Component A: The exploration of magnetic susceptibility as an indicator of topsoil loss in KwaZulu-Natal. Previous research has found that topsoil has higher frequency-dependent magnetic susceptibility than lower soil horizons.
Conclusions
The KLF values of Grid II profile G of the sample pot at four landmarks in the Bartington MS2B sensor. Magnetic susceptibility trends in three profiles from the Bonamanzi Trough, (a) Grid I profile A; (b) Grid 11 Profile F; (c) Grid 11 Profile E; and (d) KLFof Grid 11 profile E.
Introduction, aims and objectives
Introduction
Dearing et al. 1996b; Oldfield 1999; Smith 1999), African topsoil could therefore exhibit a magnetic signature that could be used as an indicator of topsoil loss.
Problem statement
Objectives
- Hypothesis
The null hypothesis states that the average of the difference between topsoil and subsoil frequency-dependent magnetic susceptibility is the same for eroded and non-eroded soil. The Alternative Hypothesis states that the mean of the difference between topsoil and subsoil will differ for eroded and non-eroded soils.
Conceptual framework
- Iron hydroxides and oxyhydroxides
The Nun hypothesis states that the average of the frequency-dependent magnetic susceptibility (XFD) of the topsoil is equal to the average of the frequency-dependent magnetic susceptibility of the subsurface. Mass-specific magnetic sensitivity is a ratio between the volume sensitivity (K) and the sample density (p) (Equation 1.2).
Formation of natural magnetic minerals
- Sedimentary rocks
- Metamorphic rocks
- Theories regarding the formation of fme-grained ferrimagnetic minerals
Long-term weathering and soil formation: leading to the concentration of primary ferrimagnetic minerals (Singer & fine 1989 cited in Dearing et al. 1996b). Because these are primary particles, they should not dominate the frequency-dependent sensitivity measurements (Dearing et al. 1996a). The large size of these particles means that they are not detected by the frequency-dependent magnetic susceptibility sensor (Dearing et al. 1996b).
Anaerobic dissimilatory bacteria: Ultrafine-grained ferrimagnetic particles such as magnetite are thought to be produced by magnetostatic bacteria under anaerobic conditions (Lovely et al. 1987). However, it has been argued that oxygen is required for the synthesis and growth of magnetite (Blakemore et al.. 1996b) argued that SP grains are not abundant in aqueous conditions, so this type of production of ferrimagnetic particles does not contribute significantly to the total content of ultrafine ferrimagnetic particles. As noted above, Dearing et al. 1996b) discredits this theory because ultrafine ferrimagnetic grains are mostly found in free-draining soils.
Thermal transformation: as a result of natural fires and crop burning, weakly magnetic iron oxides and hydroxides are transformed into highly magnetic ferrimagnetic particles, magnetite and maghemite (Kletetschka & Banerjee 1995 cited in Dearing et al. 1996b; Le Borgne 19605 & Oldfie 1999 cited 19605 ).
Influences on magnetic susceptibility
- Underlying material
- Topsoil
- Organic matter
Strong magnetic soils are visible over sedimentary and low-grade metamorphic substrates such as limestone and slate (Dearing et al. 1996b). Another important factor to note is that these areas lie to the south of the area most recently covered by glaciers, and are therefore free of 'coagulation anomalies' in the form of drift deposits (Dearing et al.1996b). The low levels of total iron in rocks such as chalk are thought to be compensated by rapid weathering (Atkinson 1957 cited in Dearing et al. 1996b) which therefore provides large amounts of iron (Moukarika et al.
Dearing et al. 1996a).It is also believed that the soil most affected by burning is well-drained (Thompson &Oldfield 1986). This can be linked to the findings that magnetite and maghemite are often found in biologically active loam rich in organic matter (Le Borgne). 1955 cited in Dearing et al. Enhancement can be measured using X-ray diffraction (Longwoth 1977 cited in Thompson et al. 1980) and other methods, including a frequency-dependent magnetic susceptibility sensor. Dearing et al. (1996a) found that pastures showed stronger pasture frequency-dependent magnetic susceptibility readings since SFM pedogen production was not as disturbed as in fields where plowing occurred.
Plowing effectively vertically distributes SFM and mixes it with the subsoil (Dearing et al. 1996a).
Frequency dependent magnetic susceptibility
This sensor measures samples in 10cc custom made plastic pots or inch long drill cores. This sensor is in the form of a portable laboratory, which makes it more convenient to work in the field and allows faster results that can be equated with environmental observation. It is used to identify the presence of ultrafine ferrimagnetic particles (Dearing 1999) that are present at the superparamagnetic and stable single domain boundary (Bartington 2001). The MS2D loop has a diameter of 185 mm and is used for the rapid assessment of surface materials such as soil, rocks and bedrock (Dearing 1999).
The loop measures approximately the top 60 mm of the surface (Bartington 2001). It is largely used for mapping and surveying (Dearing 1999), slope process studies and archaeological prospecting (Bartington 2001). In comparing data received from the different sensors, it is necessary to take into account that the measurements will vary due to the different field strengths, sample shape and the effects of demagnetization (Dearing 1999). Different sensors are used to detect the presence of specific minerals (Caitcheon 1993) and this must also be recognized when comparing data from several sensors. More accurate measurements with the Bartington sensors can be obtained in the method used to: set up the sensor, prepare the sample and measure the samples (Appendix I).
These types of conditions should eliminate any change in temperature or the presence of electromagnetic fields or strong magnetic metals (Dearing 1999).
Magnetic susceptibility measurements used in soil studies
As discussed in Section 4.2.1, the parent material contributes significantly to the range of magnetic minerals formed during pedogenesis and thus to the soil MS. A higher amount of infiltration thus increases the weathering and the final differentiation of soil profile horizons. Vegetation is important for soil because it first protects the soil surface from erosion.
The organic matter added to the soil varies with the type of vegetation, e.g. Large amounts of vegetation are likely to add large amounts of organic matter to the soil. Following these processes, layers in the soil known as horizons indicate the soil development that has occurred over time.
In this horizon, the soil has undergone a lot of pedogenesis, and as a result, there are no structural similarities between the soil in the B horizon and the parent material.
Soil processes: inputs, outputs, transformations and translocations
- Transformation
This is evident in the gray or blue-grey color of the soil that occurs during reduction (Nortcliff 1983). The change in color of the soil is evidence of the altered minerals and oxidation and reduction of iron in the material (Brady & Weil 1999; Singer & Munns 1999). Infiltration is related to the penetration of water into the soil and percolation is the passage of the water through the soil.
As water moves through the soil, it displaces water previously in the soil (Gerrard 2000). iii) Eluviation. Sodium chloride (NaCl), sodium sulfate (Na2S04) and calcium sulfate (CaS04) in a wet climate are completely washed out of the soil. Loss refers to the material lost from the soil system and includes leaching. i) Leaching.
Decaying plant residues form organic acids that are then leached into the soil by water.
Salinisation
Areas of low rainfall have a limited amount of water or dissolved minerals from the bottom of the profile, including calcium carbonate (Nortc1iff 1983). Calcium carbonate is a diamagnetic material and is thought to dilute the MS signal ( Thompson & Oldfield 1986 ; Dearing 1999 ), so calcification can be expected to reduce the MS in the profile.
Ferrallitisation and ferrugination
Limitations to the literature -South African soils
Maghemite depletion due to erosional cycles occurred largely at the coastal areas (Figure 8.1). The Lesotho Plateau is characterized by large amounts of Ti magnetite and ilmenite and various forms of Fe and Ti oxides with a poor crystalline structure (Fitzpatrick 1978). They are thought to be related to magmatic parent material or, in secondary form, from pressure or heating in the presence of organic matter (Fitzpatrick 1988). Many soil scientists have previously advocated that high concentrations of iron oxide occur in warm, humid climates found in the tropical, equatorial, or Mediterranean climates. However, iron compounds have been located in southern Africa in a wide range of different climatic environments, e.g. semi-arid zones.
The passage of the African continent through geological periods from epochs of cold periods to warmer, wetter climates has encouraged the accumulation of iron in the soil (Fitzpatrick 1988). South African soils have also been found to exhibit ferrimagnetic particles in the form of titanomaghemite produced by pedogenic weathering of titanomagnetite. African soils to those in the Northern Hemisphere that have ferrimagnetic particles mainly in the form of magnetite. In this case, burning will decrease magnetic susceptibility.
Some of KwaZulu-Natal's soils have coarse-grained topsoil as a result of eluviation (Macvicar et al.
Summary
The exploratory phase will attempt to identify whether a relationship exists between frequency-dependent magnetic susceptibility and both surface and subsurface. All sampling will be carried out in areas with well-defended topsoil, so that the relationship between frequency-dependent magnetic susceptibility and topsoil and subsoil can be correctly documented. Both samples will be examined using the Bartington MS2B dual frequency sensor which will measure the magnetic susceptibility of.
1999). The frequency-dependent magnetic susceptibility of soil from different depths can then be used to determine the signature of the topsoil and subsoil in that geological region. This is important as no previous research on frequency dependent magnetic susceptibility has previously been carried out in South African soils. Magnetic susceptibility of the soil and its significance in soil science-A review.Journal ofSoil Science.
The use of magnetic susceptibility measurements to delineate wetlands in KwaZulu-Natal, South Africa. Unpublished Master's thesis: University of Natal, Pietermaritzburg.
Appendix
- Sample size
- Sample measurement
- Introduction
The second air reading is used again for the first air measurement of the next sample. The magnetic susceptibility is determined by subtracting the average of the two air measurements from the average of the two sample measurements. It is recommended that sample containers be first measured empty to obtain an average diamagnetic susceptibility reading that can be added to the sample magnetic susceptibility.
Similarly soil from the face of the canal (outer 10 mm) was removed and samples were placed in plastic sampling bags. Three profiles (Grid I Profile D, Grid II Profile G, and Grid II Profile J) were selected based on the large within-horizon variations in the initial MS results. Large variations in magnetic susceptibility in samples from the same layer to adjacent profiles in different parts of the channel were evident for both XFD and KLF.
The KLF magnetic susceptibility measurement does not take into account the density of the sample. The KLF values of the Grid I Profile D and Grid II Profile J samples differed from the original set of MS values by a maximum of 56.5% (Table 1). Within an active, alluvial soil environment such as the BGR ditch area, MS does not appear to show layering in the soil profile.
