Evidence of slow millennial cliff retreat rates using cosmogenic nuclides in coastal colluvium

Abstract

The erosion of rocky coasts contributes to global cycles of elements over geological times and also constitutes a major hazard that may potentially increase in the future. Yet, it remains a challenge to quantify rocky coast retreat rates over millennia – a time span that encompasses the stochasticity of the processes involved. Specifically, there are no available methods that can be used to quantify slow coastal erosion (< 1 cm yr−1) averaged over millennia. Here, we use the 10Be concentration in colluvium, corresponding to the by-product of aerial rocky coast erosion, to quantify the local coastal retreat rate averaged over millennia. We test this approach along the Mediterranean coast of the eastern Pyrenees (n=8) and the desert coast in southern Peru (n=3). We observe a consistent relationship between the inferred erosion rates and the geomorphic contexts. The retreat rates are similar, 0.3–0.6 mm yr−1 for five samples taken on the Mediterranean coast, whereas two samples from vegetated colluvium have a lower rate of ∼ 0.1 mm yr−1. The coastal retreat rate of the Peruvian site currently subject to wave action is similar to the Mediterranean coast (0.5 mm yr−1), despite Peru's more arid climate. The other two Peruvian sites, which have not been subjected to wave action for tens of thousands of years, are eroding 20 times more slowly. The integration periods of the two slowest Mediterranean coast erosion rates may encompass pre-Holocene times, during which the sea level and thus the retreat rate were much lower. We explore here this bias and conclude that the associated bias on the inferred retreat rate is less than 80 %. These data show that rocky coasts are eroding 1 to 20 times faster than catchments in the same regions on average over the last few thousand years. We anticipate that this new method of quantifying slow rocky coastal erosion will fill a major gap in the coastal erosion database and improve our understanding of both coastal erosion factors and hazards.