Late Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel

International audience; We explore a new method for documenting the long term retreat rate of sea cliffs based on measurements and modeling of 10Be concentration transects across present-day shore platforms. The proposed forward numerical model relies on a scenario of sea level rise since the last deglaciation, and predicts the shape of 10Be concentration transects as a function of prescribed cliff recession and vertical coastal platform downwearing rates. Two independent transect features allow fitting the long-term recession rate model to field observations: a sharp 10Be concentration drop predicted at the former stationary location of the cliff during the last glacial period ~100 ka ago, and a characteristic dome shape whose magnitude is directly related to the recession rate of the cliff. A retreating chalk cliff site from the English Channel coast of France, at Mesnil Val, where the 7 m tidal range broadly exposes the shore platform, was selected to test the proposed method. Although retreat rates were too high to pinpoint the predicted 10Be concentration drop at the last glacial cliff position, the 10Be-concentration of the flints sampled across the shore platform are consistent with the expected dome shape. When modeled using the proper tidal range and proposed Holocene sea level rise history, the 10Be data yield a cliff retreat rate since the mid-Holocene of 11-13 cm/yr. This is consistent with a 30-year-long measurement record, strongly supporting the utility of the 10Be method.