakutsk). More evidence of the composition, age, and geo- The core section included three ice layers of dif- morphic setting of the ice complex has been obtained ferent thicknesses: wedge ice in the uppermost part at recently through core drilling in the Lena-Amga 2.5−5.0 m below the ground surface and two firn lay- Plain, at altitudes 220–250 m asl (Fig. 1), run by the ers at 12.0−17.0 m and 33.6−39.0 m separated by syn- Institute of Permafrost (Yakutsk) [Spektor and Spe- genetically frozen sandy silt and silt [Spektor et al., ktor, 2002]. 2008]. Boreholes drilled at several sites of the plain near Another hole drilled in 2011 three meters off the watersheds stripped previously unknown forms of former one provided more details of the section as it buried stratified ice which turned out to be snow re- tapped layered ice at two depth intervals of 13.5– crystallized to difference degrees (firn). The discov- 14.2 m and 23.31–24.50 m below the ground surface. ery of firn ice in drill sections, a very important out- Additionally, the upper and lower snow cover was come of the drilling project, provides clues to the Late sampled above the hole (5–10 and 30–40 cm below Pleistocene climate in the area and has additional im- the snow surface, respectively, the total snow thick- plications for the origin of the ice complex. ness at the time of 19.03.2011 being 43 cm). Copyright © 2011 V.B. Spektor, V.V. Spektor, N.T. Bakulina, All rights reserved. 16 BURIED SNOW IN THE LENA-AMGA PLAIN Fig. 1. Location map of boreholes in the Lena-Amga Plain (Central Yakutia). 1 – elevations, meters above sea level; 2 – lakes; 3 – roadway; 4 – boreholes. One more borehole, which was drilled in 2005 at Each cycle is 2–3 mm thick, with about 20 such cy- 94 km east of Yakutsk (62°04′ N, 131°33′ E), stripped cles in a banded layer, i.e., there were at least 20 year- firn buried at 2.8−8.2 m below the surface. This firn long spells of harsh climate. layer correlates with the 33.6−39.0 m interval of the The 1−3 cm thick layers of coarse-crystalline 2004 borehole in its stratigraphic position, structure, firn consist of up to 5 mm round or rectangular ice and texture. crystals with molten edges. There are pieces of colum- The ice layers in the drill section of 2004 were as nar upright ice crystals, which is typical of firn gla- follows. ciers. Uppermost layer (2.5–5.0 m): wedge ice. The Lower firn layer (33.6−39.0 m): stratified ice, stripped ice wedge is at least 30 cm wide and about slightly denser and more strongly recrystallized 2.5 m thick. Ice is transluscent and yellowish, has a than that in the upper layer. In the lower portion of vertical banded texture (1–2 cm wide bands), and en- the interval, ice layers alternate with frozen silt closes various mineral and organic lenses and elon- which, in turn, grades into lens-type or reticulate ice- gate gas bubbles. rich soil. Upper firn layer (12.0–17.0 m): thinly stratified The section of the 2011 borehole drilled near ice with very fine, fine, or less often coarse particles that of 2004 included two layers of stratified ice we (Fig. 2, a). In our view, this ice type may result from sampled: one between 13.5 and 14.2 m and the other recrystallization of buried snow. Buried snow within between 23.31 and 24.50 m below the surface. It was this depth interval is poorly compressed and weakly coarse-crystalline ice with its structure similar to that recrystallized, and shows intricate stratification. The of the lower firn layer at 33.6−39.0 m (see above). whole firn layer consists of several smaller banded The uppermost section contained wedge ice layers of (i) 3–5 cm thick pure banded firn, (ii) 2.5 m below the surface lying under massive light 1–2 cm thick banded firn with soil, and (iii) 1–4 cm brown silt and a thin soil layer at the top. Between thick brecciated and coarse-crystalline firn the ice (firn) layers, the drill hole tapped syngeneti- (Fig. 2, b). cally frozen silt and sandy silt with reticulate or mas- The compacted snow layer includes tentatively sive cryostructures in the depth intervals 5.0−12.0 distinguished annual cycles consisting of pure white and 17.0−33.6 m. Massive silt is cut in many places by snow (formed in wet and warm seasons) and grayish branching thin (a few mm to 3 cm) nearly upright snow with mineral particles (dry and cold season). cracks filled with ice. 17 V.B. SPEKTOR ET AL. Fig. 2. Middle part of 2004 drill section. a – ice cores with stratified ice (firn) from depths 12.7–13.4 m below ground surface; b – hierarchic ice stratification at depths 13.75–13.85 m below ground surface; 1 – pure banded firn; 2 – banded firn with soil; 3 – brecciated and coarse-crystalline firn. LABORATORY MEASUREMENTS The oxygen isotope composition (δ18O) of firn ice sampled from the intervals 13.5−14.1 and Isotope composition of ice and snow. The oxy- − gen and hydrogen isotope compositions of buried firn 23.4 24.41 m varies in a narrow range relative to the ice from the borehole of 2004 was studied at the Lab- SMOW standard, according to data of the IGEM δ18 oratory of Isotope and Nuclear-Physics Methods Laboratory. In the upper interval, O is − δ (Science & Technology Center, All-Russian Research from 32.25 to –29.20 ‰, and deuterium ( D) Institute of Hydrogeology and Engineering Geology, ranges from −213.5 to −236.9 ‰. The respective VSEGINGEO, Moscow, reports by V.A. Polyakov ranges in the lower interval are between −31.70 ‰ and A.F. Bobkov). The samples of ice from the 2011 and −30.32 ‰ δ18O and −234.7 to –219.9 ‰ borehole and fresh snow cover of the same year were δD. Firn from the 12.0−17.0 and 33.6−39.0 m analyzed at the Laboratory of Isotope Geochemistry intervals was analyzed at the VSEGINGEO La- and Geochronology (Institute of Geology of Mineral boratory, one sample from each interval. The Deposits, Petrography, Mineralogy, and Geochemis- results (Table 1) were similar to the δ18O and δD try, IGEM, Moscow, reports by E.O. Dubinina). The data for the 13.5−14.1 and 23.4−24.41 m inter- results are synthesized in Table 1 and in Fig. 3. vals. 18 BURIED SNOW IN THE LENA-AMGA PLAIN Table 1. δ18O and δD contents in buried firn of the Lena-Amga Plain and surface snow cover Sampling interval, m No. Deposit δ18O, ‰ δD, ‰ below ground surface Borehole 2011 1 Ice 13.50–13.58 –29.73 –213.5 2 Ice 13.62–13.70 –29.20 –214.3 3 Ice 13.72–13.80 –29.77 –223.8 4 Ice 13.82–13.90 –30.44 –232.0 5 Ice 13.92–14.00 –31.20 –236.9 6 Ice 14.05–14.10 –32.25 –232.1 7 Ice 23.41–23.44 –30.54 –219.9 8 Ice 23.51–23.56 –30.32 –223.8 9 Ice 23.62–23.68 –30.73 –226.7 10 Ice 23.71–23.78 –30.95 –230.0 Fig.