Shore platform lowering due to frost shattering during the 2009 winter at Mesnil-Val, English Channel coast, NW France
Résumé
In 2008-2009, a severe cold snap affected the otherwise temperate mid-latitude coasts of the English Channel of France. In March 2009, we gathered rock spalling observations at Mesnil Val, NW France, to document the effect of frost on platform lowering in macro-tidal environments. Six epochs of frost were recorded in two months, the two longest lasted 16 and 8 days, with minimum air temperature dropping to-9.5°C. Semi-diurnal tides flood the entire platform, imposing up to 25 freeze-thaw cycles below-2.5°C, the freezing temperature of seawater. 19 cycles occurred at neap tidal elevation lasting at most 3.5 hrs. Our integrated observations indicate that these frost cycles were responsible for a platform lowering of about 0.8±0.5 mm during a single winter. No clear spatial trend appears, nor do macroscopic chalk types clearly stand out as being more susceptible to erosion. Assuming that the long-term platform retreat model preserves a constant slope, frost shattering would then account for 10 to 20% of the expected annual platform erosion rate. Under more contrasted climates, frost is thus likely to be a prominent shaping process for rocky coastal platforms.
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