Controls on the distribution of cosmogenic <sup>10</sup>Be across shore platforms

Abstract

Abstract. Quantifying rates of erosion on cliffed coasts across a range of timescales is vital for understanding the drivers and processes of coastal change and for assessing risks posed by future cliff retreat. Historical records cover at best the last 150 years; cosmogenic isotopes, such as 10 Be could allow us to look further into the past to assess coastal change on millennial timescales. Cosmogenic isotopes accumulate in situ near the Earth surface and have been used extensively to quantify erosion rates, burial dates and surface exposure ages in terrestrial landscapes over the last 3 decades. More recently, applications in rocky coast settings have quantified the timing of mass wasting events, determined long-term averaged rates of cliff retreat and revealed the exposure history of shore platforms. In this contribution, we develop and explore a numerical model for the accumulation of 10 Be on eroding shore platforms. In a series of numerical experiments, we investigated the influence of topographic and water shielding, dynamic platform erosion processes, the presence and variation in beach cover, and heterogeneous distribution of erosion on the distribution of 10 Be across shore platforms. Results demonstrate that, taking into account relative sea level change and tides, the concentration of 10 Be is sensitive to rates of cliff retreat. Factors such as topographic shielding and beach cover act to reduce 10 Be concentrations on the platform and may result in overestimation of cliff retreat rates if not accounted for. The shape of the distribution of 10 Be across a shore platform can potentially reveal whether cliff retreat rates are declining or accelerating through time. Measurement of 10 Be in shore platforms has great potential to allow us to quantify long-term rates of cliff retreat and platform erosion.

Cosmogenic exposure dating reveals limited long-term variability in erosion of a rocky coastline

“…At present, the only way to verify the accuracy of models of long-term change of fully erosional rocky coasts is via cosmogenic radionuclide exposure dating 8,13 . Calculation of isotope concentrations across an active shore platform allows previous position(s) of the cliff to be reconstructed from their timing of exposure of the foreshore.…”

“…They have also simplified the potential role of mesoscale (10 −1 -10 0 m) block detachment stripping material from the foreshore 18,19 . While the potentially highly important role of some of these factors in reconstructing the rate and nature of rocky coast erosion has been conceptually demonstrated 13 , this has yet to be explored with field data.…”

GlobR2C2 (Global Recession Rates of Coastal Cliffs): a global relational database to investigate coastal rocky cliff erosion rate variations

“…Measuring sea cliff erosion presents a wide range of techniques. Those techniques vary significantly in terms of the following: (i) accuracy, which range from field observation and "expert" estimates May and Hansom (2003) of volume loss to precise measurements using techniques such as lidar (e.g., Dewez et al, 2013); (ii) time period surveyed, which range from twenty minutes (e.g., Williams et al, 2018) to thousands of years (e.g., Choi et al, 2012;Hurst et al, 2017;Regard et al, 2012); and (iii) the spatial extent along the coast, which ranges from tens of meters (e.g., Letortu et al, 2015) to kilometers (e.g., Hapke et al, 2009). Moreover, these measurements can be divided into three classes of methods:…”

“…The tidal range describes the variation in the height of the water surface. One consequence is that the cliff and platform undergo cyclic wetting and drying that weakens and erodes the constituting rocks (Kanyaya and Trenhaile, 2005).…”

“…Hence, platforms can be regarded as natural defences against wave attack of the cliff. The shore platform evolves under marine forcing like wave agitation and associated shear stress (e.g., Sallenger Jr et al, 2002;Stephenson and Kirk, 2000;Sunamura, 1992;Trenhaile, 2008Trenhaile, , 2009, or tide-induced wetting and drying cycles (Kanyaya and Trenhaile, 2005;Stephenson and Kirk, 2000). The second group of drivers is rock mass properties, which are believed to have a strong influence on cliff evolution (Mortimore and Duperret, 2004).…”