PERMAFROST AS A FROZEN GEOCHEMICAL BARRIER

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1 PERMAFROST AS A FROZEN GEOCHEMICAL BARRIER V. Ostroumov 1, Ch. Siegert 2, A. Alekseev 1, V. Demidov 1, T. Alekseeva 1 1. Institute of Soil Science and Photosynthesis, Russian Academy of Sciences. Pushchino, Moscow Region, Russia vostr@issp.serpukhov.su 2. Alfred Wegener Institute for Polar and Marine Research. Potsdam, Germany csiegert@awi-potsdam.de Abstract This paper considers the influence of the interface between frozen and thawed soil on the distribution of mobile forms of chemical elements in permafrost affected soils. Three levels of mobile chemical elements accumulation are present in a permafrost-affected loam (Edomic suite, Kolyma Lowland, North-East Russia): the bottom of the Holocene active layer, the upper part of the ice-rich transitive layer, and the bottom of the modern active layer. Mobile compounds migrate into depressions in the permafrost table during infiltration of the pore solution. Additional local concentration and separation of elements may occur during the subsequent freezing of the soil. Zones of concentration mark the thawing levels of perennially frozen ground at different stages of their development. The selective chemical sedimentation of chemical elements at a frozen barrier is described using the example of mobile calcium. Introduction As shown in Makarov (1993), permafrost has an important role to play as a geochemical factor. Owing to the presence of ice, frozen ground differs from thawed ground in its properties and composition. At the interface of frozen and thawed soils there is a step-like change in the chemical potentials of the pore solution components (Ershov et al., 1992). For example, at this interface the jump of electric potential (freezing potential) may achieve several tens of volts and more (SavelÕeva, 1986). The conditions under which such jumps can take place are sufficient to modify substantially the migration ability of elements and solubility of their compounds (Makarov, 1993). Permafrost soils and landscapes are also characterized by geochemical contrasts, e.g., wide distribution of surface and ground waters with lenses of fresh near-surface water and ground water with unfrozen solutions of high concentration (Streletskaya et al., 1996). Another example of geochemical peculiarities of the cryolithozone are occasional sharp rises in the mineralization of surface water related to episodes of deep thawing (Anisimova, 1996). Permafrost blocks the infiltration of surface water which concentrates in the active layer (Woo and Xia, 1995). A part of the pore water migrates into the upper horizon of permafrost and results in the ice-rich transitive layer (Shur, 1988). These and other processes may be conducive to the establishment of specific geochemical conditions and the formation of special patterns of chemical elements distribution in the upper horizons of permafrost. The problem of concentration and dispersion of chemical elements under the influence of permafrost is poorly understood. This paper examines this problem by analysing the distribution of mobile elements in permafrost affected soils. Objectives and methods To assess the role of permafrost in the distribution of mobile forms of elements under field conditions, we chose small sections within which soils had a relatively low degree of morphological variability. The first site is characteristic of the drained, gently sloping surface of a well preserved outlier of Edomic sediments. The second site is typical of hydromorphic positions and is situated in a river valley. The section is a face in the upper part of a collapsed block in sediments of the Edomic suite on the middle reaches of Bolshaya Chukochya River (in the north of Kolyma Lowland, north-western Siberia). The face was cleaned about 4 m from the slope brow within the limits of the stable part of Edomic sediments which are not involved in the on-going slope processes. The actual thickness of the thawed layer at the date of its measurement (August 5, 1994) was found to range from 0 up to 32 cm. According to the results of direct week-long observations it increased at a mean rate of about 2 mm/day. The depth of thaw is minimal at sites with a thick peat or moss cover where a portion of this cover is preserved in the frozen state (the frozen soil Ð FS on Figure 1). The thick surface organic cover contributes to a low thaw rate at this hillock. Maximum V. Ostroumov, et al. 855

2 Figure 1. Cryogenic structure, distribution of ice and active layer (section 1-94). thaw occurred under vegetation-free sites on the surface of hillocks (thawed soil Ð D on Figure 1). As a result of irregular thawing with depth, the surface of the thawed layer is an alternation of rises and hollows with planar diameters of m. In depressions of the permafrost table, the thawed soil is saturated with water. Next to the boundary of the thawed soil the frozen horizons contain middle-size ice layers and lenses whose pattern parallels the surface of the permafrost table. A horizon with an increased ice level lies at depths of 5 to cm. At sites with an elevated permafrost table, the iciness is greater due to the appearance of separate lenses of pure ice which are sometimes 10 cm thick. In depressions of the permafrost, the ice forms pure lenses 2-3 cm thick. At depths of cm (the transitive layer, Shur, 1988) the loam has a large-layered and, in some places, reticulate cryogenic texture which is gradually replaced by a thin-layered one in the downward direction. In the upper part of the frozen ground, the stretching of layers and streaks of ice repeats the form of the modern relief of the permafrost table. Starting from depth of cm and below, the streaks and layers of ice are in a virtually horizontal position, while the iciness increases gradually downwards. At a depth of 110 cm, there is pure wedge ice which fuses in its upper part with neighboring wedges and is observed to go as deep as 12 m and more. The availability of two zones of high iciness in which the ice layers and lenses have different orientations (repeating the relief of the contemporary permafrost table in the upper horizon and stretching virtually horizontally in the lower horizon) shows that thawing has occurred in two stages. The first, apparently Holocene, thawing stage formed the lower zone of high iciness at the level of the tops of the wedges. The later second stage was conducive to the appearance of an icy zone at depths of cm. It seems very likely that the icy zone emerged and continued to develop during the periods of anomalously deep seasonal thawing at the actual stage of geologic history of the Edomic sedimentary layer. We took the samples from homogeneous loam in the section. Each sample characterized a zone that differed from the neighboring zones in terms of its cryogenic structure. All samples were analyzed for the content of mobile forms of elements. To this end, we used two extracts: water extract (the ratio Òsoil:waterÓ = 1:5) and acetate extract (ph = 4.8). This dissolves most of the sorbed forms of elements, the complex compounds and part of the carbonates in soil. The ratio between the water extractable and ammonium acetate extractable elements (C Wat /C AcAm ) reflects the degree of chemical sedimentation of mobile chemicals in the soil (Geletuk and Zolotareva, 1980). The extraction scheme used is a sort of inner standard for our group, thanks to which, a large amount of comparable data on mobile forms of elements has been accumulated. The extracts were analyzed by spectral techniques for the levels of geochemically contrasting elements: Na, K, Ca, Mg (water extract) and Na, Ca (ammonium acetate extract). The moisture level in grounds of both sections as well as in the samples obtained in laboratory experiments was measured by gravimetric techniques, 856 The 7th International Permafrost Conference

3 Figure 2. Water extractable elements in the bottom of the active layer and in permafrost: (a) on a rise in the permafrost table (FS zone, Figure 1); (b) within a depression of the permafrost table (D zone, Figure 1). whereas the iciness was determined as the difference between the total moisture and the quantity of unfrozen water which is about 3% at a temperature of -8 C (Ershov et al., 1979) for the Edomic suite sediment. Results and discussion Figure 2 shows the distribution of water extractable elements in the active layer and the underlying permafrost in deposits of the Edomic suite. The zero depth is equivalent to the position of the permafrost table in Figure 2. Where the active layer is at a minimum (Figure 2a), the first local maximum of potassium content is situated directly under the permafrost table. The second one is revealed at depths of cm (upper part of transitive layer). Two contrasting maxima were situated at depths of 0-3 and cm for water extractable sodium. The maximum level of calcium is found directly under the permafrost table. There are two zones of concentration of magnesium: under the actual permafrost table (0-3 cm) and in the upper part of the transitive layer (25-32 cm). Where the active layer is at a maximum, the local maximum of potassium is situated at the bottom of the permafrost table depression (Figure 2b). The local maxima of Na, Ca, and Mg were directly under the permafrost table (0-4 cm). Their concentrations increase gradually downwards from depths of 20 to 100 cm. The common feature of different water extractable elements relative to the permafrost table is that the maximum concentrations occur in the upper layer of the frozen part of the section. The accumulation of chemical elements generally has taken place at all three levels of thawing: at the bottom of Holocene thaw truncation, in the upper part of the transitive layer, and close to the modern permafrost table. The accumulation of water extractable elements close to the permafrost table was described by Anisimova (1996), and experimental data of Naletova (1997) confirm this phenomenon. The accumulation of water extractable elements results from ion migration into the frozen soil during freezing (Naletova, 1997). The exceptions described above may be explained by individual geochemical properties of the elements. The zone of accumulation of water extractable Na (dark-gray spot, Figure 3a) coincides with the location of an accumulation of ammonium extractable form (dark-gray spots, Figure 3b). There is no chemical sedimentation of sodium in the same locations (the darkgray zone on the NaWat/NaAcAm, Figure 3c). The maximum of water extractable calcium is located in the upper part of the transitive layer (Figure 4a). The minimum content of the ammonium acetate extractable form of Ca is present here (light-gray spot, Figure 4b). The zone of accumulation of ammonium acetate extractable Ca is localized at the bottom of depression of modern permafrost table (dark-gray spot, Figure 4b). The light-gray spot on the diagram of Ca Wat /Ca AcAm (Figure 4c) reflects the existence of chemical sedimentation of Ca in the depression of the permafrost table. Redistribution of the mobile forms of elements in the zone of permafrost influence may be represented in the following way. Because of heterogeneous thaw depths, the permafrost table relief is an alternation of depressions and elevated sites. Depressions occur in areas with sparse surface cover. Furthermore, their formation in areas with reduced iciness is facilitated by low heat absorption for ice melting. Under incomplete saturation V. Ostroumov, et al. 857

4 Figure 3. (a) Water extractable sodium (NaWat), (b) ammonium acetate extractable sodium (NaAcAm) and (c) the ratio NaWat / NaAcAm. Figure 4. (a) Water extractable calcium (CaWat), (b) ammonium acetate extractable calcium (CaAcAm) and (c) the ratio CaWat/ CaAcAm. with water, pore solutions infiltrate into depressions. Such a descending percolation of pore water enhances the convective heat transfer to the front, accelerates thawing and increases its depth in depressions. The accumulation of sodium here takes place without the chemical sedimentation. The chemical sedimentation of calcium takes place inside the depressions. The selective sedimentation of chemical elements is typical for geochemical barriers. Conclusions The interface of thawed and frozen zones in soils is a geochemical barrier at which important changes in the mobility of chemical elements result in their selective 858 cryogenic concentration and chemical sedimentation. The following three levels of concentration of mobile elements are present in the upper part of loam from the Edomic suite: the bottom of the modern active layer, the upper part of the transitive layer, and the bottom of Holocene active layer. Zones of concentration are associated with depressions in the permafrost table which play the role of geochemical traps. The influx of elements to these traps occurs due to the infiltration of pore solution inside the depressions in the relief of permafrost table. Probably, additional local concentration of elements in the traps takes place during their freezing. The chemical sedimentation of elements at this frozen geochemical barrier is illustrated by the selective fixation of calcium at the bottom of trap. The 7th International Permafrost Conference

5 Acknowledgments The authors would like to express their thanks to Birgit Hagedorn, Antje Eulenburg, Uta Bastian, Heide Kraudelt, Luba Pasitskaya, Viktor Sorokovikov and Michail Novikov for their participation in chemical analysis and in fieldwork. This project was supported by the INTAS (grants and Extension). References Anisimova, N.P. (1996). An Influence of the Technogenic Cryopegs on the Temperature and Salinity of Permafrost Deposits. In 1st Conference of Russian Geocryologists, Proceedings, 1, pp (In Russian). Ershov, E.D., CheverÕev, V.G., Akimov, Yu.P. and Pachomova, T.M. (1979). The Phase Composition of Water in Frozen Grounds. Geocryological Investigations, 18, (In Russian). Ershov, E.D., Lebedenko, Yu.P., Chuvilin, E.M. and Naumova, N.S. (1992). Mass Transfer in Freezing Saline Soils. In Cryosoils, 1st International Conference of Cryopedology, Proceedings. Pushchino, pp Geletuk, N.I., Zolotareva, B.N. (1980). Use of Atomic Absorbtion Spectroscopy for the Analysis of Samples from Biosphere Compounds. Pushchino, 25 pp., (In Russian). Makarov, V. (1993). Technogenic Geochemical Fields in the Permafrost Zone - on the example of Yakutia. In Proceedings of the 6th International Conference,1, pp Naletova, N.S. (1997). Mass Transfer and Cryogenic Structures in Freezing Saline Sediments. Ph.D. Thesis, 17 p., (In Russian). SavelÕeva, E.M. (1986). Influence of the Water-Ice Phase Transistion on Hydrometeorological Processes. Ph.D. Thesis, 17 p., (In Russian). Shur, Y.L. (1988). The Upper Horizon of Frozen Ground and Thermokarst. Nauka Press, Novosibirsk, 212 p., (In Russian). Streletskaya, I.D., Ivanova, N.B. and Rivkin, F.M. (1996). The mapping of the Bovanenkov Territory, Yamal Peninsula by the Distribution of Cryopeges. In Proceedings 1st Conference of Russian Geocryologists, 1, pp (In Russian). Woo, Ming-ko, and Xia, Zhaojun (1995). Suprapermafrost Groundwater Seepage in gravelly Terrain, Resolute, NWT, Canada. Permafrost and Periglacial Processes, 6, V. Ostroumov, et al. 859