Patterns of surface downwearing on shore platforms in eastern Canada

Abstract

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.

Controls on the distribution of cosmogenic <sup>10</sup>Be across shore platforms

“…Greater rates of downwear in the intertidal zone lower the platform more rapidly in the near-shore zone, which removes 10 Be-laden rock and results in deeper water in the near-shore zone (and therefore reduced 10 Be production) than in the steady-state model runs that assume constant α. These differences may be exacerbated when taking into account the spatial distribution of other processes such as weathering, the spatial distribution of which is currently poorly understood (Porter et al, 2010). In attempting to reconstruct cliff retreat rates the elevation profile of the platform should be accounted for such that the distribution of downwear across the platform is taken into account.…”

“…Accounting for the distribution of downwear would be aided by data on the distribution of downwear rates from micro-erosion-meter measurements (e.g. Robinson, 1977;Porter et al, 2010), and there is evidence that rates measured over the short term (1-2 years) are consistent with rates measured over 3 decades (Stephenson et al, 2010. Nevertheless, it is still not clear whether it is appropriate to extrapolate these rates over centennial to millennial timescales required to accumulate measurable concentrations of 10 Be nor is it yet clear whether this result is consistent across different shore platforms around the world.…”

“…We expected the style of platform evolution to be important for the distribution of 10 Be across a shore platform. Moreover, micro-erosion meter measurements of platform downwear suggest that downwear is not uniform across the shore profile but tends to be faster in the upper intertidal zone and decline with depth (Porter et al, 2010), and to explore the influence of such a distribution on 10 Be concentrations, a dynamic morphological model was required.…”

Predicting the response of hard and soft rock coasts to changes in sea level and wave height

“…This paper does not consider the effect of climate change on rates of weathering, mass movement and other subaerial mechanisms that reduce the strength of the rock and modify the morphology of cliffs and shore platforms, or the effect of beach material in eroding or protecting rock surfaces. The model is concerned only with wave erosion and although the effect of weathering on long-term shore platform evolution has been modelled previously (Trenhaile 2005(Trenhaile , 2008, it is unlikely, because of slow rates of erosion (Stephenson and Kirk 1998;Porter et al 2010), to play an important role during this century. The term 'shore platform' or 'platform' refers to the intertidal rock surface extending from the cliff foot to the low tidal level.…”

Schmidt hammer tests across a recently deglacierized rocky coastal zone in Spitsbergen - is there a "coastal amplification" of rock weathering in polar climates?

“…On the one hand during the last three decades several geomorphological works along Atlantic Canada's rocky shorelines (e.g. Dionne and Brodeur 1988;Dionne 1989;Trenhaile and Mercan 1984;Trenhaile et al 1998;Trenhaile 2001;Trenhaile et al 2006;Porter and Trenhaile 2007;Porter et al 2010) have realized fundamental advances in our understanding of ice and frost action on the morphology of intertidal zones as well as "freezing−thawing & wet− ting−drying" influence on shore platforms and cliffs relief. Relatively few investi− gations, however, have tested the efficiency of those processes in polar settings.…”