Late Holocene sea-cliff retreat recorded by 10Be profiles across a coastal platform: Theory and examples from the English Channel and Korea

We explore a new method for documenting the long term retreat rate of sea cliffs based on measurements and modeling of Be 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 Be concentration transects as a function of prescribed cliff recession and vertical coastal platform downwearing rates. Two independent transect features allow fitting the longterm recession rate model to field observations: a sharp Be 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 Be concentration drop at the last glacial cliff position, the Beconcentration 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 Be data yield a cliff retreat rate since the mid-Holocene of 11-13 cm/yr. This is consistent with a 30year-long measurement record, strongly supporting the utility of the Be method.