-
A. Simonneau,
E. Chapron,
M. Debret, T. Winiarski,
F. Anselmetti,
M. Desmet,
C. Di Giovanni,
M. Garçon,
L. Charlet,
C. Chauvel,
M. Motellica-Heino
[show abstract]
[hide abstract]
ABSTRACT: Understanding potential consequences of the actual global warming requires mastering past environments evolution linked with anthropic and/or climate variability. High altitude proglacial lakes are key natural archives because they can accurately detect phases of glacial fluctuations and related environmental changes. Lake Blanc Huez (LBH, 2500 m asl) located at 45°N was thus selected because local glaciers extension during the Little Ice Age was restricted to the upper part of its catchment area and because nearby Saint-Sorlin glacier is instrumented since 1957 and considered as representative for the NW Alpine region.
To track palaeoenvironmental evolution and glacial fluctuations, a multiproxy approach is used. It includes seismic profiling (3.5 kHz), piston coring, radionuclide and radiocarbon dating together with a high resolution analysis of clastic and organic sedimentation (laser grain size, magnetic susceptibility, gamma-density, spectrophotometry, major and trace element measurements). Detailed organic analysis (Rock Eval pyrolysis and quantitative organic petrography) are also performed on lacustrine sediments, soils and geological formations in order to document specific organic tracers.
LBH documents the last 9800 yrs. Its upper clastic unit, corresponding to the Neoglacial period, covers the last 5400 yrs and reflects the dominating contribution of fine-grained glacial sediments and the autochthonous algal production (grey amorphous). A progressive transition towards an organic rich basal unit (Total Organic Carbon =5%) essentially made of soils tracers (red amorphous) and pollens suggests reduced glacier activity allowing pedogenesis and development of a vegetation cover in the massif before 5400 yrs cal BP. This bimodal Holocene sedimentation is in addition punctuated by smaller scale fluctuations: a glacier advance at 8200 cal BP during the Early Holocene and glacier retreats during Early Bronze Age, Iron Age, Roman period and Medieval Warm Period.
XVIII INQUA Congress, 21st–27th July, 2011, Bern; 11/2012
-
M Debret,
D Sebag,
M Desmet,
W Balsam,
Y Copard,
B Mourier,
A.-S Susperrigui,
F Arnaud,
I Bentaleb,
E Chapron,
E Lallier-Vergès, T Winiarski
[show abstract]
[hide abstract]
ABSTRACT: Colour is a fundamental property of sediment and is often used for lithographic description to determine sed-imentological structures, facies etc. However, the sedimentary information contained in this parameter is dif-ficult to extract because it is difficult to quantify. Colour can be quantified by spectrocolorimetry which provides very high resolution data quickly and non-destructively. When adapted to sedimentology, spectro-colorimeters prove to be powerful tools due to their low purchase and maintenance costs, and some are por-table and easily used in-the-field. Several methods have been used to extract sedimentological data from colorimetric spectra (first derivatives, factorial analysis, etc.). In the present study, we first provide a review of the sedimentological application of spectrophotometers and, after having described these methods, their advantages and disadvantages, we then describe a new tool called the Q7/4 diagram (abscissa L*; Ordinates 700/400 ratio). This new technique permits sedimentological units to be defined, allows the identification of dif-ferent sediment components and provides 5 distinct poles: Clayey deposits, organic rich deposits (chlorophyll a and by products), altered organic matter deposits, iron rich deposits, carbonated deposits. Coupled with the analysis of first derivative spectra, it is possible to distinguish different pigments linked to the degradation and/or nature of the organic material (Chlorophyll a, melanoidin, etc.), the state of iron oxidation (for example, hematite and goethite-like signatures) and the nature of clays. The Q7/4 diagram permits rapid acquisition of high resolution data on changes of sediment dynamics in geosystems that have been subjected to highly varied climatic/environmental conditions. The instrument is non destructive, easy to use and maintain, portable for use in the field, fast to implement, is capable of high resolution, and has a vast range of possible applications. Spectro-colorimetry appears to provide many advantages and could become an essential and robust tool for preliminary sedimentological studies.
Earth-Science Reviews 08/2011; 109:1-19. · 6.59 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In order to evaluate the historical impact of urban stormwater runoff on a lacustrine environment, eight sediment cores were collected in October 2006 in Lake Bourget (Savoie, France). Sediment samples were analysed for polycyclic aromatic hydrocarbons (PAH) and polychlorobyphenyls (PCB) in order to correlate the vertical profiles of contamination and the evolution of the urbanisation on the watershed for the last 100 years. Overall, we observe that vertical profiles of PCB and PAH concentrations in Lake Bourget are showing a downward trend, probably resulting from the pollution control measures. Concentrations of PCB and PAH measured in surface sediments are approximately 5-10 fold less than maximal concentrations found in the cores. Maximal PAH concentrations were found at the bottom of the cores ( approximately 40 cm) which approximately correspond to the beginning of the 20th century. Maximal PCB concentrations measured in cores L5 and L6 are respectively 850 and 790 microg/g dw (dryweight). The peak of PCB corresponds to the maximum of PCB production and consumption in the 1970s. In surface sediments total PCB concentrations range between 47 and 79 microg/kg dw. Those concentrations are particularly high compared to other values measured in lake sediments and highlight the problem of a local source of PCB contamination in the watershed. General trends were observed, yet the part of urban stormwater runoff in the total contamination could not be determined.
Water Science & Technology 01/2008; 57(10):1503-10. · 1.12 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Lead concentrations and isotopic ratios were measured along two well-dated sediment cores from two distant lakes: Anterne (2100 m a.s.l.) and Le Bourget (270 m a.s.l.), submitted to low and high direct human impact and covering the last 250 and 600 years, respectively. The measurement of lead in old sediment samples (>3000 BP) permits, in using mixing-models, the determination of lead concentration, flux and isotopic composition of purely anthropogenic origin. We thus show that since ca. 1800 AD the regional increase in lead contamination was mostly driven by coal consumption ((206)Pb/(207)Pb approximately 1.17-1.19; (206)Pb/(204)Pb approximately 18.3-18.6), which peaks around 1915 AD. The increasing usage of leaded gasoline, introduced in the 1920s, was recorded in both lakes by increasing Pb concentrations and decreasing Pb isotope ratios. A peak around 1970 ((206)Pb/(207)Pb approximately 1.13-1.16; (206)Pb/(204)Pb approximately 17.6-18.0) corresponds to the worldwide recorded leaded gasoline maximum of consumption. The 1973 oil crisis is characterised by a drastic drop of lead fluxes in both lakes (from approximately 35 to <20 mg cm(-2) yr(-1)). In the late 1980s, environmental policies made the Lake Anterne flux drop to pre-1900 values (<10 mg cm(-2) yr(-1)) while Lake Le Bourget is always submitted to an important flux (approximately 25 mg cm(-2) yr(-1)). The good match of our distant records, together and with a previously established series in an ice core from Mont Blanc, provides confidence in the use of sediments as archives of lead contamination. The integration of the Mont Blanc ice core results from Rosman et al. with our data highlights, from 1990 onward, a decoupling in lead sources between the high elevation sites (Lake Anterne and Mont Blanc ice core), submitted to a mixture of long-distance and regional contamination and the low elevation site (Lake Le Bourget), where regional contamination is predominant.
Journal of Environmental Monitoring 06/2004; 6(5):448-56. · 1.99 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present time-series of lead concentration and isotopic ratio in sediments from two French alpine lakes: Lac du Bourget, a suburban lake, and Lac d'Anterne, a mountain lake nearby the Mont-Blanc Range. This approach aims at differentiating “local” from “regional” inputs by comparing distant sedimentary records. $^{210}$Pb and $^{137}$Cs dating methods were validated by historical events recognition [1-2] providing confident age-depth models spanning the last 250 years. In both lakes the sediment is lead-enriched compared to upper continental crust over the whole records. The human impact reached its maximum around 1970 AD, and then presents a rapid decline. Lead concentration and isotopic composition are correlated. When compared to data from a Mont Blanc ice core [3], Lac d'Anterne time-series yields excellent correlation over the last 250 years, confirming the accuracy of both dating process and record of atmospheric lead fall-out. Record from Lac du Bourget presents a similar pattern until 1985 AD when the signal from the high elevation sites ones, both in concentrations and isotopic ratio. Our data confirm the date of 1970 as the maximum of atmospheric lead fall-out over Western Europe. Today, Lac d'Anterne still records a weak airborne contamination, while Lac du Bourget is mostly influenced by local sources.
http://dx.doi.org/10.1051/jp4:20030243.
