Quantifying erosional equilibrium across a slowly eroding, soil mantled landscape

Abstract

The textbook concept of an equilibrium landscape, which posits that soil production and erosion are balanced and equal channel incision, is rarely quantified for natural systems. In contrast to mountainous, rapidly eroding terrain, low relief and slow‐eroding landscapes are poorly studied despite being widespread and densely inhabited. We use three field sites along a climosequence in South Africa to quantify very slow (2‐5 m/My) soil production rates that do not vary across hillslopes or with climate. We show these rates to be indistinguishable from spatially invariant catchment‐average erosion rates while soil depth and chemical weathering increase strongly with rainfall across our sites. Our analyses imply landscape‐scale equilibrium although the dominant means of denudation varies from physical weathering in dry climates to chemical weathering in wet climates. In the two wetter sites, chemical weathering is so significant that clay translocates both vertically in soil columns and horizontally down hillslope catenas, resulting in particle size variation and the accumulation of buried stone lines at the clay‐rich depth. We infer hundred‐thousand‐year residence times of these stone lines and suggest that bioturbation by termites plays a key role in exhuming sediment into the mobile soil layer from significant depths below the clay layer. Our results suggest how tradeoffs in physical and chemical weathering, potentially modulated by biological processes, shape slowly eroding, equilibrium landscapes. © 2019 John Wiley & Sons, Ltd.

Carbonates in the Critical Zone

“…This system reaches topographic equilibrium when fluxes of fresh rock into the CZ are balanced by fluxes of solutes and sediments out of the channel network, resulting in a steady soil and regolith thickness. This conceptual model, in various forms, is ubiquitous throughout CZ studies (Amundson et al., 2007; Anderson et al., 2002, 2013; Brantley, Lebedeva, et al., 2017; Heimsath et al., 2020; Hilley et al., 2010; Lebedeva et al., 2010; Patton et al., 2018; Rempe & Dietrich, 2014; Riebe et al., 2017).…”

Carbonates in the Critical Zone

“…This system reaches topographic equilibrium when fluxes of fresh rock into the CZ are balanced by fluxes of solutes and sediments out of the channel network, resulting in a steady soil and regolith thickness. This conceptual model, in various forms, is ubiquitous throughout CZ studies (Amundson et al, 2007;Anderson et al, 2013Anderson et al, , 2002Brantley et al, 2017a;Heimsath et al, 2020;Hilley et al, 2010;Lebedeva et al, 2010;Patton et al, 2018;Rempe and Dietrich, 2014;Riebe et al, 2017).…”

Global rates of soil production independent of soil depth

“…1A) 5, [10][11][12][13][14][15][16] and another population of sites where it does not (Fig. 1B) [16][17][18][19][20][21][22] including a dataset collected for this study from a tropical mountain in Puerto Rico where soil production resists self-arrest even with overburden thicker than 2 meters. Soil production rates at "non-conforming" sites all exhibit random variance around a mean rate.…”

“…1B), on the other hand, are all geomorphically "stable" with respect to the local baselevel. This includes plateau surfaces 19,22 and alpine flats 20,27 , relict portions of adjusting topography 21 , low-gradient parabolic hills 18 , and post-orogenic, climatically stable landscapes 16,17 .…”