7200 years of Rhone river flooding activity in Lake Le Bourget, France: a high-resolution sediment record of NW Alps hydrology

Magnetic susceptibility (MS) was measured with high resolution (5 mm) on a 9 m long, C-14 dated core from Lake Le Bourget (Savoie, France), spanning the last 7200 years. The strong correlation (R = 0.85) of the MS with the silicate-borne suite of elements (Si, Al, Fe, Mg, K) and anti-correlation with the carbonate content (R = -0.87) allows it to be used as a proxy for the fluctuations of the abundance of riverborne elastic fraction versus authigenic carbonates in sediment. As the Rhone is the only river bringing a significant amount of silicate minerals to the coring site, the MS downstream is interpreted as a proxy of the Rhone suspended load discharge in Lake Le Bourget. This is confirmed over the last 3000 years by the good match with the evolution of hydrological activity of the Rhone as it is known through geomorphological studies of well-dated archaeological sites. Over the last 7200 years, the record is consistent with the regional record of lake water-level fluctuations. While the intensity of the MS signal might be widely affected by the human impact on soil stability, the timing of the period of enhanced hydrological activity appears to be mostly climate-related, and should thus constitute a first step toward a high-resolution (< 8 yr) continuous history of hydrological conditions in the NW Alps.