Exploring pedogenesis via nuclide-based soil production rates and OSL-based bioturbation rates

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.

Insights into biogeochemical cycling from a soil evolution model and long-term chronosequences

“…Like erosion, bioturbation also results in greater retention of unweathered minerals in the surface layers and alters soil chemical and physical properties. We have shown that bioturbation can work to alter soil profiles at a depth greater than that reached by chemical weathering alone, greatly enhancing pedogenesis, consistent with other studies (Wilkinson and Humphreys, 2005).…”

“…Cosmogenic nuclides such as in situ 10 Be have provided measures of surface erosion for hillslope soils where soil thickness is assumed to be at steady state and thus rates of soil production from bedrock balance rates of loss due to surface erosion. Erosion rates calculated from these studies lie in the range of 10 to 100 m Myr −1 (Wilkinson and Humphreys, 2005).…”

“…Bioturbation is the mixing and turnover of soil resulting from biological activity and is considered a major soil forming process (Gabet et al, 2003;Wilkinson and Humphreys, 2005;Yoo et al, 2005). Following Kirkby (1985) bioturbation is also represented in the model as a diffusive process with a depth dependent diffusivity coefficient (see Appendix).…”

“…It is assumed that the mixing intensity will decline with depth due to the decrease in faunal activity with increasing soil depth (Humphreys and Field, 1998;Wilkinson and Humphreys, 2005;Johnson et al, 2014). In the model this takes the form of an exponential relationship:…”

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“…3). Numerous data sets of mobile soil production that use cosmogenic nuclides to quantify timescales support these concepts (Wilkinson and Humphreys, 2005;Heimsath et al, 2010).…”