Research paperLate Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel
Graphical abstract
Highlights
► Holocene seacliff retreat rate is evaluated with 10Be in shore platform samples. ► Holocene platform is much less concentrated than the last interglacial one. ► 10Be concentrations display a characteristic bump (analytical solution is given). ► These features are modulated by the tide and sea-level history. ► Given the sea-level history, the retreat rate calculated is a 5–6 kyr average.
Introduction
Emery and Kuhn (1982) estimated that 75% of the world’s shoreline is rocky. Rocky seacliff collapse is therefore a major hazard in coastal environments. Evaluating this hazard requires better knowledge of cliff retreat rates. Given the episodic nature of cliff collapse events, such rates must be integrated over long enough time-spans to average over the full spectrum of events. To date, mapping the location of a receding cliff and deducing its recession rate has been accomplished largely through comparison of mapped cliff positions from aerial photographs or topographic maps (e.g., Costa et al., 2004, Dornbusch et al., 2006, Moore and Griggs, 2002). The corresponding time-span may reach two centuries in the very best cases, but is more typically of the order of 80 years. In general, as older maps and photos contain geometric inaccuracies and lower spatial resolution, the older the data source, and thus the longer the time-span, the less accurate the cliff retreat rate. Also, when trying to understand natural coastal systems, one would ideally avoid the period affected by human impacts on the system (e.g., Dornbusch et al., 2007); unfortunately, this coincides with when historical data become reliable. Measuring long-term “natural” retreat rate has therefore remained an elusive goal because of lack of information on cliff positions in the past.
Cosmogenic nuclides, and among them 10Be resulting from spallation reactions on Si and O and thus accumulating in silica-rich rocks, hold the potential to meet this challenge. Cosmogenic nuclides produced within rocks down to a few meters below the surface (in situ-produced cosmogenic nuclides) record when rocks have been exposed to cosmic rays at a given location, even when the uppermost surface rocks are removed (for example, see Gosse and Phillips (2001) for a cosmogenic nuclide primer). The production rate of cosmogenic radionuclides at depth z is given by:where the decay length scale z∗ depends upon material density, ρ (either rock or water, the respective decay length scales being zr∗ and zw∗): z∗ = Λ/ρ; P0 is the production rate at surface, which is dependent on the latitude and elevation of the sampling site, and Λ is an attenuation factor that depends upon the production reaction. According to their respective attenuation factors, the production due to spallation by neutrons, the most efficient of the processes, decays by an order of magnitude every ∼2–3 m (∼5 m under water) while the production due to the less efficient muons extends over tens of meters (Braucher et al., 2003, Kim and Englert, 2004). For a given parcel of rock, the cosmogenic nuclide concentration N evolves according to the differential equation in which production and decay are incorporated:λ being the decay constant. Cosmogenic nuclides have recently been used to quantify slowly eroding cliffs (≪10 cm/yr; Recorbet et al., 2010, Le Roux et al., 2009), but until now have not been employed to quantify recession rates of rapidly eroding coastal cliffs.
In this paper we propose a simple model for the concentrations of cosmogenic nuclides to be expected in rocks sampled across present-day shore platforms, and compare its predictions against measured concentrations to derive long-term cliff retreat rates. The model allows two ways of evaluating long-term recession rates: i) by locating where the cliff stood inactive during the last glacial period; ii) by analyzing the shape of cosmogenic nuclide concentration transects along the Holocene platform. We test our model at Mesnil-Val, a chalk cliff site along the French coast of the English Channel (“La Manche” in French).
On rocky coasts, and particularly on chalky coasts, cliffs may retreat as fast as 20 cm/yr, while the shore platform undergoes downwearing (e.g., Stephenson and Kirk, 2000a, Stephenson and Kirk, 2000b, Trenhaile and Byrne, 1986). The processes responsible include: chemical weathering during tide water level fluctuations (Duperret et al., 2005, Kanyaya and Trenhaile, 2005, Stephenson and Kirk, 2000a, Sunamura, 1992), biological action (e.g., Nesteroff and Mélières, 1967), and the mechanical action of waves (e.g., de Lange and Moon, 2005, Stephenson and Kirk, 2000b). On chalky coasts along the English Channel, these processes result in a roughly linear coastline in plan-view, and a very shallowly sloping shore platform (typically ∼1:150). This suggests that the overall effect of erosion along the coast is relatively uniform in the long term and leads us to assume that cliff erosion is in fact also steady in the long term (100–1000 yr), averaging the short term stochastic behavior associated with 0.1–100 yr recurrence of sudden collapse events (Duperret et al., 2004, Hutchinson, 2002, Mortimore et al., 2004).
The model we develop is applied to a study site located on the southern English Channel shore, at Mesnil-Val (50°03′N, 1°20′E, Fig. 1), where the cliff rises from 20 m up to 80 m, and the platform width ranges from 300 to 600 m. The average tidal range is ∼7 m. Cliff lithology is Upper Cretaceous (Turonian to Coniacian) chalk, known as the Lewes Nodular Chalk Formation (Mortimore et al., 2004). Strength varies little, resulting in a planar platform with the exception of steps that are up to 40-cm high, corresponding to alternation of hardgrounds and softgrounds (Lasseur et al., 2009) (cf. Fig. 1). The cliff at Mesnil-Val has retreated at an average rate of ∼15 cm/yr for the 1966–1995 period (Costa et al., 2004), but single collapse events can remove pieces as thick as 18 m (laser scanning observations processed by T. Dewez) that therefore correspond to roughly 120 years worth of erosion.
Section snippets
Background
With the exception of coasts situated at relatively high latitude, almost every coast on Earth was left far inland during the last glacial maximum (LGM). If distant enough from LGM ice caps, they then experienced a rapid sea-level rise from ∼−120 m 18 ka ago to roughly modern levels by ∼6 ka BP (Clark et al., 1978). This is the case for the English Channel (Edwards, 2001, Edwards, 2006, Edwards and Horton, 2000, Lambeck, 1997, Lambeck et al., 1990, Peltier et al., 2002). Assuming that the cliff
Sampling and processing
In order to test the validity of our theoretical model described above, in situ outcropping flint samples were extracted from the chalk platform along an across-shore transect, attempting to reach the furthest distance possible from the cliff (up to 600 m from the cliff, a fair weather day of strong equinox tide, cf. Fig. 1). There is one exception to this sampling scheme, as finding in situ flint was challenging. Sample MV-08 was extracted from the side of a block 80 cm below its top surface.
Discussion: how to measure the long-term retreat rate?
The numerical and simplified exposure model results presented here highlight two features that can be employed to determine long-term cliff recession rates. First, one can trace back the location of the last glacial cliff position. Second, one can fit the spatial pattern of concentration on the platform to derive cliff recession rates.
Conclusions
We propose a method to evaluate long-term cliff retreat rate based on the pattern of cosmogenic nuclide concentration across a shore platform. The method developed both analytically and numerically identifies the best conditions to reach a reliable result: the gently sloping site must have undergone moderate retreat rates (less than ∼30 cm/yr). Under these conditions, the most reliable method is to locate the seacliff position during the last glacial, indicated by a discrete step in the 10Be
Acknowledgments
C. Cavare-Hester has drawn the sketch of Fig. 2. This work benefited from a couple of years of observation by BRGM (internal project EVOLGEOM driven by TD). We thank the French SHOM/INSU RELIEFS program (“rocky coast erosion: from observation to modeling”, driven by VR) and BRGM (Bureau des Recherches Géologiques et Minières) for support. We thank D. Farber, S. Carretier, J. Martinod and D. Fink for encouraging remarks. We are indebted to A. Matmon and 3 anonymous reviewers for their
References (48)
- et al.
A complete and easily accessible means of calculating surface exposure ages or erosion rates from Be-10 and Al-26 measurements
Quaternary Geochronology
(2008) - et al.
In situ produced Be-10 measurements at great depths: implications for production rates by fast muons
Earth and Planetary Science Letters
(2003) - et al.
Production of cosmogenic radionuclides at great depth: a multi element approach
Earth and Planetary Science Letters
(2011) - et al.
Examination of surface exposure ages of Antarctic moraines using in situ produced 10Be and 26Al
Geochimica et Cosmochimica Acta
(1991) - et al.
Determination of the Be-10 half-life by multicollector ICP–MS and liquid scintillation counting
Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms
(2010) - et al.
Global changes in post glacial sea level: a numerical calculation
Quaternary Research
(1978) - et al.
Relative sea-level changes and coastal evolution at Forum Julii (Fréjus, Provence)
Comptes Rendus Geoscience
(2007) - et al.
Chalk shore platform erosion in the vicinity of sea defence structures and the impact of construction methods
Coastal Engineering
(2007) - et al.
Effect of groundwater and sea weathering cycles on the strength of chalk rock from unstable coastal cliffs of NW France
Engineering Geology
(2005) - et al.
Reconstructing relative sea-level change using UK salt-marsh foraminifera
Marine Geology
(2000)
Terrestrial in situ cosmogenic nuclides: theory and application
Quaternary Science Reviews
Production of selected cosmogenic radionuclides by muons: 1. Fast muons
Earth and Planetary Science Letters
Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons
Earth and Planetary Science Letters
Tidal wetting and drying on shore platforms: an experimental assessment
Geomorphology
Late Holocene sea-level change on Rota and Guam, Mariana Islands, and its constraint on geophysical predictions
Quaternary Research
In situ cosmogenic nuclide production of Be-10 and Al-26 in marine terraces, Fiordland, New Zealand
Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms
A new value for the half-life of 10Be by heavy-ion elastic recoil detection and liquid scintillation counting
Nuclear Instruments & Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Sea-level change along the French Atlantic and Channel coasts since the time of the Last Glacial Maximum
Palaeogeography Palaeoclimatology Palaeoecology
Estimating long-term cliff recession rates from shore platform widths
Engineering Geology
A relative water-depth model for the Normandy Chalk (Cenomanian–Middle Coniacian, Paris Basin, France) based on facies patterns of metre-scale cycles
Sedimentary Geology
Long-term cliff retreat and erosion hotspots along the central shores of the Monterey Bay National Marine Sanctuary
Marine Geology
Changes of relative sea level during the past 5000 years in the ancient harbor of Marseilles, Southern France
Palaeogeography, Palaeoclimatology, Palaeoecology
Evidence for active retreat of a coastal cliff between 3.5 and 12 ka in Cassis (South East France)
Geomorphology
CRE dating on the head scarp of a major landslide (Sechilienne, French Alps), age constraints on Holocene kinematics
Earth and Planetary Science Letters
Cited by (53)
Spatial and temporal trends in California coastal cliff retreat
2022, GeomorphologyCitation Excerpt :Numerical modelling allows better understanding of the mechanisms and feedbacks of cliff erosion, but model parameters are often abstract and hence difficult to apply for real situations, while explored timelines tend to be beyond the managerial (annual to decadal) scope (Kline et al., 2014; Limber et al., 2014; Matsumoto et al., 2016). Cosmogenic radionuclide dating can help calibrate and validate cliff erosion models and better understand how short-term erosion rates measured in the field feed into long-term cliff evolution patterns (Regard et al., 2012; Hurst et al., 2016; Swirad et al., 2020). However, the variability in cliff erosion at the local to regional scale (100–105 m) remains difficult to model because of diversity in coastal settings (e.g. topography, geology, wave climate, rainfall pattern, stage of cliff development).
Rock coast erosion: An overlooked source of sediments to the ocean. Europe as an example
2022, Earth and Planetary Science LettersRock Coasts
2022, Treatise on GeomorphologyThe precision and accuracy of measuring micro-scale erosion on shore platforms
2022, Marine Geology