Soil production and the soil geomorphology legacy of Grove Karl Gilbert

Abstract

Geomorphologists are quantifying the rates of an important component of bedrock's weathering in research that needs wide discussion among soil scientists. By using cosmogenic nuclides, geomorphologists estimate landscapes’ physical lowering, which, in a steady landscape, equates to upward transfers of weathered rock into slowly moving hillslope‐soil creep. Since the 1990s, these processes have been called “soil production” or “mobile regolith production”. In this paper, we assert the importance of a fully integrated pedological and geomorphological approach not only to soil creep but to soil, regolith, and landscape evolution; we clarify terms to facilitate soil geomorphology collaboration; and we seek a greater understanding of our sciences’ history. We show how the legacy of Grove Karl Gilbert extend across soil geomorphology. We interpret three contrasting soils and regoliths in the USA's Southern Piedmont in the context of a Gilbert‐inspired model of weathering and transport, a model of regolith evolution and of nonsteady systems that liberate particles and solutes from bedrock and transport them across the landscape. This exercise leads us to conclude that the Southern Piedmont is a region with soils and regoliths derived directly from weathering bedrock below (a regional paradigm for more than a century) but that the Piedmont also has significant areas in which regoliths are at least partly formed from paleo‐colluvia that may be massive in volume and overlie organic‐enriched layers, peat, and paleo‐saprolite. An explicitly integrated study of soil geomorphology can accelerate our understanding of soil, regoliths, and landscape evolution in all physiographic regions.

Measuring landscape evolution from inception to senescence

“…6 The application of σC has been tested on a singular idealized chronosequence, and this system provides an end member example of a transport limited system where no valley processes are present. Changes in σC value correspond with a shift in dominant transport mechanisms observed for soil-mantled landscapes 15 48 , and Richter et al 21 ) identifying landscapes that can facilitate the integrated merger of interdisciplinary methods is vital for the understanding of earth systems processes. The Cooloola Sand Mass (CSM) provides such a system.…”

“…Ridges and spurs (C < 0) have the thinnest soils whereas the hollows and valleys (C > 0) have the thickest soils 13,15 . According to commonly prescribed soil production functions (exponential or humped), production rates are the highest in thin soils and rates decrease as soil thickens 21 . Therefore, soil is primarily produced on ridges and displaced downslope to the hollows through diffusive sediment transport, whereby soil flux is solely gradient dependent (linear slopedependent transport).…”

Alluvial fan aggradation in low relief, humid‐temperate landscapes, Uwharrie National Forest, North Carolina Piedmont

“…These higher values could reflect higher degree of erosion (into less weathered bedrock) in more proximal locations to the trunk channel, or these fans could have buried existing terrace deposits, resulting in a muddled signal. Recent work cataloging exposures of colluvium accumulating high in zeroorder basins in the nearby South Carolina Piedmont(Richter et al, 2020) suggests that episodes of significant aggradation occurred throughout the late Quaternary as far back as 150 ka, however, older ages are not seen in our alluvial fan soils. All of these data are supportive of significant, episodic Quaternary hillslope erosion throughout the Piedmont.7 | CONCLUSIONSWe find that alluvial fans are common, but not ubiquitous, features of low order watersheds in the Piedmont of the south-eastern United States.…”

Warmer, Wetter Climates Accelerate Mechanical Weathering in Field Data, Independent of Stress‐Loading

“…Accurate characterizations of how climate controls weathering, the in situ breakdown of rock, are essential because weathering drives erosion and soil formation (Murphy et al., 2016; Richter et al., 2019), impacts biomes (Lu & Hedin, 2019) including the evolution of human life (Kasting, 2019), degrades civil infrastructure (Phillipson et al., 2016), and—crucially—influences the rate of atmospheric CO 2 uptake by the lithosphere, which stabilizes climate on geological time scales (e.g., Isson et al., 2020; Macdonald et al., 2019; Walker et al., 1981).…”

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Accurate characterizations of how climate controls weathering, the in situ breakdown of rock, are essential because weathering drives erosion and soil formation (Murphy et al, 2016;Richter et al, 2019), impacts biomes (Lu & Hedin, 2019) including the evolution of human life (Kasting, 2019), degrades civil infrastructure (Phillipson et al, 2016), and-crucially-influences the rate of atmospheric CO 2 uptake by the lithosphere, which stabilizes climate on geological time scales (e.g., Isson et al, 2020;Macdonald et al, 2019;Walker et al, 1981).Currently, however, published quantifications of global-climate-weathering connections (e.g., Rugenstein et al, 2019;Winnick & Maher, 2018) do not take into consideration climate's influence on the mechanical component of rock weathering, the lengthening of fractures caused by stresses at Earth's surface-hereafter referred to as "cracking." Yet chemical weathering-and most other surface processes-are limited without the breakdown and porosity facilitated by cracking (e.g.,

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