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

Abstract

The interplay of bioturbation, soil production and long‐term erosion–deposition in soil and landscape co‐evolution is poorly understood. Single‐grain post‐infrared infrared stimulated luminescence (post‐IR IRSL) measurements on sand‐sized grains of feldspar from the soil matrix can provide direct information on all three processes. To explore the potential of this novel method, we propose a conceptual model of how post‐IR IRSL‐derived burial age and fraction of surface‐visiting grains change with soil depth and along a hillslope catena. We then tested this conceptual model by comparison with post‐IR IRSL results for 15 samples taken at different depths within four soil profiles along a hillslope catena in the Santa Clotilde Critical Zone Observatory (southern Spain). In our work, we observed clear differences in apparent post‐IR IRSL burial age distributions with depth along the catena, with younger ages and more linear age–depth structure for the hill‐base profile, indicating the influence of lateral deposition processes. We noted shallower soils and truncated burial age–depth functions for the two erosional mid‐slope profiles, and an exponential decline of burial age with depth for the hill‐top profile. We suggest that the downslope increase in the fraction of surface‐visiting grains at intermediate depths (20 cm) indicates creep to be the dominant erosion process. Our study demonstrates that single‐grain feldspar luminescence signature‐depth profiles provide a new way of tracing vertical and lateral soil mixing and transport processes. In addition, we propose a new objective luminescence‐based criterion for mapping the soil‐bedrock boundary, thus producing soil depths in better agreement with geomorphological process considerations. Our work highlights the possibilities of feldspar single grain techniques to provide quantitative insights into soil production, bioturbation and erosion–deposition. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

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

“…This is explained in more detail in Román-Sánchez et al (2019). Therefore, it is very important to measure the mobile A. ROMÁN-SÁNCHEZ ET AL.…”

“…That is the case of profiles SC-9 and SC-7, for which T À W values are estimated with a high uncertainty. The use of luminescence techniques for mapping the mobile regolith-saprolite border is discussed in further detail in Román-Sánchez et al (2019). The depth of the mobile regolith is indicated in grey in Figure 6.…”

“…However, Lecroart et al (2010) noted that there is a problem associated with determining the diffusion coefficient, which can vary depending on using a short-or long-lived tracer. Currently, there are advantages in tracking singlegrains with OSL dating techniques, as used in several studies (Johnson et al, 2014, Reimann et al, 2017, Román-Sánchez et al, 2019, compared with bulk soil. Over long timescales, there are very few studies on bioturbation rates.…”

Luminescence as a Sediment Tracer and Provenance Tool

“…In their study, Johnson et al () compared both a linear and exponential depth profile for soil diffusivity and found that the exponential profile best fit the field data, which let them quantify both the diffusivity and the characteristic lengthscale of mixing. Román‐Sánchez, Reimann, et al, (),Román‐Sánchez, Laguna, et al, () merged both the downward velocity and advection‐diffusion approaches in their comprehensive study of the Santa Clotilde Critical Zone Observatory in southern Spain. Román‐Sánchez, Reimann, et al, (), Román‐Sánchez, Laguna, et al, () present a large body of work, including analytical solutions to the advection‐diffusion equation, a dataset spanning an entire hillslope, and careful analysis of the mixing and transport processes at their field sites.…”

“…Román‐Sánchez, Reimann, et al, (),Román‐Sánchez, Laguna, et al, () merged both the downward velocity and advection‐diffusion approaches in their comprehensive study of the Santa Clotilde Critical Zone Observatory in southern Spain. Román‐Sánchez, Reimann, et al, (), Román‐Sánchez, Laguna, et al, () present a large body of work, including analytical solutions to the advection‐diffusion equation, a dataset spanning an entire hillslope, and careful analysis of the mixing and transport processes at their field sites. The combined efforts of the above work are important because quantifying the soil mixing/diffusivity and lengthscales of movement opens the door for modeling other processes such as predicting the movement of nutrients and other soil properties.…”