Marc Desmet

Rhone River flood deposits in Lake Le Bourget: a proxy for Holocene environmental changes in the NW Alps, France

The Holocene evolution of Rhone River clastic sediment supply in Lake Le Bourget is documented by sub-bottom seismic profiling and multidisciplinary analysis of well-dated sediment cores. Six high-amplitude reflectors within the lacustrine drape can be correlated to periods of enhanced inter- and underflow deposition in sediment cores. Based on the synthesis of major environmental changes in the NW Alps and on the age-depth model covering the past 7500 years in Lake Le Bourget, periods of enhanced Rhone River flood events in the lake can be related to abrupt climate changes and/or to increasing land use since c. 2700 cal. yr BP. For example, significant land use under rather stable climate conditions during the Roman Empire may be responsible for large flood deposits in the northern part of Lake Le Bourget between AD 966 and 1093. However, during the Little Ice Age (LIA), well-documented major environmental changes in the catchment area essentially resulted from climate change and formed basin-wide major flood deposits in Lake Le Bourget. Up to five ‘LIA-like’ Holocene cold periods developing enhanced Rhone River flooding activity in Lake Le Bourget are documented at c. 7200, 5200, 2800, 1600 and 200 cal. yr BP. These abrupt climate changes were associated in the NW Alps with Mont Blanc glacier advances, enhanced glaciofluvial regimes and high lake levels. Correlations with European lake level fluctuations and winter precipitation regimes inferred from glacier fluctuations in western Norway suggest that these five Holocene cooling events at 45°N were associated with enhanced westerlies, possibly resulting from a persistent negative mode of the North Atlantic Oscillation.

Late-Holocene summer temperature reconstruction from chironomid assemblages of Lake Anterne, northern French Alps.

We present a chironomid-based reconstruction of late-Holocene temperature from Lake Anterne (2060 m a.s.l.) in the northern French Alps. Chironomid assemblages were studied in 49 samples along an 8 m long sediment core covering the last 1800 years. July air temperatures were inferred using an inference model based on the distribution of chironomid assemblages in 100 Swiss lakes. The transfer function has a leave-one-out cross-validated coefficient of determination (r ) of 0.88, a root mean square error of prediction (RMSEP) of 1.40°C. Despite possible biases induced by methodological aspects and the ecological complexity of the chironomid response to both climate and environmental changes, the concordance of the Lake Anterne temperature reconstruction with other Alpine records suggests that the transfer function has successfully reconstructed past summer temperature during the last two millennia. The twentieth century is the only section of the record which shows a poor agreement with other climate reconstructions and the distinct warming found in most instrumental records for this period is not apparent in the Lake Anterne record. Stocking of the lake with fish from the early twentieth century onwards was found to be a possible cause of changes in the chironomid fauna and subsequent distortion in the inferred climate signal. Evidence was found of a cold phase at Lake Anterne between AD 400 and 680, a warm episode between AD 680 and 1350, and another cold phase between AD 1350 and 1900. These events were possibly correlated to the so-called `Dark Age Cold Period (DACP), the `Mediaeval Warm Period (MWP) and the `Little Ice Age (LIA). The chironomid-based inference model reconstructed a July air temperature decrease of c. 0.7°C for the DACP and 1.3°C for the LIA compared with the temperature prevailing during the MWP.

A 300 year history of lead contamination in northern French Alps reconstructed from distant lake sediment recordsPresented as part of the Archives of Environmental Contamination at the 6th International Symposium on Environmental Geochemistry, Edinburgh, Scotland, 7?11 September 2003.

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.

Géochimie des sédiments plio-quaternaires dans l'océan Pacifique equatorial, Leg ODP 138

Plio-Quaternary sediments from ODP sites 849B and 852C (Eastern Equatorial Pacific Ocean) have been studied. A spatial and temporal comparison of the chemical composition of the sediment from a Pliocene period (3.5 to 3.3 My BP) and a Pleistocene period (550 to 350 ky BP) has been carried out. The chemical signature of the sediment is characterised by a fluctuation state of titanium, manganese, silicium, magnesium and bioactive metals (Cu, Co, Ni). The chemical sedimentation in the Eastern Equatorial Pacific Ocean is stable throughout Plio-Quaternary times. The spatial distribution of chemical elements varies strongly with the geographic location of the Intertropical Convergence Zone.

Rythmes et modalités de croissance chez quelques espèces du genre LepidodendronSternberg: Morphologie quantitative et modélisation

Resume Le genre Lepidodendron, Pteridophyte du Carbonifere, est caracterise par linsertion de ses feuilles selon unespirale a helices multiples. Les parametres de surface et de distribution des coussinets foliaires ont fait lobjet danalyses statistique et geometrique qui demontrent lexistence dun mode de croissance discret, rythme par un facteur probablement saisonnier et interprete en terme dunite de croissance. Quant au mode de ramification, cette etude montre quil est de type monopodial.