SOURCES, EMISSIONS AND CONTROL OF HEAVY METALS Chronological Evolution of Mobile Lead Isotopic Composition in Buried Soils over 3.5 Millennia as an Indicator of Increasing Anthropogenic Impact T.V. Pampura1, A. Probst2, V.A. Demkin3

1Institute of Physicochemical and Biological Problems in Soil Science RAS, Pushchino 142290, RUSSIAN FEDERATION, [email protected] 2Laboratoire d'Ecologie Fonctionnelle ENSAT/INPT, Av. de l'Agrobiopole BP 32607 AUZEVILLE TOLOSANE 31 326 Castanet-Tolosan Cedex, FRANCE, [email protected] 3Institute of Physicochemical and Biological Problems in Soil Science RAS, Pushchino 142290, RUSSIAN FEDERATION, demkin@issp..su

Abstract A chronological sequence of paleosols buried below archeologically dated kurgans (bur- ial mounds of nomadic tribes, Low steppe in ) and covering 3.5 millennia was investigated in order to evaluate the anthropogenic Pb impact on soils. Soil profile buried below a kurgan was used as an "uncontaminated" background soil. Isotopic ratios of mobile (extracted by ammonium acetate buffer, pH 4.8) and total lead were analysed, and it was shown that mobile lead isotopic ratios is the more sensitive indicator of anthropogenic impact than those ones of the total lead. Temporal trends of mobile and total lead isotopic ratios since the Bronze Age until the present were compared with correspondent trend of Pb averaged isotopic ratios in European atmospheric depositions (recorded in the peat cores) and modern Russian aerosols. A clear shift towards "less radiogenic" isotopic ratios was observed in the case of mobile lead in soils and air depositions, indicating the increasing influence of anthropogenic (ore-derived) Pb input. However, mobile lead represents only a small portion of the total pool (0.2-15%), and no substantial shift in the total Pb content or isotope ratios was found within soil chrono- logical sequence, indicating a very low level of contamination of modern soils by lead, except soils in the vicinity of the motorway (within 10 m). Key words: Lead isotopes, buried soils, peat cores, depositions, environmental recon- structions.

Introduction north - northeast of Volgograd. In the year The history of anthropogenic Pb depo- 2005 excavations of three kurgans located 5 sition in Europe over millennia is recorded km north of a village Salomatino (Kamyshin using peat and ice archives and reported in district, Volgograd region, Russia), were per- plentiful publications. However, the actual formed by a joint expedition of the IPBPSS effect of growing anthropogenic lead input RAS (Pushchino) and Volgograd State via atmosphere on soils remains unclear for University. The erection of kurgans is dated lack of an "uncontaminated" reference base- back to the Bronze Epoch (16th-15th cen- line. We propose to use as a basis for compar- turies BC: the Srubnaya culture), and the ison the paleosols preserved under the arche- Early Iron Age (late 2nd to early 3rd cen- ologically dated burial mounds (kurgans) turies: late Sarmatian Culture). Kurgans and protected from air pollution during cer- form a triangular-shaped group with the tain known period. Coupling analysis of Pb sides of 35-35-45 m. A pit of the modern ref- isotopes profile distribution in paleosoils erence soil was located close to the middle of and in their modern analogues may give a the kurgan group. In order to study the influ- new opportunity to compare sources and lev- ence of transport on soil contamination, els of metal in soils nowadays, and in the samples from A horizon (0-5 cm) were col- ancient times. lected at the distances of 2, 5 and 10 meter from a local road, and from the a major Materials and Methods motorway -Volgograd M6. The study area is located in the dry The lead concentration and isotopic steppe zone of the Low Volga, at the south composition in the total and mobile (extract- end of the Privolzhskaya Upland, 150 km ed by ammonium acetate buffer with pH 4.8)

946 15th ICHMET SOURCES, EMISSIONS AND CONTROL OF HEAVY METALS fractions were analyzed using ICPMS Agilent 2006). In our study, concentrations of 7500 Ce (LMTG-CNRS, Toulouse, France, "mobile" Pb and its fraction in the total pool and Moscow State University, Russia). varied from 2.9 to 0.02 mg kg -1, and from 15.4% to 0.22 % respectively. The highest Results and Discussion values were found in soil near the motorway, "Mobile" lead concentration could be a the lowest ones were in B2 and Dca horizons better indicator for anthropogenic Pb con- of the Bronze Age paleosol. Regular decrease tamination than total Pb, since industrially of mobile Pb concentration with depth was contaminated soils are usually enriched in a found only in the "oldest" Bronze Age soil less tightly bound Pb fraction (Jensen et al., profile. This soil has lowest concentrations

Figure 1. Chronological evolution of isotopic composition of atmospheric depositions, recon- structed using literature data on peat bogs cores: 207Pb/206Pb (a) and 208Pb/206Pb (b). Time intervals of kurgan erection (around 3500 and 1800 yr BP) are indicated by vertical lines.

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Figure 2. Comparison of chronological evolution of lead isotopic composition in soils and in atmospheric depositions (shaded regions): a - total Pb, arrow D-A indicates a shift in isotopic composition from the bedrock to the upper horizon in the soil profiles; b - mobile lead. of mobile lead within the whole profile com- total pool respectively. Probably, carbonates pared to other soils. In the Iron Age and may play a role of a trap for mobile lead, part- modern soils highest concentration and frac- ly of anthropogenic origin. The ratio of the tion of mobile Pb in the total pool were weighted mean concentrations (A+B hori- observed in lower soil horizons with highest zons) (mobile lead(ppm))/(CaCO3(%)) grows carbonates content. Regression analysis in order 3500 yr BP (0.03) < 1800 yr BP showed that carbonates content in buried (0.09) < modern soil (1.06), and could be a and modern soils explains about 49% and sign of increasing anthropogenic Pb input 55 % of the variation of mobile lead concen- within the soil chronological sequence. tration and fraction of mobile lead in the

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Isotopic composition of the mobile tent or isotope ratios was found within soil fraction of Pb was "less radiogenic" than the chronological sequence due to low fraction of total one in the case of modern and the Iron the mobile lead in the total pool. Age soils. In the case of the Bronze Age soil, there was no pronounced difference in iso- Conclusions topic composition between these two frac- 1. No substantial shift in the total Pb tions. Explanation could be that the mobile content or isotope ratios was found within lead in this soil is mostly "natural", inherited soil chronological sequence, indicating a from the bedrock, opposite to the younger very low level of contamination of the mod- soils, where its composition reflects the ern soil by lead compared to the Bronze Age influence of airborne anthropogenic (ore- soil, except soils in the vicinity of the motor- derived) Pb, accumulated preferably in the way (within 10 m). mobile form. 2. However, signs of the anthropogenic To study the relation between isotopic Pb input were found by the analysis of the composition of Pb in the atmospheric deposi- mobile lead content and isotope composi- tion and in the soil mobile fraction we com- tion: piled data for 207Pb/206Pb and 208Pb/206Pb - similar evolution trend for isotope ratios in European peat cores, available in the compositions of the atmospheric Pb (Euro- literature (Fig. 1). Using these data, the aver- pean peat bogs) and the mobile Pb in soils age isotope ratios and STD for the time inter- from the Bronze Age to the present (directed vals of kurgans erection were calculated and towards the "less radiogenic" (ore-derived Pb) compared with isotope ratios of the Pb mobile values); fraction and total pool in corresponding - an increase of the ratio mobile buried paleosols (Fig. 2). To characterize mod- lead(ppm)/CaCO3(%) in the soils A+B hori- ern Pb deposition we used data on Russian zons in order 3500 yr BP (0.03)<1800 yr BP aerosols (Mukai et al., 2001). (0.09)

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