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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.53 no.3 Concepción Sept. 2008 


J. Chil. Chem. Soc, 53, N° 3 (2008) págs: 1640-1649





1 Unité de Recherche Argües et Paléoclimats, Département de Géologie, Université de Liège, Belgium
2 Département des Sciences de la Terre et de l'Environnement, Université Libre de Bruxelles, Belgium


Two cores from tephra rich peat soils of the Chilean Lake District were investigated for mineral source and potential anthropogenic impact using elemental geochemistry, including rare earth elements, and lead isotopes. The dominant particle source to the area are the Trumaos, which are the local ando soils derived from the weathering of volcanic deposits. In Galletué, although short term events of enhanced particle inputs occur, elemental and isotopic signatures show that the Trumaos are the only particle source to the área. In San Pablo de Tregua, punctual events of crustal particle inputs are recorded in Pb enrichment factors and isotopic profiles, reflecting a long range crustal involvement. This site also records the inception of the anthropogenic in the área in recent samples of which Pb enrichment factors and lead isotopic signatures shift drastically.

Keywords: Peat soil, Trace elements, Pb isotopes, Ando soils, Lake District, Chile, Holocene


During the last decade, many researches have been focused on peat cores as archives of atmospheric deposition, providing important data on palaeoclimatic and anthropogenic history1-5. Recent studies have for example demonstrated that ombrotrophic peat (i.e. exclusively fed by atmospheric deposition) is an excellent archive to record Pb atmospheric history throughout the Holocene46.

Studies of past atmospheric deposition are more rare in minerotrophic peat despite a common occurrence relative to ombrotrophic bogs. This is principally due to possible mobilization of elements linked to the fluctuation of watertable, or leaching by groundwater7,8. However other authors9-13 have demonstrated the potential of mineralotrophic peat to record past climatic or anthropogenic changes.

In Southern Hemisphere, very few articles have investigated Holocene or anthropocene reconstruction derived from peat bog geochemistry14-17. There are however numerous geochemical data from several sites of the Antarctic ice cap18-20 are available and could be useful to decipher sources of partioles and/or periods of pollution.

In this study, we focus on minerotrophic peat soils from the Chilean Lake District (Meridional Chile, 38-40°S). In an attempt to identify the main particle source (average Upper Continental Crust, mean Chilean Southern Volcanic Zone, and Trumaos, anthropogenic source), this study aims to decipher between local and global influence on peat records, using the depth evolution of geochemical and Pb isotopic compositions of peat soils.


Site description

Two vertical cores (PB2 : 38°40'47"S - 71°12'60"S - 2.80 m; PB4 : 39°35'20"S - 71°03'78"W - 4.00 m) were drilled in peat soils from the Chilean Lake District (Fig. 1). The cores were retrieved using a Belorussian-type stainless steel D-corer21,22. Semi-cylindrical core are 50 cm long and 4.5 cm in diameter were sampled and bagged into PVC tubes and then wrapped in plástic. A pre-coring was performed to lócate the thickest sediment accumulation.

The Chilean Lake District is located at the feet of the Andes, between 38o S and 42° S. The lakes are the results of the retreat of the Andean glaciers during the Late Glacial-Holocene transition23-26. This glacial landscape provides several peat infillings. This región, part of the Chilean Southern Volcanic Zone (SVZ), has been repeatedly under volcanic activity during the Holocene. Tephras were deposited from surrounding complex active volcanoes such as the Llaima, Lonquimay, Villarrica or Mocho-Choschuenco volcanoes27. The entire área is covered by 4 to 6m thick andosoils formed by volcanic ash accumulation and weathering28,29, locally called Trumaos (Fig. 1). These Trumaos provide a large amount of local mineral particles that could be transported by wind action and trapped by peat soils.

In Galletué peat soil (PB2), vegetation is mainly composed oíJuncus sp., Carex spp., a few Sphagnum spp., and several species of Gramineaé30. In San Pablo de Tregua (PB4), Sphagnum spp. becomes abundant in the uppermost peat layers (20 cm). The actual vegetation of this mire presents an important cover of Carex spp. and Gramineaé spp. Shrubs species like Escallonia alpina, Baccharis sp.31 and Chusquea spp. are also present, indicating a drying of the área. Both sites are fed by several little streams providing a non-atmospheric contribution to the peat soils.

14C dating

Two samples from PB2 and four from PB4 have been dated either by AMS at Poznan Radiocarbon Institute (Poland), and by conventional (i.e. (ß-counting) technique at Centre for Isotope Research, University of Gróningen (The Netherlands). Results are reported in Table 1. Sediment accumulation covers more than 8500 years in PB2 and more than 11000 years in PB4.

Sediment characterization

High resolution magnetic susceptibility (HRSM), bulk density and total organic carbon (Fig. 2) were measured in both peat cores to detect specific lithologies that could induce geochemical fluctuations. HR-magnetic susceptibility was measured every 5 mm using an MS2 Magnetic Susceptibility System equipped with a MS2E spot-reading sensor (Bartington Instruments). Measurements were duplicated to assess reproducibility. HRSM exhibits sharp peaks when fine tephras are present. When tephras are coarser, their porosity prevenís good contact between the sensor and the core. Recorded values represent in this case an underestimation of the real magnetic field. Additional HRSM fluctuations are rather due to other lithological variations. Bulk density is estimated by weighting a 1 cm thick semi-cylinder of sediment preliminary dried at 105°C for 12h. Total organic carbon was measured with a CS-200 Leco analyzer (CBR Harmignies, Belgium) on dried (105°C, 12h) and milled samples. Errors on C org measurements range from 5 to 10%.

Preparation and acid digestión of peat samples

Samples were taken out from the core using 5 mi acid cleaned plástic tools. Only the central part of the semi-cylinder sampled. Samples were dried (105c for 12h) and milled using an agate mortar. The sampling was constrained by the numerous tephra falls along the profiles and remained therefore irregular and quite large (sampling resolution up to 1 sample/40 cm).

All the samples were processed in clean air cabinets (ULB, Belgium). Digestión consisted in a two-step acid dissolution in closed Savilex® beakers: (1) organic matter is digested by repeated addition of H202 30%p.a. and HN03 65% ce. sub. in a proportion of 3:1; (2) mineral matrix is then digested by addition of HF suprapur (Merck) + HN03 65% ce. sub. ± HC104 in a proportion of 1:6:0.5. The beakers were placed on a hot píate at 130°C for 4 days. After drying, 6N HC1 was added and slowly evaporated.

Elemental geochemistry

Elemental geochemistry was measured using ICP-AES (IRIS Advantage) for maj or and transition elements and ICP-MS (VG PlasmaQuad PQ2) for trace elements at MRAC-Tervuren (Belgium). Si could not be analysed because of SiF4 volatilisation during the HF-HN03 digestión procedure. Calibrations were made with In-Ru-Re-Bi internal standards (10 and 20 ppb), and two international reference materials: BCR CRM-100 beech leaves (EU certificated) and granite GA33, chosen to be representative of the current soil and volcanic samples. Accuracy varied between 1 -11 % for major elements, 1 -2% for transition metáis and 1-5% for other trace elements. Detection limits (i.e. mean + 3c) were lower than 1 ppm for maj or elements excepted Na (2.7 ppm), K (1.1 ppm) and P (7.9 ppm). Detection limits for trace metáis and other trace elements were lower than 1 and 10 ppb, respectively. To assess sample heterogeneity, replicates from both cores were analysed (PB2 - 108 cm, PB4 - 310 cm). Results for replicates are comparable within errors to the original analyses.

Lead isotope signatures

Lead fraction was separated using a one step ion-exchange chromatography on AG1-X8 resin following previouly reported chemical procedures34. To monitor instrumental mass bias, TI was added to each sample to achieve a Pb/Tl of = 5 and thus match the Pb/Tl ratio of the standard. During the analysis sessions, measurement of NBS 981 Pb standard was systematically performed after every two samples. It gave average individual error (2c, 95% confidence) on Pb isotopic analyses for Galletué (PB2) and rock samples of 0.0200 for 208Pb/2MPb, 0.0150 for 207Pb/2MPb and 0.0180 for 206Pb/204Pb (for more details see Table 4). Acids were distilled (HN03) or sub-boiled (HF, HCl, HBr). Pb total blanks for the whole procedure range between 40pg and 200pg. This is negligible relative to Pb contents in the samples (150ng < PB2 content < 900ng, 25ng < PB4 content < 8.3µg).

Lead isotopic ratios were measured on Nu-Plasma MC-ICP-MS (ULB, Belgium) during three analysis sessions. All the standard values of this study fall in these ranges. Internal laboratory reproducibility are 0.027 for 206Pb/204Pb, 0.026 for 207Pb/204Pb, 0.003 for 208Pb/204Pb, 0.1 for 207Pb/206Pb and 0.15 for 208Pb/206Pb (n~600, 2o level, 95% confidence) after several years of analyses. The total Pb beam intensities varied from 4 to 10V. Intensities during analyses of San Pablo de Tregua were sometimes lower (<1V), explaining the larger average individual errors forthose samples. A duplícate of the whole procedure (i.e. including dissolution, column exchange and analysis) on a PB2 sample (108 cm depth) was also analysed. The sample and the replícate results show differences of 13x10-3 for 206Pb/204Pb, 36x10-5 for 207Pb/206Pb and 87x10-7 for208Pb/206Pb.

Selection of the reference element

Comparable profiles of major element concentrations variations as a function of depth suggest an important imprint of the soil lithology. To compénsate for the influence of the lithology or matrix effect, trace and major element concentration values have been normalized. The peat soils of this study are located in a volcanic environment. Therefore the regional non-crustal geology of the Chilean Lake District has to be taken into account. Ti, Al, Zr, or Y are lithophile elements, well known for their resistance against weathering35,36. They are common in several minerals found in volcanic falls, such as, for example, glass shards (Al, Ti), plagioclase (Al), titanite (Ti), ilmenite (Ti), titano-magnetite (Ti), zircon (Zr). Zr and Y have been often used in literature10 as reference elements, but they are in this study sometimes under detection limit. In contrast Al appears to be the most abundant element the samples. A slight fluctuation in [Al] will therefore strongly influence the fluctuation in the corresponding [trace element]/[Al] ratio. Consequently, Ti, a conservative element35 moderately abundant in several resistant volcanic mineral phases, is chosen as the reference element. We assume in the studied área an almost entire Ti supply provided by weathering producís ofthe SVZ volcanism (i.e. Trumaos). Because such a soft material is easily transported by wind, it is the most suitable source of particles in this área. Ti flux is obtained using the following formula10:



The lithology of both cores is strongly influenced by weathering products from SVZ as the mineral residue of the samples is exclusively composed of volcanic minerals (e.g. plagioclase, pyroxene, olivine) and glass shards. Both peat soils were repeatedly blanketed by tephra falls up to 10 cm thick. These observations are consistent with the minerotrophic character of the two sites.

PB2 shows a low organic carbon content (mean Corg =5.4 %) and high density values (mean δ = 0.6). A peak in density (δ = 0.91) at 90 cm is correlated with a rapid decrease of the total organic carbon content (Corg = 2.73%) and a slight positive shift in HRSM. This shift is not due to a tephra layer (as the nearest tephra is at 95 cm depth), but to a strong clayey and mineral-rich layer (Fig. 2). Below total organic carbon is very low and density is high, reflecting the strong minerogenic character of the sediment. This mineral character becomes more important under 250 cm depth as HRSM increases significantly.

In contrast, PB4 displays an average higher total organic carbon content (mean Corg = 31.7%) and lower density (mean δ = 0.2) values relative to PB2. HRSM is also very low (always under 50 x 106 SI). This could reflect the lower contents of tephras and/or mineral particles in PB4. In this core, the Sphagnum-dominated layers become increasingly mineralised from 0 to 70 cm depth. The density therefore increases while the total organic carbon content decreases. At 71 cm depth, the occurrence of a silty layer causes a strong positive shift of density (δ = 0.37 g/cm3) and a negative shift of organic carbon content (Corg= 10.1%). Below 71 cm, profiles are relatively linear with small fluctuations between puré peat values (i.e. value of the 1 cm depth sample) and silty values (i.e. value of the 71 cm depth sample), reflecting the clayey character of the samples. At the lowermost 20 cm, HRSM increases rapidly, indicating a strong lithological change that represents the base of the peat infilling.

Major elements

Major element analysis reveáis that Al, Ca and Fe are the most abundant elements in PB2: average [Al]PB2 = 6.5 wt. %, average [Ca]PB2 = 3.5 wt. % and average [Fe]PB2 = 3.5 wt. % (Table 2). All the major elements display similar variation with depth: decreasing values from 198 cm depth, then increasing and peaking at 90 cm, finally decreasing again until the surface. Although concentrations are lower, PB4 displays the same trend in elemental abundance: average [Al]PB4 = 3.3 wt. %, average [Ca]PB4 = 0.97 wt. % and average [Fe]PB4 = 0.98 wt. %. P is often below detection limits. As in PB2, all the major elements show remarkably comparable profiles: concentrations peak slightly at 390, 310 and 160 cm depth, then decrease and peak strongly at 71 cm, to finally sharply decrease until the surface.

Trace elements

In PB2 (Table 3) Light Rare Earth elements (LREE) show a weak variation range relative to Heavy Rare Earth elements (HREE). Chondrite-normalized38 La/Lu ratio (Lach/Luch) ranging between 2.78 and 2.84. The range of variation is more important in PB4 (Lach/Luch = 0.95-9.58). However, Lach/Luch averages suggest an enrichment in LREE relative to HREE in the two peat soils. Other trace elements show comparable trends as those of REE, reflecting a strong influence of a Trumaos-SVZ particle source. However, Pb values of PB2 and PB4 show a strong negative anomaly. Based on the behavior of Pb, several potential particle sources to the peat soils could therefore be involved : Trumaos, SVZ and/or Upper Continental Crust. These potential sources will be detailed later (see section 4).

Lead isotopes

Pb Isotopic compositions in PB2 and PB4 cores are shown in Table 4. The Pb isotopic ratios for Galletué (PB2) define relatively constant profiles and small variations: from 18.556 to 18.601 for 206Pb/2MPb, from 15.602 to 15.607 for 207Pb/2MPb and from 38.462 to 38.500 for 208Pb/2MPb. Except for the three uppermost samples, the Pb isotopic compositions of San Pablo de Tregua (PB4) also vary in narrow ranges: between 18.371 and 18.655 for 206Pb/204Pb, between 15.579 and 15.669 for 207Pb/2MPb, and between 38.238 and 38.617 for 208Pb/204Pb. Isotopic ratios of the three surface samples shift strongly towards lower values (0.868 <207Pb/206Pb < 0.875 and 2.108 < 208Pb/206Pb < 2.118).


Natural vs. anthropogenic sources

Trace element concentrations normalized to chondrite values38 are reported as spider diagrams (Fig. 3). Profiles of average concentrations for the two peat soils are compared to average compositions of the Trumaos, the Chilean Southern Volcanic Zone (SVZ) soils and the average Upper Continental Crust (UCC). Trends suggest a dominant particle sources comparable to the SVZ producís and Trumaos. Galletué área (PB2) is closer to the SVZ profile than PB4. It is consistent with the influence made by the numerous tephra layers found in the core. This is also consistent with the lithology: PB2 is rather a peaty sediment rich in volcanic particles, than a puré (i.e. organic) peat. San Pablo de Tregua (PB4) core as a whole has a comparable lithology to PB2 although it displays less abundant volcanic layers.

Pb isotopic data for Galletué (PB2) show a narrow range of variation and are included within the Trumaos-SVZ isotopic data field (Fig. 4.A.). A significant shift in 206Pb/207Pb is recorded between the two samples taken below (127.75 cm) and above (108.25 cm) the Alpehué Pumice, a well known rhyodacitic volcanic layer erupted from Caldera Sollipulli at 3000 yr cal BP59. The measured isotopic ratios are shifted from the Trumaos trend to the Alpehué Pumice isotopic composition (Fig. 4. B and C). This is due to contamination by grains from Alpehué Pumice dispersed in nearby upper layers, providing a pumiceous contribution to the isotopic signature of the peaty sediment. The shift shown by the sub-surface sample (3 cm depth) is too small (e.g. 206Pb/204Pb = 18.564 ± 0.0014) regarding to Pb isotopic ratios in actual Chilean cities aerosols60 (206Pb/204Pb = 17.00 ± 0.02) to invoke any anthropogenic contribution to the Pb isotopic composition for the subsurface sample of Galletué core. The shallowest sample has probably been taken under the anthropogenically contaminated layers. Note that it was not possible to sample the subsurface layers due to contamination by sparse tephra grains. Moreover, in cores from Lake Galletué, no anthropogenic influence is recorded in terms of fossil fuel pollution61. Anthropogenic partióles originating from a long distance source are negligible61.

Variation in Pb isotopic ratios ranges are ten times wider for San Pablo de Tregua (PB4) relative to PB2. Sediments sampled above 20 cm depth display isotopic ratios (206Pb/207Pb < 1.16, 208Pb/206Pb > 2.10) strongly shifted towards anthropogenic field (Fig. 4. D. and E) here reported by Actual Chilean industrial aerosols59. As it was already suggested by the high Pb E.F., Pb isotopic signatures of the three uppermost samples (1, 3, and 20 cm) reflect conjugated inputs of both SVZ volcanic partióles and actual industrial aerosols. 206Pb/207Pb ratios were reported to vary between 1.12 and 1.14 associated Pb E.F. higher than 60 in Antarctic ice core samples1920 aging from 1950 A.D. to 1987 A.D. Moreover, ice samples dating from the early 1900's A.D. to about 1940 A.D. show preferentially 206Pb/207Pb varying between 1.15 and 1.18, and lower Pb E.F. Therefore, despite a lack of dating for our samples, we could assume ayoung age (1940 A.D.-2000 A.D.) for the two uppermost samples of PB4 on the basis of their low 206Pb/207Pb ratios (1.12- 1.14), strong Pb enrichment and extremely high Pb E.F. With a 206Pb/207Pb ratio of 1.15 and lower Pb E.F. (13), the third sample (16 cm depth) could be rather older. However, further sampling and dating of PB4 core would have been necessary to specify the exact age of this anthropogenic influence, but tephra grains contaminating surface samples peat make this sampling very tricky.

Changes in partiole fluxes and Crustal involvment

PB2 samples show a Al/Ti profile with an opposite trend relative to Nd/Ti and Pb/Ti profiles (Fig. 5). All the normalized profiles are included within the range of Trumaos and SVZ values, except for Al/Ti. This latter profile ranges between (1) the Trumaos and/or the SVZ and (2) the average Upper Continental Crust Al/Ti values, suggesting a possible crustal contamination. Pb E.F. profile (calculated with Trumaos reference) suggests a unique mineral source for PB2. Nd/Ti and Pb/Ti profiles suggest a mineral particle source with a volcanic geochemical signature reflecting the Trumaos and/or SVZ compositions, as already shown by spider diagrams. Trumaos are easily eroded and carried by wind. It is therefore assumed that the SVZ signature is due to the Trumaos inputs rather than particles issued from solid volcanic rocks weathering or direct volcanic supply.

In PB4, Ti-normalized profiles of Al and Nd are roughly comparable. As a whole, the profiles are included within the composition ranges of SVZ and Trumaos. However, three peaks show significantly high ratios exceeding the SVZ and Trumaos composition ranges. They correspond to high Al/Ti values between at about 360 cm depth and high Al/Ti and Nd/Ti values between 210 and 246 cm depth (Fig. 5). These compositional shifts suggest a possible crustal particles input. Moreover, Pb/Ti and Pb E.F. profiles show also higher values trending towards crustal composition between 210 and 246 cm depth but not at 360 cm depth. Pb E.F. also shows higher values (approximately 3 and 5.5, respectively) between 210 cm and 246 cm but again not at 360 cm depth. These observations suggest involvement of a crustal particle source. The strong peak clearly observed at 71 cm depth in the Ti, Ti flux, and Pb E.F. profiles, is missing in Ti-normalized profiles. This contrast suggests that the 71 cm depth silty layer is mainly composed of volcanic derived material (Trumaos). Pb/Ti and Pb E.F. profiles display natural background (i.e. Trumaos) in the lower part of the core, while the uppermost 20 cm of sediments are strongly enriched inPb.

Figure 6 presents Ti concentrations, Ti fluxes and particulate fluxes for the two peat soils. Galletué (PB2) samples show extremely high fluxes in Ti and particles, averaging 0.93 g/m2/yr and 158 g/m2/yr, respectively. For comparison, in Swiss peat bogs10, the Younger Dryas is recorded by a strong dust peak averaging 11.5 g/m2/yr while the natural background dust flux is less than 0.5 g/m2/yr. At 90 cm depth, a shift in [Ti] is correlated with a strong peak in Ti flux and particulate flux. Between 130 and 180 cm, a slight increase in [Ti] is also recorded in the three profiles. However, 147Sm/144Nd signatures calculated from Sm and Nd concentrations (for more details, see figure 7 caption) of the Galletúe (PB2) samples confirm that the dominant source of atmospheric particles to the peat soils is related to the SWZ-Trumaos component (Fig. 6). The possible contribution of a crustal sources for the 90 cm depth sample, previously suggested by the Al/Ti ratio, is inconsistent with the 147Sm/144Nd results. The high values recorded by the PB2 sample reflect abundant inputs of Trumaos and other volcanic weathering particles inputs.

In San Pablo de Tregua (PB4), flux values are globally lower relative to PB2. [Ti] flux and total particular flux average 0.18 g/m2/yr and 31 g/m2/yr, respectively. In the three profiles of PB4, a peak is located at 71 cm depth (Fig. 6). 147Sm/144Nd data mainly plot in the SVZ isotopic field (Fig. 6) except for five samples (130 cm, 210 cm, 246 cm, 270 cm and 310 cm). Samples 160 cm and 310 cm depth may reflect contamination by a crustal input as they are at the limit of the SVZ and Upper Continental Crust isotopic fields (Fig. 6). In addition they display opposite trends in Pb/Ti and Pb E.F. profiles. However, sample at 160 cm depth shows negative shifts (Pb/Ti = 2 x 10-3, Pb E.F. = 1.6) while sample at 310 cm shows positive ones (Pb/Ti = 7 x 10-3, Pb E.F.= 5.6). Therefore, it is doubtful to interpret them as contaminated by crustal particles. Samples from 210 cm, 246 cm and 270 cm depth are distinguishable from the others by significant lower 147Sm/144Nd values, reflecting potential contribution of a crustal source. This is especially consistent for the 210 and 246 cm samples because they present strong peaks in Al/Ti, Nd/Ti, Pb/Ti and Pb E.F. Those observations suggest a possible involvement of a long range crustal dust source during corresponding deposition period, strongly influencing 147Sm/144Nd signatures of these layers.

Summary and conclusions

We investigated peat cores from Chilean lake district. In this región, continental archives are frequently affected by volcanic falls. The availability of soft ash-derived Andosoils (i.e. Trumaos) material provides a continuous particle rain input in the Lake District. Short-term high fluxes of particles from various origins appeared to reflect punctual events. Cores from Galletué and San Pablo de Tregua peat soils (Chilean Lake District, South Chile) record ca 6000 and 10000 years of atmospheric particle deposition. Those peat soils are characterized by a minerotrophic matrix rich in volcanic dust with variable organic contení. The use of major and trace elements data as well as Pb isotopes allow to decipher between three the particles categories supplied to the peat soils: a) SVZ weathering products (Trumaos) (0<Pb E.F.<2 and 1.185<206Pb/207Pb<1.195) providing a great amount of erodible material easily transported by wind, and constituting the local and main particles source; b) a mix of SVZ weathering products and long range crustal particles (2<Pb E.F.<6 and 147Sm/144Nd <0.105 to 0.115), only recorded in San Pablo de Tregua; c) an anthropogenic undifferentiated source (Pb E.F>6 and 207Pb/206Pb>0.865) only recorded in the uppermost 20 cm of San Pablo de Tregua (PB4).

In Galletué (PB2) core, a clayey layer induces a peak in several elemental profiles at 90 cm depth. Particularly, the total particle flux reaches 158 g/m2/yr, which is extremely high. This particular clayey level could suggest a climatic change corresponding to a warmer period with a main volcanic atmospheric dust supply. Since this period, no crustal aerosol input is detected.

In San Pablo de Tregua (PB4) core, the particle source below 20 cm depth is the SVZ-Trumaos source. Above this depth, the signature is a mix between SVZ-Trumaos and the industrial sources, reflecting the inception of the anthropogenic influence. In PB4, two periods of Pb pollution could be distinguished by comparison with isotopic values from the Antarctic ice cap1920. The sample at 16 cm could be related to the beginning of the 20th century while the two uppermost ones reflect more likely the isotopic signature of the period between 1950 and 1987. Moreover the isotopic signatures between 210 cm and 246 cm suggest punctual crustal involvement, possibly linked to a long range particle transport.


We would like to thank O.S.T.C. project EV/12/10B (Coord. M. De Batist, RCMG Gent, Belgium), TNQUA and Lefranc Foundation (ULg) for field financial support. Analyses were partially financed by the Belgian National Found for Scientific Research (F.N.R.S.). We thank especially J. De Jong (ULB), J. Navez and L. Monnin (MRAC, Tervuren) for helpful laboratory and analytical assistance, and Sébastien Bertrand and Lourdes Vargas for field assistance. We are also grateful to M. Stürm, E. Chapron (ETH-Zurich) and F. Charlet (RCMG Gent, Belgium) for giving us the possibility to use HR-magnetic susceptibility system. This manuscript has received the kind improvement of D. Weis, M. Streel, María Gehrels and two anonymous reviewers. During this investigation, Francois De Vleeschouwer was a F.R.I.A. (F.N.R.S.) fellow.



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(Received: August 16, 2007 - Accepted: March 17, 2008)

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