SEASONALITY OF ROCKWALL THERMAL REGIMES IN A TEMPERATE CLIMATE

In temperate climates, exposed rock outcrops experience cool winters and warm summers that make them susceptible to a variety of thermal weathering processes. Studies of thermal weathering have focused on the winter (e.g., freeze-thaw processes) or summer months (e.g., thermal shock and fatigue), yet few have explored the seasonality of these processes on an annual basis. Moreover, at the annual scale, data are often recorded at large time increments which are insufficient to evaluate the occurrence of thermal shock. Here we present a unique 18-month record of rock surface and fracture temperatures at 1-minute temporal resolution recorded on rockwalls along the Niagara Escarpment in Hamilton, Ontario. Outcrops differing in aspect and lithology were instrumented with thermistors inserted into pre-existing fractures and affixed to the nearby rock surface. The range of fracture and rock surface temperatures, and the incidence of rapid temperature changes capable of inducing thermal shock (>2°C) is greatest across all sites in the shoulder (spring and fall) seasons. In the winter, diurnal and prolonged oscillations about 0°C, which may induce freeze-thaw processes, predominate. This suggests that there is a distinct seasonality to weathering mechanisms and intensity on exposed rock faces in a temperate climate; the shoulder seasons experience the highest intensity of thermal weathering, whereas freeze-thaw mechanisms predominate throughout the winter. Aspect moderates the intensity of weathering. On southeast-facing sites which receive high insolation, more frequent rapid temperature changes and shorter, diurnal-scale freeze-thaw oscillations are recorded than at west- and east-facing sites. The magnitude of surface-fracture temperature gradients is also seasonally variable. However, across all seasons, we observed a diurnal reversal of surface-fracture temperature gradients that suggest pre-existing fractures play an important role in moderating weathering processes.