Spatio-temporal variability in the distribution of biogenic particles over a cool temperate carbonate intertidal flat (Mont Saint Michel bay, France)

Coastal barriers represent around 15% of the world’s oceanic shorelines, occurring in energetic, wave-dominated environments, and play an important role as early warning indicators of environmental changes. They are typically composed of coarse-grained siliciclastic particles, but in many cases biogenic carbonates also represent a significant proportion of the bulk sediment. Shelly chenier ridges are lag-concentrations of mollusc skeletal remains separated by prograding intertidal mudflats. They have been identified along several non-tropical coasts, such as U.K., or western Korea, among others. This study focuses on shelly cheniers of the hypertidal Mont Saint Michel bay, which is located within the cool temperate shallow carbonate Normandy Brittany Gulf (western English Channel). Previous works have focused on the hydrodynamic behaviour of biogenic particles with flume tank experiments, and on the internal architecture of cheniers imaged by sub-surface radar. The aim of this study is to focus on 1) meso-scale spatial variability in the distribution of mollusc skeletal remains across the 5-km wide intertidal mudflat, and 2) identify the processes involved in the spatial distribution of cheniers along the coast. A bathymetric survey of the intertidal flat was carried out in 1957 with a mono-beam echo sounder. More than 4500 soundings have been digitised and interpolated to reconstruct a spatially-explicit bathymetry of the study area at that time, which was compared with a LiDAR survey carried out in 2002. The 1957-2002 sediment budget is weak but globally positive (+13 cm). Difference bathymetry map shows a cross-shore pattern of alternating ca. 1 km wide corridors of erosion (up to50 cm) and accretion (up to +50 cm). Erosion corridors coincide with areas of chenier development on the coast, as evidenced by sets of aerial photographs taken between 1947 and 2014. Molluscs have been sampled in the first 5 cm of surface sediment at 25 stations scattered along 5 cross-shore transects corresponding to contrasting values of sediment budget. Live and dead individuals (> 1mm) have been sorted, identified, counted and weighed separately. Bivalve molluscs account for more than 95% of the biogenic carbonate particles found on the intertidal mudflat. Short-lived intertidal species clearly dominate both the live and death assemblage, including Cerastoderma edule, Macoma balthica and Scrobicularia plana. Diversity of the live assemblage is very low (6 species), resulting in a poor live-dead agreement related to incorporation in the death assemblage of subtidal species transported out of habitat (Abra alba), and to time-averaged accumulation of rare taxa, in accordance with previous works. High amounts of fragmented shell hash in surface sediments (1 to 2 kg.m-2) were found in erosion corridors. These results suggest that wave and current propagation across the intertidal flat is focused along corridors, favouring the fragmentation of shells and their coastward transportation, and leading to preferential areas of chenier accumulation. High-resolution numerical hydrodynamic modelling is now being tested to identify the factors controlling corridor pattern of propagation.