Conceptual model of fracture-limited sea cliff erosion: erosion of the seaward tilted flyschs of Socoa, Basque Country, France

International audienceSea cliff morphology and erosion rates are modulated by several factors, including rock control that reflects both lithology and rock structure. Erosion is anticipated to preferentially exploit “fractures”, broadly meant as any discontinuity in an otherwise continuous medium, where the rock mass is weakest. Unpicking the direct control of such fractures on the spatial and temporal pattern of erosion remains, however, challenging. To analyze how such fractures control erosion, we monitored the evolution of a 400 m long stretch of highly-structured sedimentary cliffs in Socoa, Basque Country, France. The rock is known as the Socoa Flysch Formation. This formation combines decimeter-thick turbidites composed of repeat triplets of medium to strong calcareous sandstone, laminated siltstones and argillaceous marls. The sequence plunges at 45° into the sea with a shore-parallel strike. The cliffs are cross-cut by two normal and reverse fault families, with 10 – 100 m alongshore spacing, with primary and secondary strata bound fractures perpendicular to the bedding, which combined delimit the cliff rock mass into discrete blocks that are exploited by erosion process. Erosion, and sometimes plucking, of such beds and blocks on the cliff face was monitored using ground-based Structure-from-Motion (SfM) photogrammetry, over the course of 5.7 years between 2011 and 2017. To compare with longer time change, cliff-top retreat rate was assessed using SfM-orthorectified archive aerial photographs spanning 1954 - 2008. We show that the 13,250 m² cliff face released 4500 blocks exceeding 1.45 ×10-3 m3, removing a total volume of 170 m3. This equates to an average cliff erosion rate of 3.4 mm/yr, which is slightly slower than the 54-year-long local cliff top retreat (10.8 ± 1.8 mm/yr). The vertical distribution of erosion reflects the height of sea water inundation, where the maximum erosion intensity occurs ca. 2 m above high spring-tide water level. Alongshore, the distribution of rockfall scars is concentrated along bed edges bounding cross-cutting faults; the extent of block detachment is controlled by secondary tectonic joints, which may extend through several beds locally sharing similar mechanical strength; and, rockfall depth is always a multiple of bed thickness. Over the longer-term, we explain block detachment and resultant cliff collapse as a cycle. Erosion nucleates on readily exploitable fractures but elsewhere, the sea only meets defect free medium-strong to strong rock slabs offering few morphological features for exploitation. Structurally delimited blocks are quarried, and with sufficient time, carves semi-elliptic scars reaching progressively deeper strata to be eroded. Lateral propagation of erosion is directed along mechanical weaknesses in the bedding, and large episodic collapses affect the overhanging slabs via sliding on the weak marl beds. Collapse geometry is confined to one or several triplets of turbidite beds, but never reaches deeper into the cliff than the eroded depth at the foot. We contend that this fracture-limited model of sea-cliff erosion, inferred from the Socoa site dynamics and its peculiar sets of fractures, applies more broadly to other fractured cliff contexts, albeit with site specific geometries. The initiation of erosion, the propagation of incremental block release, and the ultimate full failure of the cliff, have each been shown to be fundamentally directly controlled by structure, which remains a vital control in understanding how cliffed coasts have changed in the past and will change in the future

Quantification of a rock platform bioerosion by the sea urchin Paracentrotus lividus (Lamarck, 1816) the Basque Coast case (Bay of Biscay)

The French Basque Coast is an actively eroding rocky coast. In the study area it is made of flysch with a circa 40 degrees-dip; it is exposed to an average significant wave height of 1.8 m and peak  period of 9.6 s. It is mesotidal (spring tide of ~4m). The current rocky shore platform is carved into the flysch layering. We focus on sea urchin Paracentrotus lividus contribution to rock shore erosion. Indeed, this sea urchin has a burrowing behaviour. An ecological study has been conducted for stock assessment, from shore to 10 m-deep. It indicates that sea urchins are burying their own shelter and are not reusing former ones. Below the depth of 10 m, sea urchins densities are very low and burial behavior is considered insignificant. Estimates of sea urchin density, and biomass are provided, making possible to evaluate the average erosion rate along this coast due to burying. It is of the order of 0.17 mm/yr between 0 and 3 m below sea level, ~0.05 mm/a between 3 and 5 m-deep, ~0.02 mm/a between 5 and 8 m below sea level, and finally drops to negligible under 8 meters below sea-level. Close to the 0 level, it is thus between 30 and 5% of the expected value (evaluated to be 0.5-3 mm/a after 5-10cm/a cliff retreat rate and a 1-2degrees platform dip). The overall sea urchin contribution to shore platform erosion is not negligible

L'érosion des côtes rocheuses : une source de sédiments vers l'océan méconnue. Exemple à l'échelle de l'Europe

International audienceCurrent assessments of the continent-to-ocean sediment budget assume that river discharge provides 11-21 Gt/a (72-89%) of the global sediment flux to the ocean. The remaining 11-28% supposedly comes mainly from glacially-derived sediments and airborne dust. Until recently, the contribution from rock coasts was estimated to represent 0.4 Gt/a, representing only 2-4% of the total flux. For the first time, and using the most complete global compilation of sea cliff recession rates in Europe, this study evaluates the rock coast contribution to sediment flux. We show that this sediment flux has been largely underestimated, and reveal that cliff derived sediment supply is only three times less than the solid discharge of rivers (111 ± 65 vs. 290 Mt/a) for Europe. This new estimate of the rock coast erosion should be included in future studies on the evolution of the surficial Earth system.Les évaluations actuelles du bilan sédimentaire continent-océan supposent que le débit fluvial fournit 11-21 Gt/a (72-89%) du flux sédimentaire mondial vers l'océan. Les 11-28% restants proviendraient principalement des sédiments d'origine glaciaire et de des poussières appotées par le vent. Jusqu'à maintenant, la contribution des côtes rocheuses était estimée à 0,4 Gt/a, soit seulement 2-4% du flux total. Pour la première fois, et en utilisant la compilation globale la plus complète des taux de recul des falaises maritimes en Europe, cette étude propose une évaluation de la contribution de l'érosion des côtes rocheuses au flux sédimentaire. Nous montrons que ce flux sédimentaire a été largement sous-estimé, et révélons que l'apport sédimentaire issu des falaises est seulement trois fois moins important que le débit solide des rivières (111 ± 65 vs. 290 Mt/a) pour l'Europe. Cette nouvelle estimation de l'érosion des côtes rocheuses devrait être incluse dans les futures études sur l'évolution des enveloppes superficielles de la Terre