High-altitude varve records of abrupt environmental changes and mining activity over the last 4000 years in the Western French Alps (Lake Bramant, Grandes Rousses Massif)

Abstract

Two twin short gravity cores and a long piston core recovered from the deepest part of proglacial Lake Bramant (Grandes Rousses Massif, French Alps), under and overlying a large slump identified by high-resolution seismic profile, allow the investigation of Holocene natural hazards and interactions between human activity and climatic changes at high-altitude. Annual sedimentation throughout the cores (glacial varves) is identified on photographs, ITRAX (high-resolution continuous microfluorescence-X) and CAT-Scan (computerized axial tomography) analyses and is supported by (1) the number of dark and light laminations between dates obtained by radionuclide measurements ( 137Cs, 241Am), (2) the correlation of a slump triggered by the nearby AD 1881 Allemond earthquake (MSK intensity VII) and of a turbidite triggered by the AD 1822 Chautagne regional earthquake (MSK intensity VIII), (3) the number of laminations between two accelerator mass spectrometry (AMS) 14C dates, and (4) archaeological data. In Lake Bramant, dark layers are coarser, contain less detrital elements, but more neoformed elements and organic matter content. These darker laminations result from calm background sedimentation, whereas the lighter layers are finer and rich in detrital elements and reflect the summer snowmelt. Traces of mining activity during the Roman civilization apogee (AD 115–330) and during the Early Bronze Age (3770–3870 cal BP) are recorded by lead and copper content in the sediments and probably result from regional and local mining activity in the NW Alps. Warmer climate during the Bronze Age in this part of the Alps is suggested by (1) two organic deposits (4160–3600 cal BP and 3300–2850 cal BP) likely reflecting a lower lake level and smaller glaciers and (2) evidence of a different vegetation cover around 2500 m a.s.l. The onset of clastic proglacial sedimentation between 3600–3300 cal BP and since 2850 cal BP is synchronous with periods of glacier advances documented in the Alps and high-lake levels in west-central Europe. This major change in proglacial sedimentation highlights the development of a larger St. Sorlin glacier in the catchment area of Lake Bramant.