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Dep. of Geography, 314 Natural Science Building, Michigan State Univ., East Lansing, MI 48824-1115
* Corresponding author (weisenbo{at}msu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: AAO, acid ammonium-oxalate • CBD, citrate–bicarbonate–dithionite • CD, sodium citrate-dithionite • EDS, energy-dispersive x-ray spectroscopy • MMFE, Michigan Model of Fragipan Evolution • SEM, scanning electron microscopy
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Though not developed based on soils in the Great Lakes region, there exist widely cited fragipan genesis models (e.g., Harlan et al., 1977; Karathanasis, 1989; Bryant, 1989; Smeck et al., 1989). These models aim to explain (i) the physical processes involved in fragipan genesis, such as self-weight collapse followed by physical ripening (Bryant, 1989), or (ii) the physical and chemical processes involved in fragipan bonding by agents, such as translocated silica (Harlan et al., 1977) or Si-rich aluminosilicates (Karathanasis, 1989) or precipitation of acid-weathering products (Smeck et al., 1989), or (iii) both (James et al., 1995). Formation and development of these binding agents have been proposed to occur in association with lithologic, hydrologic, or weathering discontinuities in the solum (Harlan et al., 1977; Karathanasis, 1989; Smeck et al., 1989).
The purpose of this study was to (i) identify and describe pedogenic processes important to fragipan evolution based on some Michigan soils, formed in till, that have variable fragipan expression, (ii) infer fragipan genesis based on data from these soils, linked to existing pedogenic models, and (iii) evaluate our findings in terms of these models so as to develop a new, synthesizing, and integrating model of fragipan evolution. Although developed from and for Michigan soils, our model may have, theoretically, wide applicability.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Undisturbed, bulk samples were collected from all protofragipan and fragipan horizons for micromorphological description using scanning electron microscopy (scanning electron microscopy, SEM; see Weisenborn and Schaetzl, 2005). All SEM observations and imaging were conducted on a JOEL JSM-6400V SEM (Joel Inc., Boston) equipped with a VANTAGE Digital Microanalysis and Imaging System (by Thermo NORAN, Thermo Electron Corporation, San Jose, CA) for energy-dispersive x-ray spectroscopy (EDS) analysis. The EDS analysis was used to semiquantitatively evaluate the elemental composition of targeted features in select SEM samples. Spectra reflect the elemental composition of a 5-µm3 volume at the sample surface; thus, sufficiently thick features were sought for analysis so as to reduce the influence of underlying material in the collected spectra. Because of the sample coatings required for SEM imaging, all spectra indicate the presence of carbon and gold.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Key morphological properties of the protofragipan and fragipans are based on field observations (Weisenborn and Schaetzl, 2005). All protofragipans and fragipans have firm to very firm moist consistence; brittle failure; weak, thick platy to strong, coarse subangular blocky structure; fine vesicular pores; redoximorphic features; slake; and exhibit reduced root presence or root restriction. All three profiles exhibit evidence of oxyaquic conditions; episaturation is due to seasonally perched zones of saturation. Predominantly eluvial protofragipan and fragipan horizons tend to lack clay coats/films and contain variable amounts of albic materials that form glossic horizons when well developed. Eluvial protofragipan and fragipan colors (high value, low chroma) indicate moderate to strong eluviation processes. Tonguing of the eluvial fragipans' albic materials into the underlying fragipan horizon suggests that eluvial fragipan horizons contain zones of degradation. Predominantly illuvial fragipan horizons exhibit clay coats or films and flows on ped faces and vertical root channels, and clay bridging between sand grains.
Physical and Chemical Properties
On the basis of data presented in Weisenborn and Schaetzl (2005), these soils have formed in sandy loam to loamy parent materials. The protofragipan and fragipan horizons are dominated by very fine to medium sand or silt, with variable (2.2 to 28.4%) clay contents. Predominantly illuvial fragipans are finer textured than their eluvial counterparts. Each of the soils contains at least one lithologic discontinuity in proximity to the protofragipan or fragipan. All protofragipan and fragipan horizons have higher bulk density values than their overlying horizons; fragipan horizons also have higher bulk density values than their underlying horizon(s). Protofragipan and fragipan pH values are lower than horizons directly above and below them. The Munising soil formed in acidic glacial parent materials, while the parent materials for the Feldhauser and Glennie soils were calcareous.
While there are no obvious associations among these horizons and sodium citrate-dithionite (CD)- and acid ammonium-oxalate (AAO)-extractable forms of Fe, Al, and SiO2 in our protofragipan and fragipans, the presence of a fragic-property agent that contains these elements in combination, however, is not precluded by the data (Weisenborn and Schaetzl, 2005). Furthermore, soil extraction data can be used to infer the presence of certain solid-phase forms in the soil fraction. Extraction-data ratios have been used specifically to investigate the role of Si, Al, and Fe in fragipan development, particularly as they relate to fragipan binding agents and strength (Norfleet and Karathanasis, 1996; Duncan and Franzmeier, 1999). For example, fragipan strength has been related to molar Si/(Si + Al) ratios > 0.5, associated with an amorphous aluminosilicate binding phase (Norfleet and Karathanasis, 1996) or silica polymer bridging agent (Duncan and Franzmeier, 1999). Duncan and Franzmeier (1999) also used the citrate–bicarbonate–dithionite (CBD)-extracted Al/(Al + Fe) ratio to evaluate Al-substitution in Fe-oxide minerals in soils with fragipans. In the Feldhauser, Munising, and Glennie soils, the (SiO2)d/[(SiO2)d + Ald] ratio exceeds 0.5 in the fragipans and their underlying horizons (Fig. 1) . This ratio approaches 0.5 in the Feldhauser protofragipan and exceeds 0.5 in the underlying horizons. Although we did not test soil strength, values > 0.5 in the fragipans seem consistent with the findings of Norfleet and Karathanasis (1996) and Duncan and Franzmeier (1999). This finding suggests that silica or silica polymers may be associated with fragipan strength in the Munising and Glennie soils (Duncan and Franzmeier, 1999). The Ald/(Ald + Fed) ratio is highest in horizons above the Munising and Glennie fragipans (Fig. 1). The Feldhauser soil, however, has higher ratios above and below the protofragipan. Elevated ratio values above the fragipan in the three soils studied seem consistent with Duncan and Franzmeier's (1999) work.
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Closely packed fabrics were observed in all of the protofragipan and fragipan horizons studied. While some of the closely packed fabrics are associated with groundmasses (Fig. 2A; Fig. 4A in Weisenborn and Schaetzl, 2005), others are associated with voids as coatings or infillings (Fig. 2B, 2C). Silt concentrations similar to those reported by Collins and O'Dubhain (1980) and Thompson (1980) were especially noticeable in SEM images (Fig. 2B, 2C). These void coatings or infillings contain closely packed silt grains that sometimes appear oriented and/or microsorted.
Intergrain bridging was observed in the protofragipan and fragipans largely as meniscus-like bridges that are either (i) composed of or coated with a clay-sized material that appeared to be amorphous based on its surface morphology (Fig. 2D) or (ii) composed of oriented clay-sized particles or clay minerals (or both) (Fig. 2E). Other larger bridges are composed primarily of silt-grains with some clay-sized particles (Fig. 4C in Weisenborn and Schaetzl, 2005).
Of the void coatings that we observed in protofragipan and fragipan horizons, some had relatively smooth surfaces that seemed to obscure the topography of the underlying fabric; these were composed of either layers of strongly parallel-oriented clay plates (Fig. 3A) or a veneer of clay-sized material that may be amorphous (Fig. 2C). Since the coatings were observed along voids in closely packed fabrics of the protofragipan and fragipans, their formation most likely occurred after the collapse of the fragipan zone. The void coating in Fig. 3B has a surface morphology unlike the others discussed; it is composed of clay-sized particles and appeared very densely assembled with wavy edges, possibly because of the presence of smectite (Smart and Tovey, 1982a, 1982b; Bisdom et al., 1990) or related mixed-layer clay minerals, stress (Magaldi et al., 1994), or wetting-drying processes (Smart and Tovey, 1982a). The EDS spectra of some of the intact voids show that Si consistently produced the highest number of collected x-ray counts, followed by Al, and then by Fe, K, and Mg in variable order (Fig. 3). Calcium was detected in most of the spectra, as was Ti, but with very low count numbers. Norton et al. (1984) found plasma separations to consist of Si, Al (both with substantial peak-counts), and variable peak-counts for K, Ca, and Fe. Payton (1993a) reported the presence of similar elements in both ferriargillans (clay coatings enriched in Fe oxides or hydroxides) and gray, grainy degraded void coatings: relatively high peak-counts for Si and Al, moderate peak-counts for Fe and K, and low peak-counts for Ti. We were not able to determine the chemical formulas for these coatings due to the semiquantitative nature of EDS; thus, we do not know the type of clay materials or minerals that may be associated with them. Yassoglou and Whiteside (1960), however, conducted x-ray diffraction on related soils with fragipans in northern Michigan and reported that illite and chlorite (individually and probably as mixed-layered or interlayered arrangements) are the main clay minerals, which weather to form smectite and vermiculite.
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Interpretations
Compiling a model that accounts for the complexities of this type of soil system is daunting, and perhaps risky. Nevertheless, a goal of this research was to identify the major, current models of fragipan genesis that best apply and explain the fragipan properties presented for our soils, and to use those models and our data as a foundation to develop a more comprehensive model of fragipan evolution for soils in Michigan and the Great Lakes region. In that light, borrowing in part from Bryant (1989), fragipan genesis can be viewed in terms of three components: (i) parent-material properties; (ii) physical processes associated with the development of high bulk density, closely packed fabrics, and structure; and (iii) physical and chemical processes associated with the development of fragipan bonding. A model that attempts to explain fragipan genesis must account for as many of these components as possible. A combination of existing models seems to best accomplish this goal.
Models developed to explain fragipan development in Great Lakes' soils vary in their ability to explain fragipan genesis. While we did not find evidence to support the inheritance of fragipan properties from a soil's parent material as proposed by Miller et al. (1971a)( 1971b), we did observe evidence of wetting-drying processes in fragipan formation, and clearly found that our fragipans do degrade. Data presented here also meet many of the necessary conditions involved in the model of Miller et al. (1993), even though it does not seem to explain the genesis of our protofragipan and fragipans. Likewise, the Olson and Hole (1967) model may not explain fragipan genesis in our soils, but does help to accentuate the similarities among the Wisconsin and Michigan soils containing fragipans. Of all the models proposed for fragipan genesis in the Great Lakes region, that of Yassoglou and Whiteside (1960) seems most consistent with the data presented for the Feldhauser, Munising, and Glennie soils. Close-packing of skeleton grains and meniscus-like intergrain bridges, as well as the influence of wetting-drying processes and eluviation in portions of the protofragipan and fragipans, were observed in all of the soils. Similarly, the involvement of Al in fragipan bonding, as proposed by Yassoglou and Whiteside, cannot be ruled out. The degree to which gravitational forces, root pressures, or freeze-thaw processes function as agents of compaction is, however, unclear.
In the end, two models seem to best provide a foundation for explaining fragipan genesis in some Michigan soils. Bryant's (1989) model, involving self-weight collapse (hydroconsolidation) and physical ripening (desiccation), and the weathering discontinuity model of Smeck et al. (1989) appear to compliment one another in terms of explaining fragipan development (Smeck and Ciolkosz, 1989; James et al., 1995). More importantly, these two models appear to explain the development of our protofragipan and fragipans. The two models' complimentary nature is clear: Bryant's (1989) model addresses components (i) and (ii) above and the physical aspect of (iii) above, while the weathering discontinuity model (Smeck et al., 1989) addresses the chemical aspect of component (iii) above. Used together, they provide (i) the prerequisite conditions for fragipan-genesis initiation, (ii) mechanisms for fabric modification of a specific zone at depth within a parent material or soil, (iii) a mechanism for the more permanent imprinting of fragic properties onto that zone, and (iv) mechanisms for the genesis of bonding agents that supplement and support the structural fabric of the fragipan zone and (progressive) development of fragipan expression (Weisenborn, 2001). We have more-or-less combined these two models (with a few additions or modifications) to make inferences about fragipan evolution in a larger context, one that includes whole-profile genesis. We have additionally provided the mechanisms for regressive fragipan development based on a handful of papers (i.e., Hallmark and Smeck, 1979; Langohr and Pajares, 1983; and Payton, 1993b). The end result is the MMFE, which provides one possible scenario for fragipan evolution in loamy, upland soils in Michigan and nearby regions (Fig. 4) .
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Progressive Fragipan Development
Working first within Bryant's (1989) model, parent material is deposited, most likely with a high water content. The parent material must allow water to move and drain through it. Hence, it may either be uniform, as in loessial soils, or it may contain a textural or lithologic discontinuity, as in our soils (Weisenborn and Schaetzl, 2005). The parent material must also meet specific, initial criteria to be conducive to self-weight collapse and physical ripening, including: (i) loose, granular-contact structure; (ii) loam texture with approximately 5 to 40% clay (clay content can vary depending on clay mineralogy and distribution in the collapsible zone) (Assallay et al., 1998); (iii) contains fine material (i.e., bonding agents in any form) that weakens or compacts when wet; (iv) thick enough so that at some depth compressive stress from overlying materials is sufficient for self-weight collapse (Bryant, 1989; Assallay et al., 1998); (v) lacks a persistently high water table or zones of saturation; and (vi) low coefficient of linear extensibility. Our parent materials meet most, if not all, of these criteria. Regardless of whether these criteria are met at the time of deposition or developed via pedogenic processes, they must be met if a collapsed zone is to form and persist (Fig. 4).
Pedogenesis, including early processes such as melanization, leaching, and acidification, begins after the parent material has been deposited. Some early pedogenic processes may be prerequisite for the parent material to satisfy the above criteria. For example, the parent material for the Glennie soil is dense and calcareous, requiring acidification, leaching, pedoturbation, and perhaps even lessivage to develop a loose fabric capable of rearrangement and collapse. It is also conceivable that these soils may have been influenced by frost action following deposition—a process that has been invoked to account for disturbances of fragipan structure and fabric in glaciated regions (e.g., Collins and O'Dubhain, 1980; Langohr and Vermeire, 1982; Langohr and Pajares, 1983; Van Vliet-Lanoë, 1985; Nettleton et al., 1994). Although we have only observed weak evidence that supports the influence of frost action (e.g., vesicle-like pores, silt accumulations, microsorting and reorganization), we cannot exclude it.
If all the necessary parent material criteria are met, explicit fragipan genesis may be initiated with wetting of the potential collapse-zone to (or near to) saturation (Fig. 4). Indeed, in many tills (Bryant, 1989), or colluvium as James et al. (1995) found, it is likely that the sediment was initially deposited as a slurry, or in a very wet or saturated state. Saturation of the tills creates a potential collapse-zone, in which clay bonding is weakened. The zone's loose, granular-contact structure no longer has the support needed to remain open (Rogers et al., 1994). These conditions are conducive for the self-weight collapse of a zone at an optimal depth within the profile. Even if the materials above and below this zone met these criteria, they still experience too little or too much compressive stress, respectively, for fabric rearrangement and self-weight collapse to occur. Therefore, the newly collapsed zone has a well-defined upper boundary and a diffuse lower boundary (Assallay et al., 1998). The number of grain contacts is increased in the collapsed zone, which may in turn impede internal drainage, making it a morphologic discontinuity within the profile (Assallay et al., 1998). Essentially, self-weight collapse essentially rearranges and closely packs the fabric in the collapsed zone.
Next, if the solum desiccates to a depth sufficient to affect the collapsed zone, physical ripening can occur (Fig. 4). The collapsed zone may exhibit sufficient fragic soil properties at this point to be called a protofragipan. Physical ripening has been invoked to explain fabric reorientation in the form of granular close-packing, clay-bridging, and void coatings (e.g., Bryant, 1989; Rogers et al., 1994; Assallay et al., 1998). Attou and Bruand (1998) demonstrated experimentally that close fabrics associated with fragipans can be formed after a single desiccation event if clays were dispersed before the event. They also reported, however, that the formation of clay bridges and void coatings required multiple wetting-drying cycles. Additional wetting-drying cycles encourage successive saturation of voids, dispersion of clays, desiccation, and further fabric rearrangement. The layered and compound nature of void coatings and bridges in the protofragipan and fragipans appear to confirm the morphologic expression of multiple wetting–drying cycles in these soils (Weisenborn and Schaetzl, 2005).
Differentiation of the upper and lower sequa begins around this time, as the profile becomes increasingly acidic, initiating lessivage (and possibly pervection) and clay mineral decomposition (Fig. 4). The profile thickens and an illuvial Bt horizon begins to form. Either occurring contemporaneously with or immediately following clay translocation, podzolization is initiated in the upper solum (Schaetzl, 1996; Bockheim, 2003).
A weathering discontinuity in the form of a weathering front (Smeck et al., 1989) becomes more strongly developed as the degree of pedogenesis in the upper sequum increasingly contrasts with that of the unleached but oxidized C horizon (Fig. 4). By this time, the protofragipan occurs in the transition zone between the weathered upper sequum and the relatively unweathered parent material. As time progresses, acid weathering products get increasingly eluviated from the upper sequum. Because of the protofragipan's ability to impede internal drainage, these weathering products, primarily hydrous oxides of Al and Si (Smeck et al., 1989), become hydrologically stranded in the protofragipan zone and interact with components of the less weathered lower sequum. Precipitation of amorphous materials in the protofragipan may be enhanced by desiccation as well as variations in pH or base saturation at the discontinuity. On the basis of micromorphological observations of the protofragipan and fragipans, this amorphous material occurs in the form of intergrain bridges and void coatings [Fig. 2D; Fig. 4F, 4H in Weisenborn and Schaetzl (2005)].
According to Smeck et al. (1989), the addition of an amorphous bonding agent to the protofragipan fulfills the final requirement of fragipan development. If, at this time, the protofragipan also meets the thickness and volume criteria specified by the Soil Survey Staff (1999), it is considered a fragipan. Fragipan expression may, however, increase with time as close-packing (via coarse grain reorientation, and void coatings and infillings), intergrain bridging by clays, and amorphous-material bonding become better developed (Smeck et al., 1989).
As fragipan expression increases, the fragipan becomes increasingly effective as an internal drainage impediment, eventually becoming an aquitard during seasonally wet periods (Fig. 4). Episaturation increases in duration as the fragipan's density increases and porosity decreases. It may also increase in frequency as the fragipan becomes able to impede water from rainfall events throughout the year (Calmon et al., 1998; McDaniel et al., 2001).
Regressive Fragipan Development
All three soils exhibit eluvial characteristics in a portion of the protofragipan or fragipan (Weisenborn and Schaetzl, 2005). The eluvial nature of these horizons is related to their position in the lower sequum; they exist as the eluvial member of eluvial-illuvial couplings. A portion of this eluvial character in the fragipan soils, however, may be additionally associated with fragipan degradation. Degradation has been reported to occur at various locations within a fragipan (Yassoglou and Whiteside, 1960; Hallmark and Smeck, 1979; Langohr and Vermeire, 1982; Miller et al., 1993; Payton, 1993b; Ciolkosz et al., 1995; Lindbo et al., 2000). Furthermore, degradation of eluvial fragipans and clay-rich underlying horizons has also been observed in bisequal soils similar to Glennie and Munising (Yassoglou and Whiteside, 1960; Miller et al., 1993).
Processes involved in fragipan degradation (Fig. 4) are likely akin to those invoked for Bt horizon degradation, based on the morphological similarity of these two phenomena. Eluviation and microerosion of clay have been invoked for degradation of Bt horizons (e.g., Smeck et al., 1968; Bullock et al., 1974; Langohr and Vermeire, 1982) and fragipans (Payton, 1993b). Additionally, degradation in the Glennie and Munising soils may be related to seasonal saturation and drying in the upper fragipan (Ransom et al., 1987; Payton, 1993b; Miller et al., 1993; James et al., 1995; Lindbo et al., 2000). Eluviation and microerosion of soil constituents (possibly Fe and clay from void coatings and intergrain bridges) clearly has occurred in the upper portion of the fragipan or in voids along structural-unit faces (Weisenborn and Schaetzl, 2005). Degradation may start as irregular tongues of clay-impoverished and bleached eluvial material that extend into the Bt or Bx horizon, only to gradually penetrate more deeply along areas of preferential flow (Langohr and Vermeire, 1982). Eventually, tongues of albic materials engulf the underlying horizon, leaving only remnants of the former Bt horizon (Soil Survey Staff, 1999; Lindbo et al., 2000). The Glennie (E/B)x and Munising 3(E/B)x' are predominantly glossic, eluvial horizons composed of albic materials that tongue and interfinger into the underlying Btx or 3Bt horizons, respectively.
Fragipan degradation in the Glennie and Munising soils is probably influenced by a number of conditions that develop with time. Because the fragipan essentially acts as a barrier, it restricts many pedogenic processes to overlying horizons, possibly until these processes are able to overcome the fragipan's ability to impede them. Consequently, processes associated with the regressive pedogenic pathway in the fragipan predominate over those associated with progressive development. Episaturation and resulting changes in redox status also occur at the upper boundary of the fragipan. The combination of these processes and conditions (e.g., wetting-drying, redox conditions and processes, eluviation, microerosion, lessivage, and pervection) promote fragipan degradation. A degradational zone begins to form at the fragipan's upper boundary or along preferred drainage pathways and, with time, becomes increasingly thicker and glossic—all at the expense of the fragipan. This zone exhibits evidence of Fe and clay remobilization and eluviation, microerosion of void coatings and infillings, and destabilization of void walls (Fig. 2F; 4G in Weisenborn and Schaetzl, 2005).
The fragipan may conceivably become completely degraded or even destroyed as clay and amorphous bonding materials are remobilized and translocated, and the solum gets more completely pedoturbated. The result of fragipan destruction, and thus the end point of fragipan evolution in the soils studied, may be a well-developed bisequal solum with a horizon sequence of A-E-(Bhs, Bs)-E'-Bt-C. Fragipan genesis is not likely to be reinitiated in the resulting profile unless it meets the prerequisite conditions for self-weight collapse. Two apparently different end points for fragipan evolution have been observed and described. One scenario involves the downward thickening of an albic eluvial horizon (containing a few, isolated fragipan remnants and Fe-Mn nodules) at the expense of the illuvial fragipan (Lindbo et al., 2000), while the other assumes that the illuvial fragipan is converted to a Bt horizon, requiring at least 120000 yr (Ciolkosz et al., 1995; Ciolkosz and Waltman, 2000).
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The MMFE involves the self-weight collapse of a (soil or) parent material, followed by physical ripening of the collapsed zone. The closely packed fabric and intergrain bridging in the collapsed zone are retained, creating a protofragipan. Amorphous bonding agents, such as forms of Al and Si, then precipitate in the protofragipan due to its position near a weathering-front discontinuity between the highly weathered upper solum and the less weathered parent material. The resulting fragipan becomes better expressed with pedogenic development. Fragipan degradation is initiated by an increasingly prominent, seasonally perched zone of saturation above the fragipan. With time, processes associated with fragipan degradation and translocation of materials to lower parts of the profile exceed those associated with progressive development, and the fragipan is gradually destroyed. The result of fragipan destruction and, thus the end point of fragipan evolution in Great Lakes region may, therefore, be a thick bisequal soil that lacks a fragipan. Subsequent fragipan genesis is not likely to reinitiate in the residual profile.
The MMFE may help to explain fragipan evolution in other loamy soils or parent materials that met the initial, prerequisite conditions set by the model. Thus, the MMFE may be particularly applicable to fragipans in the northern Great Lakes region or other areas with similar loamy soils or parent materials, forest vegetation, and deglacial histories.
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ACKNOWLEDGMENTS |
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Received for publication December 11, 2002.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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on scanning electron microscopy (SEM) images. Italicized peak labels represent elements that are associated with the sample coating or may be present in the SEM beam-chamber. v = void.