The influence of weathering processes on microtopographic changes of sandstone under simulated upper intertidal conditions from hourly to weekly scales

Abstract

Micro-scale surface changes of bedrock on shore platform have been monitored with micro-erosion meters (MEMs) and show significant downwearing over several (≥2) years. At short-term temporal scale of hours to days, rock surface behaviors are reversible and do not generate net surface lowering. Therefore, there is a temporal gap between the two temporal scales of surface behaviors over which grain detachment must occur. In this study, an upper intertidal zone (2-h immersion with subsequent 2-week exposure) was simulated in the laboratory to investigate the fast erosion rate at high elevation identified from the Otway coast in south-eastern Australia using MEMs.During the experiments, the microtopography of rock surface was monitored 2-hourly during the daytime period (0−12 h) on Day 1 (beginning), 8 (middle) and 14 (end) of the 14-day simulation to understand the cumulative effect of short-term surface changes on rock weathering and erosion at longer timescales in order to provide insights into the role of rock surface dynamics in shore platform evolution. Through manipulation of the salt content of water as well as temperature and humidity in the controlled measures, the experiment was also designed to test the primary weathering process in driving short-term surface changes at this particular tide level. Results show the primary role of temperature-induced salt crystal volume change in generating the short-term rock surface movements on high tidal to supratidal rock. Although the cumulative effect of short-term surface behaviors on rock microtopography appeared to be observed over longer timescales, it still remains uncertain if this drives rock decay at the weekly scale. Future studies are required to monitor the rock surface over a sufficiently long period (up to months) to determine the temporal threshold of erosion and understand the linkage between short-term surface changes and rock decay.