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

Micro-scale surface changes of bedrock on shore platform have been widely monitored with micro-erosion meters (MEMs) and traversing micro-erosion meters (TMEMs) over different temporal scales. At short-term temporal scale of hours to days, rock surface behaviors are characterized by dynamic fluctuations without net surface lowering, while significant downwearing are commonly observed over several (≥2) years. Therefore, there is a temporal gap between the short-term (hours to weeks) and long-term (months to years) surface behaviors over which grain detachment is suggested to occur. In this laboratory study, a sandstone was placed in a simulated upper intertidal condition, in which the rock sample experienced 2-h immersion with subsequent 2-week exposure over a 14-day spring tidal cycle. By running this spring tidal cycle for 6 times (12 weeks) over the experiment, the microtopography of the rock surface was monitored before and after the 2-h immersion period and at a weekly scale over the exposure period of each tidal cycle to understand the influence of tidal immersion and weekly surface changes on rock weathering and erosion at longer monthly timescales. This is to provide insights into the association between rock surface dynamics (weathering) and grain removal (erosion) to identify the temporal threshold for rock decay to initiate. Immersed under sea water and deionized water respectively, the rock sample was then exposed in an environmental chamber through manipulation of temperature and air humidity in controlled measures, generating four different treatments in the experiment. This design also aims to test the primary weathering process in driving the fast downwearing of shore platform observed at the upper intertidal zone along the Otway coast. Results showed that significant microtopographic changes of −0.392 to +0.484 mm were observed on the sandstone surface at the weekly scale. These weekly surface changes were initiated with dynamic surface movements of lower magnitude operating at finer hourly timescales. Rock decay occurred with surface rising movement at the weekly-monthly scale. This could be caused by repeated short-term surface changes associated with weathering processes during the exposure period, generating loose/weakened grains which are subsequently removed by waves during the following tidal immersion period. As more occurrences of rock decay over longer monitoring period, the heterogeneous pattern of rock surface changes at hourly