Quantifying processes of pedogenesis using optically stimulated luminescence

Abstract

New analytical techniques have opened up the possibility of addressing rates of soil processes quantitatively. Here, we present the results of an investigation into the use of single‐grain optically stimulated luminescence (OSL) dating to derive rates of soil mixing in the top 50 cm of soil profiles from two toposequences situated in the Werrikimbe National Park in Australia. Of 500 single grains analysed from each sampled depth increment, less than 25% provided a finite age, with the rest of the grains either non‐responsive or dose‐saturated. This proportion of finite‐age grains tended to decrease with soil depth. Median ages of quartz grains increased down the soil profile, with topsoil ages of up to 500 years and subsoil ages of up to 5000 years. Few ‘younger’ grains were found deeper in the profile and few ‘older’ grains near the soil surface. These trends suggest that pedoturbation is resulting in vertical transport of grains through the profile, but that there is a distribution of transport distances, with a poor probability of large transport distances from surface to subsoil or vice versa compared with a more frequent movement of grains to and from the surface in the uppermost 10–35 cm. The calculation of a single age for each soil horizon was unachievable as each horizon contained a heterogeneous mixture of grains with varying histories of transport to and from the soil surface. Soil mixing was confirmed along both toposequences studied. However, the occurrence of minor mixing rates did not lead to a homogenization of the topsoil and adjacent horizons. We postulated that mixing velocities were mostly related to flora at our study site. Vertical soil mixing rates of 0.5 and 0.2 mm year−1 were calculated from the distribution of finite single‐grain ages.

Bioturbation and erosion rates along the soil‐hillslope conveyor belt, part 2: Quantification using an analytical solution of the diffusion–advection equation

“…The diffusion-advection model used in this study offers a more accurate representation of actual soil reworking processes compared to linear soil reworking rates, calculated by dividing depth by particle age, as reported by earlier luminescence studies (Heimsath et al, 2002;Stockmann et al, 2013), and later corrected by NSF to obtain effective soil reworking rates (SR eff ) (Román-Sánchez et al, 2019). Figure 7a shows how these linear effective soil reworking rates (Equation (2)) compare to the optimized diffusion coefficient (Equation (4)).…”

“…Many studies have used TCN dating for measuring long-term sediment fluxes, but this technique is laborious and expensive, especially when applied over many points of the landscape. Heimsath et al (2002) and Stockmann et al (2013) used single-grain optically stimulated luminescence (OSL) dating techniques but only determined linear mixing rates, i.e. Very often, bioturbation rates have been determined by simple methods such as measuring the amount of surface casts, biomounds, or tree falls (Meysman et al, 2006).…”

Resolving the integral connection between pedogenesis and landscape evolution

“…Besides radionuclides, analyses of optically stimulated luminescence (OSL) and stable isotopes can be used to track soil and landscape forming processes. For example, Wilkinson and Humphreys (2005) and Stockmann et al (2013) used OSL to estimate bioturbation mixing rates, and Balesdent et al (1987) used the stable isotope 13 C to measure the turnover time of organic matter derived from plants with a C4 photosynthetic pathway, and thus to calibrate a C-model. The vertical distribution of radionuclides in soils can be used for parameterizing soil processes such as bioturbation, mixing by plowing, erosion and clay migration.…”

Bioturbation and erosion rates along the soil‐hillslope conveyor belt, part 1: Insights from single‐grain feldspar luminescence

“…To be specific, different studies have reported a clear feedback between chemical weathering and physical erosion rates (Riebe et al, 2003;Larsen et al, 2014). Heimsath et al (2002) have shown that single-grain optically stimulated luminescence (OSL) dating offers an efficient solution for quantifying pedogenic processes such as soil mixing, erosion or deposition rates at a centennial to millennial resolution, and the approach was adopted in a number of studies in different environments (Stockmann et al, 2013;Johnson et al, 2014;Kristensen et al, 2015;Gliganic et al, 2016). Thus, there is an urgent need in soil geomorphology for complementary reconstruction methods to: (i) quantify soil fluxes related to bioturbation, (ii) elucidate soil process rates at a higher temporal resolution than is currently possible with TCN and (iii) study soil-landscape processes under non-steady-state conditions.…”