Paper No. 246-7
Presentation Time: 4:10 PM
MILLENNIAL HISTORY OF SEA CLIFF RETREAT DERIVED USING COSMOGENIC 10BE REVEALS A RECENT ACCELERATION IN EROSION
Observed retreat of rocky coastlines is limited to the relatively short timescales that historical records exist for (< 150 years). During this time, human-induced acceleration in sea level rise started, and humans began modifying the coastal environment. There is little knowledge of rates of coastal erosion and cliff retreat prior to these relatively brief historical records. Cliff retreat rates are expected to accelerate if influenced by stormier climates and rising sea level. Predicting potential future coastal change requires us to establish baseline conditions averaged over longer periods. Here we present analysis of sea cliff retreat during the Holocene at chalk cliffs in southeast England using cosmogenic isotopes. We determine millennial-scale rates of sea cliff erosion from concentrations of 10Be measured from in situ flint samples collected from two transects across coastal platforms. Using a Monte Carlo approach, we deploy a numerical model of 10Be accumulation on an evolving coastal profile in order to determine the most likely history of cliff retreat rate during the Holocene. The model accounts for variation in 10Be accumulation with tides and sea-level rise, and takes into account platform downwear and topographic shielding by adjacent cliffs. We find that cliff retreat rates during the Holocene were significantly slower (2-6 cm yr-1) than those derived from recent historical observations (15-25 cm yr-1). Modelled accumulation of 10Be requires retreat rates that increase rapidly in recent times. The causes of this acceleration potentially include a response to increased recent (last 500 year) storm activity and a changing sediment budget. Loss of beach protection at the toe of the cliffs may result in a nonlinear response and rapid erosion. This may be further influenced by human modification of the coastal sediment budget through construction of sea defences, flood defenses and aggregate extraction.