Climate controls on coupled processes of chemical weathering, bioturbation, and sediment transport across hillslopes

Abstract

Most hillslope studies examining the interplay between climate and earth surface processes tend to be biased towards eroding parts of landscapes. This limitation makes it difficult to assess how entire upland landscapes, which are mosaics of eroding and depositional areas, evolve physio‐chemically as a function of climate. Here we combine new soil geochemical data and published 10Be‐derived soil production rates to estimate variations in chemical weathering across two eroding‐to‐depositional hillslopes spanning a climate gradient in southeastern Australia. At the warmer and wetter Nunnock River (NR) site, rates of total soil (–3 to –14 g m‐2 yr‐1; negative sign indicates mass loss) and saprolite (–18 to –32 g m‐2 yr‐1) chemical weathering are uniform across the hillslope transect. Alternatively, the drier hillslope at Frog's Hollow (FH) is characterized by contrasting weathering patterns in eroding soils (–30 to –53 g m‐2 yr‐1) vs. depositional soils (+91 g m‐2 yr‐1; positive sign indicates mass addition). This difference partly reflects mineral grain size sorting as a result of upslope bioturbation coupled with water‐driven soil erosion, as well as greater vegetative productivity in moister depositional soils. Both of these processes are magnified in the drier climate. The data reveal the importance of linking the erosion–deposition continuum in hillslope weathering studies in order to fully capture the coupled roles of biota and erosion in driving the physical and chemical evolution of hillslopes. Our findings also highlight the potential limitations of applying current weathering models to landscapes where particle‐sorting erosion processes are active. Copyright © 2018 John Wiley & Sons, Ltd.

Patterns in Soil Chemical Weathering Related to Topographic Gradients and Vegetation Structure in a High Andean Tropical Ecosystem

“…A change in weathering intensity along topographic transects can be associated with (1) physical transport of weathered material downslope and/or (2) hillslope hydrology controlling spatial patterns in soil and subsoil water fluxes. Lateral redistribution of soil particles along slope can lead to an overall increase in soil residence time and degree of weathering downslope (Green et al, ; Yoo et al, ): when eroded soil particles are transported downslope, they can be reincorporated in the soil matrix and continue to weather chemically (Wackett et al, ). Along the 10 toposequences, we observed no differences in soil depth, volumetric deformation or soil acidity, and only marginally significant differences in base cation mass transfer coefficients (Table ).…”

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

“…Sklar et al, 2017). Although bioturbation is increasingly recognized as being a key process in soil formation and landscape evolution (Wilkinson and Humphreys, 2005;Wilkinson et al, 2009;Wackett et al, 2018) as it significantly influences the disintegration of the saprolite and bedrock and actively mixes the soil or mobile regolith layer, current reconstruction methods are unsuitable for quantitatively comprehending those key process relationships. Although bioturbation is increasingly recognized as being a key process in soil formation and landscape evolution (Wilkinson and Humphreys, 2005;Wilkinson et al, 2009;Wackett et al, 2018) as it significantly influences the disintegration of the saprolite and bedrock and actively mixes the soil or mobile regolith layer, current reconstruction methods are unsuitable for quantitatively comprehending those key process relationships.…”

“…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). Although bioturbation is increasingly recognized as being a key process in soil formation and landscape evolution (Wilkinson and Humphreys, 2005;Wilkinson et al, 2009;Wackett et al, 2018) as it significantly influences the disintegration of the saprolite and bedrock and actively mixes the soil or mobile regolith layer, current reconstruction methods are unsuitable for quantitatively comprehending those key process relationships. 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.…”

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

“…Hillslopes can be seen as conveyor belts as suggested by Anderson et al (2013), in analogy to river systems (Kondolf, 1994), onto which soil is loaded at a particular rate, depending on their formation, and transported downwards depending on erosion and bioturbation processes. Wackett et al (2018) revealed the importance of linking erosion-deposition with bioturbation for an understanding of the physical and chemical evolution of hillslopes. However, uprooting of trees, mound generation and constant mixing of soils trigger a significant net downslope movement on hillslopes.…”

“…However, uprooting of trees, mound generation and constant mixing of soils trigger a significant net downslope movement on hillslopes. Wackett et al (2018) revealed the importance of linking erosion-deposition with bioturbation for an understanding of the physical and chemical evolution of hillslopes. This study measured and compared these processes in two hillslopes spanning a climate gradient from a warmer and wetter climate to a drier one.…”