Marlène Lavrieux

6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)

A 19 m long sedimentary record retrieved in Lake Aydat (French Massif Central) covers the last 6700 yr at a high resolution. A multiproxy approach (density, magnetic susceptibility, XRF, Rock-Eval, pollen and non-pollen palynomorph analyses and a historical archives study) was used to characterise and propose a model of sedimentation. The high deposition rate results from the combined effects of the high suspension load of the river, autochthonous production and the narrow shape of the incised fluvial valley dammed by a lava flow c. 8550 years ago. Two contrasted periods (6700±200 to 3180±90 cal. BP, and 1770±60 cal. BP to present) were characterized. The lower unit (mid Holocene) displays a fine and regular lamination and holds a single, major, flood deposit. This unit is capped by an erosive mass-wasting deposit triggered c. 1770±60 cal. BP. The upper unit (late Holocene) is made of organic-rich and fine-grained faintly laminated sediment, with numerous interbedded flood deposits and diatom blooms. The sedimentation was principally controlled by climatic forcings until c. 1100 cal. BP, accompanied by detrital events linked to human activities around the lake. Then, a more detrital input attested by numerous and recurrent flood deposits can be linked to the intensification of a persistent anthropogenic impact on the catchment. Two phases of lake eutrophication are highlighted: 1200–1130 cal. BP, as a result of increased anthropogenic pressure, and the current phase that could have started c. 150 cal. BP.

Sedimentary cannabinol tracks the history of hemp retting

Hemp (Cannabis sp.) has been a fundamental plant for the development of human societies. Its fibers have long been used for textiles and rope making, which requires prior stem retting. This process is essential for extracting fibers from the stem of the plant, but can adversely affect the quality of surface waters. The history of human activities related to hemp (its domestication, spread, and processing) is frequently reconstructed from seeds and pollen detected in archaeological sites or in sedimentary archives, but this method does not always make it possible to ascertain whether retting took place. Hemp is also known to contain phytocannabinoids, a type of chemicals that is specific to the plant. Here we report on the detection of one of these chemicals, cannabinol (CBN), preserved in a sediment record from a lake in the French Massif Central covering the past 1800 yr. The presence of this molecule in the sedimentary record is related to retting. Analysis of the evolution of CBN concentrations shows that hemp retting was a significant activity in the area until ca. A.D. 1850. These findings, supported by pollen analyses and historical data, show that this novel sedimentary tracer can help to better constrain past impacts of human activities on the environment.

Free fatty acids in Lake Aydat catchment soils (French Massif Central): sources, distributions and potential use as sediment biomarkers

PurposeEighteen soils were sampled in the Lake Aydat catchment in order to analyse free fatty acid (FA) content; FAs are considered to be among the most amenable biomarkers to mobilisation by runoff waters. The majority of the study area has soil cover consisting of grasslands or forest since the 2nd World War, although some covers having changed more recently.Material and methodsThe soil studied all developed on volcanic rocks (andisols). The bulk organic matter (OM) content of the samples was characterized by Rock-Eval (RE) pyrolysis. The FAs were determined by gas chromatography–mass spectrometry (GC-MS) analysis of isolated and derivatized (methylation and trimethylsilylation) FA fractions.Results and discussionFew low molecular weight compounds (LMW; i.e., <C20) were detected; FA distributions were dominated by even numbered-carbon high molecular weight (HMW; ≥C20) normal FAs and difunctionalized FAs that included: dicarboxylic acids (diFAs), n-alkylcarboxylic acids (nFAs), and α- and ω-hydroxycarboxylic acids (αHOFAs and ωHOFAs). The distributions and abundances of HMW terms of all families (which can be all considered as representative of terrestrial OM source) displayed only slight differences. These differences were rationalized by the following ratios: (C26:0 + C28:0)/ΣCeven nFAs, C22/C24 di-FAs, and C20−/C20+ω-HOFAs). Soils from areas that had changed use recently consistently displayed intermediate ratio values typical of their double inheritance. All grassland soils and some samples from intermediate areas contained notable amounts of the bile acid deoxycholic acid that testifies to their present or recent use for cattle breeding.ConclusionsDespite the variety and the abundance of all HMW FAs in soils, work done previously on Lake Aydat sediments found only nFAs (Stefanova, M. and Disnar, J. R. 2000. Composition and early diagenesis of FAs in lacustrine sediments, Lake Aydat (France). Org Geochem 31, 41–55). These results suggest we should question the importance of the watershed contribution, the source (plant or soil) and mode of transportation of the FAs to the lake sediments.

From medieval land clearing to industrial development: 800 years of human-impact history in the Joux Valley (Swiss Jura)

The Joux Valley (Swiss Jura Mountains) has a rather unusual history of human occupation, characterized by tardive but extensive settlement since the Late Middle Ages, followed by an intensive period of industrial development. To estimate the links between human activities and environmental consequences, sediment cores were retrieved in Lake Joux and submitted to a multiproxy analysis (high-resolution photographs, magnetic susceptibility, density, x-ray fluorescence, grain size, organic geochemistry, 14C, 210Pb and 137Cs dating). The diversity of anthropication phases, defined from historical data, is clearly recognized in the lake archive. The record suggests the region was mainly under climatic influence until the end of the 13th century. The growth of settlements in the valley and the associated massive deforestation is recorded by increasing terrestrial inputs, reflecting large-scale soil destabilization, which subsequently persists despite the transition from farming to industrial activities. Autochthonous contributions then dominate the record, both in response to climatic and anthropogenic influences. Construction works conducted at the outlet of the lake affected water flow, sedimentation and aquatic community (macrophytes, ostracods) dynamics. The substantial increase of anthropogenic heavy metals (Fe, Zn, Pb) recorded during the 20th century could reflect the development of the watch-making industry in the area, as well as the use of leaded gasoline. Historical information facilitated interpretation of the observed paleolimnological evolution in the context of varied coexisting human activities. This study highlights the importance of applying an integrated paleolimnological-historical approach in order to establish clear links between well-defined human activities and their subsequent environmental responses through time.