08 Jan 2024
 | 08 Jan 2024
Status: this preprint is open for discussion.

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

Rémi Bossis, Vincent Regard, Sébastien Carretier, and Sandrine Choy

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, the geomorphic and climatic contexts. The retreat rates are similar, 0.3–0.5 mm yr-1 for five samples taken on the Mediterranean coast, whereas one sample located on a cape and two samples from a vegetated colluvium have a lower rate of ~0.1 mm yr-1. The coastal retreat rate of the drier Peruvian coast is slower at 0.05 mm yr-1. Although the integration periods of these erosion rates may encompass pre-Holocene times, during which the sea-level and thus the retreat rate were much lower, we conclude here that the associated bias on the inferred retreat rate is less than 80 %. 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.

Rémi Bossis, Vincent Regard, Sébastien Carretier, and Sandrine Choy

Status: open (extended)

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  • RC1: 'Comment on egusphere-2023-3020', Luca C Malatesta, 24 Feb 2024 reply
Rémi Bossis, Vincent Regard, Sébastien Carretier, and Sandrine Choy
Rémi Bossis, Vincent Regard, Sébastien Carretier, and Sandrine Choy


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Short summary
The erosion of rocky coasts occurs episodically through wave action and landslides, constituting a major natural hazard. Documenting the factors that control the coastal retreat rate over millennia is fundamental to evidencing any change in time. However, the known rates to date are essentially representative of the last few decades. Here, we present a new method using the concentration of an isotope, 10Be in sediment eroded from the cliff to quantify its retreat rate averaged over millennia.