No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
The convenient fiction of steady-state soil thickness
Abstract
Steady-state regolith or soil thickness (SSST), whereby surface removals are approximately balanced by production of new soil by bedrock weathering, is a common assumption in most models of hillslope and landscape evolution. SSST is also a fundamental assumption in the use of cosmogenic radionuclides (CRN) to estimate erosion and weathering rates. The steady-state concept is based on feedbacks between soil thickness and weathering at the base of the regolith, such that (sometimes after an optimal or threshold thickness is achieved), thicker soils lead to lower weathering rates (and vice versa). SSST is thus only applicable to soils formed chiefly from weathering of the underlying bedrock, where sufficient time has elapsed for regolith accumulation, and where effects of processes other than weathering and surface removals on thickness are negligible. Even within this domain, the widespread occurrence of deep weathering profiles, regolith stripping, and inherited regolith features makes SSST problematic as a conceptual model for pedogenesis or weathering profile development. The ratio of soil thickness to total weathering profile thickness is proposed as a simple index of steady state. Steady-state profiles formed on weathered bedrock should exhibit ratios close to unity. Data from the Cumberland Plateau region of eastern Kentucky show that soil/weathering profile ratios in shallow (< 1.5 m) profiles formed on sandstone may reach or approach unity, but are generally < 1. Geotechnical core data show depths to bedrock of 2 to > 20 m, and generally significantly greater than soil thicknesses in the region, suggesting ratios <<1. However, while evidence shows that SSST is likely rare and not a viable conceptual framework for assessing soil and weathering profile development, deviations from SSST may have limited influence on results of CRN-based estimates of erosion and weathering and simulation model results. This is because in the landscape settings where SSST is typically assumed, and over the customary time scales involved, rates of denudation and weathering are very small compared to regolith thickness, such that imbalances do not materially affect results of calculations. Steady-state in development of soil, regolith, or weathering profile development thus represents a convenient fiction facilitating the use of some models and tools. The potential pitfalls arise from the possibility that the utility of SSST as a convenient fiction in some contexts may be mistaken for a realistic representation of the dynamics of pedogenesis and weathering profile evolution.
... Changes in environmental factors such as climate or land use can trigger multiple pedogenic pathways that affect earlier developed soil properties (Targulian and Bronnikova, 2019). Also, changes in denudation, erosion and deposition can disrupt the reaching of a steady state soil (Phillips, 2010). Therefore, conceptualization, and quantification, of soil evolution under varying initial and boundary conditions, e.g. ...
... When the rates of advection and diffusion are equal, the upward transport of clay by bioturbation equals the amount of downward translocation by water; the clay-depth profile of the soil occurs in steady state and will not change substantially. Steady-state circumstances are however rare in natural soil systems (Phillips, 2010). Swanson and Swanston, 1977;West et al., 2013). ...
... After this initial change, all pedogenic pathways turn divergent and approach a new steady state (SOM stocks, Figure 6.1B&D) or approach a steady rate of change (depth to Bt, Figure 6.1A&C). The model simulations show that a steady state in soil formation can indeed be reached, but only for fast changing properties under stable circumstances, and it is thus a rare occurrence (Phillips, 2010;Sauer, 2015). ...
Soils provide numerous functions to support natural and human life. Soils and their functions develop over long timescales (decennia to millennia) under influence of environmental properties and drivers such as water flow, vegetation type and topography of the landscape. At the same time, these environmental properties develop too, often under influence of soil properties and processes. This interactive co-evolution of soils and the landscape forms a complex system that can aggravate, or diminish, rates and direction of soil-landscape evolution. In the Anthropocene, a proposed geological Epoch where humans are the main forcing actors, soil-landscape evolution changed substantially under influence of anthropogenic processes, such as deforestation and tillage. In current intensively managed agricultural landscapes in undulating settings, rates of anthropogenic erosion far exceed rates of natural soil development, leading to severe soil and land degradation. Sustainable nature-based land management is crucial to counteract this degradation, and to preserve and restore soil functions for the environment and future generations. The aim of my thesis is to identify and quantify how soils and landscape have evolved and possibly co-evolved during the transition from natural land cover to intensive land management in the Anthropocene.
The first part of this thesis (Chapter 2-3) aims at reconstructing the impact and rates of anthropogenic landscape change on complex agricultural fields. As study site I use the landscape laboratory CarboZALF D. CarboZALF D is a kettle-hole catchment of 4 ha with elevation differences up to 8 meters, located in north-eastern Germany. The catchment is characterized by complex small-scale topography, heterogeneities in the hydrological system and a long history of agricultural use. The colluvium in the closed kettle hole catchment provides a complete geo-archive of landscape change. In Chapter 2 we reconstruct the paleosurface of study site Carbo-ZALF-D prior to the anthropogenic erosion. We used an extensive dataset of soil descriptions, which enabled a detailed spatial estimate of erosion and deposition by estimating erosion based on soil profile truncations and deposition based on colluvium thickness. The paleosurface shows a high variation in topographic properties and suggests that natural soils and landscapes contain considerable spatial heterogeneity. In Chapter 3 we reconstruct the rates of deposition in Carbo-ZALF-D using Optically Stimulated Luminescence (OSL) dating. We present a novel methodology to apply OSL dating in colluvial sediments, where the soil chronology gets disturbed by reworking by ploughing after deposition. Our results show a 100-fold increase in deposition rates, starting around 5000 years ago. This increase does not solely represent increased erosion in the catchment, but is also caused by indirect effects of agricultural drainage. The kettle hole shows a complex spatiotemporal pattern of colluvial infilling and landscape evolution, which we were only able to reconstruct using a high OSL sampling density and extensive soil geomorphic research.
The second part of this thesis aims at simulating the evolution of soils and landscapes under varying climatic and anthropogenic forcing. In Chapter 4 we review the role of water as dominant driver in natural soil and landscape evolution and its potential as driver in simulations with soil-landscape evolution models (SLEMs). Water plays a pivotal role in soil and landscape evolution, by transporting and transforming soil material and facilitating vegetation growth. In turn, surface and subsurface flow paths of water are controlled by soil and landscape properties. The co-evolution of soils, topography and the hydrological system is essential for understanding the response of soils and landscapes to changes in climate. However, this co-evolution can currently not be simulated over long timescales with SLEMs due to several conceptual and methodological challenges. We provide partial solutions for these challenges. In Chapter 5 we utilize these partial solutions to develop our SLEM HydroLorica. HydroLorica simulates soil and landscape evolution with various dynamic drivers such as water flow, vegetation type and land use. We included additional essential processes such as tree throw, soil creep and tillage. We use HydroLorica to simulate the evolution of soils and landscape under various rainfall and land-use scenarios for an artificial undulating landscape. The results show that in natural systems, rainfall amount is the dominant factor controlling soil and landscape heterogeneity, while for agricultural systems landform explains most of the variation. The cultivation of natural landscapes increases soil heterogeneity, but also increases correlations between soil and terrain properties. Our results confirm that humans have become the dominant soil forming factor in intensively managed landscapes.
In the third part of this thesis (Chapter 6), I synthesize the findings from the research chapters to meet the objectives of this thesis. I critically evaluate the developed reconstruction methods in Chapters 2 and 3 and compare them with other potential methods. The development of HydroLorica in Chapters 4 and 5, with water flow as explicit driver and with increased process coverage, is a big step forward in soil-landscape evolution modelling. A combination of reconstruction and simulation methods is essential for developing and testing hypotheses of soil-landscape co-evolution. Soil-landscape evolution in natural and intensively managed landscapes have different characteristics due to different driving forces and dominant processes. In natural landscapes, soils develop to patterns where individual soils might be disturbed occasionally, but where the average properties are stable. In intensively managed landscapes, disturbance rates are much higher than in natural settings. As a consequence, slowly developing soil properties degrade, while fast-developing soil properties can form a new equilibrium. The co-evolution of soils and landscapes that occurs in natural settings is often controlled by biotic processes. In agricultural settings, humans control vegetation type and aggravate erosion processes through tillage. As a consequence, co-evolution does not occur in the sense that it does in natural settings, because interactions between landscape components are missing. However, the management of soils and landscapes is often adapted to counteract unintended changes to soils and landscapes under earlier management. In intensively managed landscapes, land management may thus co-evolve with the rest of the landscape.
... However, many analyses and simulation models are based on a two-stage sequence whereby weathering turns bedrock into regolith (or saprolite), and pedogenesis turns regolith into soil (e.g., Gabet and Mudd, 2010;Riebe et al., 2017). In our own previous work this has been explicit in some cases (Phillips, 2010(Phillips, , 2018 and implicit in others (Phillips et al., 2005). ...
... This is justifiable as a simplifying model assumption regardless of its applicability at a given site. A more nuanced understanding, however, would help explain the infrequency of the steady-state soil and regolith thickness (at the scales of interest here) implied by the production function (e.g., Phillips et al., 2005;Phillips, 2010;Tye et al., 2011;Zollinger et al., 2016;Yu and Hunt, 2017), and the occurrence of very deep weathering profiles with active weathering still occurring at the bedrock interface (e.g., Hill et al., 1995;Migon and Lidmar-Bergstrom, 2002;Carmo and Vasconcelos, 2004;Arias et al., 2016;Jiang et al., 2018). Weathering fronts may also be more directly related to surface topography. ...
... P is the formation of soil or solum material from saprolite, etc. The amount of transformation over the evolution of the profile is ΔW1, ΔW2, ΔP, and includes volume expansion or contraction and mass additions as well as weathering and mass losses (see Johnson, 1985;Johnson et al., 2005b;Phillips, 2010). For simplicity, we treat here in situ weathering profiles with negligible surface erosion or deposition, though in reality weathering and erosion or deposition often occur concurrently. ...
A distinct boundary between unweathered and weathered rock that moves downward as weathering proceeds—the weathering front—is explicitly or implicitly part of landscape evolution concepts of etchplanation, triple planation, dynamic denudation, and weathering- and supply-limited landscapes. Weathering fronts also figure prominently in many models of soil, hillslope, and landscape evolution, and mass movements. Clear transitions from weathered to unweathered material, increasing alteration from underlying bedrock to the surface, and lateral continuity of weathering fronts are ideal or benchmark conditions. Weathered to unweathered transitions are often gradual, and weathering fronts may be geometrically complex. Some weathering profiles contain pockets of unweathered rock, and highly modified and unmodified parent material at similar depths in close proximity. They also reflect mass fluxes that are more varied than downward-percolating water and slope-parallel surface processes. Fluxes may also be upward, or lateral along lithological boundaries, structural features, and textural or weathering-related boundaries. Fluxes associated with roots, root channels, and faunal burrows may potentially occur in any direction. Just as pedology has broadened its traditional emphasis on top-down processes to incorporate various lateral fluxes, studies of weathering profiles are increasingly recognizing and incorporating multidirectional mass fluxes. Examples from karst systems may also be useful, where concepts of laterally continuous weathering fronts, rock-regolith boundaries, and water tables; and an assumption of dominantly diffuse downward percolation are generally inapplicable. We also question the idea of a single weathering front, and of a two-stage process of weathering rock to regolith, and transforming regolith to soil. In many cases there appears to be three stages involving conversion of bedrock to weathered rock, weathered rock to regolith, and regolith to soil.
... Under ideal circumstances, it is possible for most of the sites in a given region to have erosion rates similar or equal to the local soil production function, a necessary condition for the existence of an equilibrium landscape (Heimsath et al., 2010). As pointed out by Phillips (2010), steady-state soil depths are a 'convenient fiction', because, particularly at sites where soil erosion and production rates are small, steady state is unlikely to be achieved. Although the question regarding how to distinguish between steady-state and non-equilibrium landscapes is of interest in geomorphology, our focus here will be on the reliability of the steady-state assumption at a point. ...
... The purpose of this work was to check whether, in contrast to the exponential formulation of soil production, the proposed power law decay of soil production with depth allows an inference to be made regarding the relevance of the steadystate assumption for real soils. Following Phillips (2010) we focus on sites with low erosion rates. If soil production declines according to a power law rather than according to an exponential, it is possible that such sites will not have reached steady-state conditions and that the soils will still be deepening, albeit at very slow rates. ...
... A model of soil production tied to solute transport-limited chemical weathering has been applied to address the question of whether common assumptions regarding steady-state conditions of soil columns are justified. In the present study we find that steady-state conditions are more nearly attained at higher erosion rates, consistent with an earlier suggestion by Phillips (2010), but we do not conclude that this result will necessarily translate to other studies or other regions. In less arid regions, for example, soil production function values will tend to be larger, and steady-state conditions can be attained Figure 5. Observed soil depth versus predicted soil depth at all seven study sites. ...
Soil depth and soil production are highly complicated phenomena, generated from a complex interaction of physical, biological and chemical processes. It has nevertheless become increasingly clear that soil formation rates are closely related to chemical weathering rates. Somewhat paradoxically it is likewise becoming apparent that such biogeochemical reactions as slowly transform rock to soil, are limited by physical processes, such as flowing water and the formation of fractures. We have formulated a theoretical approach that relates soil formation rates to chemical weathering rates, and those, likewise, to solute transport rates. For such a theoretical framework to be relevant, the solute transport rates cannot equal those of the flowing water, as is the case in Gaussian solute transport. Rather, solute transport must be slowed in accord with heavy-tailed solute arrival time distributions. The inference is that the traditional advection-dispersion equation formulation for solute transport is inadequate in the typically heterogeneous geologic media that weather to form soils. Here we examine the implications of this soil production model on the assumption of the approach to steady state. Particularly at slow erosion rates we find that many soil columns are not in equilibrium. This tendency may be accentuated in dry climates.
... As early as 1909, Gilbert [1909] pointed to long-exposed convex hillslopes and argued that the rate of production of regolith at depth should equal the rate of erosion. Geomorphological models of landscape evolution commonly assume this steady state condition implicitly or explicitly and are thus based on assumed feedbacks between erosion and bedrock weathering [Anhert, 1987;Tucker and Slingerland, 1994;Odoni, 2007;Heimsath et al., 1997;Burke et al., 2007;Phillips, 2010;Hilley et al., 2010;Gabet and Mudd, 2009]. Another feature of many of these models is treatment of weathering only at the bedrock interface. ...
... Several models of geomorphological evolution incorporate a soil production rate function similar to equation (43) [Anhert, 1987;Tucker and Slingerland, 1994;Burke et al., 2007;Phillips, 2010;West, 2012;Odoni, 2007], but this is based only on empirical evidence. As highlighted by Odoni [2007], a common feature of many of these models is treatment of weathering only at the bedrock interface, without a description of weathering of rocks or minerals in their path from B to S. Recent works consider the transformation of bedrock to saprolite and of saprolite to regolith/soil separately [Phillips, 2010;Burke et al., 2007] or account for weathering across the full thickness using a mass balance [Gabet and Mudd, 2009]. ...
... Several models of geomorphological evolution incorporate a soil production rate function similar to equation (43) [Anhert, 1987;Tucker and Slingerland, 1994;Burke et al., 2007;Phillips, 2010;West, 2012;Odoni, 2007], but this is based only on empirical evidence. As highlighted by Odoni [2007], a common feature of many of these models is treatment of weathering only at the bedrock interface, without a description of weathering of rocks or minerals in their path from B to S. Recent works consider the transformation of bedrock to saprolite and of saprolite to regolith/soil separately [Phillips, 2010;Burke et al., 2007] or account for weathering across the full thickness using a mass balance [Gabet and Mudd, 2009]. They also assume a regolith production rate that decreases exponentially with the thickness, as well as assuming steady state conditions. ...
We propose models for calculating the rate of chemical weathering of minerals as a function of depth in the weathering zone. A macropore network is assumed to be responsible for the transport of mobile water, which removes soluble weathering products at the interface of that network and the matrix. Conditions of infrequent rainfall (A) and of very frequent rainfall (B) are separately modeled, but both lead to a volumetric weathering rate with the general form , where the amplitude ν0 and the equilibration length ξ depend on the pore geometry of regolith and on parameters describing transport across the macropore-matrix interface (A) or mineral dissolution (B). This is obtained with no assumption of steady state for regolith evolution. Extrapolating these endmembers into intermediate conditions, the model is consistent with the exponential decay of regolith production rate versus depth reported by several authors and yields ξ in a variety of regolith types in the range from centimeters to meters. The velocity of the regolith-bedrock interface also shows an exponential decay with weathering zone thickness and is enhanced by bedrock fractures when compared to models of unfractured bedrock. When the residence time of fluid in the weathering zone, τf, is large, the bulk weathering rate Rb is inversely proportional to this time, and for small τf, Rb reverts to the laboratory rate. Application to compiled data from several sites leads to estimates of a dissolution rate constant and specific surface area consistent with those of albite.
... Erosion of surface material stimulates weathering, thus maintaining a steady-state soil thickness. The history and critiques of this concept are given by (Humphreys and Wilkinson, 2007;Phillips, 2010b;Phillips et al., 2019). ...
... However, the emergent perspective shows that the characteristic forms can occur without any goal functions, steady-state equilibrium or otherwise. Additional examples include the morphology of fluvial channels and development of fluvial channel networks (Phillips, 2010b;2011;Smith, 2010). More broadly, Corenblit et al. (2008) discussed how biological evolution and complex abiotic-biotic feedbacks leads to emergence of landscape patterns, without any stipulation of goal functions. ...
This chapter focuses on the (not necessarily final) destination of landscape evolution—the attractors that landscapes may move toward and the goal functions that govern these trajectories. Single-outcome concepts posit that landscape systems move toward a single self-perpetuating state. These include notions of progression toward climax or mature forms, stable equilibrium conditions, or self-organized critical states. Multi-outcome models include notions of alternative stable states, nonequilibrium systems, and unstable attractors. As they evolve, landscapes have plasticity defined by their degrees of freedom, and constraints imposed by limits on energy, matter, and geographical space. These can be described using concepts of a multidimensional resource or landscape evolution space. Goal functions for landscape evolution are generally based on increasing fitness, often assessed in terms of optimality hypotheses, which systems strive toward maximizing or minimizing some aspect of energy and/or mass flux. Many of these are directly or indirectly related to the least action principle and maximum entropy production. Apparent goal functions can generally be explained on the basis of emergent phenomena. Landscape systems cannot aspire to anything in a literal sense, and there exist no laws that dictate trends toward the optimal states. However, if these optimal states are associated with advantages in the formation, survival, and replication of landscape components, then trends toward the optima will frequently be observed. Emergence and general principles of selection can tie together the majority of the concepts of attractors and goal functions in landscape evolution.
... Erosional removal of surface material stimulates erosion, thus maintaining a steady-state soil thickness. The bestknown formal expression of these feedbacks is from Carson and Kirkby (1972), whose framework is employed in numerous soil, hillslope, and landscape evolution models to the present (see critical reviews by Humphreys and Wilkinson, 2007;Phillips, 2010). ...
... Extensive evidence exists of nonsteady-state soil thickness (see, e.g. Phillips, 2010;Phillips et al., 2019;Samonil et al., 2020) and two of the key assumptions of the model (often called the soil production function) have been debunked. First was the assumption that weathering of underlying rock and surface removals are the only significant influences on thickness. ...
An approach to landscape and Earth surface system evolution is outlined based on the inseparability of landform, soil, and ecosystem development, versus the traditional semi-independent treatment of geomorphic, ecological, pedological, and hydrological phenomena. Key themes are the coevolution of biotic and abiotic components of the environment; selection whereby more efficient and/or durable structures, forms, and patterns are preferentially formed and preserved; and the interconnected role of laws, place factors, and history. Existing conceptual frameworks for evolution of geomorphic, soil, ecological, and hydrological systems are reviewed and contrasted with the integrated approach.
... Toward the surface, weathered rock is transformed to highly weathered rock or saprolite, and then to soil. This is sometimes simplified to a two-sequence whereby weathering turns bedrock into regolith (or saprolite), and pedogenesis turns regolith into soil (e.g., Gabet and Mudd, 2010;Phillips, 2010;Eckerer et al., 2016;Riebe et al., 2017) The soil production function, discussed earlier, implies eventual conversion of weathered material to soil, maintained as a steady-state thickness. The concept of weathering-or transport-limited systems (Carson and Kirkby, 1972), not surprisingly, has different implications for those two cases ( Table 1). ...
... Consider a case where soil directly overlies fresh bedrock (T s > 0;T t, T w ≈ 0; T s /T wp ≈ 1). These observations are only compatible with scenario 2, and have in fact previously been used as a criterion for steady-state soil thickness (Phillips, 2010). For another example, take a case where there are significant thicknesses of all layers, with relative thicknesses of T w > T s > T r . ...
Evolution of weathering profiles (WP) is critical for landscape evolution, soil formation, biogeochemical cycles, and critical zone hydrology and ecology. Weathering profiles often include soil or solum (O, A, E, and B horizons), non-soil regolith (including soil C horizons, saprolite), and weathered rock. Development of these is a function of weathering at the bedrock weathering front to produce weathered rock; weathering at the boundary between regolith and weathered rock to produce saprolite, and pedogenesis to convert non-soil regolith to soil. Relative thicknesses of soil (Ts), non-soil regolith (Tr) and weathered rock (Tw) can provide insight into the relative rates of these processes at some sites with negligible surface removals or deposition. Scenarios of weathering profile development based on these are developed in current study. We investigated these with ground penetrating radar, electrical resistance tomography, and seismic profiling at three old growth forest sites in the Czech Republic, on gneiss, granite, and flysch bedrock.
We found that the geophysical methods – which generated thousands of separate measurements of Ts, Tr, Tw – to produce good estimates. The weathered rock layer (sensu lato) was generally the thickest of the weathering profile layers. Mean soil thicknesses were about 0.64–0.75 m at the three sites, with typical maxima around 1.5 m. Non-soil regolith thicknesses averaged about 2.5 m on the gneiss site and 1.2–1.4 at the other sites. Weathered rock had a mean thickness of 7 m at the gneiss site (up to 10.3), 4.6 at the granite site, and 3.4 on flysch. Results indicate that weathering at the bedrock weathering front is more rapid than conversion of weathered rock to regolith, which is in turn more rapid than saprolite-to-soil conversion by pedogenesis on all three bedrock types. No evidence was found of steady-state soil, non-soil regolith, or weathered rock thicknesses or evolution toward steady-state. Steady-state would require that weathering rates at the bedrock and/or regolith weathering fronts decline to negligible rates as profiles thicken, but the relative thicknesses at our study sites do not indicate this is the case.
... While the steady-state perspective has made large-scale weathering simulations tractable, it has limited applicability in regions that were glaciated or subject to intense periglacial deposition during the Last Glacial Maximum (LGM) (Boyle et al., 2013;Hilley & Porder, 2008;Houlton et al., 2018;Phillips, 2010). Glacial deposits alone cover 17.6% of the land surface, while alluvial and eolian deposits-many of which are associated with the LGM-comprise an additional 23.3% and 7.5% of the land surface respectively (Börker et al., 2018). ...
... In particular, the model represents the effect of climate on weathering zone thickness using an empirically derived "soil production function," which is a geomorphic concept describing the boundary between the mobile upper fraction of the regolith and the underlying rock or saprolite (Heimsath et al., 1997;Pelletier & Rasmussen, 2009). The soil production function is inappropriate for modeling deep weathering in saprolite, which is controlled by hydrologic factors that are not necessarily linked to soil thickness (Phillips, 2010). In many landscapes weathering can extend tens of meters below the lower boundary of the soil (Brantley et al., 2013;Hewawasam et al., 2013;Turner et al., 2003). ...
Global‐scale models of rock‐derived nutrient availability often assume that physical erosion drives soils toward an approximate “steady state” over geologic timescales. By definition, steady‐state models do not represent landscape age—that is, the time elapsed since soil formation is initiated by major erosional or depositional events. We hypothesize that this steady‐state assumption has large consequences on estimates of soil fertility because landscape age can mediate the retention of mobile elements in soil, particularly in low‐relief landscapes and humid climates. We quantified the effect of landscape age on soil fertility by estimating Na retention in soils across the United States and explicitly resolving landscape age in regions that experienced significant deposition or glacial retreat after the Last Glacial Maximum (LGM). We then used a simple one‐compartment model to simulate soil formation and weathering, comparing predictions that incorporated landscape age with those based on the steady‐state assumption. We found that soils formed in LGM deposits in low‐relief, humid settings generally retain 10 times more Na than soils formed outside of LGM deposits. Furthermore, the model that accounted for landscape age outperformed a steady‐state model across the United States and increased globally averaged estimates of Na retention by 17%. These results reinforce the idea that landscape age is a major control on weathering and should not be ignored in simulations of nutrient cycling.
... To facilitate parameter estimation and model application, several studies have carried out theoretical derivations of the parameters in process-based models by applying the local steady-state concept which is a convenient assumption in most models of hillslope and landscape evolution, and is also a fundamental assumption in the use of cosmogenic radionuclides to estimate soil production rates (Phillips, 2010;Yu and Hunt, 2017). For example, based on the steady-state assumption, Hurst et al. (2012) estimated erosion rates using hilltop curvature, and Chiang and Hsu (2006) calibrated their model's parameters by using soil thickness measurements on ridges. ...
... The steady-state concept which is a convenient assumption in most models of hillslope evolution (Phillips, 2010) is adopted for parameter estimations. If we assume a long-term balance between local soil production and erosion, i.e., that an equilibrium between the production rate and the divergence of sediment transport on hillslopes ( Fig. 1) has been established, the soil thickness is time-independent; that is, ∂h/ ∂t = 0, and ...
The spatial distribution of soil thickness plays a critical role in upland hydrological and biogeochemical processes. Process-based models are thought to have great potentials for predicting soil thickness. However, applications of process-based approaches have been greatly limited by parameter estimation, which typically requires specialized laboratories. In this study, on the basis of field works, an approach for parameter estimation under steady-state assumption is developed using linear and nonlinear soil transport models. Under each model, parameter, namely the ratios between the maximum soil production rate and the diffusion coefficient can be determined. Moreover, the optimal simulation time is derived by a power function of the diffusion coefficient derived under the linear model. The method is then used in a 0.31ha headwater hillslope in the Hemuqiao catchment, China. According to the suggested method, parameters are determined using soil thickness data collected at 45 different locations on the hillslope, and then soil thickness is predicted. We find the distribution of predicted soil thickness and the root mean square error between the measured and predicted soil thickness for all the soil pits are all the same with the estimated ratios in both models. Furthermore, the derived power function between the optimal simulation time and the diffusion coefficient is verified and the optimal simulation time is determined. Comparisons of simulations and field observations indicate that both models can achieve comparable results, although the procedure developed using the linear model is more suitable for the study area. Furthermore, the linear model is more efficient in parameter estimation, and thus is simpler to employ in practical applications. With our framework, field studies are constrained either on ridges or sideslopes, thus reducing the aimless hunting for sampling sites. This framework provides a useful approach for applying process based geomorphic models in real catchments.
... Limitations of the soil chronosequence approach include the inability to give numeric ages of surfaces, the often-limited number of soil pits for describing surfaces, variability within the soils forming on surfaces assumed to have formed at the same time, degradation of surfaces over time which especially impacts older surfaces, and assumptions that soil development is entirely progressive (Bockheim, 1980;Harrison et al., 1990;Eppes, 1999;Phillips, 2010). Soils have been utilized in many locations throughout the Rocky Mountains to fit moraine sequences into a previously established glacial sequence (Richmond, 1963;Berry, 1986;Birkeland et al., 1987Birkeland et al., , 1988Wesling, 1988;Birkeland et al., 1991;Dahms, 2002). ...
... Pinedale and post-Pinedale glaciations are well documented throughout the entirety of the Rocky Mountain physiographic region (Hack, 1943;Richmond, 1960Richmond, , 1962Richmond, , 1963Birkeland, 1973;Mahaney 1973Mahaney , 1975Mahaney , 1978Berry, 1976;Birkeland et al., 1979;Benedict, 1981Benedict, , 1985Hall and Heiny, 1983;Berry, 1983;Wesling, 1988;Shroba, 1991;Dahms 2002;2010;2018;Pierce, 2003;Davis et al., 2009). Pinedale glacial moraines have been extensively dated utilizing cosmogenic exposure techniques and obsidian-hydration techniques, as well as through correlation utilizing soils, lichenometry, and clast weathering techniques. ...
... Geomorphological models usually consider empirical relations between the erosion rate at the land surface and the regolith thickness and assume a steady state evolution in which the erosion rate equals the soil production rate at the interface with saprolite or bedrock (Anhert, 1994;Heimsath et al., 1997;Burke et al., 2007;Odoni, 2007;Gabet and Mudd, 2009;Hilley et al., 2010;Phillips, 2010). On the other hand, geochemical models describe the spatial variation of physical and chemical parameters that locally depict the state of the regolith (Li et al., 2017). ...
... Eq. (38) shows that s R decreases when v E increases, so that the regolith becomes thinner as v E approaches v c . In the geomorphology literature, the instability of thin regolith has been considered but bedrock weathering rates have generally been assumed to be humped functions of the thickness (Anhert, 1994;Phillips, 2010). However, this assumption is not necessary in reactive transport models; for instance, Lebedeva et al. (2010) and Li et al. (2014a) have already shown that regolith was removed when v E exceeded a certain value. ...
Models for weathering and regolith formation are generally built on the assumption of constant rates of water advection in the zone of water-saturated pores, and constant water content of those pores, but it is common that weathering occurs in the water-unsaturated zone where lateral flow occurs. Thus, water content in pores varies with depth. Here we model mineral weathering profiles while accounting for depth-dependent water content. Like previous models, a mineral equilibrates with water over a length ξ that depends on dissolution and advection rates, but a new lengthscale λ is introduced to describe the decrease of water content with depth. Steady states of the regolith thickness can be attained for any finite λ and non-zero velocity vE of erosion at the land surface. The type of mineral depletion profile developed over geological timescales depends on coupling between weathering and erosion: for slow erosion, a completely developed profile (CDP) is observed, in which the mineral-depleted zone at the top of the regolith has thickness of order λ; as vE increases, there is a transition to an incompletely developed profile (IDP), in which partial mineral depletion at the land surface is constrained to a narrow range of velocities; when vE exceeds the advance rate of weathering under far from equilibrium conditions, the profile transitions to an unstable regime that exposes bedrock. In general, the reaction front (RF) thickness and the velocity where CPD transitions to IDP depend on the interplay of both water infiltration and chemical equilibration over the timescale of water residence in regolith. The RF thickness roughly equals a correlation length χ defined as half the harmonic average of ξ and λ. In cases of limited water infiltration, water-mineral equilibration is achieved within the length λ, so that the RF thickness is controlled by hydrological properties and is independent of dissolution and advection rates. In the opposite endmember, water infiltrates to large depths and the effects of physical and chemical parameters on RF thickness are the same as in previous geochemical models. The relaxation time for reaching a steady state is shown to be ∼χ/vE. We discuss the effects of physical and chemical parameters in CDPs and IDPs in those endmembers and show an application to a CDP in granitic regolith.
... With respect to regolith thickness, while the thresholds limiting the upper limits are less clear, and oscillatory behavior appears less common, thicknesses may also vary from steadily increasing thickness to complete regolith stripping, with evidence for steadystate relationships (addition of new material by weathering or input via deposition roughly equals removals) relatively uncommon (see Phillips, 2010b and references therein). ...
... For instance, the assumption of steady-state soil thickness often employed in soil and landscape evolution models and underlying some dating methods is often violated, and is not an accurate representation of soil and regolith processes and evolution. However, the fact that steady-state thickness is not a truth statement about soils or regolith may have little or no effect on the efficacy and utility of some models and methods based on the assumption (Phillips, 2010b). ...
This document is a collection of Jonathan Phillips’ Geoscience Blog posts from its inception (29 May 2014) through 2 July 2017. The major sections include (1) How it's Done; (2) Earth Surface System Theory 1: Equilibrium & Otherwise; (3)Earth Surface System Theory 2: Nonlinear Dynamics, Complexity, Self-Organization, Power Laws; (4) Earth Surface System Theory 3: Optimality & Selection; (5) Forest Biogeomorphology; (6) Climate & Sea-Level Rise; (7) Coevolution; (8) Rivers & Streams; (9) Environmental Management; (10) Geomorphology; (11) Soil, Regolith & Karst
... However, this approach mostly requires the assumption that denudation and production are balanced and that the soil thickness remains in steady-state. According to Phillips (2010), the assumption of steady-state conditions for soil, regolith, or weathering profile development may lead to unrealistic representations of the dynamics of pedogenesis and weathering profile evolution. Steady-state assumptions should only be applied on surfaces that have been formed on more or less homogeneous bedrock and have not recently been eroded or subjected to colluvial deposition (Phillips, 2010). ...
... According to Phillips (2010), the assumption of steady-state conditions for soil, regolith, or weathering profile development may lead to unrealistic representations of the dynamics of pedogenesis and weathering profile evolution. Steady-state assumptions should only be applied on surfaces that have been formed on more or less homogeneous bedrock and have not recently been eroded or subjected to colluvial deposition (Phillips, 2010). Consequently, steady-state assumptions might not be valid for Alpine hillslope soils that developed only 'recently' (c. last 20 ky) on unconsolidated sedimentary parent material. ...
Spatially discontinuous permafrost conditions frequently occur in the European Alps. How soils under such conditions have evolved and how they may react to climate warming is largely unknown. This study focuses on the comparison of nearby soils that are characterised by the presence or absence of permafrost (active-layer thickness: 2 – 3 m) in the alpine (tundra) and subalpine (forest) range of the Eastern Swiss Alps using a multi-method (geochemical and mineralogical) approach. Moreover, a new non-steady-state concept was applied to determine rates of chemical weathering, soil erosion, soil formation, soil denudation, and soil production. Long-term chemical weathering rates, soil formation and erosion rates were assessed by using immobile elements, fine-earth stocks and meteoric 10Be. In addition, the weathering index (K + Ca)/Ti, the amount of Fe- and Al-oxyhydroxides and clay minerals characteristics were considered. All methods indicated that the differences between permafrost-affected and non-permafrost-affected soils were small. Furthermore, the soils did not uniformly differ in their weathering behaviour. A tendency towards less intense weathering in soils that were affected by permafrost was noted: at most sites, weathering rates, the proportion of oxyhydroxides and the weathering stage of clay minerals were lower in permafrost soils. In part, erosion rates were higher at the permafrost sites and accounted for 79 – 97% of the denudation rates. In general, soil formation rates (8.8 – 86.7 t/km2/y) were in the expected range for Alpine soils. Independent of permafrost conditions, it seems that the local microenvironment (particularly vegetation and subsequently soil organic matter) has strongly influenced denudation rates. As the climate has varied since the beginning of soil evolution, the conditions for soil formation and weathering were not stable over time. Soil evolution in high Alpine settings is complex owing to, among others, spatio-temporal variations of permafrost conditions and thus climate. This makes predictions of future behaviour very difficult. This article is protected by copyright. All rights reserved.
... In that case, h ≡ 0 and ℎ ̅ ≡ ℎ, and the uplifting speed in [3] can be replaced with Pr. The weathering profiles are often assumed to have reached a steady state, that is, the soil production rate equals to erosion rate (Phillips, 2010). Under such an assumption, the soil production rate does not change with time (as long 180 as the tectonic setting and climate have not changed), and the exposure time is simply z/Pr. ...
Silicate weathering, which is of great importance regulating global carbon cycle, has been found to be affected by complicate factors including climate, tectonics, vegetation, and etc. However, the exact transfer function between these factors and silicate weathering rate is still unclear, leading to large model-data discrepancies of the CO2 consumption associated with silicate weathering. Here we propose a simple parameterization for the influence of vegetation cover on erosion rate to improve the model-data comparison based on a state-of-the-art silicate weathering model. We found out that the current weathering model tends to overestimate the silicate weathering fluxes in the tropical region, which can hardly be explained by either the uncertainties in climate and geomorphological conditions or the optimization of model parameters. We show that such an overestimation of tropic weathering rate can be rectified significantly by considering the shielding effect of vegetation cover on the erosion rate of the leached soils considering that the geographic distribution of such soils is coincident with regions with the highest leaf area index (LAI). We propose that the heavy vegetation in the tropical region likely slows down the erosion rate, much more so than thought before, through reducing extreme stream flow in response to precipitation. The silicate weathering model thus revised gives a smaller global weathering flux which is arguably more consistent with the observed value and the recently reconstructed global outgassing, both of which are subject to uncertainties. The model is also easily applicable to the deep-time Earth to investigate the influence of land plant on global biogeochemical cycle.
... This same review also suggests microtopography within uplands that protect against surface erosion can lead to thick regolith (Migon & Thomas, 2002). These ideas have been adopted by several more recent studies with similar outcomes (Clair et al., 2015;Lebedeva & Brantley, 2013;Phillips, 2010;Rempe & Dietrich, 2014). The expression of soil morphologic features was indicative of landscape stability in uplands and instability at lowland positions. ...
Abstract Rooting in deep regolith enables forests to withstand seasonal and annual precipitation shortfalls. Despite its ecological importance, spatial patterns in regolith thickness within forest ecosystems are scarcely documented. Regolith thickness was estimated at 66 sites throughout a 543‐ha watershed in the southern Sierra Nevada by hand auger to point of failure or a maximum depth of 7.5 m, describing a minimum thickness estimate. Regolith consists of 1–2 m of soil overlying thick and porous weathered granodiorite. Depth to auger failure ranged from 1.52 to an indeterminate depth beyond 7.5 m. A total of 27 points exceeded 7.5 m depth. Normal, lognormal, and gamma data distribution models were fitted to observations to extrapolate thickness across the watershed and estimate thicknesses beyond the measurement limitation. Predictions for the 95th percentile of regolith thickness varied substantially; 26.05 m for lognormal, 16.87 m for gamma, and 9.56 m for normal. Considering any best fit model, >55% of the watershed area was deeper than 5 m. Depth classes were formed to evaluate the extent to which topography is associated with spatial trends in regolith thickness. Spatial patterns were related to two covariate proxies (distance from stream channel and topographic wetness) with the general landscape trend of shallow depth classes (7.5 m) in uplands. The normalized difference vegetation index signatures over the late stages of a 5‐year drought were greener in the lowlands. In contrast, upland forests displayed widespread tree die‐off, suggesting deep water storage does not maintain forests over long‐term drought.
... Furthermore, field verification only focuses on locations of high indicative soil damage potential to prove the truth of the potential damage [30]. When in the field, the independent system soil observations at 10 representative sites of the dimensions of solum thickness, surface rock, degree of water release, and taking soil samples at a depth of 0-20 cm for laboratory review [31][32][33]. ...
Currently, land use is considered intensive for various purposes that affect the soil as the main series of land and the environment. The other side of the soil in Kalimantan is naturally formed from material that is poor in nutrients so it is not fertile and acidic. This study attempted to evaluate the status of soil damage to the carrying capacity for biomass production in Tanjung Selor District. The overlay analysis of land slope, rainfall, soil type and land cover in the form of a map produces indicative areas of low, medium and high damage. High damage indicative areas were selected for verification, observation and soil sampling to obtain soil damage parameter data, carried out in March 2020. Analysis of the relative frequency score of the damaged parameters aims to determine the status of carrying capacity and soil damage. The results of the study based on 10 soil damage parameters obtained a score of 6 with the status of lightly damaged soil damage, the main factor being soil acidity (R.I-a) with a high carrying capacity of 1,684 ha. The acidity factor with a pH of <4.5 (very acidic) has the most effect, 80% is damaged, but is relatively easy to repair. Efforts to improve cultivated plants are stressed by soil acidity by using soil amendments to raise the pH above the minimum range that is more suitable to increase biomass production and carrying capacity, namely agricultural lime and compost followed by N, P and K fertilization as needed.
... This point becomes increasingly important because paleosols may be preserved preferentially during one part of the supercontinent cycle (Dzombak, 2021). The relative relationships may have been different during stable cratonic periods (e.g., Millot et al., 2002) and varied as alluvium-or regolith-parented soils (rather than bedrock-parented) became more common (e.g., Phillips, 2010;Norton et al., 2014). Therefore, as our understanding of those factors evolves, preservation biases due to geomorphic evolution should be reconsidered. ...
Although continental weathering intensity has been invoked as a primary control on biogeochemistry, tectonics, and the carbon cycle throughout geologic history, it remains poorly quantified over Earth’s history. As a direct product of continental weathering, paleosols (fossil soils) offer unique insight into past weathering intensity, but they remain underused in efforts to constrain terrestrial weathering patterns over geologic time. Here, we compile the largest terrestrial weathering record to date, comprising 248 paleosol and weathering profiles that span three billion years. We analyze a suite of weathering indices to test common hypotheses around state-changes in terrestrial weathering intensity due to atmospheric changes and terrestrial biosphere expansion. Contrary to commonly invoked assumptions, we find that these weathering indices reflect consistent average terrestrial weathering intensity through time. No unidirectional state changes in average weathering intensity, as have previously been hypothesized, are detectable in the record. However, Phanerozoic paleosols preserve an increase in the total range of Chemical Index of Alteration (CIA) values, with the increased CIA range driven by the appearance of high-CaO paleosols. We compare the paleosol weathering record to weathering intensities recorded by select fluvial sandstones and diamictites.
We interpret the overall stability of the continental weathering record as reflecting the baseline level of weathering from which the Earth system deviates during periods of perturbation (i.e., major climate transitions, rapid tectonic activity). With consistent weathering intensity over geologic timescales, the record supports subaerially-emerged continental area as a critical control on total potential erosional flux and nutrient flux to the oceans. The paleosol community should work to build an even more complete database of paleosol geochemistry to allow more nuanced analyses of terrestrial weathering through time.
... That characteristic follows the notion that preferential flow may be predetermined or influenced by the preexisting structure and water flow tends to adopt the most efficient ways (Hardie et al., 2011). Emergent feedbacks result in expanding these ways during pedogenesis (Phillips, 2010). ...
The work investigated the paleoenvironmental significance of kaolinite-rich weathering fronts developed during the Cretaceous in stabilized dune fields of Sanfranciscana Basin in Brazil. To this, was conducted a multi-scale analysis from the outcrops of paleosols to mineral grains and soil pores. The macro and micromorphology of paleoweathering fronts allow recognizing weathering features and estimating the porosity and hydraulic parameters (permeability, capillary pressure, and saturated hydraulic conductivity). Scanning electron microscope analyses were conducted to investigate surface microtextures of quartz grains for weathering process characterization based on dissolution microfeatures. Energy-dispersive X-ray spectrometry was utilized to identify chemical elements of clay minerals. The major and trace elements were analyzed using a Philips X-ray fluorescence (XRF) spectrometer (PW 2440). Our results showed that weathering fronts expand through preferential water paths (pipe and finger flow) in bedding planes in response to textural and porosity contrasts related to the bimodality (i.e., the difference of grain size distribution between overlapped sedimentary structures) of aeolian deposits. The dissolution features present in quartz grains, such as etching, pitting, total dissolution of grains, iron coatings, and booklets of well-developed kaolinite, reveal an increase in humidity and the pedogenetic kaolinization process typical of a humid tropical environment. These weathering patterns coincide with cooling events between the Albian and the Cenomanian report in the literature. Our study indicates that during the Cretaceous, the Sanfransicana Basin underwent climatic amelioration and a rise in water table level, which likely enhanced dune stabilization and the formation of an extensive geomorphic surface.
... Over time, the soil properties approach an equilibrium or steady state, where the SLDS and complexity remain relatively constant. The model simulations show that a steady state in soil formation can indeed be reached, but only for faster changing properties, such as SOM stocks, and under stable circumstances with little to no external perturbations, and it 170 is a rare occurrence (Phillips, 2010;Sauer, 2015). In the agricultural phase, the changes in the soil properties equal or exceed those of the initial soil development. ...
Soils and landscapes can show complex, non-linear evolution, especially under changing climate or land use. Soil-landscape evolution models (SLEMs) are increasingly equipped to simulate the development of soils and landscapes over long timescales under these changing drivers, but provide large data output that can be difficult to interpret and communicate. New tools are required to analyse and communicate large model output. In this work, I show how spatial and temporal trends in previously published model results can be summarized and conceptualized with evolutionary pathways, which are possible trajectories of the development of soil patterns. Simulated differences in rainfall and land use control progressive or regressive soil development and convergence or divergence of the soil pattern. These changes are illustrated with real-world examples of soil development and soil complexity. The use of evolutionary pathways for analysing the results of SLEMs is not limited to the examples in this paper, but they can be used on a wide variety of soil properties, soil pattern statistics and models. With that, evolutionary pathways provide a promising tool to analyse and communicate soil model output, not only for studying past changes in soils, but also for evaluating future spatial and temporal effects of soil management practices in the context of sustainability.
... The common assumption is that soil stocks in pristine systems are in equilibrium, i.e., the rate of soil loss via erosion is the same as the rate of new soil formation. It was shown that for mature soils, formed from uniform bedrock parent material, soil formation and soil erosion are practically at steady state (Phillips, 2010). In these pristine systems, the sediment fluxes in rivers are dominantly controlled by climate and topography (Ludwig et al., 1996). ...
Studies on sediment export from tropical forest watersheds are scarce. Of the assessments that do exist, most are of larger rivers or are model-based and lack validation with measured data. Understanding the mechanisms of sediment export dynamics in forested headwaters is important for assessing downstream effects and as a baseline for net impacts of land-use change. To that end, we quantified annual total suspended sediment (TSS) yields in forested headwater catchments of two major forest types in central Africa (tropical lowland forest and subtropical Miombo woodland) and analyzed turbidity-discharge hysteresis over one hydrological year. We measured TSS yields of 0.24 ± 0.09 t ha⁻¹ yr⁻¹ in the Miombo woodland and 0.25 ± 0.05 t ha⁻¹ yr⁻¹ in the lowland forest catchment. The Miombo woodland experienced similar TSS yields as the lowland forest despite a shorter, five-month, rainy season and lower annual precipitation. In the Miombo forest, sparser vegetation cover, seasonal fires that remove understory vegetation and high rainfall intensity during the rainy season therefore resulted in similar TSS yields. As a result of these differences in vegetation and rainfall, approximately 68% of TSS was exported during storm events in the Miombo woodland and 30% in the lowland forest. Both sites showed mainly clockwise hysteresis (positive hysteresis index) patterns of sediment export. In the Miombo woodland, the hysteresis index (i.e., the magnitude and direction of hysteresis) increased with the ongoing rainy season, indicating source limitation already after one month of rain. In the lowland forest, the predominant clockwise hysteresis was more likely caused by the increasing contribution of baseflow during the falling limb of an event, whereas during the rising limb there is a quick flushing of surface material available in the forest. These findings based on hysteresis analysis were further supported by C:N ratio and δ¹³C analyses of particulate organic matter (POM). POM C:N ratios increased and δ¹³C signatures decreased with increased discharge in the lowland forest, indicating the mobilization of topsoil sediments during rain events. In contrast, the Miombo exhibited no shifts in C:N ratios nor in the δ¹³C signature. Despite the pristine nature of these forests and their assumed negligible sediment yields, our results demonstrate that erosion is a significant loss process in tropical forests and call for future research to examine its role in forest functioning and biogeochemical cycling.
... Burke et al., 2007; Dixon et al., 2012). The key problem here is that the assumption of steady-state soil (saprolite) thickness, which underlies many theoretical and mathematical models of slope evolution, is invalid for deeply weathered lands, with inherited saprolite cover (Phillips, 2010). A recent monograph by Willgoose (2018) offers a state-of-the-art of modeling approaches to landscape evolution, with special emphasis of the role of soils and regolith. ...
Weathering processes are not confined to surface conditions. Thick weathering mantles are ubiquitous around the world and show various ages, from Mesozoic to Quaternary. Of considerable geomorphological importance is stripping of pre-weathered materials that exposes an etched surface. Etchsurfaces at different stages of evolution are present around the globe, and current surface development in low latitudes is mainly through episodic etchplanation. Inselbergs, multi-convex relief, and topographic basins are geomorphic signatures of landscape development through etching and stripping.
... This point becomes increasingly important because paleosols may be preserved preferentially during one part of the supercontinent cycle, when one part of the weathering/erosion relationship may be true (see section 4.5.2). The relative relationships may have been different during stable cratonic periods (e.g., Millot et al., 2002) and varied as alluvium-or regolith-parented soils (rather than bedrockparented) became more common (e.g., Phillips, 2010;Norton et al., 2014). Therefore, as our understanding of those factors evolves, preservation biases in the paleosol record should be reconsidered. ...
The co-evolution of the terrestrial biogeochemical cycle, the atmosphere, and the marine biosphere remain relatively poorly understood, with outstanding questions surrounding terrestrial-marine links, climate, and tectonics. In particular, the terrestrial sediment source (i.e., soils) remains understudied relative to the marine sediment sink, with the source essentially defined by the record in the sediment sink rather than being considered equally important. Both the sediment source and sink need to be well-constrained in order to understand global biogeochemical changes. Additionally, interpretations of trends in paleosol (fossil soil) geochemistry are only loosely constrained by large-scale modern soil chemical variability, limiting our ability to assess potential changes in biogeochemical cycling through time. This dissertation focused on two primary goals: improving quantitative constraints on terrestrial biogeochemical cycling and weathering over geologic time, and improving our ability to accurately interpret those records by understanding both modern context and what the paleosol record actually represents. To address these goals, I analyzed the geochemical composition of soils and paleosols (fossil soils) over the past three billion years. Because soils form in the ‘critical zone’—the intersection of the biosphere, geosphere, and atmosphere at Earth’s surface—they record surficial conditions more directly than other geologic records, providing valuable insight into past climates, atmospheres, and ecosystems. After providing generalized, quantitative constraints on geochemical and weathering variability in modern soils (Chapter II), I used the paleosol record to test for state changes in soil P (Chapter III) and weathering intensity (Chapter IV) on land during key biogeochemical transitions. I also explored a variety of processes that could bias the distribution of paleosols through space and time (e.g., preservation, sampling), which needs to be better constrained in order to interpret paleosols accurately. In modern soils, I found weaker than expected relationships between soil P and Fe geochemistry and key environmental factors (climate, vegetation, parent material), but weathering intensity, the presence of vegetation, and P concentrations were related. The weak relationships could be due to the continental rather than localized scale of analysis. While the latter might have provided predictive relationships between soil chemistry and soil-forming factors, a highly-localized scale is often not considered in deep-time biogeochemical modeling. In paleosols, I found that both the P composition and weathering intensity have been stable through time. Discrete, state changes in P composition or weathering intensity—as have been hypothesized based upon marine records—were not recorded. A discrete change was present in the concentration of Ca in paleosols, which increased in the Phanerozoic, perhaps reflecting a shift in pedogenic processes as vascular, rooting plants evolved. Roots and vascularity allowed plants to colonize more arid environments and facilitated the formation of pedogenic carbonate—an important C sink. Therefore, while the advent of land plants may not have led to a global state change in either terrestrial P retention or weathering intensity, plants facilitated the growth of the soil C sink. Because weathering intensity is consistent through time, other factors (e.g., land area, erosion rates) would have been dominant controls on marine nutrient supply through time, with shorter-term perturbations in weathering intensity occurring before returning to the stable baseline. Finally, the distribution of paleosols through time is uneven, with more paleosols being more common (a) towards the present and (b) during peaks in zircon ages, suggesting a formation and/or preservation bias related to the supercontinent cycle.
... Roering et al. (1999) made this assumption for a number of catchments in Oregon and tested postdictions of a hillslope transport law using measured high-resolution profiles. It must be noted that the equilibrium assumption has attracted criticism on theoretical grounds (Phillips, 2010), and that, in many settings, hillslope profiles and catchments are clearly in disequilibrium (e.g., Densmore et al., 2003;Tomkin, 2009). At the very least, use of the equilibrium assumption must be clearly defended. ...
This chapter reviews quantitative modeling of landscape evolution. Quantitative modeling is contrasted with conceptual or physical modeling, and four categories of model studies are presented. Procedural studies focus on model experimentation. Descriptive studies use models to learn about landscapes in general. Postdictive and predictive try to correctly simulate the evolution of real landscapes, respectively in the past (with calibration) or in the future (with calibrated models). After classification of 322 landscape evolution studies in these categories, we find that descriptive studies are most common, and predictive studies are least common. Procedural studies have focused on production methods for digital landscapes, spatial resolution effects, the role of sinks and depressions and calculation schemes for flow routing. Descriptive studies focused mainly on surface-tectonic interactions, sensitivity to external forcing, and the definition of crucial field observations from model results. Postdictive and predictive studies operate mainly in time-forward mode and are increasingly validated using independent data. Overall, landscape evolution modeling has progressed to the extent that non-experts are able to easily use modern models, and are commonly used in inversion schemes to obtain the most likely (set of) inputs to produce known topographies. This development will likely continue, with more attention for interactions with ecology and soils over short (ca, ma) timescales, and with climate over long (Ma) timescales.
... However, the success of the broader model in replicating observations does not validate the reality of the steady-state assumption. Phillips (2010Phillips ( , 2011a discussed this in the context of soils and geomorphology. Mimicry can also result from skillful "knob twiddling." ...
Landscape evolution often occurs over long-time scales that do not allow for direct observation and measurement. This chapter reviews approaches for observing, inferring, and reconstructing evolutionary trajectories. These include direct observation and monitoring (e.g., observatories), simulation models, and historical reconstruction. The latter encompasses documentary evidence, dating techniques, paleoenvironmental indicators, and inferential methods such as space-for-time substitutions and contemporary inferential indicators.
... For example, sometimes with relatively thin covers, increasing regolith thickness promotes weathering (i.e., has a positive relationship), and there is no convergence to a single steady-state thickness. The system is dynamically unstable and may experience pseudo-random changes in regolith thickness anywhere between zero and the threshold at which thickness inhibits rather than promotes bedrock weathering (Furbish and Fagherazzi, 2001;Phillips, 2010). ...
Thresholds are ubiquitous in Earth surface systems and fundamental to landscape evolution. They occur at the level of process mechanics, and at the broader level of landscape system states, and may be fuzzy or crisp in their occurrence and/or the ability to measure or define them. Five main types of thresholds occur: force vs. resistance, storage capacities, relative rates of linked processes, saturation and depletion effects, and limiting factors. Tipping points and regime shifts are types of thresholds that occur at the landscape level (or broader) and are abrupt. As virtually all landscape systems are strongly influenced by thresholds, and many are threshold-dominated, landscapes and ESS are nonlinear, opening up possibilities for complex phenomena that do not occur in linear systems. One of these is dynamical instability, which can be both a consequence (via nonlinearity) and a cause of thresholds. Instability is a cause of thresholds in the case of system-level meta-thresholds. These are shifts in the positive or negative effects, or in the relative magnitudes, of interactions within the system. They can result in switches between dynamically stable and unstable modes, often manifested as convergent or divergent evolution.
... Soil scientists and soil engineers are continuously exploring the possibility of getting reliable soil thickness utilizing the information about the soil formation process and terrain attributes (Moore and Burch, 1986;Moore et al., 1991;Kuriakose et al., 2009;Tesfa et al., 2009). Different researches have used stochastically-based models, focusing on directly building the relationship between the thickness of the soil and a set of influencers that can be easily estimated in the area (Moore et al., 1993;Zhu, 2000;Kuriakose et al., 2009;Phillips, 2010;Wang et al., 2010;Wang et al., 2018). However, stochastically-based models require large number of soil thickness observation points and soil related attribute database, and those database are treated in statistical models such as multiple regression method (Ziadat, 2010;Cavazzi et al., 2013) or machine learning algorithms such as random forest and artificial neural networks (Lagomarsino et al., 2017;Han et al., 2018). ...
Soil thickness is a major parameter to better understand slope stability, surface erosion, groundwater storage, and vegetation growth. In this study, the main focus is the development and application of a relative relief (RR)-based spatial soil thickness model. Intensive field works were also carried out to gather ground-truthing soil thickness data using traditional drilling and excavation methods. The spatial distribution of soil thickness obtained from the RR model was validated with the results of the field measurements, and compared with the predictions derived from S and multiple linear regression (MLR) models, which are already known in the literature. In this study, we tested how raster resolutions (5, 10, 20, 30, 50, 60 and 90 m) influence the spatial prediction of soil thickness. Based on the comparison between the predicted soil thickness and the measured soil thickness, a map of 10 m resolution contributed reasonable delineation of soil thickness over the study area. A comparison of the predicted results was performed using the agreement coefficient (AC) which showed that the RR model has a better predictive ability (AC = 0.970) than the S (AC = 0.945) and MLR (AC = 0.710) soil thickness models. The results indicate that an adjustment to the soil thickness and spatial resolution can significantly improve the modelling efficiency.
... When the rates of advection and diffusion are equal, the upward transport of clay by bioturbation equals the amount of downward translocation by water; the clay-depth profile of the soil occurs in steady state and will not change substantially. Steady-state circumstances are however rare in natural soil systems (Phillips, 2010). Our simulations do not show steady-state circumstances, because in our simulations there is always lateral transport of soil material that continuously changes slope and terrain properties and affects the soil's clay balance, complicating the achievement of a steady state. ...
Humans have substantially altered soil and landscape patterns and properties due to agricultural use, with severe impacts on biodiversity, carbon sequestration and food security. These impacts are difficult to quantify , because we lack data on long-term changes in soils in natural and agricultural settings and available simulation methods are not suitable for reliably predicting future development of soils under projected changes in climate and land management. To help overcome these challenges, we developed the HydroLorica soil-landscape evolution model that simulates soil development by explicitly modeling the spatial water balance as a driver of soil-and landscape-forming processes. We simulated 14 500 years of soil formation under natural conditions for three scenarios of different rainfall inputs. For each scenario we added a 500-year period of intensive agricultural land use, where we introduced tillage erosion and changed vegetation type.
Our results show substantial differences between natural soil patterns under different rainfall input. With higher rainfall, soil patterns become more heterogeneous due to increased tree throw and water erosion. Agricultural patterns differ substantially from the natural patterns, with higher variation of soil properties over larger distances and larger correlations with terrain position. In the natural system, rainfall is the dominant factor influencing soil variation, while for agricultural soil patterns landform explains most of the variation simulated. The cultivation of soils thus changed the dominant factors and processes influencing soil formation and thereby also increased predictability of soil patterns. Our study highlights the potential of soil-landscape evolution modeling for simulating past and future developments of soil and landscape patterns. Our results confirm that humans have become the dominant soil-forming factor in agricultural landscapes.
... One of the caveats in testing soil development models is that it is often uncertain whether the observed soil had reached a steady-state depth (Phillips, 2010). A steady state in this respect means that long-term erosion rates equals long-term soil production, which results in minimal long-term change in its depth (dH/dt = 0). ...
Climate drives the coevolution of vegetation and the soil that supports it. Wildfire dramatically affects many key eco‐hydro‐geomorphic processes, but its potential role in coevolution of soil‐forest systems has been largely overlooked. The steep landscapes of southeastern Australia provide an excellent natural laboratory to study the role of fire in the coevolution of soil and forests, as they are characterized by temperate forest types, fire frequencies, and soil depths that vary systematically with aridity. The aims of this study were (i) to test the hypothesis that in Southeastern Australia, fire‐related processes are critical to explain the variations in coevolved soil‐forest system states across an aridity gradient and (ii) to identify the key processes and (iii) feedbacks involved. To achieve these aims, we developed a numerical model that simulates the coevolution of soil‐forest systems which employ eco‐hydro‐geomorphic processes that are typical of the flammable forests of southeastern Australia. A stepwise model evaluation, using measurements and published data, confirms the robustness of the model to simulate eco‐hydro‐geomorphic processes across the aridity gradient. Simulations that included fire replicated patterns of observed soil depth and forest cover across an aridity gradient, supporting our hypothesis. The contribution of fire to coevolution increased in magnitude with aridity, mainly due to the higher fire frequency and lower post‐fire infiltration capacity, increasing the rates of fire‐related surface runoff and erosion. Our results show that critical feedbacks between soil depth, vegetation, and fire frequency dictate the trajectory and pace of the coevolution of flammable temperate forests and soils.
... Topographic equilibrium is the situation where landscapes experience equal rates of uplift and erosion, leading to no net change in altitude or morphology (Thorn and Welford 1994). The concept has been criticized on the grounds that driving factors such as climate and uplift rate can rarely be assumed stable over the timescales needed to achieve equilibrium (Phillips 2010). Path dependence adds to that criticism because of its implication that a series of landslides, causally related to each other, can accelerate mass transport and denudation from one hillslope above uplift rates, whereas similar nearby slopes remain relatively stable, perhaps below uplift rates, because they did not experience the initial landslide. ...
Landslides often happen where they have already occurred in the past. The potential of landslides to reduce or enhance conditions for further landsliding has long been recognized and has often been reported, but the mechanisms and spatial and temporal scales of these processes have previously received little specific attention. Despite a preponderance of qualitative and anecdotal evidence, analysis has been limited. As a result, there is little consensus on the meaning of terms such as landslide repetition, recurrence, and reactivation. This source of confusion is evident when such terms are also used to describe systems where landsliding is prevalent but unrelated to landslide history. Recent findings, partly based on a rare multi-temporal landslide inventory for an area in Italy, show that the impacts of earlier landslides affect a substantial fraction of landslides, that landslides following earlier landslides differ from those that do not, and that accounting for the effect of previous landslides can improve susceptibility assessments. These findings await confirmation in other landslide-prone landscapes but show that consecutive landsliding deserves more attention, which requires consistent terminology. No such terminology is presently available, and we therefore propose it in this manuscript. We use the term “uncorrelated landsliding” to describe situations where landslides are common, but where a correlation with environmental variables such as terrain steepness is not implied. We propose “correlated landsliding” to describe situations where landslides are common and correlations with environmental variables exist, and “path-dependent landsliding” to describe situations where causal relations exist between consecutive landslides, for instance, when landslides occur at the scarp of previous landslides. These are situations where past landslides impact future landslides. Within the path-dependent category, we distinguish three subcategories based on the spatial distance between earlier and later landslides: “reactivation” or “continuation” if essentially the same material recommences or continues to slide, “local activation” if an earlier slide causes changes in a local hillslope that cause a later slide, and “remote activation” if an earlier slide causes changes elsewhere in the landscape that cause a later landslide. We use this proposed set of terms to outline some prominent knowledge gaps and potential research questions.
... Soil as living skin of the earth provides numerous services to nature and society. However, rates of soil formation are subject of ongoing debate as quantitative estimates often rely on steady-state assumptions (Phillips, 2010;Stockmann et al., 2014). Periglacial slope sediments are considered as main parent material for soils in hilly and mountainous environments of the temperate zone, which have not been covered by ice during the last glacial period (Kleber and. ...
Slope deposits with aeolian silt admixture are a widespread parent material of soils in the temperate zone but may be neglected when rates of soil production are quantified. The concept of periglacial cover beds differentiates slope deposits with or without aeolian silt admixture; yet there is a remaining debate on processes and the timing of their formation. A previous study done by us at Mt. Ślęża, SW Poland, concluded that slope deposits with variable aeolian silt admixture, or its lack, have a significant influence on the pathway of soil formation. The present work builds upon this finding, by adding further granulometric and micromorphological data from three representative profiles along a toposequence, in order to refine our understanding of local slope deposits and soil formation. Additionally, seven numerical ages using luminescence dating provide a chronological framework for our reconstructions and allow linking the forming processes of these pedosedimentary records to regional palaeoenvironmental conditions. The oldest aeolian deposits are of Middle Pleistocene age (> 280 ± 19 ka) with interlayered palaeosol (marine isotope stage [MIS] 9 or older). Late Pleistocene slope deposits encompass the maximum loess thickness and are dated to MIS 2. Luminescence ages from the upper layers indicate shallow reworking, which we tentatively correlate to the Younger Dryas (YD). Two profiles with thick loess mantles have strong clay illuviation features, presumably formed during the Holocene. However, weak clay illuviation in the third profile with a thin loess mantle (having an age of YD) over granite regolith seems to have occurred before the Holocene, as only fragmented clay coatings (probably MIS 2 pedogenesis) could be found.
... At least one researcher (Phillips, 2010) has argued that steady states where 'surface removals are approximately balanced by production of new soil by bedrock weathering' are rare in natural systems. Nonetheless, even Phillips argues that a steady state is a 'convenient fiction facilitating the use of some models and tools'. ...
We present a model of chemical reaction within hills to explore how evolving porosity (and by inference, permeability) affects flow pathways and weathering. The model consists of hydrologic and reactive‐transport equations that describe alteration of ferrous minerals and feldspar. These reactions were chosen because previous work emphasized that oxygen‐ and acid‐driven weathering affects porosity differently in felsic and mafic rocks. A parameter controlling the order of the fronts is presented. In the absence of erosion, the two reaction fronts move at different velocities and the relative depths depend on geochemical conditions and starting composition. In turn, the fronts and associated changes in porosity drastically affect both the vertical and lateral velocities of water flow. For these cases, estimates of weathering advance rates based on simple models that posit unidirectional constant‐velocity advection do not apply.
In the model hills, weathering advance rates diminish with time as the Darcy velocities decrease with depth. The system can thus attain a dynamical steady state at any erosion rate where the regolith thickness is constant in time and velocities of both fronts become equal to one another and to the erosion rate. The slower the advection velocities in a system, the faster it attains a steady state. For example, a massive rock with relatively fast‐dissolving minerals such as diabase – where solute transport across the reaction front mainly occurs by diffusion – can reach a steady state more quickly than granitoid rocks in which advection contributes to solute transport. The attainment of a steady state is controlled by coupling between weathering and hydrologic processes that force water to flow horizontally above reaction fronts where permeability changes rapidly with depth and acts as a partial barrier to fluid flow. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.
... When the rates of advection and diffusion are equal, the upward transport of clay by bioturbation equals the amount of downward translocation by water; the clay-depth profile of the soil occurs in steady state and will not change substantially. Steady-state circumstances are however rare in natural soil systems (Phillips, 2010). Our simulations do not show steady-state circumstances, because in our simulations there is always lateral transport of soil material that continuously changes slope and 475 terrain properties and affects the soil's clay balance, complicating the achievement of a steady state. ...
Abstract. Humans have substantially altered soil and landscape patterns and properties due to agricultural use, with severe impacts on biodiversity, carbon sequestration and food security. These impacts are difficult to quantify, because we lack data on long-term changes in soils in natural and agricultural settings and available simulation methods are not suitable to reliably predict future development of soils under projected changes in climate and land management. To help overcome these challenges, we developed the HydroLorica soil-landscape evolution model, that simulates soil development by explicitly modelling the spatial water balance as driver of soil and landscape forming processes. We simulated 14500 years of soil – formation under natural conditions for three scenarios of different rainfall inputs. For each scenario we added a 500-year period of intensive agricultural land use, where we introduced tillage erosion and changed vegetation type.
Our results show substantial differences between natural soil patterns under different rainfall input. With higher rainfall, soil patterns become more heterogeneous due to increased tree throw and water erosion. Agricultural patterns differ substantially from the natural patterns, with higher variation of soil properties over larger distances and larger correlations with terrain position. In the natural system, rainfall is the dominant factor influencing soil variation, while for agricultural soil patterns landform explains most of the variation simulated. The cultivation of soils thus changed the dominant factors and processes influencing soil formation, and thereby also increased predictability of soil patterns. Our study highlights the potential of soil-landscape evolution modelling for simulating past and future developments of soil and landscape patterns. Our results confirm that humans have become the dominant soil forming factor in agricultural landscapes.
... In a threshold landscape, such an equivalence can only reflect a spatial or long-term average. The relevance of steady state landscapes has been called into question (Phillips, 2010;Yu & Hunt, 2017a), even though an equivalence of soil production and soil erosion rates is often assumed. Nevertheless, the investigation of Yu and Hunt (2017a) indicated that, while slowly evolving landscapes in arid continental interiors were unlikely to be in steady state, tectonically active regions were much more likely to conform to steady state conditions, at least if erosion processes were largely gradual. ...
The importance of gradual erosion relative to landsliding depends predominantly on the slope angle. One factor of critical influence in landsliding along with slope angle and slope shape is the soil depth. Understanding soil depth development on steep topography is fundamental for understanding and predicting the occurrence of landsliding at threshold landscapes. We develop a model to predict soil depth .
If erosion is a gradual process, soil depth increases until the soil production rate no longer exceeds the erosion rate, and steady-state is reached. The predicted soil depth (x) is proportional to the ratio of the infiltration to the erosion rate. Identifying a predictive result for erosion as a function of slope angle (S) allows a test of both the erosion and soil production models with field observations. The same theoretical approach to soil production should be applicable when the principal erosion process is shallow landsliding. After landslides, soil recovery initially follows our predicted power-law increase in time, though with increasing time background erosion processes become important. At a time equal to a landslide recurrence interval, the soil depth can exceed the steady-state depth by as much as a factor 2. By comparing predicted and observed x(S) results, we show that the accessed result for erosion as a function of slope angle is accurate. Soils deeper than the depth predicted at the landslide recurrence interval are beyond the stability limit. This result suggests an important practical relevance of the new soil production function.
Keywords: soil production function, landslides, threshold slope, percolation theory
... Catchment-wide denudation rates are not only reflecting soil erosion and accumulation, but may also include mass wasting (Norton et al., 2010). Another problem is that often steady-state conditions are assumed to calculate denudation or soil production rates; an assumption that may lead to unrealistic representations of the dynamics of pedogenesis and weathering profile evolution (Phillips, 2010) if the requirements for a steadystate are not fulfilled. Soil denudation rates are rarely estimated from profile data using in situ 10 Be (e.g. ...
Two principal groups of processes shape mass fluxes from and into a soil: vertical profile development and lateral soil redistribution. Periods having predominantly progressive soil forming processes (soil profile development) alternate with periods having predominantly regressive processes (erosion). As a result, short-term soil redistribution — years to decades — can differ substantially from long-term soil redistribution; i.e. centuries to millennia. However, the quantification of theses processes is difficult and consequently their rates are poorly understood. To assess the competing roles of erosion and deposition we determined short- and long-term soil redistribution rates in a formerly glaciated area of the Uckermark, NE Germany. We compared short-term erosion or accumulation rates using 239+240Pu and long-term rates using both in situ and meteoric cosmogenic 10Be. Three characteristic process domains have been analysed in detail: a flat landscape position having no erosion/deposition, an erosion-dominated mid-slope, and a deposition-dominated lower- slope site. We show that the short-term mass erosion and accumulation rates are about one order of magnitude higher than long-term redistribution rates. Both, in situ and meteoric 10Be provide comparable results. Depth functions, and therefore not only an average value of the topsoil, give the most meaningful rates. The long-term soil redistribution rates were in the range of -2.1 t ha-1yr-1 (erosion) and +0.26 t ha-1yr-1 (accumulation) whereas the short-term erosion rates indicated strong erosion of up to 25 t ha-1yr-1 and accumulation of 7.6 t ha-1yr-1. With such an approach, temporally-changing processes can be disentangled, which allows the identification of both the dimensions of and the increase in soil erosion due to human influence.
... The framework above does not assume that soils or weathering profiles generally approach steady state, and was introduced in an article calling into question the common assumption of steady state thickness (Phillips, 2010). It does provide a crude index of whether steadystate thickness is present or being approached, however. ...
Tree roots have biogeomorphic engineering effects on epikarst weathering and soil deepening. This is investigated using a system model describing the interactions among biogeomorphic effects of roots, weathering, and soil-epikarst development. The model shows that the system is dynamically unstable when roots are limited by subsurface accommodation space and water availability, and weathering is moisture limited. Instability indicates relatively rapid, unstable growth of epikarst cavities and soil, driven by positive feedbacks. However, when belowground rooting space and moisture are no longer limiting, and weathering is reaction-limited, the system is dynamically stable, indicating steady state or slow growth of epikarst and soils. Results suggest an important role for biogeomorphic ecosystem engineering (BEE) by tree roots in soil and epikarst development, but that BEE is self-limiting. When moisture storage and supply for both plants and dissolution are adequate and sufficient root space is available, BEE effects become negligible. Supportive data and field observations from the Inner Bluegrass region of Kentucky indicate that BEE effects of trees can produce favorable conditions for tree growth, with these effects becoming negligible as soil thickness increases sufficiently.
... For example, Šamonil et al. (2016) found 214-225 m 3 ha À1 of soil transported by uprootings on an outwash in Michigan, and Phillips et al. (2017) calculated 263 m 3 ha À1 on flysch in the Czech Republic. Our results support the idea of a crucial role of treethrows in lateral biogeomorphological processes in some forest ecosystems (Gabet et al., 2003;Gallaway et al., 2009;Gabet and Mudd, 2010;Phillips, 2010). The average volume of single treethrow pit-mound pair reached 2.8 m 3 in Boubin, which is slightly above the average of previously published studies volumes of individual treethrow pit-mounds from 0.1 to 5.6 m 3 (see review by Pawlik, 2013). ...
The role of biomechanical effects of trees (BETs) in ecosystem and landscape dynamics is poorly understood. In this study, we aim to (i) describe a widely applicable methodology for quantifying the main BET in soil, and (ii) analyze the actual frequencies, areas and soil volumes associated with these effects in a mountain temperate old-growth forest.
The research took place in the Boubínský Primeval Forest in the Czech Republic; this forest reserve, predominated by Fagus sylvatica L. and Picea abies (L.) Karst., is among the oldest protected areas in Europe. We evaluated the effects of 4000 standing and lying trees in an area of 10.2 ha from the viewpoint of the following features: tree uprooting, root mounding, bioprotection, trunk baumsteins (rock fragments displaced by trunk growth), root baumsteins, stump hole infilling, trunk and root systems displacements, depressions formed after trunk fall, stemwash, and trunkwash.
BETs were recorded in 59% of standing and 51% of lying dead trees (excluding the pervasive soil displacement by thickening trunks and roots and the infilling of decayed stumps). Approximately one tenth of the trees showed simultaneous bioprotective and bioerosion effects. Different tree species and size categories exhibited significantly different biomechanical effects.
A bioprotective function was the most frequent phenomenon observed, while treethrows prevailed from the viewpoint of areas and soil volumes affected. The total area influenced by the BETs was 342 m2ha-1. An additional 774 m2ha-1 were occupied by older treethrow pit-mounds with already decayed uprooted trunks. The total volume of soil associated with the studied phenomena was 322 m3ha-1, and apart from treethrows, volumes of the living and decaying root systems and bioprotective functions predominated. Other processes were not so frequent but still significant for biogeomorphology.
... The erosion will approximate the weathering front to the surface, accelerating the soil production. When the soil production rate is similar to the erosion rate, the system is in equilibrium (Pelletier and Rasmussen, 2009;Phillips, 2010), or in a steady-state soil thickness (∂ h/∂ t = 0). This assumption is convenient to modelling, but might not reflect the current reality. ...
Soil thickness is an important soil characteristic changing over space and time. In this study, we used a me-chanistic soil landscape models to predict soil thickness and show it under development over time. The study was conducted in an 8,118 ha area in Vale dos Vinhedos, Rio Grande do Sul State, Brazil. Different soil production functions (SPF) combined with a landscape evolution model (LEM) were explored. The SPF calculated the soil production rates and LEM calculated erosion and deposition patterns. We evaluated two types of model. Model 1 was used to predict the current soil thickness. The model equals the erosion estimations (by a LEM) to the soil production rate (by a SPF). Three types of SPF were tested, based on a spatial variation of soil moisture. A steady-state condition was assumed, considering soil production rates similar to erosion rates. The model simulated erosion events to 1 year, using a Digital Elevation Model (DEM). A soil survey with observed soil thickness was used to validate the different models. Model 2 used the soil thickness estimation from Model 1 to simulate the soil thickness changes up to 100 kyr, considering the balance between soil production rate and soil eroded or deposited. The soil thickness changes were evaluated in different landscape positions. In Model 1, the linear correlation between observed and predicted soil thickness varied between 0.25 and 0.49, with higher linear correlation in models using soil moisture data. The RMSE under different models varied between 34 cm and 37 cm. Overall, soil depth was more accurately predicted in the upland areas than in the valley bottom areas. Model 2 suggested that the soil thickness variation largely depended on the landscape position. The average soil thickness changed from initial 67 cm (0 Kyr) to 103 cm (100 kyr).
... For instance, the assumption of steady-state soil thickness often employed in soil and landscape evolution models and underlying some dating methods is often violated, and is not an accurate representation of soil and regolith processes and evolution. However, the fact that steady-state thickness is not a truth statement about soils or regolith may have little or no effect on the efficacy and utility of some models and methods based on the assumption (Phillips, 2010b). ...
Metanarratives are critiqued and even rejected by many geographers and geoscientists. Yet, despite the inescapable role of geographical and historical contingency in physical geography, metanarratives are helpful, perhaps even necessary, in part because equifinality is common in Earth surface systems (ESS). Similarity of forms and patterns implies a possible single underlying cause. However, by definition the similar outcomes of equifinality are not the result of the same underlying processes, indicating that any encompassing construct must be in the form of a metanarrative. An effective metanarrative need not be strictly true, but should be useful in explanation, and its implications subject to empirical verification. Metanarratives should also be simplifying rather than complexifying. An example proposed here is the principle of efficiency selection: the most efficient pathways and modes of mass and energy flux are preferentially preserved and enhanced. This explains and unifies optimality principles proposed for a variety of ESS. Efficiency selection is testable based on observations and simplifying in that it encompasses a number of situations with a single concise proposition. According to the principle of efficiency selection, apparent optimality in ESS is neither teleological nor deterministically inevitable, but rather an emergent property.
... Finally, it is important to note that the notion of steady state for weathering and erosion is perhaps a 'convenient fiction' (Phillips, 2010). Steady state is nonetheless often assumed and is probably useful in temperate regions such as the Piedmont rocks discussed throughout this paper (Pavich et al., 1989). ...
Both vertical and lateral flows of rock and water occur within eroding hills.Specifically, when considered over geological timeframes, rock advects vertically upward under hilltops in landscapes experiencing uplift and erosion.Once rock particles reach the land surface, they move laterally and down the hillslope because of erosion.At much shorter timescales, meteoric water moves vertically downward until it reaches the regional water table and then moves laterally as groundwater flow. Water can also flow laterally in the shallow subsurface as interflow in zones of permeability contrast. Interflow can be perched or can occur during periods of a high regional water table.The depths of these deep and shallow water tables in hills fluctuate over time. The fluctuations drive biogeochemical reactions between water, CO2, O2, and minerals and these in turn drive fracturing. The depth intervals of water table fluctuation for interflow and groundwater flow are thus reaction fronts characterized by changes in composition, fracture density, porosity, and permeability.The shallow and deep reaction zones can separate over meters in felsic rocks.The zones act like valves that reorient downward unsaturated water flow into lateral saturated flow. The valves also reorient the upward advection of rock into lateral flow through solubilization. In particular, groundwater removes highlysoluble, and interflow removes moderately soluble minerals.As rock and water moves through the system, hills may evolve toward a condition where the weathering advance rate, W, approaches the erosion rate, E.If W = E, the slopes of the deep and shallow reaction zones and the hillsides must allow removal of the most soluble, moderately soluble, and least soluble minerals respectively. A permeability architecture thus emerges to partition each evolving hill into dissolved and particulate material fluxes as it approaches steady state.
... If the theory of steady-state thickness holds true, this implies that within an area of constant or minimally variable climate and geology, thickness should be closely related to topography, and soil taxonomical units should be formed in homogeneous large patches closely related to major topographical forms. Even now, "Gilbert's humped soil production function" has supporters (see Small et al., 1999;Anderson 2002) as well as critics, and its validity is much discussed (Gabet and Mudd, 2010;Phillips, 2010). Carson and Kirkby (1972) concluded that "Gilbert's steady-state soils" are actually metastable and the model does not operate at the landscape scale. ...
Biota–soil interactions in natural ecosystems are the subject of considerable research. Our hypothesis is that individual trees play a significant role through biomechanical and biochemical disturbances affecting soil formation in temperate forests, resulting in a complex spatial pattern of disturbance regimes and a close relationship between disturbance histories and soil units.
In Žofínský Prales (Czech Republic) – the fourth oldest, continuously protected reserve in Europe and the first site of global research network SIGEO (Smithsonian Institution Global Earth Observatories) in continental Europe – we compared extensive dendrochronological, soil and pit–mound microtopography data both temporally and spatially from an area of anthropogenically unaffected 42 ha collected from 2008–2012. These data sets differ in terms of information complexity and length of memory: tree cores contain complex information about the disturbance history of the past 350 years, footprints of disturbances from the uprooting of a specific tree can persist 1700 years, and soils represent an extensive composite phenotype
that has been developing for at least the entire postglacial period (10 500 years).
On average, 6.18–13.41% of the canopy on individual soil units was disturbed per decade. Even though the "backbone" of key events in the development of the forest ecosystem remained the same (e.g. the 1870s, 1880s and 1980s), the internal structure of disturbance history often differed among soil units; the most exceptional were Gleysols and Histosols, where important feedback from soil to trees was expected. However, the characteristics of treethrow dynamics as well as the frequencies of stronger releases in core series also significantly differed along a gradient of terrestrial soil weathering and leaching (Haplic Cambisols – Dystric Cambisols – Entic Podzols – Albic Podzols). These results suggest the existence of several disturbance regimes within the forest, controlling fine-scale pedodiversity.
Global soil thickness is only about 1 m. Its spatial distribution is nevertheless crucial in many hydrological and ecological processes, and it also determines hillslope stability and channel initiation in geomorphological fields. Due to its significant spatial heterogeneity, it is difficult to obtain the soil thickness distribution on a catchment scale based on existing soil survey databases, geophysical investigations, or empirical models. Therefore, it is urgent to develop a process-based model for soil thickness prediction. In this study, methodologies and theories were comprehensively reviewed, and the applicability of different soil production and soil transport models were evaluated. This study pointed out that the mechanism of soil production by chemical weathering is still unclear and is a theoretical bottleneck restricting the development of soil thickness evolution models. Moreover, the methodology of the model still needs to be further developed, and it is urgent to develop and improve the parameter estimation methods and the adoption of equation forms for describing soil production and soil transport in such models upon applications. From our analysis, we inferred that a hybrid model combining stochastic and process-based models as well as mathematical physically-based methods for determining parameters may help solve many difficulties faced in model applications. Finally, we discussed the possible integration of soil thickness evolution models and soil pedogenesis models based on the theoretical frame of catchment coevolution for predicting soil thickness, texture, layering and organic carbon content variation in the landscape.
Soils and landscapes can show complex, nonlinear evolution, especially under changing climate or land use. Soil-landscape evolution models (SLEMs) are increasingly equipped to simulate the development of soils and landscapes over long timescales under these changing drivers, but provide large data output that can be difficult to interpret and communicate. New tools are required to analyze and visualize large model outputs. In this work, I show how spatial and temporal trends in previously published model results can be analyzed and visualized with evolutionary pathways, which are possible trajectories of the development of soils. Simulated differences in rainfall and land use control progressive or regressive soil development and convergence or divergence of the soil pattern. These changes are illustrated with real-world examples of soil development and soil complexity. The use of evolutionary pathways for analyzing the results of SLEMs is not limited to the examples in this paper, but they can be used on a wide variety of soil properties, soil pattern statistics and models. With that, evolutionary pathways provide a promising tool to analyze and visualize soil model output, not only for studying past changes in soils, but also for evaluating future spatial and temporal effects of soil management practices in the context of sustainability.
Hilly upland landscapes are cloaked in a thin layer of soil derived primarily from the underlying parent material. This soil, home to much of Earth's terrestrial biota, is physically transported by slope-dependent processes driven by bioturbation, freeze-thaw and wet-dry cycles, as well as chemical weathering. Understanding how Earth's surface evolves under changing climatic, tectonic, or anthropogenic forces thus includes understanding how these physically mobile upland soils are produced and transported. Creep subsumes transport processes thought to be linearly proportional to slope and significant effort can go into quantifying it. This chapter provides a short overview of the conceptual framework for models of soil creep, as well as results from some studies that have characterized external controls on creep rates across a range of landscapes. Soil in this context is also termed regolith by some workers and is produced and transported by very different processes than those active on agricultural or lowland landscapes. While creep sensu stricto may occur across a wide variety of landforms and sediment types, we focus here on creep as a process that transports soils on hilly landscapes. More For the studies covered we summarize the important findings, and we conclude by emphasizing that despite extensive measurement difficulties in quantifying creep, it is an important hillslope process requiring continued examination.
Weathering plays an important role in the evolution of hillslopes. It decreases strength of a rock mass and hence contributes to slope failures by fall, slide, or topple. Accelerated weathering of soft rock under hard caprock disturbs slope equilibrium and results in long-term escarpment retreat. In deeply weathered terrains, patterns of mass movements are directly related to the weathering grade. Minor mid-slope landforms such as boulders, tors, and flared slopes are in most instances products of differential weathering and removal of weathering products. The significance of weathering is conceptualized in the traditional distinction between weathering-limited and transport-limited slopes.
Landscapes and soils are subjected to changing environmental conditions, resulting in a non-linear evolution over time. Capturing the variations of surface denudation and soil erosion over geological timescales remains challenging due to the lack of suitable archives. Common denudation studies using cosmogenic nuclides and catchment-wide approaches provide only average denudation or erosion rates. However, the natural denudation variations over time are mostly neglected. In addition, soil formation can also change in dependency of soil production and erosion rates or aeolian additions-a fact that has not had much data collected over millennia due to the limitations of present-day investigation techniques. The fragmented and incomplete knowledge of these temporal fluctuations hinders an understanding of how landscapes and soils respond to environmental changes. This study tackled this knowledge gap using modern surface dating techniques (surface exposure dating-10 Be) within the well-defined conditions of the granite upland plateau of the Sila Massif (Calabria, Italy). In pursuit of unravelling soil erosion history, specific rock formations, so called "tors", were investigated as potential soil erosion and landscape denudation archives. Tors are large residual rocks having a height of some meters that are still attached to the underlying bedrock. It was hypothesised that the by surface exposure dating quantified exhumation speed of tors reflects the surrounding surface lowering and associated soil erosion. The exhumation theory was not only tested on tors but also on detached bedrock (boulders) and exposed rock-outcrops along steep slopes (scarps). In addition, alternative soil age estimation techniques were explored and allochthonous soil material (volcanic glass) was geochemically traced to its original location and time of input for additional correlations with local soil formation ages. The multi-method approach for soil-age estimations revealed that local radiocarbon ages provide information about minimum soil ages of about 14,000 years. Semi-quantitative dating with chemical weathering indices resulted in a soil age span from 16-67 ka, averaging at 45 ka. Geochemical fingerprinting of volcanic glass allowed a stratigraphic correlation of volcanic eruption sequences of distant volcanos. Using chemical mass balances and laser ablation on thin sections showed that large proportions of the volcanic deposits derived from Lipari (Aeolian Islands) and a minor part from Etna (Sicily). This correlation framed certain soil horizons to be older than 30,000 years but younger than 92,000-81,000 years. The postulated exhumation approach deciphered for the first time the temporal evolution of surface denudation and soil erosion over Holocene to Pleistocene time. Among the three investigated landforms tors provided the most consistent denudation results and covered the longest exposure time-span (100,000 years). Hence, the exhumation history of granite tors in the upland plateau vastly enabled the determination of continuous soil erosion rates over geological timescales. Linking of the deciphered soil erosion and surface denudation variation II with paleoenvironmental data resulted in a local surface evolution model and an event chronology. The role of excepted soil-forming factors (e.g. topography, vegetation, climate) was thereby captured for the first time over a continuous geological timescale up to the Pleistocene. Slope angles and vegetation density appear to regulate soil erosion intensity, while climate transitions can initiate relatively abrupt increases. In detail, extremely low rates of mostly below 100 t km −2 year −1 (about 0.12 mm year-1) were detected prior to 21,000 years. During these conditions soil formation seemed to have occurred, being also stimulated by the volcanic ash input. During the transition from the Pleistocene to the Holocene (cool-dry to warm-humid), soil erosion rates increased up to a maximum of about 0.31 mm year-1 at slopes and 0.16 mm year-1 at planar surfaces. Since this peak phase at about 17,000-5,000 years, soil erosion appears to have decreased back to ~0.12 mm year-1. However, modern soil redistribution rates obtained from fallout radionuclide investigations (239+240 Pu) suggest soil erosion rates of the last five decades to be ≥ 1,000 t km −2 year −1 (about 1.22 mm year −1). These rates are far above the tor-derived surface denudation rates and the natural rates of soil production. The continuation of the local anthropogenic influence of the area will definitely lead to a distinct decrease in soil depth and with time to a rockier surface. In conclusion, the exhumation approach tested on tors detected soil erosion and surface denudation variation over an unprecedented timescale and enabled a linkage to influential environmental factors. Tors proved to be a very suitable archive that retain information about surface evolution and its fluctuation over a geological timescale. Further explorations of tors as archive can help to decrease the knowledge gap of surface denudation and soil erosion variations of the past from regional to potentially global scales.
Granitic weathering profiles display highly diverse morphologies, reflecting the complex relationships between climate and weathering rates. Some profiles exhibit abrupt transitions from fresh bedrock to highly weathered material over short (<1 m) distances, while others exhibit only limited weathering extending 10s of meters into the bedrock. Although granitic weathering processes have been well studied, the controls on profile morphology and weathering rates within granitic, and many other lithologies remain poorly understood; these are likely influenced by a range of both intrinsic and extrinsic factors, which in turn will have crucial implications for understanding, for example, climate-weathering feedbacks. In this study we present multi-scale elemental and mineralogical analyses of a >30 m granitic weathering profile from the cool, temperate, Lysina catchment in the NW Czech Republic, from which we calculated mass transfer, weathering indices, and mineral specific weathering rates. The Lysina profile exhibits limited weathering extending >30 m into fractured bedrock, dominated by albite weathering at a rate of 9.3 × 10⁻¹⁷ mol m² s⁻¹.
To identify environmental and geological controls on weathering front morphology and chemical weathering rates, Lysina was compared to previously published granitic weathering profiles from around the world. Weathering front morphology and weathering rates were calculated for the additional sites from published data and were correlated to mean annual precipitation (MAP), mean annual temperature (MAT), and erosion rates, with MAP having the strongest relationship. Higher MAP likely promotes lower saturation indices in pore waters, allowing weathering reactions to occur further from equilibrium. Comparison of erosion rates amongst the granitic catchments revealed an inconsistent effect on chemical weathering rates, but high erosion rates may promote weathering by reducing the thickness of the regolith and exposing the bedrock to reactive fluids. Mean annual temperatures appear to only have significant impacts on weathering fronts in environments with high precipitation and high erosion rates. Fractured bedrock profiles (Lysina and Río Icacos) have higher weathering intensities, than the other sites studied here. High connected porosity in fractured rocks enhances water movement allowing more efficient removal of weathering products, thus reducing thermodynamic saturation, increasing weathering rates, and producing sharper weathering gradients. These findings indicate that CO2 drawdown on geological timescales is also likely to be governed by precipitation rates, as well as temperature, and that much of the climate-significant weathering may occur within very narrow zones of the Earth's surface.
Arable soils are critical resources that support multiple ecosystem services. They are frequently threatened, however, by accelerated erosion. Subsequently, policy to ensure their long-term security is an urgent societal priority. Although their long-term security relies upon a balance between the rates of soil loss and formation, there have been few investigations of the formation rates of soils supporting arable agriculture. This paper addresses this knowledge gap by presenting the first isotopically constrained soil formation rates for an arable (Nottinghamshire, UK) and coniferous woodland hillslope (Shropshire, UK). Rates ranged from 0.026 to 0.096 mm yr⁻¹ across the two sites. These rates fall within the range of previously published rates for soils in temperate climates and on sandstone lithologies but significantly differed from those measured in the only other UK-based study. We suggest this is due to the parent material at our sites being more susceptible to weathering. Furthermore, soil formation rates were found to be greatest for aeolian-derived sandstone when compared with fluvially derived lithology raising questions about the extent to which the petrographic composition of the parent material governs rates of soil formation. On the hillslope currently supporting arable agriculture, we utilized cosmogenically derived rates of soil formation and erosion in a first-order lifespan model and found, in a worst-case scenario, that the backslope A horizon could be eroded in 138 years with bedrock exposure occurring in 212 years under the current management regime. These findings represent the first quantitative estimate of cultivated soil lifespans in the UK.
Recent modeling and comparison with field results showed that soil formation by chemical weathering, from bedrock or unconsolidated material, is limited largely by solute transport. Chemical weathering rates are proportional to solute velocities. Nonreactive solute transport described by non-Gaussian transport theory appears compatible with soil formation rates. This change in understanding opens new possibilities for predicting soil production and depth across orders of magnitude of time scales. Percolation theory for modeling the evolution of soil depth and production was applied to new and published data for alpine and Mediterranean soils. The first goal was to check whether the empirical data conform to the theory. Secondly we analyzed discrepancies between theory and observation to find out if the theory is incomplete, if modifications of existing experimental procedures are needed and what parameters might be estimated improperly. Not all input parameters required for current theoretical formulations (particle size, erosion and infiltration rates) are collected routinely in the field; thus, theory must address how to find these quantities from existing climate and soil data repositories, which implicitly introduces some uncertainties. Existing results for soil texture, typically reported at relevant field sites, had to be transformed to results for a median particle size, d50, a specific theoretical input parameter. The modeling tracked reasonably well the evolution of the alpine and Mediterranean soils. For the Alpine sites we found, however, that we consistently overestimated soil depths by approximately 45%. Particularly during early soil formation, chemical weathering is more severely limited by reaction kinetics than by solute transport. The kinetic limitation of mineral weathering can affect the system until 1kyr to a maximum of 10kyr of soil evolution. Thereafter, solute transport seems dominant. The trend and scatter of soil depth evolution is well captured, particularly for Mediterranean soils. We assume that some neglected processes, such as bioturbation, tree throw, and land use change contributed to local reorganization of the soil and thus to some differences to the model. Nonetheless, the model is able to generate soil depth and confirms decreasing production rates with age. A steady state for soils is not reached before about 100 kyr.
Obtaining accurate soil depth information is critical to improving how we assess the health and manage of soil resources that contribute to sustainable management of agricultural lands. While there are many techniques to assess soil characteristics, using ground penetrating radar (GPR) to determine soil depth has received little attention. This study aimed to determine the suitability of GPR for obtaining accurate soil depth information over a 10 km intervals grid system in a generally flat grasslands ecosystem in the Sichuan Province of China. Geographic information system (GIS) and geostatistical techniques were used to map the spatial distribution of soil depth across the field site. Images created from GPR were filtered using DC removal and automatic gain control, and log‐transformation was used to transform the raw data in order to conform a normal distribution. The soil depth data were spatially interpolated across the field site using the geostatistical techniques of (semi‐) variogram and ordinary kriging (OK), then ground‐truthed and validated via comparison with traditional methods and previously collected data. A total of 39 random data points (ruler‐measured and GPR data) were selected to evaluate the accuracy of the GPR, and results showed that the difference were within 3 cm of the actual soil depth in 93% of all samples, and within 5 cm in all samples (R2 = 0.914). Results confirmed that this GPR reflection technique has the potential to precisely and quickly measure soil depth over large areas and under variable topography, contributing to the body of technical information that can help inform soil management policy for sustainable agriculture. The spatial distribution map of soil depth produced with the aid of OK demonstrated the accuracy and non‐destructive features of GPR, which is able to provide a more detailed map of soil depth than methods used in previous grassland soil depth studies.
The Deccan Trap region exhibits an erosional landscape over a relatively ancient and stable Deccan shield. The Quaternary history of the area has been reconstructed on the basis of evidence from alluvial deposits occurring along the major rivers. However, recent investigations have revealed that evidence for geo-environmental change during the Quaternary Period is also contained in the colluvial deposits that occur in the foothill zones. The colluvial deposits, ranging in thickness from 1 to 10 m, invariably occupy gently inclined pediment slopes. The sediments are presently deeply dissected by gullies, and the process of colluviation has almost ceased. These deposits are best preserved in the semi-arid parts of the region. Detailed textural, geochemical and stratigraphical studies at four different sites reveal similar input processes, the slight variations being attributed to local environmental factors. Scanning electron microscopy studies of some grains indicate marginal contribution of aeolian processes al the time of deposition. Mesolithic artefacts and a few U/Th dates indicate that the colluviation took place during the Late Quaternary. The properties of the deposits suggest relatively high energy conditions as well as a remarkable variability in the intensity of hillslope processes. These properties are indicative of semi-arid conditions during which the regolith was stripped from devegatated hillslopes and was deposited on the pediments. A variety of evidence indicates that the period of colluviation coincided with arid conditions during the Last Glacial Maximum. The geomorphological and archaeological evidence also indicates that incision by gully systems was initiated during the early Holocene humid phase. The environmental conditions deduced for the study area are similar to those reported for other parts of the intertropical zone. (C) 1997 by John Wiley & Sons, Ltd.
A high degree of soil variability over short distances and small areas is common, particularly in forest soils. This variability is sometimes, but not always, related to readily apparent variations in the environmental factors that control soil formation. This study examines the potential role of biomechanical effects of trees and of lithological variations within the parent material in explaining soil diversity in the Ouachita Mountains of Arkansas. The diversity of soils on Ouachita sideslopes is high, and the soil series vary primarily with respect to morphological properties such as soil thickness and rock fragment content. Soils vary considerably within small more-or-less homogeneous areas, and richness-area analysis shows that the overall pattern of pedodiversity is dominated by local, intrinsic (within-plot) variability as opposed to between-plot variability. This is consistent with variation controlled mainly by individual trees and local lithological variations. Given the criteria used to distinguish among soil types, biomechanical as opposed to chemical and hydrological effects of trees are indicated. Results also suggest divergent evolution whereby the pedologic effects of trees are large and long-lived relative to the magnitude of the initial effects and lifespan of the plants.
Considers some misconceptions related to weathering processes, catenas, vertical or lateral movements, landscape lowering, equilibrium, and the relevance of past and present climates, and stresses the need for care in both pure and applied regolith research.
Instability - the failure of particular landforms or process-response relationships to persist in the face of environmental change - has been widely observed in geomorphic phenomena. This is often manifested as a symptotic instability, which implies exponential divergence of system states away from an equilibrium following a change or perturbation. Asymptotic instability may indicate chaos - complexity which can arise from the nonlinear dynamics of completely deterministic phenomena. A mass balance model of hillslope evolution, the explicit basis for Ahnert's slope evolution model and the conceptual basis for all comprehensive slope evolution models, is examined with regard to its stability and the possibility of chaotic behavior. The mass balance is unstable. The analysis suggests that chaotic behavior is possible in the evolution of slope regolith cover. Chaos, which occurs at high rates of debris removal relative to weathering rates, implies that spatial and temporal complexity in landscape evolution can occur independently of environmental heterogeneity and stochastic forcings. -from Author
Several supergene ferromanganese wad deposits, mined on small scale for industrial applications, are developed on dolomite of the Neoarchaean Malmani Subgroup of the Transvaal Supergroup in South Africa. At the West Wits Gold Mine on the plateau of the Witwatersrand escarpment near Johannesburg, the ferromanganese wad is developed in manganiferous dolomite of the Oaktree Formation at the base of the Malmani Subgroup. The wad represents an ancient saprolite developed in the dolomite below a major unconformity with incised valleys filled by ferruginous silty mudstone and gravels containing abundant reworked ferromanganese soil nodules. The saprolite is up to 80 m thick. The incised valley deposits are cut by a second erosion surface below which another palaeosol is developed with characteristics of a ferric podzol. Large root marks extend from the top of this palaeosol through the ferruginous silty mudstone channel-fill deposits, into the underlying ferromanganese wad, up to depths of several tens of metres. The root marks are filled with yellow Kalahari sand. The second erosion surface is flat and draped by a poorly sorted pediment stone lag. Stones on the pediment were derived from the underlying channel-fill succession. The pediment is in turn overlain by a dark yellowish brown sandy transported soil (the Hutton soil). Finally the Hutton soil is incised by modern stream erosion and overlain by a thin modern soil covered by grass. The ferromanganese wad in the saprolite at the base of the incised valley succession is mainly composed of amorphous manganese- and iron-oxyhydroxides containing some crystalline birnessite, lithiophorite, and haematite, as well as subordinate quartz, muscovite, and kaolinite. Original sedimentary bedding in the dolomite is partly preserved in the saprolite, although mass balance calculations suggest that the saprolite has undergone 60 to 70% compaction in the transformation of dolomite to wad. Carbonaceous chloritic shale beds interbedded with the Oaktree dolomite are altered to kaolinitic clays in the saprolite. Ferromanganese soil nodules present in the incised valley-fill are composed of a nucleus of older abraded or broken nodules concentrically overgrown by several coatings of cryptomelane ± goethite. The saprolite and overlying erosion surfaces, sediments, and soil profiles provide new information on the Post-Gondwana tectonic and climatic history of the Witwatersrand plateau. This plateau forms the watershed between rivers draining into the Indian and Atlantic Oceans from the Highveld of South Africa. The ferromanganese wad apparently formed after the break-up of Gondwanaland, perhaps during the late Cretaceous to early Tertiary African cycle of erosion. Conditions must have been humid and warm to have allowed deep chemical weathering with wad formation, kaolinitization, and lateritization. The incised valley succession is correlated with the post-African I cycle of erosion, which took place between 30 and 2.5 Ma. During this period the climate became more arid with development of savannah-type vegetation and trees with very deep tap roots. The arid conditions climaxed with the formation of the pediment stone lag on top of the incised valley-fill succession and influx of Kalahari sand in the middle Pliocene. It is thought that the pediment developed at the start of the post-African II cycle of erosion in the Pliocene. After that the climate apparently became more humid, resulting in reworking of Kalahari sediment into the Hutton soil. This soil is at present being incised by stream erosion and covered by grassland below which a thin modern humic soil is developed.
Soil catenas crossing various archaeological objects (ramparts, ancient settlements, burial mounds, etc.) with an age of 360-1800 years have been studied in the sub-zones of dark chestnut soils and typical, ordinary, and southern Chernozems in the East European Plain. It is found that topography-induced variations in the thickness of zonal soil types are not always distinct. Often, the thickness of soil profiles does not depend on the slope gradient. In this context, it is supposed that the rate of the formation of humus horizons on slopes exceeds that on leveled surfaces. A comparative analysis of the thickness of humus horizons along soil catenas offers excellent possibilities for the determination of the rates of erosion and pedogenesis. The methodology of quantitative assessment of the potential rate of soil formation on slopes is suggested.
The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10Be and 26Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10Be and 26Al on the surfaces of emergent tors.
We present model results suggesting that a physical erosion–bedrock weathering feedback is responsible for the development of isolated bedrock knobs (tors/inselbergs) that often punctuate otherwise smooth pediments of homogeneous basement lithology. Tors and larger, more heavily jointed and morphologically complex exposures, inselbergs, may arise as a consequence of fluctuations in rainfall and sediment transport conditions combined with a bedrock weathering mechanism that depends on regolith thickness. Hydrogeochemical considerations and field observations in arid, granitic environments suggest that the relationship between weathering rate and regolith thickness exhibits a maximum for a finite thickness of cover. We have encapsulated this simple erosion-weathering feedback in a numerical model simulating arid/semiarid landscape evolution that produces low-sloping pediments punctuated by tors. Tors form during periods of higher effective moisture, resulting in local base level incision and regolith thinning on pediments, invoking a transition in which mantled surfaces lower at rates exceeding the bare bedrock weathering rate. This condition favors the emergence and growth of tors in areas covered by regolith thickness less than a threshold value. Subsequent shifts in climate or local base level that restore sediment surface lowering rates less than the bare bedrock weathering rate will lead to a progressive decrease in tor height and, ultimately, their disappearance. Thus, according to this model, tors in arid environments represent possibly transient features related to fluctuations in climate or local transport conditions rather than palimpsests of an ancient landscape derived from differential subsurface weathering followed by regolith stripping.
The identification and extent of etch surfaces depends primarily on the preservation of regolithic remnants and the coincidence in level of weathering front and surface. The occurrence of bornhardts, corestone boulders, flared slopes, basins, gutters and pitting on covered weathering fronts demonstrates that they can be initiated in the subsurface, and where they are preserved on exposed surfaces, particularly in assemblages of such features, they constitute sound evidence of an etch origin. In some instances the location of the front and hence of the related etch surface is determined by subtle variations in degree of weathering. There is some suggestion that the front advances episodically rather than continuously.
Uplift of the Himalayas has been proposed to have locally accelerated chemical weathering, thus leading to enhanced CO2 sequestration and global cooling. This hypothesis assumes that rapid erosion exposes fresh, highly reactive minerals at Earth's surface. Empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results, where the nature of the relationship is dependent on the weathering regime of the sampled landscapes. We derive a quantitative model that defines this relationship across the range of weathering regimes, from supply-limited to kinetically limited conditions. The model matches trends in field data collected by others and reconciles apparently conflicting results. The model also demonstrates that, as erosion rates increase, potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith. We conclude that the relationship between weathering and erosion is one of diminishing returns, in which increases in erosion rate lead to progressively smaller increases in weathering rate; indeed, at the highest erosion rates, weathering rates may decline. The ability, therefore, of accelerated uplift and erosion to stimulate greater CO2 sequestration may be significant in landscapes eroding at rates of 10(0)-10(2) t km(-2) yr(-1). However, where erosion rates are greater than 10(2) t km(-2) yr(-1), increases in denudation may not be matched by increases in chemical weathering. Finally, our results suggest that watersheds with regolith thicknesses of similar to 0.5 m will yield the greatest solute fluxes.
Landscape evolution is modeled widely using a simple creep law for complex processes of sediment transport. Here, field data show how a new transport model, combined with an exponential soil production law, better captures spatial variations of soil thickness on hillslopes. We combine parameterizations of simple and depth-dependent creep with overland flow to predict soil thickness and suggest how soil distribution evolves in response to climatic and tectonic forcing. We present an empirical expression for the response time of the system to external forcing that shows strong dependence on relief and is independent of soil production rate. We suggest that this parameterization may be used to quantify upland carbon storage and removal and predict impacts of deforestation or rapid climatic changes.
Interpretations of regolith and soil thickness in the context of landscape evolution are typically based on the notion that thickness is controlled by the interaction of weathering rates and erosion and tuned to topography. On sideslopes of the Ouachita Mountains, Arkansas, however, there is a high degree of local spatial variability that is largely unrelated to topography. This indicates nonequilibrium in the sense that there is no evidence of a balance between rates of weathering and removal, as is postulated in some conceptual models in geomorphology and pedology. Johnson's soil thickness model is applied as an alternative to interpret local variations in regolith thickness. At the study sites, regolith thickness is not generally related to slope, curvature, elevation, or pedogenic development in the solum. This indicates that variability in thickness is related chiefly to processes and controls acting in the lower regolith, below the solum. The primary controls of variability are local lithological variation, variable structural resistance associated with fractures and bedding planes in strongly tilted Paleozoic sedimentary parent material, and point-centered pedological influences of trees. A steady state regolith may be relatively rare. Results of this study suggest that an equilibrium regolith thickness is most likely in uniform lithology with a high degree of lithologic purity, less likely in interbedded sedimentary rocks, and more unlikely still if the latter are titled and fractured. Equilibrium thickness would also be more likely where the effects of bioturbation are more areally uniform (as opposed to the point-centered effects of individual trees) and where the biomantle is above the weathering front.
https://deepblue.lib.umich.edu/bitstream/2027.42/155803/1/Taylor_et_al_1997_Relation_between_soil.pdf
Data from soil and weathering profiles developed on constructional landforms in the Coastal Plain province of the Eastern United States indicate that there are positive correlations between specific physical and chemical properties of altered parent material and ages of associated landforms. Diagnostic properties of profiles developed on essentially unmodified constructional surfaces include depth of oxidation, percentage of labile minerals, thickness of solum, thickness of argillic horizon, clay mass, clay mineralogy, rubification, iron chemistry, and bulk chemistry. The depth of oxidation, the thickness of the solum, and the total thickness of the argillic horizon increase with the age of the landform. The percentage of labile minerals decreases with age and increases with depth from the surface. There is also an inverse relation between soil (surface) age and pedon variability, the younger surfaces showing more variability in pedon characteristics than the older surfaces do. -from Authors
This chapter aims to provide a review of the stratigraphic concepts of paleopedology. To achieve this both Quaternary and pre-Quaternary examples are used, and are integrated to provide a framework for use in understanding paleosol sequences, especially in the thick alluvial sequences so common in the geological record. In addition soil formation associated with major periods of landscape evolution (such as at major unconformities) will also be briefly discussed. -from Author
The intermontane basin of Jelenia Góra in the West Sudetes is renowned for its numerous noble residences and landscape parks, established in the 19th century. Collectively, the parks form a unique cultural heritage, which is now being promoted as one of the most valuable assets of the region and a target for tourists. One of the reasons why parklands may have been set up with such a success is local geomorphology. Deep selective weathering of granite followed by stripping of the regolith has revealed a multi-concave (multi-basin) granite topography, with isolated conical and domed hills, enclosed elementary basins, tors, clefts, scattered boulders, overhangs and caves, and minor surface features such as weathering pits and flutes. They have been skilfully incorporated into the planned landscapes around the residences, adding to their scenic appeal. In the past few decades the granite parklands of the Jelenia Góra Basin were largely neglected and suffered deterioration, but since the early 1990s projects to restore the parklands to their original glory are under way. This provides an excellent opportunity to promote geomorphology alongside the more recognized cultural heritage.
Inselbergs in the Sudetes are landforms which have been evolving through continuous adjustment to and exploitation of lithological and structural contrasts within bedrock. Most inselbergs are associated with granite massifs and only a minor proportion has developed upon other lithologies, whilst many of the lithologies in the Sudetes have not given rise to inselbergs at all. Inselbergs testify to prolonged landsurface lowering, accomplished in alternating sequences of deep weathering (etching) and stripping. Although many of them are inherited features of pre-Miocene age, some have been evolving through the Neogene up to Recent times. There are no clear indicators of climatic control upon the development of inselbergs.
If soil depth is defined by the depth of organism activity, the generalized concept of useful soil depth is much too shallow. While climate and geologic features combine to limit the extent of biologic activity in some soils, this review indicates many instances where such activity continues to great depths. Arbitrarily selecting a 1.5m lower limit to the solum, the authors review reports of plant root, root symbiont, and vertebrate and invertebrate activity below this depth. -from Authors
Soil profile thickness of mineral soils reflects the relative amounts of deepening, upbuilding, and removals that have occurred during the evolution of the soil. Deepening is the downward migration of the lower soil boundary into fresh unweathered material. Upbuilding refers mainly to surface sedimentation and subsequent pedogeneticization of allochthonous deposits, but also includes autochthonous organic matter production in mineral soils. Removals include material losses via erosion, mass wasting, pervection and leaching. The relationships of these processes are examined in the context of three pathways of soil evolution progressive, regressive, and static pedogenesis. Using these concepts, soil thickness processes are traced in a hypothetical model of a dynamically evolving surface soil. Three examples of actual landscapes, two from California and one from the Sahara, are given and linked to the hypothetical model to augment a conceptual framework for viewing soil thickness processes as aspects of pedogenesis. -from Author
Place-to-place variation within soil units influences the study and use of soil, but is seldom formally acknowledged in soil maps and descriptions. Research on soil variation could be improved by increased coordination of experimental designs and by application of statistical methods suitable for analysis of place-to-place variation, including statistical functions such as spatial autocorrelation. Eventually it may be possible to estimate spatial variation in unstudied soils by means of easily observable landscape characteristics used as surrogates for soil variability. -Author
Evidence is presented demonstrating that in areas of varied tectonic settings - shield, orogenic, platform - relief amplitude has increased over periods of 60-100 My. Slope steepening is characteristic of many of the areas discussed. Of the recognised models of landscape evolution Crickmay's model involving Unequal Activity is closest to that described here. But whereas Crickmay attributed areal inequalities in erosion to disparities between the erosive capacities of major streams and their tributaries draining divides, the present model is based largely in structure, and its control of shallow groundwaters, weathering and erosion, and in consequent reinforcement effects. This model involving increasing relief amplitude readily accommodates the very old palaeosurface remnants reported from various parts of the world. -Author
Reports tests of alternative hypotheses in the Wilsons Promontory, an area of Devonian granite. The granite had been unroofed by the Permian, and a palaeoplain existed in Trias-Jura times, below which the granite continued to be deeply weathered to the Lower Cretaceous. The resultant profile was at least 300 m thick, and consisted of regular zones. The area was faulted and uplifted in mid-Cretaceous times, since when it has experienced erosional stripping. This supports hypotheses suggesting that landscape evolution in areas of deep weathering can occur with a single period of deep weathering followed by a distinct stage of regolith stripping. -from Authors
Landscape evolution of Australia is on the same time scale as global tectonics
and biological evolution. In places, actual landforms and deep weathering
products are hundreds of millions of years old. Much of Australia has a
landscape resulting from stripping of weathered rock after an earlier period
of very deep weathering. Other regions have sequential landforms that provide
a natural laboratory where we can work out the biogeochemistry of the past.
Landforms and regolith reveal the long evolution of groundwater in Australia.
Lateral movement of groundwater is of paramount importance. The effects of
past climates are stored in the landscape. They show that the present is not
the key to the past, and former environments must be worked out from
consistent internal evidence rather than the application of models based on
present-day conditions. Inorganic chemistry alone is inadequate to explain
many earth materials, and biology, especially microbiology, has a very
significant role. Recent and present-day processes also affect the landscape,
and it cannot be assumed that because the landscape and regolith are old the
soils are old. Many regions have a complex regolith cover that shows modern
processes working on inherited materials.
pdf
, The following values have no corresponding Zotero field:
ID - 268
pdf
, The following values have no corresponding Zotero field:
ID - 653
A model for the evolution of weathered landsurfaces in Uganda is developed using available geotectonic, climatic, sedimentological and chronological data. The model demonstrates the pivotal role of tectonic uplift in inducing cycles of stripping, and tectonic quiescence for cycles of deep weathering. It is able to account for the development of key landforms, such as inselbergs and duricrust-capped plateaux, which previous hypotheses of landscape evolution that are based on climatic or eustatic controls are unable to explain. Development of the Ugandan landscape is traced back to the Permian. Following late Palaeozoic glaciation, a trend towards warmer and more humid climates through the Mesozoic enabled deep weathering of the Jurassic/mid-Cretaceous surface in Uganda during a period of prolonged tectonic quiescence. Uplift associated with the opening South Atlantic Ocean terminated this cycle and instigated a cycle of stripping between the mid-Cretaceous and early Miocene. Deep weathering on the succeeding Miocene to recent (African) surface has occurred from Miocene to present but has been interrupted in the areas adjacent to the western rift where development of a new drainage base level has prompted cycles of stripping in the Miocene and Pleistocene.
The production of regolith is a fundamental geomorphic process because most surface processes transport only unconsolidated material. We use concentrations of the cosmogenic radionuclides (CRNs) and in regolith and bedrock to deduce the rate of production of regolith on an alpine hillslope in the Wind River Range, WY. These calculations are based on a theoretical model which we develop here. This model shows that it is important to consider dissolution of regolith in regolith production and in basin-averaged erosion rate studies. Rates of production of regolith are uniform along the hillslope and the mean rates for the entire hillslope deduced from and are 14.3±4.0 and 13.0±4.0 m Ma−1, respectively. Rates of production of regolith deduced from concentrations in regolith-mantled bedrock support the rates deduced from regolith concentrations. In the alpine environment examined here, the rate of production of regolith beneath ∼90 cm of regolith is nearly twice as fast as the average rate of production of regolith on bare rock surfaces, which Small et al. [Small, E.E., Anderson, R.S., Repka, J.L., Finkel, R., 1997. Erosion rates of alpine bedrock summit surfaces deduced from in situ and . Earth and Planetary Science Letters 150, 413–425] previously documented. Rock-mantled with regolith probably weathers more rapidly than bare rock because the water required for frost weathering is limited on bare rock surfaces. Because the hillslope examined here is convex with constant curvature and regolith production and thickness are uniform down the slope, the regolith volume flux must be proportional to the local slope of the hillside. Therefore, our results are consistent with Gilbert's [Gilbert, G.K., 1909. The convexity of hilltops. Journal of Geology 17, 344–350] steady state hillslope hypothesis. If tor height and the difference between rates of weathering on bare and regolith-mantled rock provide a fair estimate of the age of summit flats, steady-state hillslope conditions have been attained in less than several million years.
Subsurface solutional pathways make limestone terrains sensitive to changes in soil properties that regulate flows to the epikarst. This study examines biogeomorphic factors responsible for changed water movements and erosion in fluviokarst slopes deforested 200 years ago along the Kentucky River, Kentucky. In this project, infiltration and water content data from forest and fescue grass soil profiles were analyzed within a detailed overview of system factors regulating hillslope hydrology. Results show that grass has growth and rooting characteristics that tend to create a larger volume of lateral water movement in upper soil layers than occurs under forests. This sets up the current emergent pattern of erosion in which water perches at grass slope bases and overwhelms pre-existing epikarst drainage. Tree roots are able to cause solution at multiple discrete points of entry into fractures and bedding planes, increasing storage capacity and releasing sediment over time. Grass roots do not enter bedrock, and their rooting depth limits diffuse vertical preferential flow in root channels to above one meter. In the areas dense clay soils, flow under grass is conducted sideways either through the regolith or at the bedrock surface. Rapid flow along rock faces in hillslope benches likely moves fines via subsurface routes from the hillslope shoulders, causing the exposure of flat outcrops under grass. Lower growing season evapotranspiration also promotes higher grass summer flow volumes. Gullying occurs at sensitive points where cutters pass from the uphill grassed area into the forest, or where flow across the bedrock surface crosses grass/forest boundaries oriented vertical to the slope. At these locations, loss of the protective grass root mat, coupled with instigation of tree root preferential flow in saturated soils, causes soil pipes to develop. Fluviokarst land management decisions should be based on site-specific slope, soil depth, and epkarst drainage conditions, since zones sensitive to erosion are formed by spatial and temporal conjunctions of a large number of lithologic, karst, soil, climate, and vegetation factors. This study shows that it is the composite of differing influences created by forest and grass that make forests critical for soil retention in high-energy limestone terrains.