Holocene development of soils in response to environmental changes: the Novosvobodnaya archaeological site, North Caucasus

Catena 41 Ž2000. 237–248 www.elsevier.comrlocatercatena

Holocene development of soils in response to environmental changes: the Novosvobodnaya archaeological site, North Caucasus Alexander L. Alexandrovskiy ) Institute of Geography, Russian Academy of Sciences, Staromonetnyi per. 29, Moscow, 109017, Russia

Abstract Soil-archaeological studies in North Caucasus indicate that the expansion of broad-leaved deciduous forests from mountains to foothills and piedmont plain took place about 3000 years ago, after the Middle Holocene steppe stage. At this time, steppe Chernozems transformed into Luvisols under forests. Often the Luvisol profiles preserve the relict humus horizon and paleokrotovinas Žmole tunnels. inherited from the chernozemic stage. Within the same period, normally developed soils evolved on the surface of burial mounds: Luvisols on mounds composed of loess and chernozemic material and Rendzinas and Luvisols with a calcareous horizon on loamy mounds with a limestone paving Žstone armour. used to strengthen the mounds. The rates of profile differentiation and some pedogenic processes have been calculated. In the eluvial horizons of Luvisols developed on loess or charnozemic material, the rate of clay loss was up to 7–8 g my2 year. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Soil; Novosvobodnaya; Holocene period; Luvisols

1. Introduction Soils on the surface of burial mounds of different ages and those buried under the mounds are valuable for studying the trends and rates of pedogenic processes during the Holocene period ŽParsons et al., 1962; Ivanov, 1992.. Soils of different ages Žsoil chronosequences. have been studied mainly on natural geomorphic surfaces, such as the morainic deposits of mountain glaciers, lacustrine and river terraces and coastal sandbars ŽStevens and Walker, 1970; Gennadiev, 1978; Bettis, 1994.. However, soil evolution on loess and covering loams within forest and forest-steppe regions has been investigated )

Tel.: q7-095-959-0028; fax: q7-095-959-0033. E-mail address: [email protected] ŽA.L. Alexandrovskiy..

0341-8162r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 0 0 . 0 0 1 0 5 - 3

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less. This reflects the lack of sufficiently complete chronosequences of Holocene soils on natural geomorphic surfaces composed of loamy material and a failure to recognise the value of soils associated with burial mounds. In this connection, the soil chronosequences we have studied in the northwest Caucasus are of special interest. They have developed on the surfaces of burial mounds composed of loess and other rocks, and date from different periods in the second half of the Holocene. The soils allow us to trace soil evolution in response to climatic and environmental changes. The character of soils buried under the mounds and the presence of relict humus horizons in surrounding surface Žbackground. soils indicate a sharp change in the Holocene environment: a steppe stage with Chernozems was replaced by a forest stage with Luvisols ŽAlexandrovskiy and Birina, 1987.. Similar patterns of soil evolution induced by environmental changes have been inferred for the Russian Plain ŽAlexandrovskiy, 1996., Central Europe ŽBork, 1983. and North America ŽRuhe, 1974.. However, there is no agreement on the time when these changes took place. According to Bork Ž1983., the steppe stage ended in the first half of Holocene; but Buol et al. Ž1963. believed that it ended in the second half of the Holocene. The character of soil formation during the Atlantic period also remains uncertain. According to the traditional Blitt–Sernander scheme, this period was relatively humid, but some palaeosol data ŽCzerney, 1965; Karavaeva, 1978; Alexandrovskiy, 1996. and pollen studies ŽSerebryannaya, 1994. indicate mainly arid conditions in Russia during the Atlantic period. The history of Holocene climate in the North Caucasus was very complicated. Palaeobotanical data suggest that a relatively arid steppe environment persisted here during the first half of Holocene. A more humid climate, which favoured the invasion of forests over the steppe, was then established in the Late Holocene ŽGalushko, 1976.. According to Yakovlev Ž1915., soil evolution in the northwest Caucasus was determined by an increase in climatic humidity and passed through three main stages: Ža. the chernozemic stage, Žb. a stage of gradual degradation of Chernozems with the appearance of vertic properties, and Žc. the stage of Chernozem transformation into grey forest soils with a second humus horizon. Similar sequences of events were established for the whole Caucasian region ŽZakharov, 1935.. Paleosol data suggest that the foothills were marked by stronger changes in soil development than the piedmont plain, where steppe ecosystems persisted throughout the Holocene ŽAlexandrovskiy, 1996.. However, shortterm fluctuations of climate may have corresponded with changes in vegetation and pedogenic processes. Soils formed on dated surfaces and buried soils of different ages have special significance for the study of Holocene soil evolution ŽGennadiev, 1990.. In this paper, we report a study of soil evolution based on surface and buried soils of different ages near the Novosvobodnaya settlement, south of Maykop city in the North Caucasus.

2. Materials and methods The Novosvobodnaya site is located within the foothills of the northern macroslope of the West Caucasus under an oak-beech forest at 550–600 m a.s.l. ŽFig. 1.. The

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Fig. 1. Locations of sites and biomes in northwest Caucasus. ŽI. Modern steppe and flood plain; ŽII. Late Holocene forests on the Piedmont; ŽIII. Late Holocene forests on the Foothills; ŽIV. Middle and Late Holocene forests on the mountains. Ž1. Modern boundaries of biomes; Ž2. Middle Holocene boundary of forests; Ž3. mountains; Ž4. Investigated sites.

soil-forming rocks are brown loams with some admixture of sand. Carbonates are found at depths of more than 1.5 m. The soils developed from loams are classified as grey forest soils ŽLuvisols.. Rendzinas occur on the outcrops of calcareous rocks. Mean annual temperature reaches q108C and mean annual precipitation is about 800 mm. Typical Chernozems occur under burial mounds 5000–5500 years old and up to 8 m high. About 3000–3500 years ago forests spread down from the mountains to the foothills and piedmont plain because of an increase in humidity. The initial Middle Holocene steppe Chernozems were then degraded under the forests and were transformed into Luvisols. Soil carbonates were leached away and the soils were acidified. However, the lower part of the chernozemic humus horizon with krotovinas was partly preserved within the newly formed Bt horizon of the Luvisols. The initial Chernozems, slightly changed by diagenesis, were studied under the burial mounds, and were compared with surface soils on mounds composed of different materials and also the surface soils of surrounding areas not affected by construction of the burial mounds Žthe background soils.. The following soil variants were studied: 1. Mature Luvisols formed on burial mounds up to 5–7 m in height and up to 60 m in diameter, with ages of 4500–5500 years, which are composed of Chernozem, loess, eluvial horizons of soils, and alluvial deposits of various lithological compositions;

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2. Rendzinas and Luvisols with a calcareous horizon formed on mounds composed of loess sediments with an artificial limestone paving, which was constructed on the top and sides of the mounds to strengthen them; 3. Chernozems buried under mounds of the Bronze Age; 4. Degraded Chernozems and Luvisols buried under younger mounds; 5. Interglacial ŽMikulinskaya epoch. soils buried within the loess deposits; 6. Surface Žbackground. Luvisols of surrounding areas with the second humus horizons inherited from the chernozemic stage of soil development and with krotovinas and carbonate concretions; 7. Luvisols of truncated surfaces from which the material for the construction of burial mounds was taken; 8. Leached Chernozems buried under limestone paving and dolmens. Standard methods for studying surface and buried soils were used ŽIvanov and Aleksandrovskiy, 1987.. The soils of different ages were compared with respect to properties such as the thickness and degree of profile development and the extent of illuvial accumulation in different parts of soil profile, using analytical data for humus content and composition, carbonate content, particle-size distribution and radiocarbon dating. The 14 C ages of humic acids were obtained by liquid scintillation counting.

3. Results 3.1. Stages of soil eÕolution Modern Luvisols of the North Caucasus contain relict features of the Middle Holocene stage of soil development, such as the second humus horizon, palaeokrotovinas and carbonate concretions in the subsoil ŽFig. 2, profile 156s.. The age of the second humus horizon, based on 14 C dating of humic acids, is as great as 7130 " 40 years ŽIGAN-1084, Table 1.. This horizon and paleokrotovinas within the Bt horizon of the modern Luvisols represent the lower part of the paleochernozemic profile. Its upper part has degraded under the later forest vegetation. Sometimes the second humus horizon is completely degraded in the soil profile. Increase in climatic humidity has led to the appearance of gleyic features in the B horizons of Luvisols. The background soils that were overlain by limestone paving in the Bronze Age have preserved the humus profile of Middle-Holocene Chernozems, but are depleted of carbonates to depths of 1.5–2.0 m. The soils under burial mounds of different ages represent the Middle Holocene Chernozems and the first stages of their degradation after the appearance of forest vegetation. Non-degraded typical Chernozems buried under Bronze Age Ž3.5–5.5 ka BP. mounds resemble the Chernozems of the modern steppe zone located on the piedmont plain, 30–50 km to the north of the Novosvobodnaya site. The 14 C age of humic acids from the Chernozem buried under one of the oldest mounds ranged from 6450 " 100 to 9780 " 580 BP ŽIGAN-1213, 1156 to 1154, Table 1.. These data indicate

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Fig. 2. Cross-section of burial mounds and distribution of clay content in surface and buried soils.

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Table 1 Radiocarbon dates of humic acids in buried and surface Žbackground. soils at the Novosvobonaya site Žtime of burial 5500 BP. Ža. Buried Chernozem

Žb. Background Luvisol

Lab code ŽIGAN.

Soil horizon; deptha Žcm.

14

C age Žyear BP.

Lab code ŽIGAN.

Soil horizon; depth Žcm.

14

1213 1156 1155 1154

A11 0–15 A12 25–35 AB 55–65 BA 75–85

6450"100 7105"200 8240"330 9780"580

1086

A1E 0-20

93"9 b

1084

BtAh 80-100

7130"40

C age Žyear BP.

a

Depths of buried soil horizons are given from the level of buried surfaces. In the sample of soil that was affected by nuclear bomb14 C, the 14 C age is determined according to the method of Cherkinsky and Brovkin Ž1993.. b

that the Chernozemic stage of soil development started at the beginning of the Holocene. Also, dates from similar depths ŽIGAN-1154 and 1084, Table 1. suggest that humus in the second humus horizon of the background soils has experienced some rejuvenation Žup to 2650 years during the last 5500 years, or 50%.. Diagenetic processes under the burial mounds have considerably altered the buried Chernozems. Their humus content has decreased, their pH has increased and diagenetic carbonates have appeared in the humus horizon Že.g. profile 156b, Table 2.. However, stable soil features, such as the horizon differentiation of the profile and the particle-size distribution, have remained unchanged. In particular, the initial calcareous horizon with a high CaCO 3 content is well preserved, and the distribution of clay Ž- 0.001 mm. through the profile retains a typical chernozemic pattern. The soils buried under younger mounds and ramparts constructed after development of forest vegetation are represented by degraded Chernozems and Luvisols with relict humus horizons and paleokrotovinas. 3.2. Soil deÕelopment on dated surfaces The surface soils studied were developed on mounds dating from 1.0–5.5 ka BP under forest vegetation. Luvisols with strongly differentiated profiles occur on the mounds composed of loess with an age of 3500 years Žprofiles 166, 153 154; Fig. 2, profile 166, Table 2.. These soils are depleted of carbonates and have an acid reaction, especially in the lower part of their profiles. A thick eluvial horizon depleted of clay and a well-developed Bt horizon with distinct illuviation cutans and increased clay content have developed. Similar soils occur on the mounds constructed of chernozemic soil material ŽFig. 3.. Rendzinas form on the mounds composed of calcareous loams and clay with limestone paving debris. They are identical to the Rendzinas formed on outcrops of limestone within the area. On the mounds composed of loess and chernozemic material and overlain by limestone paving, the additional input of CaCO 3 from limestone has impeded leaching

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Table 2 Soil analytical results hh: the relict humus of the second humus horizon. Horizon depth, cm

Humus, %

pH H 2O

CaCO 3 , % KCl

166. LuÕisol on a mound constructed from loess AE 0–23 4.5 6.4 5.3 EA 23–51 1.6 6.4 4.7 B1E 51–67 0.8 5.7 3.9 B1t 67–80 0.9 5.5 3.8 B2t 80–112 0.9 5.5 3.8 B3t 112–152 0.6 5.6 3.9

Exchangeable cations, mer100 g Hq

Ca2q

Mg 2q

0.4 0.8

18.0 9.2

3.2 2.1

2.4

14.1

4.5

1.3

13.9

4.8

158. LuÕisol on a mound composed of chernozem with a limestone paÕing A 0–10 3.0 5.2 4.3 – 0.2 13.7 EA 10–25 3.2 5.5 4.6 – 0.1 12.8 AEB1 25–40 5.3 6.2 5.4 – 0.05 22.0 EB 40–60 1.7 5.2 3.9 – 0.5 14.5 B11t 60–80 1.7 4.9 3.7 – B12t 80–100 1.3 5.5 4.1 – 0.3 22.3 B2k 100–140 1.8 8.1 6.9 3.3 156b. Typical Chernozem buried under burial mound 5000 BP A 0–40 2.4 7.9 6.4 0.5 AB 40–60 1.5 8.2 6.5 0.1 BA 60–90 0.9 8.4 6.6 0.1 B1 90–100 0.7 8.5 6.6 0.1 B2ca 100–140 0.4 8.9 7.2 9.5 156s. LuÕisol on the surrounding surface A 0–15 5.3 6.6 5.8 A 15–20 3.0 5.9 5.0 AE 20–30 1.7 5.9 4.9 EB 30–50 0.7 5.5 4.2 BEhh 50–60 0.8 5.5 4.2 B1thh 60–80 1.6 5.5 4.2 B2thh 80–100 1.2 5.6 4.2 B2t 100–130 6.0 4.7 B3tg 130–150 6.8 5.1

1.6 1.3 2.2 1.4 1.0

Clay - 0.001 mm, %

12 10 20 36 42 39

21 15 16 28 31 37 27

22.7 25.2 22.5 21.5

2.9 4.0 3.0 3.5

28 29 29 29 25

0.05

18.3

2.9

0.07 0.26 0.27

8.5 8.9 13.2

1.6 1.9 2.6

0.08

25.5

3.6

13 12 12 21 30 44 47 44 42

and other profile differentiation processes. A Rendzina with two calcareous horizons has formed on the mound with a thicker limestone paving. The lower calcareous horizon occurs at a depth of more than 120 cm. The upper, weakly developed calcareous horizon is confined to the base of the limestone paving at a depth of 50 cm. A Luvisol ŽProfile 158. with well developed A1 and E horizons, but with a thin Bt horizon and relatively shallow carbonate separations in fissures, has formed on the mound with a thin limestone paving ŽTable 2.. This soil has higher pH values and a somewhat greater humus content than the background Luvisols.

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Fig. 3. The trends of soil development on burial mounds and neighbouring areas on different rocks.

Luvisols are also found on the burial mounds and ramparts composed of carbonate-free rocks and constructed about 3.5–2.0 ka BP. The younger surfaces are occupied by less developed soil profiles. Thus, the soils of burial mounds, whose age does not exceed 1000 years and which were constructed from the humus horizon of Luvisols, do not have distinct morphological differentiation. This can be explained by the fact that the soil-forming substrate Žhumus horizon of the Luvisols. remains in equilibrium with the environment; much longer periods are required for its transformation.

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4. Discussion 4.1. Stages of soil eÕolution Paleopedological data indicate that the territory studied experienced strong environmental changes during the Holocene. Archeological and radiocarbon dates of buried Chernozems indicate that the soils have passed through two different stages of pedogenesis: the steppe stage Ž10–3.5 ka BP. and the forest stage Ž3.5–0 ka BP.. Thus, in the Early and Middle Holocene steppe landscapes extended up to a height of 700 m. The construction of burial mounds that buried the steppe Chernozems started at the end of the steppe stage. Surface soils of adjacent areas developed for some time under steppe vegetation. The deciduous broad-leaved forests expanded from the mountains over the footslopes Ž50–100 m a.s.l.. about 3500 years BP and fundamentally altered the trend of pedogenetic processes: Chernozems started to transform into Luvisols. First, degraded Chernozems with a thin eluvial horizon appeared, then Luvisols with a thick eluvial horizon, a relict second humus horizon and paleokrotovinas developed. In conditions of good drainage, the relict features in Luvisols can be completely destroyed. The rates of soil transformation were relatively high and the profiles of initial Chernozems seem to have been very susceptible to environmental change. In contrast to Chernozems, the soils that developed on calcareous rocks were much more stable. They have preserved the properties acquired during the steppe stage of soil formation until the present time, except for some decrease in humus content. Transformation of Chernozems into Luvisols was also retarded under limestone paving. Inputs of CaCO 3 from the limestone neutralised the acidity of the soils and slowed down the degradation of humic substances and the textural differentiation. The surface soils studied are relatively stable. They are unchangeable ŽRendzinas., or change very slowly Žleached Chernozems under limestone paving., or have already passed through the stage of strong transformation ŽLuvisols with degraded relict humus horizons.. The last soils are still subject to some transformation, but its rate is very slow. 4.2. Soil deÕelopment on the surface of burial mounds Mature soils very similar to the background soils developed on the surfaces of burial mounds during the forest stage that started at 3.5 ka BP. The specific conditions on the tops of the mounds Žmore rapid runoff and thus a drier environment compared with level background surfaces. did not exert a significant influence on these soils. Near the boundary between the modern forest and steppe zones, where annual precipitation is lower and the precipitationrevaporation ratio is slightly more than 1.0, the Luvisols that developed on the tops of burial mounds have thin profiles and weakly expressed textural differentiation. Mean annual precipitation at the Novosvobodnaya site is considerably greater, and soil climatic conditions here are more favourable for development of Luvisols. The character of the parent material has played a more significant role in the direction of pedogenetic processes than the topography of burial mounds. The soils on the mounds composed of loess and chernozemic material were subject to relatively rapid leaching of

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carbonates, after which the development of the Luvisol profile took place. Microclimatic conditions under the forest canopy favoured leaching by acidic solutions, which led to degradation of humus to a great depth; as a result, the chernozemic material has become similar to the loess. Simultaneously, lessivage and the formation of a texturally differentiated soil profile has taken place. During the initial stages of development, the soils on the tops of the mounds may preserve the initial quasi-stable state because of the reserves of weatherable minerals. Later, the intensity of profile differentiation increases, and after 2000–2500 years the soil with well-developed textural differentiation reaches the quasi-equilibrium state with the environment. The soils on the truncated surfaces around the burial mounds Ži.e. the surfaces from which material was taken to construct the mounds. are characterised by similar Luvisolic profiles. These also must have begun development from the truncated Chernozems or from loess substrate about 3500 years ago. The Rendzinas that developed on the mounds composed of calcareous loams and clays with limestone debris are the most stable Žsteady-state. soils, even in conditions of sharp environmental changes ŽFig. 3.. Two types of quasi-stable soils ŽRendzinas and specific Luvisols. developed on the mounds composed of loess and chernozemic material overlain by limestone paving. The Rendzinas have two separate calcareous horizons: the main one at depths of 1–1.5 m developed as a result of leaching of carbonate from the topsoil, and a thin calcareous horizon has developed in the upper part of the profile by leaching of carbonates from the paving. The Luvisols have a thick humus horizon and shallow depth to carbonate and have developed on the mounds with a thinner limestone paving than those with Rendzinas. Pedogenic changes that took place after the construction of mounds were irreversible. After complete destruction of the limestone paving and exhaustion of the reserves of CaCO 3 , the soils will be subject to stronger leaching, with development of mature Luvisols on brown loams. 4.3. The rate of the processes of textural differentiation On the background surfaces and burial mounds the textural differentiation of Luvisols occurred within initially homogeneous Žwith respect to clay content. Chernozems. The total clay removed from the eluvial horizon of Luvisols reaches 210–240 kg my2 , and accumulation of clay in the Bt horizon reaches 115 kg my2 ŽAlexandrovskiy and Birina, 1987.. The remainder of the clay has been completely removed from the soil profile. Hence, the mean annual loss of clay from the eluvial layer was up to 7–8 g my2 , whereas the rate of clay accumulation in the Bt horizon was at least two times less. As there were some short drier periods during the Late Holocene forest stage, when steppe vegetation could have redeveloped, the rates of carbonate leaching and clay loss were probably not constant. 5. Conclusions The development of Luvisols in the foothills of northwest Caucasus started near the end of the Middle Holocene Ž3.5 ka BP., after an increase in climatic humidity and

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expansion of forest vegetation from the mountains over the foothills previously occupied by steppes. The forest margin moved by up to 50 km, and in this zone numerous burial mounds constructed by steppe nomad tribes of the Bronze Age became covered by forest. The Novosvobodnaya site underwent strong environmental changes, and the transformation of Middle Holocene steppe Chernozems into forest Luvisols here required less than 3000 years. During this period, the carbonates were completely leached from the soil profile, distinct textural differentiation developed and gleyic features appeared in the soils. The upper part of the Chernozems was transformed into the A1, E and EBt horizons of Luvisols, but the lower part was preserved in the Luvisol profiles as relict second humus horizons and as paleokrotovinas within the Bt horizon of the Luvisols. Soil development on dated surfaces of burial mounds during the last 5500 years was characterised by different trends and rates of pedogenic processes that mainly depended on the character of the substrate. The presence of carbonates in the parent material impeded the development of Luvisols. The soils on the mounds composed of loess and chernozemic material with low carbonate contents were soon depleted of carbonates, after which textural differentiation began. In contrast, the soils on calcareous rocks do not show any features of illuviation and are classified as Rendzinas. Soils occupying a transitional position between Rendzinas and Luvisols, such as leached Rendzinas and Luvisols with a calcareous horizon at 1–1.5 m depth, developed on the mounds with limestone paving and can be considered quasi-stable soils. After destruction of the limestone paving and depletion of this additional CaCO 3 , they will be subjected to active leaching, and should finally evolve into well developed Luvisols. Luvisols that developed on the mounds composed of loess and chernozemic material, on background surfaces and on surfaces near the mounds truncated more than 2000–3000 years ago are very similar morphologically and are characterised by the same degree of profile differentiation. Their eluvial horizons have lost about 210–240 kg my2 of clay, and the rate of clay loss was up to 7–8 g my2 yeary1 . References Alexandrovskiy, A.L., 1996. The Holocene evolution of soil cover of the Russian Plain. Eurasian Soil Sci. 28 Ž3., 20–32. Alexandrovskiy, A.L., Birina, A.G., 1987. Evolution of grey forest soils in the foothills of North Caucasus. Pochvovedenie 8, 28–39, Žin Russian.. Bettis, E.A. III, 1994. Soil morphologic properties and weathering zone characteristics as age indicators in Holocene alluvium in the Upper Midwest. In: Holliday, V.T. ŽEd.., Soils in Archaeology: Landscape Evolution and Human Occupation. Smithsonian Institution Press, Washington, pp. 119–144. Bork, H.R., 1983. Die Holozane Relief-und Bodenentwicklung in Lossgebieten. Catena Suppl. 3, 1–93. Buol, S.W., Hole, F.D., McCracken, R.J., 1963. Soil Genesis and Classification. Iowa State University Press, Ames, IA. Cherkinsky, A.E., Brovkin, V.A., 1993. Dynamics of radiocarbon in soils. Radiocarbon 35 Ž3., 363–367. Czerney, P., 1965. Bodenkundliche Untersuchungen an einer fossilen und einer rezenten Schwarzerde. Albrecht-Thaer-Archiv. 9 Ž10., 867–881. Galushko, A.I., 1976. Analysis of the flora of the western part of Central Caucasus. In: Flora Severnogo Kavkaza i Voprosy Istorii. wFlora of the Northern Caucasus and Problems of its Historyx Stavropol State University Press, pp. 5–130, Žin Russian..

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