Freeze-thaw and shore platform development in Gaspé, Québec

Downwearing rates were measured on shore platforms at about 200 transverse micro-erosion meter (TMEM) stations, over periods ranging from 2 to 6 years. There were seven study areas in eastern Canada. The platforms were surveyed and a Schmidt Rock Test Hammer was used to measure rock hardness. More than 1200 rock samples from three of the study areas were also subjected each day, over a 3 year period, to two tidal cycles of immersion and exposure, which simulated the central intertidal zone. A further 840 samples were subjected to longer periods of exposure and immersion, over a 1 year period, which represented different elevations within the upper and lower intertidal zone, respectively. These experiments suggested that tidally generated weathering and debris removal is an effective erosional mechanism, particularly at the elevation of the lowest high tides. In the fi eld, mean rates of downwearing for each study area ranged from 0·24 mm yr −1 to more than 1·5 mm yr . Rates tended to increase with elevation in the fi eld, with maxima in the upper intertidal zone. This trend in the fi eld cannot be attributed entirely to the tidally induced weathering processes that were simulated in the laboratory, and must refl ect, in part, the effect of waves, frost, ice, and other mechanisms. It is concluded that there are no strong spatial downwearing patterns on shore platforms, and that downwearing rates in the intertidal zone are the result of a number of erosional mechanisms with different elevationeffi cacy characteristics. Furthermore, even if only one or two mechanisms were dominant in an area, any resulting relationship between downwearing rates and elevation would be obscured or eliminated by the effect of variations in the chemical and physical characteristics of the rocks.

Section: Discussionmentioning

confidence: 99%

Patterns of surface downwearing on shore platforms in eastern Canada

Porter

Trenhaile

Prestanski

et al. 2010

“…Frost shattering has some influence on shore platform erosion; it is controlled by the ambient temperature and modulated by tide buffering (Harris and Ralph, ; Trenhaile and Rudakas, ; Robinson and Jerwood, , ). Frost shattering is a generic term used to describe the brecciation effect of frost on a rock surface.…”

Section: Introductionmentioning

confidence: 99%

Shore platform lowering due to frost shattering during the 2009 winter at mesnil Val, English channel coast, NW France

Dewez

Regard

Duperret

et al. 2015

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 2 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 h. 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. Copyright © 2015 John Wiley & Sons, Ltd.

“…The operation of freeze-thaw weathering in seawater has been examined (e.g. Trenhaile and Rudakas, 1981;Robinson and Jerwood, 1987), but where tidal patterns have been simulated in the laboratory rock temperatures are not typically recorded (e.g. Trenhaile and Mercan, 1984) or freshwater has been used to control for the presence of salts in slaking experiments (e.g.…”

Section: Introductionmentioning

confidence: 99%

Rock warming and drying under simulated intertidal conditions, part I: experimental procedures and comparisons with field data

Coombes

2011

The wetting-drying and warming-cooling behaviours of rock and stone are known to influence the nature and rate of weathering. The way materials warm-up and dry-out also influences their suitability as biological substrata. While rock thermal behaviours have been measured under controlled laboratory conditions, previous experiments have largely been restricted to terrestrial simulations due to practical constraints. Where efforts have been made to simulate intertidal conditions, expansion and contraction of rocks or rates of breakdown (i.e. sediment production and weight loss) have been measured, while detailed observations of thermal and drying behaviours have rarely been made.A simple, semi-automated procedure is described that enabled measurement of surface temperatures and desorption (evaporative water loss) for different material types (rock and concrete) under simulated semidiurnal tide conditions. Some preliminary results are presented illustrating the types of data that were obtained, and comparisons are made with temperature data collected on a rock platform in the UK to assess the ability of the procedure to adequately represent field conditions.