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Loess transportation surfaces in west-central Wisconsin, USA
- Randall J. Schaetzl
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- Quaternary Research , First View
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- 27 December 2023, pp. 1-17
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The concept of a loess transportation surface portends that saltating sands deflate silt/dust and send them into suspension. This process continues until a topographic barrier stops the saltating sand, allowing loess deposits to accumulate downwind. This paper reports on loess transportation surfaces in west-central Wisconsin, USA. During the postglacial period, cold, dry conditions coincided with strong northwesterly winds to initiate widespread saltation of freely available sands, deflating any preexisting loess deposits. Large parts of the study area are transportation surfaces, and lack loess. Loess deposits were only able to accumulate at “protected” sites—downwind from (east of) topographic barriers, such as isolated bedrock uplands and the north-to-south flowing Black River. Loess in locations from these barriers is thicker (sometimes >5 m) than would be expected, and in places has even accumulated above preexisting loess deposits. For example, downwind (east) of the Black River, most of the low-relief landscape is covered with ≈40–70 cm of silty loess, even though it is many tens of kilometers from the initial loess source. Upwind of the river, on the transportation surface, the low-relief landscape is only intermittently mantled with thin, scattered deposits of silty-sandy eolian sediment, and generally lacks loess.
A sediment-mixing process model of till genesis, using texture and clay mineralogy data from Saginaw lobe (Michigan, USA) tills
- Randall J. Schaetzl, Christopher Baish, Patrick M. Colgan, Jarrod Knauff, Thomas Bilintoh, Dan Wanyama, Michelle Church, Kevin McKeehan, Albert Fulton, Alan F. Arbogast
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- Quaternary Research / Volume 94 / March 2020
- Published online by Cambridge University Press:
- 21 February 2020, pp. 174-194
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We present a sediment-mixing process model of till genesis based on data from surface tills of the Saginaw lobe terrain in lower Michigan. Our research uses a spatial approach to understanding glacial landsystems and till genesis. We sampled calcareous till at 336 upland sites and at 17 sites in lacustrine sediment of the Saginaw Lake plain. The loamy tills have bimodal grain-size curves, with a fine-texture mode near the silt–clay boundary and a sand mode. Spatial grouping analysis suggests that tills can be divided into six groups, each with different textures and clay mineral compositions that vary systematically down-ice. The similarity among groups with respect to the silt–clay mode and clay mineralogy argues for a common origin for the fines—illite-rich lacustrine sediment of the Saginaw Lake plain. Fine-textured sediments were probably entrained, transported, and deposited down-ice as till, which also becomes sandier and enriched in kaolinite, reflecting increasing mixing with shallow sandstone bedrock with distance from the lacustrine clay source. Clayey tills on the flanks of the Saginaw terrain may reflect proglacial ponding against nearby uplands. A process model of progressive down-ice mixing of preexisting fine lake sediments with crushed/abraded sandstone bedrock helps to better explain till textures compared with a purely crushing/abrasion process model.
Approaches and challenges to the study of loess—Introduction to the LoessFest Special Issue
- Randall J. Schaetzl, E. Arthur Bettis III, Onn Crouvi, Kathryn E. Fitzsimmons, David A. Grimley, Ulrich Hambach, Frank Lehmkuhl, Slobodan B. Marković, Joseph A. Mason, Piotr Owczarek, Helen M. Roberts, Denis-Didier Rousseau, Thomas Stevens, Jef Vandenberghe, Marcelo Zárate, Daniel Veres, Shiling Yang, Michael Zech, Jessica L. Conroy, Aditi K. Dave, Dominik Faust, Qingzhen Hao, Igor Obreht, Charlotte Prud’homme, Ian Smalley, Alfonsina Tripaldi, Christian Zeeden, Roland Zech
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- Quaternary Research / Volume 89 / Issue 3 / May 2018
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- 11 May 2018, pp. 563-618
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In September 2016, the annual meeting of the International Union for Quaternary Research’s Loess and Pedostratigraphy Focus Group, traditionally referred to as a LoessFest, met in Eau Claire, Wisconsin, USA. The 2016 LoessFest focused on “thin” loess deposits and loess transportation surfaces. This LoessFest included 75 registered participants from 10 countries. Almost half of the participants were from outside the United States, and 18 of the participants were students. This review is the introduction to the special issue for Quaternary Research that originated from presentations and discussions at the 2016 LoessFest. This introduction highlights current understanding and ongoing work on loess in various regions of the world and provides brief summaries of some of the current approaches/strategies used to study loess deposits.
Eolian sand and loess deposits indicate west-northwest paleowinds during the Late Pleistocene in western Wisconsin, USA
- Randall J. Schaetzl, Phillip H. Larson, Douglas J. Faulkner, Garry L. Running, Harry M. Jol, Tammy M. Rittenour
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- Quaternary Research / Volume 89 / Issue 3 / May 2018
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- 30 October 2017, pp. 769-785
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Our study adds to the Quaternary history of eolian systems and deposits in western Wisconsin, USA, primarily within the lower Chippewa River valley. Thickness and textural patterns of loess deposits in the region indicate transport by west-northwesterly and westerly winds. Loess is thickest and coarsest on the southeastern flanks of large bedrock ridges and uplands, similar in some ways to shadow dunes. In many areas, sand was transported up and onto the western flanks of bedrock ridges as sand ramps, presumably as loess was deposited in their lee. Long, linear dunes, common on the sandy lowlands of the Chippewa valley, also trend to the east-southeast. Small depressional blowouts are widespread here as well and often lie immediately upwind of small parabolic dunes. Finally, in areas where sediment was being exposed by erosion along cutbanks of the Chippewa River, sand appears to have been transported up and onto the terrace treads, forming cliff-top dunes. Luminescence data indicate that this activity has continued throughout the latest Pleistocene and into the mid-Holocene. Together, these landforms and sediments paint a picture of a locally destabilized landscape with widespread eolian activity throughout much of the postglacial period.
Optical ages on loess derived from outwash surfaces constrain the advance of the Laurentide Ice Sheet out of the Lake Superior Basin, USA
- Randall J. Schaetzl, Steven L. Forman, John W. Attig
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- Quaternary Research / Volume 81 / Issue 2 / March 2014
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- 20 January 2017, pp. 318-329
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We present textural and thickness data on loess from 125 upland sites in west-central Wisconsin, which confirm that most of this loess was derived from the sandy outwash surfaces of the Chippewa River and its tributaries, which drained the Chippewa Lobe of the Laurentide front during the Wisconsin glaciation (MIS 2). On bedrock uplands southeast of the widest outwash surfaces in the Chippewa River valley, this loess attains thicknesses > 5 m. OSL ages on this loess constrain the advance of the Laurentide ice from the Lake Superior basin and into west-central Wisconsin, at which time its meltwater started flowing down the Chippewa drainage. The oldest MAR OSL age, 23.8 ka, from basal loess on bedrock, agrees with the established, but otherwise weakly constrained, regional glacial chronology. Basal ages from four other sites range from 13.2 to 18.5 ka, pointing to the likelihood that these sites remained geomorphically unstable and did not accumulate loess until considerably later in the loess depositional interval. Other OSL ages from this loess, taken higher in the stratigraphic column but below the depth of pedoturbation, range to nearly 13 ka, suggesting that the Chippewa River valley may have remained a loess source for several millennia.
The loess cover of northeastern Wisconsin
- Randall J. Schaetzl, John W. Attig
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- Quaternary Research / Volume 79 / Issue 2 / March 2013
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- 20 January 2017, pp. 199-214
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We present the first study of the distribution, genesis and paleoenvironmental significance of late Pleistocene loess in northeastern Wisconsin and adjacent parts of Michigan's Upper Peninsula. Loess here is commonly 25–70 cm thick. Upland areas that were deglaciated early and remained geomorphically stable preferentially accumulated loess by providing sites that were efficient at trapping and retaining eolian sediment. Data from 419 such sites indicate that the loess was mainly derived from proglacial outwash plains and to a lesser extent, hummocky end moraines within and near the region, particularly those toward the east of the loess deposits. Most of the loess was transported on katabatic winds coming off the ice sheet, which entrained and transported both silt and fine sands. The loess fines markedly, and is better sorted, distal to these source regions. Only minimal amounts of loess were deposited in this area via westerly winds. This research (1) reinforces the observation that outwash plains and end moraines can be significant loess sources, (2) provides evidence for katabatic winds as significant eolian transport vectors, and (3) demonstrates that the loess record may be variously preserved across landscapes, depending on where and when geomorphically stable sites became available for loess accumulation.
Hornblende etching and quartz/feldspar ratios as weathering and soil development indicators in some Michigan soils
- Leslie R. Mikesell, Randall J. Schaetzl, Michael A. Velbel
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- Quaternary Research / Volume 62 / Issue 2 / September 2004
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- 20 January 2017, pp. 162-171
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Weathering can be used as a highly effective relative age indicator. One such application involves etching of hornblende grains in soils. Etching increases with time (duration) and decreases with depth in soils and surficial sediments. Other variables, related to intensity of weathering and soil formation, are generally held as constant as possible so as to only minimally influence the time–etching relationship. Our study focuses on one of the variables usually held constant–climate–by examining hornblende etching and quartz/feldspar ratios in soils of similar age but varying degrees of development due to climatic factors. We examined the assumption that the degree of etching varies as a function of soil development, even in soils of similar age. The Spodosols we studied form a climate-mediated development sequence on a 13,000-yr-old outwash plain in Michigan. Their pedogenic development was compared to weathering-related data from the same soils. In general, soils data paralleled weathering data. Hornblende etching was most pronounced in the A and E horizons, and decreased rapidly with depth. Quartz/feldspar ratios showed similar but more variable trends. In the two most weakly developed soils, the Q/F ratio was nearly constant with depth, implying that this ratio may not be as effective a measure as are etching data for minimally weathered soils. Our data indicate that hornblende etching should not be used as a stand-alone relative age indicator, especially in young soils and in contexts where the degree of pedogenic variability on the geomorphic surface is large.
OSL ages on glaciofluvial sediment in northern Lower Michigan constrain expansion of the Laurentide ice sheet
- Randall J. Schaetzl, Steven L. Forman
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- Quaternary Research / Volume 70 / Issue 1 / July 2008
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- 20 January 2017, pp. 81-90
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We report new ages on glaciofluvial (outwash) sediment from a large upland in northern Lower Michigan—the Grayling Fingers. The Fingers are cored with > 150 m of outwash, which is often overlain by the (informal) Blue Lake till of marine isotope stage (MIS) 2. They are part of an even larger, interlobate upland comprised of sandy drift, known locally as the High Plains. The ages, determined using optically stimulated luminescence (OSL) methods, indicate that subaerial deposition of this outwash occurred between 25.7 and 29.0 ka, probably associated with a stable MIS 2 ice margin, with mean ages of ca. 27 ka. These dates establish a maximum-limiting age of ca. 27 ka for the MIS 2 (late Wisconsin) advance into central northern Lower Michigan. We suggest that widespread ice sheet stabilization at the margins of the northern Lower Peninsula, during this advance and later during its episodic retreat, partly explains the thick assemblages of coarse-textured drift there. Our work also supports the general assumption of a highly lobate ice margin during the MIS 2 advance in the Great Lakes region, with the Fingers, an interlobate upland, remaining ice-free until ca. 27 ka.
12 - Models and Concepts of Soil Formation
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- Soils
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- 12 January 2024
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- 06 April 2015, pp 283-320
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Summary
Soils are complex. They exist at the interface of the lithosphere, atmosphere, and biosphere and function as integral components of the hydrosphere. They inherit, react to, and affect (seemingly simultaneously) all of these realms. Erosion, burial, climate change, biomechanical movement and mixing processes, water table effects, inputs of eolian dust, microclimatic effects of aspect and topography, and innumerable other nuances of the soil-forming environment all interact to form the most complex of natural systems – soil. Soil plays a key role in global energy, water, and geochemical cycles (Bockheim and Gennadiyev 2000; Fig. 12.1). To top it off, soils have no single end point toward which they are developing. Every one of Earth's soil bodies is on its own individual journey to a destination that may be impossible to envision. How fun!
Question: How can we possibly make sense of this commotion and sensory overload? The answer: By using conceptual models that help us understand the soil system and distinguish a signal from all the noise (Minasny et al. 2008, Bockheim and Gennadiyev 2010). Conceptual models are essential tools of science; they are simplified descriptions of natural systems (Drury and Nisbet 1971). Rather than being precise mathematical descriptions that can be solved with ample data, they are used to help put soil information into perspective and provide insight into the system interrelationships, process linkages, and nuances of pedogenesis and soil geomorphology (Jenny 1941b, Cline 1961, Dijkerman 1974, Conacher and Dalrymple 1977, Burns and Tonkin 1982, Phillips 1989, 1993b, Hoosbeek and Bryant 1992, Johnson et al. 1990). Models provide a way to organize, simplify, and enumerate the factors that affect soil systems and processes (Smeck et al. 1983, Bockheim et al. 2005). They help to view things in ordered ways, organize our thoughts, and provide a conceptual framework within which to consider facts (Johnson and Watson- Stegner 1987). Some models are complex, but most try to take the complex and make it simple. They help us see the big picture. By their very nature, models are simplifications of reality. In fact, sometimes the simpler the model, the better.
Unfortunately, models can also serve as mental blinders, preventing us from seeing aspects of reality that do not necessarily fit with the model. They can constrain our viewpoint on the world and force us to see it through a certain type of conceptual lens.
Soils
- Genesis and Geomorphology
- 2nd edition
- Randall J. Schaetzl, Michael L. Thompson
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- 12 January 2024
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- 06 April 2015
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In its first edition, Soils established itself as the leading textbook in the fields of pedology and soil geomorphology. Expanded and fully updated, this second edition maintains its highly organized and readable style. Suitable as a textbook and a research-grade reference, the book's introductory chapters in soil morphology, mineralogy, chemistry, physics and organisms prepare the reader for the more advanced treatment that follows. Unlike its competitors, this textbook devotes considerable space to discussions of soil parent materials and soil mixing, along with dating and paleoenvironmental reconstruction techniques applicable to soils. Although introductions to widely used soil classification systems are included, theory and processes of soil genesis and geomorphology form the backbone of the book. Replete with more than 550 high-quality figures and photos and a detailed glossary, this book will be invaluable for anyone studying soils, landforms and landscape change anywhere on the globe.
9 - Weathering
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- Soils
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- 06 April 2015, pp 165-180
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Summary
An important step in the formation of soil from rock involves weathering of the rock into smaller and/or chemically altered parts (Yatsu 1988). Weathering is the physical and chemical alteration of rocks and minerals at or near the Earth's surface, produced by biological, chemical, and physical agents (in actuality, by their combination), as they adjust toward an equilibrium state in the surface environment (Pope et al. 2002). Few soils form directly from bedrock. More often soils develop after an intermediate step in which weathering processes break rock down in situ, or geomorphic processes comminute and erode the rock. This intermediate step forms various types of regolith, or rock overburden, which are then acted upon by pedogenic processes to form soil (see Chapter 13). In the end, rocks become discolored, are structurally altered, acquire precipitates of weathering by-products, and experience collapse as a result of weathering. In this section we examine the main components and processes of weathering – a discussion that logically precedes our discussion of soil parent materials in Chapter 10. Excellent reviews of weathering are found in Ollier (1984), Yatsu (1988), Pope et al. (1995), Bland and Rolls (1998), and Hall et al. (2012).
In various degrees, rocks are physically and chemically unstable at the Earth's surface, and hence they weather, because the surficial environment is far different from the one in which they formed. For most rocks, the surface (soil) environment is colder, with less pressure and increased amounts of oxygen, water, and biota, than their formative environment, be it in a volcano's cooling magma, below the seafloor, or deep within the crust. For this reason, rocks, minerals, and soils are typically the most weathered at the surface and progressively less weathered with depth (April et al. 1986).
As noted in Chapter 4, primary minerals are those that crystallize as magma cools from high temperatures. Over long periods, primary minerals are unstable in soils and weather to secondary minerals, commonly clay minerals. In short, the essence of weathering is the breakup of rock and the formation of secondary minerals from the inherited (primary) minerals, as rocks are changed into forms that are more stable at the Earth's surface.
Several other processes are almost always associated with weathering, e.g., erosion (the wearing away of rocks or sediments/soils) and transport (the movement of those same materials), which collectively are termed denudation.
8 - Soil Classification and Mapping
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- 06 April 2015, pp 111-164
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Summary
Soil classification and mapping are as old as farming (Michéli and Spaargaren 2011). By 8000 BC, farmers in Europe had already determined where the better agricultural soils were and had preferentially settled on them. By 2000 BC, evidence of farming communities exists for the fertile black soils formed on the Deccan Plateau of India, but not in areas where these soils are absent (Shchetenko 1968). Clearly, early peoples were capable of assessing soil character – the essence of soil classification – and were able to use this information to differentiate the productivity of soils across the landscape. Creating physical maps of this soils knowledge would occur later. Even then, a system whereby soils could be differentiated from each other, based on their characteristics, was needed, just as it is today. Captured in this statement is the essence of soil classification.
The essence of any classification system is to place a name on a “definable entity,” in this case, a basic soil unit. Once this is done, it is possible to arrange it in an orderly system and establish its interrelationships with related entities and establish its value (Beckmann 1984). Most importantly, though, it allows for communication about the entities being classified.
Users of classification schemes agree that a name should convey the unit's unique and defined range of characteristics, whether it be a plant, e.g., Pinus sylvestris; rock, e.g., arkosic sandstone; or soil, e.g., an Aeric Fragiaquept. For soils, that range of characteristics may involve physical properties such as clay content of the B horizon, thickness of the A horizon, or color, among others. Over time, these ranges are altered and adjusted as new and more complete information about the universe of soils (or plants or rocks) accrues. Thus, most classification systems are open-ended, and periodic updates are assumed.
Before we try to classify a soil, we must first define it. We agree with Johnson (1998a), who defined soil as organic or lithic material at the surface of planets and similar bodies that has been altered by biological, chemical, and/or physical agents. But to classify soils, it is essential that we also agree on some sort of singular or basic unit (Johnson 1963). In many disciplines, it is relatively easy to determine the basic unit that is being classified: a plant, a rock, a virus.
3 - Basic Concepts: Soil Horizonation … the Alphabet of Soils
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- 06 April 2015, pp 29-49
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Summary
Soil Horizons
Most of the Earth's surface contains soils, and most soils have discernible, genetic horizons, that is, genetic layering. Horizons are what distinguishes soil material from nonsoil sediment. Soil horizons may be absent in areas where erosion rates exceed the rates of soil development, as on steep slopes. In other places, horizons may not occur because they have not yet formed in young sediment, or because the sediments are essentially inert material devoid of nutrients that would support vegetation. Regardless, the presence of one or more genetic horizons indicates the existence of a soil, and vice versa.
Early soil scientists used terms like soil layer, vegetable mould, stratum, and level to describe these genetic layers (Tandarich et al. 2002). For almost a century (Shaw 1927), a layer formed by pedogenic processes that is more or less parallel to the soil surface has been called a soil horizon. Moreover, because its origin is pedogenic, referring to it as a genetic horizon is also appropriate. Anderson (1987, 56) described soil horizons as the “working aggregates of the whole (soil) system, and, like the organs of an organism, … generally adapted for the performance of specific functions.” In this chapter, we describe the basic characteristics of the major soil horizons. These types of horizons are different from and not to be confused with diagnostic horizons, which are defined for the purposes of soil classification and will be discussed in Chapter 8.
Types of Soil Horizons
Soil horizons generally form within unconsolidated materials on geomorphically stable surfaces that have been subaerially exposed for a sufficient length of time. In fact, pedologists often use the presence of well-developed horizons as an indication that the surface below which the soil is forming has been relatively stable for a considerable period (see Chapter 14). Horizons form as material is translocated (upward, downward, or laterally) within the profile or as it is transformed (chemically or physically) in place (Simonson 1959; see Chapter 12). Pedogenic processes tend to form distinct horizons within the upper mantle of unconsolidated materials, i.e., certain types of horizons are often associated with a certain suite of pedogenic processes (see Chapter 13).
Ever since Vasili Dokuchaev and Nikolai Sibirtsev introduced them at the 1893 World's Columbian Exposition in Chicago, soil horizons have been divided into a few types of master horizons.
11 - Pedoturbation
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- Soils
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- 06 April 2015, pp 232-282
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Summary
Pedoturbation, a word popularized by Francis Hole (1961), is synonymous with soil mixing. The mechanisms and vectors by which physical mixing is accomplished are many, and they operate from microscopic scales at which crystals grow and deteriorate, to larger mixing processes associated with uprooted trees, to massive termite mounds and solifluction lobes. The importance of pedoturbation has traditionally been underemphasized by many soil and earth scientists. Nonetheless, it is ubiquitous, and its importance to soil and landscape genesis is becoming increasingly documented. Pedoturbation may promote horizonation and physical organization in a soil, but it can also be a regressive (mixing) process that promotes disorder. In short, although it is a form of mixing, pedoturbation is not, as we shall see, always synonymous with homogenization.
Pedoturbation affects soil genesis and its developmental pathways almost continually, but it often is little noticed. Knowledge of pedoturbation is vital for the study of preexisting stratification, such as in archaeology (Wood and Johnson 1978, Rolfsen 1980, Stein 1983, Bocek 1986, McBrearty 1990, Balek 2002) and sedimentology, as well as for those who study pedogenic layering, e.g., soil scientists or geomorphologists (Johnson et al. 1987). For example, geological lithologic discontinuities can be blurred or completely masked by pedoturbation processes. Alternatively, layering formed by pedoturbation processes can be mistakenly attributed to geological processes that operated prior to pedogenesis. Pedoturbation plays a critical role in maintaining macroporosity in most soils, which in turn aids in infiltration and retards runoff and erosion. Physical mixing of organic matter in surface litter layers into the underlying soil is largely accomplished via pedoturbation. In short, most pedogenic pathways are in some way affected by pedoturbation.
Many traditional soil genesis studies have focused on ascertaining how and why the soil profile (an ordered state) has developed. Comparatively few studies, however, have stressed or examined the converse – the formation of disorder (haploidization, simplification) from an otherwise pedologically ordered soil, i.e., one with horizons (see Chapter 12). Still fewer have examined the preservation of disorder by pedoturbation, or the formation of soil order by pedoturbation. This oversight is the impetus for this chapter.
Expressions of Pedoturbation
In its various forms (Table 11.1), pedoturbation is studied either by observing the process, such as ants moving soil particles (Pérez 1987b), or by examining and interpreting the end products of pedoturbation (Baxter and Hole 1967, Schaetzl 1986b, Cox et al. 1987a, Johnson 2002, Robertson and Johnson 2004).
17 - Conclusions
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- 06 April 2015, pp 638-640
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Summary
The Importance of Soils
With the exception of ice-covered surfaces and areas of bare bedrock, soils of one kind or another exist on almost every part of Earth's landsurface. The wide variety and range of expression that soils have are truly remarkable, especially when one considers that this plethora of colors, mineral assemblages, and horizon types and sequences is due to the interaction of only a few (five) soil-forming factors. In mathematics, 5 is a small number; 55, which (minimally) reflects the complex interactions among those five factors, is considerably larger (3,125). So it is in soils, as those five soil-forming factors can team together in myriad ways to form a world of soils that is complex, spatially diverse, and manifaceted. Unraveling and explaining that world, or at least some of the better-understood parts of it, have been our goals. Indeed, in the context of its complexity, when we think about it, it should be clear that we still know relatively little about soil and how it functions as a natural system. Yaalon (2000) paraphrased Leonardo da Vinci as stating, “Why do we know more about distant celestial objects than we do about the ground beneath our feet?”
Almost all of our food and sustenance are from the soil. As the saying goes, “If you eat, you are involved in agriculture,” and, we would add, you are also dependent upon the soil. Most of the oxygen that we breathe originates from plants rooted in the soil. Soil is also one of the best natural filters we have. What would our world be like without soil?
The name Adam is from the Hebrew adama, which means earth, or soil. Adam's name, in the Bible's book of Genesis, is meant to capture humankind's intimate link with the soil, to which we are tied while we live, and to which we return upon our death (Hillel 1991). Remember Francis Hole's expression, TNS? Temporarily Not Soil – that is what we are. And what all terrestrial life on this planet is. Clearly, soils have been significant to humankind for as long as we have existed and will be here long after we are gone. The importance of soils cannot be overestimated. Their value cannot be overstated.
Glossary
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- 06 April 2015, pp 735-770
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References
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- 06 April 2015, pp 641-734
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5 - Basic Concepts: Soil Chemistry
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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Summary
Soils are multiphase systems in which solids, liquids, gases, and colloids collectively interact. The solids in soils are crystalline or poorly crystalline minerals as well as organic matter. The liquid is water. Gases include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), methane (CH4), and water vapor (H2O). And colloids are minerals, humified organic matter, and many kinds of microorganisms that are generally <1 μm in diameter. Although some chemical and biochemical reactions in soils occur in a single, homogeneous phase, e.g., the liquid or solid phase, most reactions occur at the boundary between two phases, e.g., at the gas–liquid interface or at the liquid–colloid interface. Alternatively, they could be coupled with reactions that occur in more than one phase. Because both the architecture and the biological activity of soils vary by horizon and landscape position, the speed and direction of chemical and biochemical reactions also vary in complex and fascinating ways.
The Liquid Phase in Soils
Most chemical reactions in soils occur in the liquid phase, i.e., the soil solution, or at the interface between the liquid and solid phases. The soil solution consists of water in which cations, anions, ion pairs, small organic molecules, and gas molecules are dissolved and in which colloids are suspended. The chemical composition of the soil solution usually varies seasonally and depends on how much water is in the soil, on the minerals present, and on plant nutrient uptake. In many soils, Ca2+, Mg2+, Na+, and K+ (referred to as base cations) are the most abundant cations, whereas the dominant anions include HCO3-, Cl-, and SO42- (Table 5.1). Other cations and anions are present at relatively low concentrations. Most trace metals in solution, such as Fe or Al, occur in a variety of species that are determined by the pH of the solution and by the abundance of complexing anions or ligands. Soluble complexes of ions may be charged, e.g., Fe(OH)2+ or uncharged, e.g., CaCO3°.
The likelihood that any soluble species will react with another (i.e., its chemical activity) depends on its charge, size, and the concentrations of all other species that are in the solution (indexed by the solution's ionic strength). Ions shield one another from participating in chemical reactions, so the activity of any particular ion decreases as the ionic strength of the solution increases.
6 - Basic Concepts: Soil Physics
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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Summary
Soil physics is the branch of soil science that deals with the physical properties and processes of soils. Soil physics is generally concerned with the state and movement of matter and energy in soils; hence, most soil physicists study the movement of water in soils, and the changes in soil temperature, over time and space. Soil physical properties such as water content, texture, and structure, as well as soil physical processes such as water retention and transport, soil temperature and heat flow, and the composition of the soil atmosphere (mainly O2 and CO2), all affect weathering and soil genesis. Therefore, in this chapter we discuss the basic concepts of soil physics that are necessary as background to the discussions of soil genesis and geomorphology in more depth that follow.
Soil Water Retention and Energy
Many pedogenic processes begin and end with the flow of water in soils (see Chapters 13 and 14). Water is the main agent by which solids and ions are transported within soils. Knowledge of the forces acting on water flow is, therefore, important for understanding soil genesis, not to mention soil use and management.
Water is retained in soils in two ways. Adsorbed water molecules are retained at or very close to the surfaces of soil particles. They are held there by attractive forces acting between the water molecules and the surface, or between water molecules and ions near the particle surface. Water that is absorbed is taken into the pores of a solid (soil, mineral, rock particle, or organic substance) and is retained by the surface tension of the water molecules interacting with one another in small pores. Water retention in soils, in both adsorbed and absorbed forms, increases with increasing contents of clay and organic matter because of the affinity of those solids for water.
Liquid water has high surface tension because the H atoms in each H2O molecule form strong hydrogen bonds to the O atoms of neighboring water molecules. The polar, hydrogen-bonding nature of water also leads to a strong, adhesive attraction between water molecules and most soil particles. For example, water molecules can form H-bonds with OH groups on oxide minerals and at the edges of clay minerals, as well as with NH and OH groups on soil organic matter. In addition, water molecules strongly solvate cations that are adsorbed by soil minerals.
14 - Soil Geomorphology and Hydrology
- Randall J. Schaetzl, Michigan State University, Michael L. Thompson, Iowa State University
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- Book:
- Soils
- Published online:
- 12 January 2024
- Print publication:
- 06 April 2015, pp 445-525
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Summary
Introduction to Soil Geomorphology
Geomorphology is the study of landforms and the evolution of the Earth's surface. Because soils are so strongly linked to the landforms upon which they develop, a discipline that dealt with those relationships eventually emerged: soil geomorphology. Popularized by Pete Birkeland in the 1970s and 1980s, soil geomorphology came into its own with the publication of a number of books by Birkeland and other scholars during this time, all devoted to the topic and helping to define the field (Mahaney 1978, Gerrard 1981, 1992, Richards et al. 1985, Daniels and Hammer 1992, Paton et al. 1995). In 1999, Birkeland had defined soil geomorphology as the study of soils and their use in evaluating landform evolution, age, and stability, surface processes, and past climates. Wysocki et al. (2000) more broadly defined it as the scientific study of the origin, distribution, and evolution of soils, landscapes, and surficial deposits, and the processes that create and alter them. McFadden and Knuepfer (1990) emphasized the linkages between pedogenic and other surficial processes, in their definition of the field. Perhaps the definition we like best was presented by John Gerrard in 1992: Soil geomorphology is an assessment of the genetic relationships between soils and landforms.
Yes, soils and landforms develop together. Many times, it just makes sense to study them together. Soil geomorphology is designed to examine and elucidate the nature of that genetic dance. But it is a two-way street. Soils are affected by landforms, and through their developmental accessions and features, they in turn influence geomorphic evolution. Most importantly, pedogenic processes are variously dependent and intertwined with slope processes, e.g., erosion and sedimentation. And on top of it all, the influence of landforms on the flow of water – across the soil surface and belowground – impacts soils markedly. This is the essence of soil geomorphology – putting all these interrelationships together.
Historical Background
Soil geomorphology was first studied in its own right by the U.S. National Cooperative Soil Survey (NCSS) program in the 1930s. At that time, interests had developed among geographers, geologists, and soil scientists on the relationships between soils and landforms (Effland and Effland 1992, Holliday 2006). Acknowledging the merit in this type of approach, the NCSS adopted soil geomorphology as a paradigm for studying soil landscapes.