Holocene land-use evolution and associated soil erosion in the French Prealps inferred from Lake Paladru sediments and archaeological evidences

Abstract

International audienceA source-to-sink multi-proxy approach has been performed within Lake Paladru (492 m a.s.l., French Prealps) catchment and a six-meter long sediment sequence retrieved from the central lacustrine basin. The combination of minerogenic signal, specific organic markers of autochthonous and allochthonous supply and archaeological data allows the reconstruction of a continuous record of past human disturbances. Over the last 10000 years, the lacustrine sedimentation was dominated by autochthonous carbonates and the watershed was mostly forest-covered. However, seven phases of higher accumulation rate, soil erosion, algal productivity and landscape disturbances have been identified and dated from 8400-7900, 6000-4800, 4500-3200, 2700-2050 cal BP as well as AD 350-850, AD 1250-1850 and after AD 1970. Before 5200 cal BP, soil erosion is interpreted as resulting from climatic deterioration phases toward cooler and wetter conditions. During the Mid-Late Holocene period, erosion fluxes and landscape disturbances are always associated with prehistorical and historical human activities and amplified by climatic oscillations. Such changes in human land-used led to increasing minerogenic supply and nutrients loading that affected lacustrine trophic levels, especially during the last 1600 years. In addition, organic and molecular markers document previously unknown human settlements around Lake Paladru during the Bronze and the Iron Ages

Tracking Holocene glacial and high-altitude alpine environments fluctuations from minerogenic and organic markers in proglacial lake sediments (Lake Blanc Huez, Western French Alps)

“…Generally, the isolated OM fraction is obtained after the elimination of carbonate and silicate phases by hydrochloric and hydrofluoric acids attacks. This method allows to identify and quantify the major organic constituents (Di Giovanni et al, 1998Noël et al, 2001;Sebag et al, 2006;Simonneau et al, 2013aSimonneau et al, , 2013bSimonneau et al, , 2013c Combaz (1964) and is based on the incorporation of a known mass of a standard (Cupressus pollen) in organic concentrate after acid attacks. QOP allows the quantification of organic material inputs into modern environments in mass per gram of sample.…”

“…pollens and spores) and non-palynomorphs microfossils, such as lignocellulosic fragments (TLC, figure 5B) with different aspects depending of their degree of alteration (from non-alterated to burnt particles defined by irregular boundaries and internal small dark vesicles observed in reflected light) and red colloidal amorphous particles with diffuse limits and no internal structure (rAP, figure 5B). All lacustrine sediment samples only differ from the watershed ones by the presence of grey amorphous particles (gAP, figure 5B) corresponding to product of algal growth in the lake waters (Patience et al, 1995;Simonneau et al, 2013aSimonneau et al, , 2013b) and rhombohedral pyrite suggesting some periods of anoxic conditions (), in particular throughout the sedimentary unit U4.…”

“…Conversely, gyttja deposits in proglacial lacustrine environments are rich in organic matter (OM) but poor in minerogenic material and reflect periods without any significant glacier activity in the drainage basin (Nesje et al, 2001). Because OM in lakes can have different origins (either algal production, soil erosion or bedrock erosion) and since OM accumulation and preservation can reflect climatic regimes and global climatic conditions (Lallier-Vergès et al, 1993;Ariztegui et al, 1996;Sifeddine et al, 1996, Simonneau et al, 2013a. It is relevant to clearly quantify and qualify the different sources of OM accumulated in a proglacial lacustrine sequence.…”

Distribution of miliacin (olean-18-en-3β-ol methyl ether) and related compounds in broomcorn millet (Panicum miliaceum) and other reputed sources: Implications for the use of sedimentary miliacin as a tracer of millet

“…In contrast, miliacin was the sole detectable PTME in the sediments of lake Le Bourget (Jacob et al, 2008a;2008b; and lakes Ledro and Paladru (Simonneau et al, 2013), and in Ukrainian paleosols that were thought to have been cultivated for P. miliaceum (MotuzaiteMatuzeviciute et al, 2013). In conjunction with other evidence, the presence of miliacin in these cases was interpreted as a tracer for the cultivation of P. miliaceum in the catchment area.…”

“…Because miliacin is resistant to degradation, changes in its content in sediments were then used to infer when millet cultivation began and subsequent fluctuation in its production in the catchment (Jacob et al, 2009). Since these pioneering studies, miliacin has also been detected in the sediments from other lakes (Simonneau et al, 2013) and in paleosols (Motuzaite-Matuzeviciute et al, 2013) where it was interpreted as a tracer of former millet cultivation.…”