The drivers of hourly-scale surface changes on shore platforms
AffiliationSchool of Geography
Document TypeMasters Coursework thesis
Access StatusOnly available to University of Melbourne staff and students, login required
Subaerial weathering is a key process in the formation of shore platforms, with downwearing dominating the semi-horizontal intertidal rock surfaces. Over decadal to millennial scales this downwearing lowers platforms to mean low water spring tide elevations. On daily to hourly timescales, however, swelling at micro-scale (<mm) is also commonly observed. This topographic change is likely a precursor to granular disintegration of the rock surface by generating stress. Endolithic lichens on shore platforms have been hypothesised as a driver of this short-term change, but the influences of environmental factors (e.g. air temperature and humidity) are poorly understood. In this study, surface change was monitored over 2-hourly intervals for 3.5 days on a supratidal sandstone at Marengo, Victoria, Australia to quantify the precise short-term dynamics of shore platforms. These field measurements were complemented by controlled laboratory experiments on the same lithology to precisely quantify the role of biofilms and environmental conditions in driving hourly-scale surface change. At Marengo, rock surface behaviour was observed at different temporal scales. The study surface was dynamic at the two-hourly scale with falling and rising of -222 to 357 microns being the most frequent change trends. There was no net elevational change on the rock surface over a day, however net contraction was observed at the multi-day scale. Short-term surface micro-topography change over 24-hours was found to be divided into three distinct periods: falling (06:00-12:00), rising (12:00-20:00) and stable (20:00-06:00). Spatial heterogeneity, at the centimetre scale, was also demonstrated across a rock surface with contraction and expansion occurring concurrently. Micro-topography and related variations in aspect appeared to be critical in determining this variation. Endolithic lichens are suggested as the main mechanism of surface change on supratidal sandstone at Marengo, as evidenced by a higher magnitude and more alternations of expansion and contraction on the colonized versus bare rock surface. By influencing water content of lichen hyphae, rock surface swelled with increasing humidity and shrunk with decreasing humidity. An inverse pattern was apparent in response to thermal variations as the colonized biofilms were desiccated. The bioerosive role of endolithic lichens is therefore found to be very important for erosion of supratidal bedrock on shore platforms.
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