No abstract

New dating techniques are available for soil scientists to test fundamental pedogenic ideas. Recent developments in applications of terrestrial in situ cosmogenic nuclides (TCN) from bedrock and saprolite allow the derivation of soil production rates, at scales ranging from local (sub-hillslope) to catchment wide, generally averaged over timescales of 104–105 years. Where soil depths are relatively constant over time, soil production rates equal transport rates and are thus essential to establishing sustainable erosion rates. TCN also allow the form of the soil production function to be compared to theoretical models—a difficult task previously. Furthermore, parameterised soil production functions can now be incorporated into numerical surface process models to test landscape evolution ideas. Bedrock and saprolite conversion to soil is demonstrably dependent on the overlying soil depth, and there is general agreement that weathering declines exponentially beyond maximum soil production, consistent with theory. Whether maximum soil production occurs under a finite or non-existent soil cover at particular sites remains unresolved. We suggest that, in general, soil production from saprolite declines exponentially with increasing depth, while production from bedrock follows a humped function. Estimates of the role of flora, fauna and processes such as freeze–thaw that mix soil mantles to depth, have been limited prior to optically stimulated luminescence (OSL) dating techniques. Recently derived OSL mixing rates extend the magnitude of previous partial, short-term bioturbation rates. In fact, bioturbation appears to be the most active pedogenic process operating in many soils, with freeze–thaw environments a noted exception. Although bioturbation far outweighs soil production, it does not always lead to homogenisation as is often reported. We maintain that the above-ground component of bioturbation, i.e. mounding, may alone, or particularly when combined with particle sorting via rainwash processes, lead to horizonisation and texture contrast soils in those materials that can be sorted such as mixtures of sand and clay. Together, TCN- and OSL-based estimates of hillslope soil transport and bioturbation, suggest significant rates of downslope soil mantle movement coupled with rapid mixing, contrary to in situ soil development models.

No 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...