Contemporary versus long‐term denudation along a passive plate margin: the role of extreme events

Abstract

Short-term (contemporary) and long-term denudation rates were determined for the Blue Mountains Plateau in the western Sydney Basin, Australia, to explore the role of extreme events (wildfires and catastrophic floods) in landscape denudation along a passive plate margin. Contemporary denudation rates were reconstructed using 40 years of river sediment load data from the Nattai catchment in the south-west of the basin, combined with an analysis of hillslope erosion following recent wildfires. Long-term denudation rates (10 kyr-10 Myr) were determined from terrestrial cosmogenic nuclides, apatite fission track thermochronology and post-basalt flow valley incision. Contemporary denudation rates average several times lower than the long-term average (5·5 ± ± ± ± ± 4 mm kyr − − − − −1 versus 21·5 ± ± ± ± ± 7 mm kyr − − −Recent studies have compared long-term denudation rates (10 kyr-10 Myr) determined from cosmogenic nuclides and apatite fission track thermochronology (AFTT), with contemporary rates (1-100 yr) calculated from stream gauging, sediment rating curves and sediment trapping (see, e.g., von Blanckenburg, 2005). Whilst some studies have observed similar short-term and long-term rates of denudation indicating steady state erosion (Bierman and Caffee, 2001;Matmon et al., 2003;Nichols et al., 2005), many have found either elevated contemporary rates attributable to human impact and land use change (Hewawasam et al., 2003;Gellis et al., 2004), or lower rates thought to be explained by the absence of high-magnitude, low-frequency (extreme) events in records that span only decades (Kirchner et al., 2001;Schaller et al., 2001). To substantiate the former, paired catchment type investigations have been conducted using undisturbed environments to give reasonable estimates of natural contemporary rates (Brown et al., 1998; von 1014 K. M. Tomkins et al. Blanckenburg et al., 2004). For the latter, determination of sediment yields from hypothetical extreme events has proved to be more problematic and, as a result, so has extrapolation of these contemporary records to long-term landscape evolution. In a study of denudation rates from the Rocky Mountains in Idaho, USA, Kirchner et al. (2001) found that modern sediment yields measured over 10 -84 years from stream gauging and sediment trapping were on average 17 times lower than the long-term sediment yield. They concluded that the mismatch in rates was the result of sediment delivery being dominated by extreme erosional events triggered by external forces such as severe storms and wildfires, which occur at time intervals greater than the length of the modern record. In south-eastern Australia, weather extremes including drought, floods and wildfires are a dominant characteristic of the landscape. Very low rates of contemporary denudation have been reported (Bishop, 1984; Wasson, 1994; Wasson et al., 1996), despite significant increases as a result of European settlement and land use change (Wasson, 1994). However, it is possible that these short-term records have not capt...

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