Copyright © 2009 IG SB, Siberian Branch of RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.gnr.2009.06.004

Geography and Natural Resources 30 (2009) 113–118

Formulation of the problem

The taiga type of natural environment has a multifunc-tional role in the biosphere. In view of the ongoing global spontaneous and anthropogenic changes in natural environ-ment [1], the study of the dynamics and trends of geosystems in the main zonal areas of the territory and of large geomor-phological formations is becoming increasingly important. One of the representatives of Russia’s plain-taiga regions, the West-Siberian region, is reputed to have a high degree of swamping (as high as 75%). According to M. I. Neishtadt [2], this process, evolving in the Holocene at a rate of 100 km2 per year, is able to encompass the entire territory of the plain. Its present-day landscape structure and the dynamical state of boreal vegetation of the unique Ob-Irtysh formations of the Ural-Siberian phratry [3] is dominated by water-accu-mulative processes, on the one hand, and by runoff-forming processes, on the other.

Accumulation and spatial redistribution of sedimentary deposits, and their lithological and lithochemical inhomoge-neity are associated with the activity of the Irtysh and Ob. Because of the functional significance of this powerful river system, the marginal territory of the North-Asian subconti-nent is called the Ob-Irtysh physical-geographical region [4]. The floodplain provinces of the same name was identified in

the pattern of its geographical zoning from geologo-geomor-phological attributes [5, 6].

Of particular natural-ecological and resource value for the region are the southern-taiga dark-coniferous (with ele-ments of the nemoral flora) forests of relatively well drained, near-watershed surfaces with deep podzolic (with the second humic horizon) soils on residual-calcareous loessal loams. Directional changes of these native forests manifest them-selves in extreme (for the dark-coniferous geom. of optimal development) facies: denudational facies, and paludic facies. As a consequence of the erosional processes in the shore area of high terraces and concurrent swamping of their plakor surfaces, the areas of these unique forests are decreasing [7]. They are degrading in some eroded areas, and the watershed bog massifs merge together with the bottom of river valleys in which peat blocks collapse.

The dynamics and trends of dark-coniferous forests were rather thoroughly explored at the Nizhneirtyshsky geographical permanent station in the area of the Tobolsk mainland [8-10]. It is significant that zonal forest facies occur under harsh hydrological-climatic conditions of the plain-taiga Ob-Irtysh region, on relatively elevated surfaces of its extensive valley. They are characterized as alluvium-phobic forests and constitute the final stages of alluviogenic successions [11].

Our intention is to examine the formation, development and resistance of the valley varieties of forest geosystems, identify the various forms of their zonal and local attributes, and, in general, to study the dynamical and evolutionary be-havior of plain-taiga landscape.

* Corresponding author. E-mail address: nechaeva@irigs.irk.ru (E. G. Nechayeva)

Evolution of the plain-taiga geosystems of Western Siberia

E. G. Nechayeva*

Institute of Geography SB RAS, Irkutsk

Received 19 August 2008

Abstract

The role of the Ob-Irtysh valley in the dynamics and evolution of the soil-earth materials factor of taiga-plain landscape is considered. The study demonstrates the mechanisms of operation of the soil-earth materials factor that is responsible for the formation of zonal for-est facies along the valley which undergo swamping at the final stage of their functioning. The progressive development of this regional process leads to marked changes of the biogeochemical situation, and to a deterioration of habitat quality. The problem is complicated by technogenic impacts on the landscape sphere of the oil and gas production complex of Western Siberia.

Keywords: valley geosystems, dynamics, zonal phenomena, regional swamping, landscape-geochemical changes, technogenic impacts, evolution.

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Development of zonal phenomena in the river valleys

The priority role played by the river system in struc-tural-functional organization of the landscape under consid-eration, and in its preservation as the taiga type of natural environment calls for an in-depth analysis of the dynamical state of valley-taiga geosystems. Local formation of forest facies in the extensive floodplain component of landscape ensures stability of its hydrological regime as well as natural diversity as the livelihood condition of the valley portion of the forest-bog territory of Western Siberia. These forest facies within the southern-taiga subzone are predominantly spruce-fir and Siberian stone pine-spruce-fir forests, while in the mid-dle-taiga zone they are fir-Siberian stone pine and spruce-fir-Siberian stone pine forests. They all refer to the green-moss group with the standing volume of 300-500 m3/ha, and with the age of stand of 200-250 years or more [11].

Reliable information on stability potential of the val-ley geosystems, the length of the periods of their dynam-ics and trends is contained in the soil and earth materials layer. The characteristic property of “memory” is imprinted in their polygenetic structure and the soils and peat layers buried under alluvial deposits. It provides a means to recon-struct spatial and temporal changes in landscape-forming processes induced by the operation of the mechanism of the river system (or, more exactly, alluvial sedimentation in this case). The ecological conditions of environment undergoing changes at particular locations create preconditions for zonal forest formation and regional manifestations. These proc-esses occur during the formation of valley-forest facies on old-alluvial facies that moved out of the meadow flooding regime, with conifers in the forest stand and podzol forma-tion in soils predominating. Regional processes include the swamping processes of swales of mature and old-floodplain massifs.

The determining factor of formation of forest facies on the old-alluvial massifs is provided by the water regime of their soil-earth materials layer governed by it granulometric composition. Of considerable importance is also the ground-water table, and the possible underflooding of the sides of the crests during floods. Pine facies occur on tall crests with pre-dominantly supes-sand sediments. The conditions of heavier granulometric composition develop dark-coniferous facies.

In the Tobolsk-Tsingalinsk natural floodplain area as the most representative for the Lower-Irtysh region and stretch-ing over the floodplain from the lower reaches of the Tobol river to the lower reaches of the Konda river [12], we investi-gated a representative (for the left-bank old-floodplain mas-sif in the bend of the Irtysh) facies in a state of a weakened alluvial regime as a consequence of the meandering river channel to the right. Here, the layer of earth materials of one of the old-near-channel crests at a depth of 203 m is repre-sented by blue-grey sandy loams with black silty interlayers with the inclusion of remnants of roots. These deposits give evidence of a long-lasting meadow-floodplain development stage of the massif during the Holocene period. The indica-

tor of an abrupt change in the alluvial regime is provided by the overlying layer of different-grain size alluvial sands (about one meter). They are overlaid by the upper layer of fine dispersed alluvium (1.2 m) characterizing the transition of the previous active stage of dynamics of the floodplain geosystem to the stage of moderate, weakened sedimentation of solid material. This mineral substratum, after the massif had moved out of the regime, developed the southern-taiga small-leaved dark-coniferous facies with fir trees in the first tree layer.

In a similar spruce-fir (wit the inclusion of birch and as-pen) facies of the old-floodplain crest in the lower reaches of the Konda, the fine dispersed layer overlapping alluvial sands is twice as small in thickness, or 60 cm. The persisting single pine trees of about 300 years old in the first tree layer indi-cate that previously there was a pine facies on sand deposits. With the subsequent transition of this crest to the alluvial regime and with the sand layer incorporating fine fractions, the forest-vegetation conditions became more favorable for dark-coniferous forest. However, its subsequent function-ing is limited by the nearby (about one meter) groundwater and swamping at the contact with adjacent spaced between crests.

Along the Ob valley, within the middle-mountain sub-zone, on low crests of old-floodplain surfaces, such as the Agansky bank, there occur birch-pine (with Siberian stone pine and spruce) facies and, in the second layer, subshrubs-lichen and small-grass-birch (with larch) green-moss facies, while on flattened crests there occur dark-coniferous-birch (with larch) green-moss forests. Their soils, with the water pH-values of 4-5 in the upper part have features of eluvial-illuvial differentiation of matter, whereas in the lower part with the water-pH values of about 6 there are features of the old-lacustrine-alluvial genesis.

Dark-coniferous forests on bank-like old-near-channel surfaces evolve also in conditions with a predominantly loamy alluvial layer without abrupt layer-wise changes in its granulometric composition. This is exemplified by the fir-Siberian stone pine (with spruce, birch and aspen) horsetail-small grass-green moss facies in the floodplain of the Baibal-akovskaya river branch at the confluence of the Irtysh with the Ob. Here, under the layer of podzolic light loam (30 cm) is a far darker (with black spots caused by the abundance of charcoals) layer about 50 cm in thickness, of moderate loam with finely nutty structure. These attributes are indicative of the forest-meadow and the preceding meadow dynamical de-velopment stages of the facies.

As a result of a subsequent intensification of the alluvial process involving surface sedimentation of larger particles and with the crest leaving the meadow-flooding regime, it has acquired the present-day forest-vegetation conditions. The lower-lying silty-oozy, heavily gleyified layer (about 4 m) characterizes the Holocene epoch of sedimentation under geochemical conditions differing from the contemporaneous conditions. The changes imply the transition of the neutral and weakly alkaline (of the calcium type) development stage

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of landscape-geochemical environment to the weakly acidic and acidic organo-mineral stage, with the typomorphic role of iron and heavy metals forming part of its group [13].

A thick peat layer occurring in the contiguous inter-bank low is represented at a depth of 1-2 m by a well decayed grass-bog mass, which is indicative of a long-lasting period of peat extraction in a relatively favorable situation for trans-formation of plant remains.

The peculiarities of the development in the past and of the present state of the taiga-floodplain facies located in the area of old floodplains are due not so much to the biotic regu-larities of successions of plant communities as to the soil and earth materials factors dependent, in turn, on channel processes, the role of which is attenuated in the course of al-luvial buildup of floodplain surface. In the case of formation of the various modifications of such facies, common to this process is the presence (predominantly in the sand layer of old-floodplain crests) of a layer of fine fractions which create favorable forest-vegetation conditions for dark-coniferous tree species. This layer usually lies in the upper (and, more rarely, in the deep-seated) part of the earth-materials layer of the crests at the final stage of their alluvial growth.

Zonal portions of valley-taiga landscape occur also on (relatively uniform in granulometric composition) silty-oozy deposits, yet with a rather clearly pronounced water runoff in this area. On the whole, dark-coniferous facies of old-floodplain massifs can supersede the pine facies following sedimentation of the fine dispersed layer overlying the layer of sand alluvium as well as the meadow-floodplain facies following the burial of their surface under the layer of silt sediments and the termination of the alluvial regime.

The aforementioned valley-forest facies are unstable as a consequence of the transformation of their zonal-invariant properties under the effect of the regional swamping process. The peats occurring in the inter-bank lows are characterized by layered alluvial silt covering and, accordingly, by high content of mineral matter (over 30%). With the increasing thickness of the silty-peat layer (the eutrophic and, then, mesotrophic development stage), it enters the oligotropic-paludic process spreading to the contiguous forest crests which are represented not only by dark-coniferous facies but also by pine facies on sedimentary supes soils. They show a clearly pronounced zonality in the form of a podzolic pro-cess complicated by an enhanced soil-earth materials hydro-morphism.

Thus, in the Surgutsky natural district on the old near-channel crests along the right bank of the Ob, in the pine lichen-green moss-foxberry facies there occur illuvial-iron gley podzols. At the depth where contrasting redox condi-tions emerge, there occur dense new formations in the form of ortsand plates, cemented by iron oxides. They, having the role of a retainer, create surface overmoistening of the crests’ supes soils thereby contributing to the encroach-ment of peat-bog formations from the inter-bank lows. The evolving peaty-illuvial-iron-humus podzols with the water Ph-value of about 4 characterize the entry of the pine facies

into the stage of mesotrophic and, subsequently, high-level swamping. Pine stands with a maximum age of 300 years are destroyed in this case. Natural degradation of forest geo-systems is enhanced by the anthropogenic factor including burns, and tree felling where logs are used in laying out win-ter trunk roads on swampy surface, especially in the valley on the left bank of the Irtysh and Ob.

Based on the discussion above, it can be said that under the conditions of the West-Siberian plains the buffer role of forests cannot withstand the high-level swamping process, so that the forested crests are gradually overlapped by the sphagnum-moss mantle. This process results in bog spaces stretching from the Trans-Uralia to the contemporary valleys of the Irtysh and Ob which have “evolutionarily” reworked this territory. And only at the tops of old-alluvial zandr outli-ers, amidst moss bogs, do there occur lichen pine forests.

Based on analyzing the history of swamping of the West-Siberian taiga and its present state, the future regressive-marsh evolution of peatlands is predicted, with their tran-sition to bog water bodies, which entails swamping of dry valleys [14]. As a result, valuable lands are decreased in the area, and the runoff-draining function of rivers is attenuated. To guard against these spontaneously irreversible processes, an effective method would be to carry out preventive ame-lioration of the swamping inter-bank lows adjacent to forest facies at the contact with valley-forest massifs.

Natural tendencies of landscape-geochemical changes

The dynamics of matter in boreal landscapes is induced by their leading functional carbon-calcium complex. Its in-tegral expression, namely, the result of the interaction of the biotic (organic) and lithogenic (mineral) components of en-vironment, involves hydrocarbonate-calcium surface water of taiga landscapes. The importance of the hydrocarbonate ion (HCO3

-) is well represented by three critical elements for biosphere which form this main ion for water, while calcium (Ca2+) prevailing among cations is characterized as the “main metal of living matter which, in the case of decomposition of plant remains, enters in small amounts the waters” [15, p. 211]. These ions fulfill the function of landscape self-clean-ing and are readily accessible to the plants.

The elements of the carbon-calcium complex combine to form an equilibrium system for a soluble calcium compound and an insoluble calcium compound: Ca(HCO3)2 ↔ CaCO3.Prominent among taiga landscapes is calcium hydrocarbonate while calcium carbonate holds the lead in steppe landscapes. Directional transformation of one of these compounds to the other, depending on environmental conditions, leads to an evolutionary change of dynamical stages of landscape. The aforementioned ions are also highly informative as regards the dynamics and directional changes inside landscapes. Thus, the HCO3

- and Ca2+ concentrations in the river waters of the region vary spatially and over a year within 0.6–6.0 mg-eq/dm3, while in the surface waters they decrease to 0.1–1.0 mg-eq/dm3.

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It was established that with the value of alkalinity and hardness of the surface waters in excess of 0.5 mg-eq/dm3,the forests experiencing waterlogging still can retain their resistance, while in the case of less than 0.3 mg-eq/dm3 the forest facies transform to bog facies. In forests undergo-ing swamping, a decrease in quantitative indicators of the carbon-calcium complex is accompanied by the shift of the reaction of the medium toward the values of acidic condi-tions, from pH 4–6 for forest soils to 2–4 to laggs (high bogs). Changes along this direction have a substantial effect on the operation of the evolutionarily established migratory-accumulative system of landscape, resulting in a decrease in mobility of some elements and, accordingly, in an increase in mobility of the others.

Swamping of forest territories drastically alters the land-scape-geochemical situation. First and foremost there is taking place a change of the calcium-nitrogen type of bio-geochemical turnover of forests for the silicon-nitrogen type of high bogs. According to the analytic for more than 500 vegetation samples, the calcium concentration in the ash is 135-275 (averaging 200) g/kg for tree species, 20-200 (90) for meadow-grass vegetation, and 7-120 (45) g/kg for the sphagnum-peat layer. The silicon concentrations in this se-quence, on the contrary, increase and are, respectively, 6–20 (10) g/kg, 25-310 (140), and 145-370 (260) g/kg. There are still more contrasting conditions for the forest and bog ge-osystems which provide the following values of the Ca/Si ratio: 15-23 for tree vegetation, and only 0.1-0.3 for sphag-num-moss cover.

The aforementioned regularity is supported, according to the data on absolute concentrations of elements, by their relative indicators, or the coefficients of biogenic migration (Cb). The value of the calcium Cb in forests averages 23, and it decreases in high bogs to 8; in silicon, with its typi-cal values of Cb < 1, this coefficient increases, respectively, from 0.1 to 0.7.

The integral expression of the decrease in biogeochemical activity of calcium and of the enhancement of this role for silicon in the case of taiga landscape swamping is provided by the concentration of these elements in surface waters. The cal-cium content in the region’s river waters, according to the data of analysis of 240 samples, is 9-135 (averaging 55) mg/dm3,in the bog waters (100 samples) 7-90 (30) mg/dm3, and for silicon, respectively, 2-26 (10), and 6-59 (22) mg/dm3.

The aforementioned evidence for the hydrocarbonate-calcium complex and for the calcium/silicon ratio in natu-ral components as informative elements of landscape-geo-chemical changes as a consequence of swamping of the taiga geosystems reveal a serious problem related to the ongo-ing deficit of vitally important calcium. Some contribution is also made by the podzol-forming process in the region’s soils, and by the removal of alkaline-earth elements with the ion runoff.

Another problem is associated with iron. The coefficient of its biogenic migration in moss bogs (0.9) is by a factor of three higher than that in forests, and the coefficient of wa-

ter migration (1.2) is higher by more than an order of mag-nitude. In the course of swamping of the taiga territory the tendency of the values of the Ca/Fe ratio in biogenic objects for a decrease from 31-45 in forests to 1-12 in bogs, and from 20 in river waters to 3 in bog waters is indicative of the evolution of the landscape-geochemical environment toward a decrease in functional activity of calcium, and toward an enhancement of this role for iron.

Equally informative is the Fe/Mn ratio. While iron acts as a typomorphic element of hydromorphic localities, man-ganese is a typical taiga-specific heavy element of optimally humidified forest geosystems. The functional and indication differences of these elements are clearly seen in the Fe/Mn values for wood ash (0.2-0.5), grass ash (0.5-2), and moss-bog ash (16-50). The range of values from <1 to 50 of this in-dicator of substance-dynamical state of plain-taiga landscape accommodates the entire spectrum of its subdivisions.

The regional biogeochemical problem of iron intensifica-tion is aggravated by the same tendency as regards the ele-ments forming part of its group (copper, zinc, nickel, cobalt, titanium, vanadium, and chromium) as well as lead repre-senting heavy, toxic metals for the biota. Thus, the mean val-ues of the coefficient of water migration for forests and high bogs, respectively, are 2.5 and 10.5 for copper, 1.3 and 7.0 for zinc, 0.4 and 1.8 for nickel, 01 and 0.3 for cobalt, 0.2 and 1.2 for vanadium, 0.1 and 0.5 for chromium, and 0.5 and 4.5 for lead. The chief reason behind the higher water-migration indicators for high bogs is the acid reaction of their envi-ronment, and the abundance of organic matter that produces with many elements migratory forms of compounds.

The principal reason behind the aforementioned direc-tional landscape-geochemical changes in the course of the progressive bog-forming process is the abrupt decrease in activity of the metabolism of organic compounds. Observa-tional evidence for the Lower Irtysh region suggests that the CO2 liberation rate from forest soil surface is 0.8-2.0 (aver-aging 1.4) kg/ha per hour, and for high bogs it is 0.2-1.0 (0.4) kg/ha per hour. Such a reduction of the supply of carbon di-oxide as the source of photosynthesis of vegetation to the ground-level atmosphere of territories experiencing swamp-ing should be regarded as a limiting factor of bioproductivity on the regional scale.

As regards the optimization regulation of the substance-dynamical state of plain-taiga landscape, amelioration meas-ures aimed to intensify the biogeochemical cycles of carbon and calcium would be effective.

Technogenic changes in landscape-geochemical environment

In addition to the natural factors that are responsible for the biogeochemical unfavorable situation of the plain-taiga region of Western Siberia plus the occurrence of endemic diseases of animals and humans [16], the natural environ-ment is experiencing the technogenic pressure from the oil production complex predominantly in the Middle Ob region.

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Most of its facilities, situated on low floodplain terraces and even in the floodplain, form part of the group of increased risk (see the figure). This refers, in particular, to the first (dis-covered in 1961) Megionskoye oil field at the old-floodplain massif between the Mega and Mulka branches of the Ob.

During high floods the imperfect engineering protection of the drilling platforms with inadequate water proofing and with no recultivation of the used areas do not save the situ-ation – the water environment still receives oil and proc-ess fluids referring to toxicity class II-IV. Given the natu-ral content of hydrocarbons in the region’s surface waters in the range of 0.01-0.20 mg/dm3, the concentration of oil products in the waters of the middle Ob is 0.1-0.4 mg/dm3

[17], whereas the MAC for the waters of fishery utilization is 0.05 mg/dm3.

The object of complete technogenic transformation of the water environment is represented by the region’s larg-est, waterlogged Samotlor oil field. The water body of Lake Samotlor was transformed to the infrastructure of the oil pro-duction complex, with oil extracted from a depth of down to 2.6 km. As a result of the impact of salt-containing drilling fluids and grouting mortars, the hydrocarbonate class of the group of calcium was replaced by the chloride class of the group of sodium (Cl- up to 2 g/dm3, and Na+ 0.1-0.2 g/dm3).No sulfate pollution was detected. Overall, the total miner-alization of the lake (650 mg/dm3) became nearly an order of magnitude higher than that for the surface waters of the Middle Ob region.

The products from the oil production complex exert on the natural environment an integral effect thereby disturb-ing the physicochemical conditions for the operation of the migratory-accumulative mechanism of differentiation of matter. First and foremost this applies for the alkaline-acidic conditions. In oil-polluted soils, pH is increased by 1-2 units. At the natural background of the acidic and weakly acidic soil environment, the pH-values of technogenic matter (bot-tom substratum of settling basins, technogenic pulp spilling over the surface, etc.) reach alkaline values, 8.9-9.1.

Analytic data suggest that in the fuel oil-polluted grass and moss mantles, the concentrations of some heavy metals (Ti, V, Cr, and Ni) are increased, with a concurrent abrupt decrease in concentrations of alkaline-earth and biogenic elements (Ca, Mg, Sr, P, Mn, and Cu) because of the decline in intensity of the metabolic processes in plants under the oil film.

Changes in the subterranean and other environments are also clearly pronounced. In, for example, the geologi-cal environment in the case of deep-drilling operations there is taking place the absorption of process fluids to produce flows between different aquifers. Sometimes there occur dis-charges of pressure brines to the surface.

The spontaneous transformations of landscape structure considered above are enhanced dramatically by technogenic impacts from thousands and thousands of drilling platforms. To maintain them in good conditions in order to avoid over-moistening, backfilling is used, which results in the burial of soil and plant cover. Mechanical disturbances of swamped surfaces lead to an enhancement of their hydromorphization. Combustible materials and lubricants, and also the numerous accidents on oil pipelines [18] are responsible for large-scale forest and peat fires, and for the concomitant irreversible hy-drothermal and, in general, geoecological changes in unsta-ble landscape realm of the West-Siberian plain.

Conclusion

The analysis of the various forms of zonal and regional manifestations in the extensive West-Siberian valley geosys-tem has demonstrated its integration role in the structural-functional organization of plain-taiga landscape. The evolu-tionary-genetic coupling of the alluvial and zonal-regional structures of the valley geosystems is the key element of the currently important and potential integrity of plain-taiga landscape. In our view, it is the guiding principle in forecast-ing-geographical research.

The ongoing intense swamping of this territory engen-ders a number of problems. Among them, evolutionary landscape-geochemical changes are highly important, along with the decrease of the river runoff and in bioproductivity of the territory. They involve, first of all, disturbances of the alkaline-acidic equilibrium, i.e. the shift of environmental parameters toward their acidic values. Given this situation, there is taking place a clearly pronounced tendency toward a decrease in the intensity of the metabolism processes of or-ganic matter, bio-water-migration activity, and in concentra-tions of alkaline-earth elements in natural components, with a concurrent increase in this activity, and with an accumula-tion of iron, a number of heavy metals, silicon, and lead in the environment.

The regional biogeochemical problem, caused by the deficiency in the landscape components of vitally important calcium, and, on the other hand, by the excessive amount of toxic elements of the group of iron as well as of lead, is exacerbated by the technogenic factor of environmental im-pacts from the extraction and transportation of subterranean Fig. Development of the oil field in the valley of the middle Ob.

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hydrocarbon raw materials in the region. Furthermore, it is of utmost importance to pose the question as to whether the economic effect of this sector of the industry is comparable with the ecological damage it is inflicting to the landscape realm, and to the population.

To prevent a further reduction in biodiversity across the territory in connection with the natural-evolutionary proc-esses, and with the techogenesis, it would be appropriate to undertake hydrotechnical amelioration measures in areas at the contact of forest and bog geosystems as well as to ar-range a monitoring of the set of the aforementioned informa-tive indicators of substance-dynamical state and evolution tendencies of plain-taiga landscape.

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