| 28 Oct 2025
Coastal-Cosmo-Model (CCMv1): a cosmogenic nuclide model for rocky coastlines
Abstract. Understanding the long-term evolution of rocky coasts requires models that can account for complex interactions between exposure, erosion and sea level, constrained by empirical observations. This paper introduces Coastal-Cosmo-Model version 1 (CCMv1), a modular forward modelling framework designed to reconstruct coastal histories from in situ cosmogenic nuclide concentrations. CCMv1 integrates community-standard production rate calculations and allows flexible inversion of platform histories using discrete erosion and exposure parameters. The model includes four sub-models—inheritance, zero erosion, down-wearing only, and cliff retreat with down-wearing—enabling users to test hypotheses of increasing complexity. Crucially, CCMv1 can be applied to both eroding and non-eroding coastlines, offering a means to investigate the dominant controls on rocky shore histories for different settings. A demonstration using a published dataset from shore platform shows that CCMv1 effectively reproduces measured nuclide concentrations and supports a history of Holocene cliff retreat. CCMv1 provides an adaptable and hypothesis-driven framework for exploring rocky shore histories, with potential for future development to incorporate probabilistic optimisation and additional nuclide systems, and implementation for testing complex (multi-stage) erosion histories or relative sea-level histories.
This manuscript provides a welcome new addition to efforts to model shore platform evolution (and coastal cliff retreat rates) using in situ produced cosmogenic radionuclides (CRN). It contributes to a growing body of literature on the topic. I suggest adding an additional paragraph within the introduction to more fully summarise the relatively small number of contributions in the field to date.
The model formulation appears to be grounded in established cosmogenic nuclide theory. A key novel contribution of this new model (CCM) is that it provides a clear hypothesis testing framework. Rather than seeking a single solution between morphological development and observed CRN concentrations, it allows users to systematically step through multiple hypotheses that increase in complexity. I think this will be a valuable addition to existing tools to model rock coast evolution.
Four sub-models are provided: inheritance, zero erosion, down-wearing only, and cliff retreat with down-wearing. Similar to previous work (e.g. Hurst et al., 2016), the inheritance sub-model calculates cosmogenic inheritance based on a sample taken from a site that represents how much inherited 10Be is in the rock prior to exposure. If a feasible value is found for the site, that value is then used to correct platform nuclide concentrations prior to modelling the erosion history. The zero erosion sub model represents a null hypothesis in which the test is whether CRN observations can be explained solely by sea-level change. In this model there is no cliff retreat (backwear) or vertical shore platform erosion (downwear). The downwear model then tests vertical erosion without cliff retreat, and the final model is the most complex (and the most likely in nature!) in that it involves both backwear (cliff retreat) and downwear (vertical platform erosion). It would be interesting to understand a little more about whether there would be any value in a backwear only model. Downwear only makes sense, because it makes it possible to rule out a no-backwear scenario. Perhaps backwear only doesn't make sense, because downwear would be expected if backwear is also occurring, but is there value in isolating what a backwear only signal would look like? Perhaps it is possible to set the downwear rate to zero in the combined backwear/downwear model?
The CCM model usefully integrates with CRONUScalc and a global calibration dataset. It also includes an optimisation framework, and while it doesn't go as far as previous work (Shadrick et al., 2021) in respect to techniques such as multi-objective and Bayesian optimisation, the overall model framework means that the optimisation function (multidimensional unconstrained nonlinear minimization using Nelder-Mead within MATLAB) can help to solve for variable backwear and downwear erosion rates through time.
Similar to previous work, three downwear scenarios are considered: constant, increasing or decreasing (relative to some specified present-day erosion rate). Three backwear scenarios are also considered: accelerating, decelerating or constant. In theory, if cliff retreat is wave-driven then widening platforms through time should result in a non-linear decline in erosion rates, although this is complicated by Holocene sea level variability (eg see Trenhaile 2010). In future work, in addition to the decelerating scenario, it would be interesting also to allow testing of a non-linear deceleration.
The overall value of the new modelling approach is demonstrated with a useful real-world comparison using the dataset of Swirad et al. (2020) from North Yorkshire. Ultimately the lower panel of Figure 6 appears to be a compelling demonstration that the combined effects of backwear and downwear have driven the evolution of this rock coast environment. The discussion section notes that the model does not actually simulate the physical drivers of erosion, but in helping to untangle the relative contribution of backwear and downwear, the model does present a new tool that should help in broader efforts to understand process controls on rock coast evolution.
Additional references cited:
Trenhaile, A. S. (2010). The effect of Holocene changes in relative sea level on the morphology of rocky coasts. Geomorphology, 114(1-2), 30-41.