Climatic variability in the northwestern Alps, France, as evidenced by 600 years of terrigenous sedimentation in Lake Le Bourget

Abstract

Cores recovered from periglacial Lake Le Bourget deep basin (northwestern Alps) were investigated to examine the influence of the ‘Little Ice Age’ (LIA) on terrigenous lacustrine sedimentation. Growing glaciers in the regional watershed induced catastrophic Rhone river floods and major underflow deposits in the deep basin during the early fifteenth, the sixteenth and the mid-eighteenth centuries. The LIA is characterized by a decrease in deposition from interflows from AD-1550 to 1740 and an increase in deposition from underflows from AD-1550 to 1800. On one hand, spectral analyses of the laminations in interflow deposits reveal 4-5 years cyclicities from AD-1440 to 1550, as well as 7-8 and 13-14 years cyclicities from AD-1740 to 1870; on the other hand, spectral analyses of a clay mineral ratio reflecting underflow deposits highlight 45-50 years cyclicities from AD-1550 to 1800. These pluriannual, decadal and pluridecadal periods are typical of the North Atlantic Oscillation (NAO). A NAO-like period in our data would be a consequence of periodical variations in rainfall and snow accumulation during late autumn and winter over Lake Le Bourget's watershed.