Chapter 1: Introduction, aims and objectives
4.3 Influences on magnetic susceptibility
4.3.1 Underlying material
5. Thermal transformation: due to natural fires and crop burning, iron oxides and hydroxides that are weakly magnetic are converted to highly magnetic ferrimagnetic particles, magnetite and maghaemite (Kletetschka & Banerjee 1995 cited in Dearing et al. 1996b; Le Borgne 1955 & 1960 cited in Oldfie1d 1999). (This is further discussedinSection 4.3.2).
6. Abiological weathering of Fe(!!): results in oxidised magnetite or maghaemite, evident in synthetic experiments (Maher & Taylor 1987 cited in Dearing et al.
1996b; Maher & Taylor 1988). It is important to note that SFMs (secondary ferrimagnetic minerals) will also eventually weather away but if a new iron supply is available new SFMs will form (Dearinget al. 1996b).
7. Formation ofmicrocrystals of maghaemite or magnetitefrom weakly magnetic iron oxides and hydroxides :this occurs due to reduction-oxidation cycles that take place in pedogenesis. This process is not well understood (Thompson& Oldfield 1986).
Dearinget al.1996b adds a further category of "anaerobic microbial Fe reduction", but this can be seen as an overlap of the theory related to anaerobic dissimilatory bacteria.Conclusively only thermally transferred minerals can be seen to have a significant contribution to XFD and therefore is the only viable theory of formation of fine-grained ferrimagnetic minerals.
Research carried out in the United Kingdom has found that strongly magnetic soils with ultrafine-grained ferrimagnetic particles occur over material that has no concentration of primary ferrimagnetic particles (Maher & Taylor 1988). Strongly magnetic soils are evident over sedimentary and low-grade metamorphic substrates such as limestone and slate (Dearing et al. 1996b). This is evident in areas consisting of slate in Devon in the United Kingdom (Maher & Taylor 1988). Another important factor to note is that these areas lie south of the area most recently covered by glaciers, and thus are free of 'igneous erratics' that occur 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) thus supplying high amounts of iron (Moukarika et al. 1991 cited in Dearing et al. 1996b).This is done by releasing iron in the ionic form that is either followed by oxidation leading to the precipitation of iron,or iron is taken into soil solution and is then precipitated in another section of the profile. Both of these rely on the oxidative conditions of the soil (Maher 1986). Usually soil with large amounts of calcium carbonate have low magnetic susceptibility values, but when dissolution occurs large amounts of magnetic minerals are left in the giving a high magnetic susceptibility value (Dearinget al.1996b). This shows that magnetism is masked by by minerals diamagnetic in nature, such as calcium carbonate (Thompson & Oldfield 1986; Dearing 1999).Although this is inconsistent with De long et at. (1998) who found higherXFD values in calcium carbonate rich soil.
Iron supply does not totally control soil magnetic susceptibility as other factors, such as weathering, also play an important role (Dearing et at. 1996b). Weathering relies on climatic controls in order to supply iron to the soil. Maher (1986) states that although the macroclimate may control the general release of iron, the "micro-environment pedoclimate" is important in determining its final composition. In soils the weathering processes that are dominant include hydrolysis and solution, and these are largely driven by precipitation (Dearinget al.1996b).
4.3.2 Effects of burning
Oldfield (1999) advocates that if sufficient paramagnetic or imperfect anti-ferromagnetic iron is burnt at a temperature of greater than 400°C then the magnetic properties of the
and loss of an electron, respectively. Reduction conditions occur due to heating and rapid cooling, causing magnetite to form. Oxidation then takes place, allowing maghaemite to result in a thin soil layer under the fire (Jonscher 1975; Kletetschka& Banerjee 1995 cited in Dearing et al. 1996b; Le Borgne 1955 & 1960 cited in Oldfield 1999). Maher (1986) includes organic matter in the combustion of the soil as an important factor in creating reducing conditions in the soil pores (Figure 4.3).
Produces reducingsoil pore atmosphere
Reoxidises to maghaemite as
air entersand coolsthe soil
Poorlydrained soils with increased temperature forms
maghaemite
Figure4.3: The formation ofmaghaemite due to burning (after Scheffer et al. 1959 cited in Maher 1986).
Research relating burnt soils to high XFD%has been done extensively. Studies of sediment cores from Lago di Origlia, South Switzerland have found that XFD% peaks correspond to charcoal peaks and thus catchment fires that caused viscous and SP grains to be present in the material (Oldfield 1999). A study in North Wales showed that soil, with no input of primary ferrimagnetic particles, burnt in forest fires had a maximum Xvalue of 36lJ.m3kg-l compared to aXvalue ofO.13lJ.m3kg-l in unburnt areas (Maher 1986).
Dearing et al. (1996b) argue against burning as a major contributor towards increasing magnetic susceptibility in soils. Firstly, they argue that if crop burning causes an increase in the value ofXFD, then arable land should have higher frequency dependent magnetic susceptibility than ley grasslands. However, the converse of this was found (Dearing et al.
T.Barker:The exploration of magnetic susceptibility as an indicator for topsoil loss in Kwazulu-Natal 30
1996b) as it was not taken into account that cultivation disrupts the pedogenic processes in the soil that result in the production of ultrafine-grained ferrimagnetic particles (Dearinget al.1996a).Dearing et al.(1996b) propose as their second argument the fmdings of a study at Rothamsted Experimental Station where straw on adjacent plots was burnt or acted as control plots. The topsoil measurements in this study showed no significant differences in frequency dependent magnetic susceptibility following eight years of burning. Problems associated with these findings are that the burning was controlled, and natural fires are uncontrolled and thus may reach a higher temperature that could be conducive to the formation of ultrafme ferrimagnetic particles. The straw had also been cultivated and thus would be more spread out and less dense than natural vegetation, which may also contribute to higher temperatures in natural fires than those on cultivated land, lowering the production of ferrimagnetic particles. Dearing et al. (1996a) found that soil samples exposed to burning in the laboratory caused the 'XFD%value to reach a high measurement of 12%.However this doesn't consider the interaction of environmental factors.
There is a strong interaction between burning, iron available, temperature and organic matter present (Thompson & Oldfield 1986; Dearing et al. 1996a).It is also thought that the soils most affected by burning are well drained (Thompson &Oldfield 1986).This can be linked to the findings that magnetite and maghaemite are often found in biologically active topsoil that is rich in organic matter (Le Borgne 1955 cited in Dearing et al. 1996a;
Maher & Taylor 1988). This is related to the need to create a reducing atmosphere in the soil pores (Figure 4.3). Research has also found that in burnt areas the highest magnetic susceptibility readings were found around tree trunks, and this is related to the accumulation of organic matter (Maher 1986). A field experiment was carried out by burning soil profiles with an organic horizon to show the importance of organic matter because higher magnetic susceptibility measurements were obtained from the organic horizon than the inorganic horizon.The soil beneath the ashed organic layer was pink in colour (Longworth et al. 1979 cited in Thompson & Oldfield 1986), indicative of the change in composition of the iron oxides. Burnt topsoil can result in a range of colours: black,grey, pink or bright orange, depending on the amount of magnetic minerals formed.
This occurs concurrently with a shift in multidomain magnetite to a high concentration of viscous and SP magnetite that occurs with a growing haematite component. This particular experiment found that mostly magnetite resulted (Thompson& Oldfield 1986), which goes
burning. His theory can also be criticised for only examining maghaemite's formation though burning and anaerobic reduction (Jonscher 1975) when maghaemite occurs in smaller proportions in the environment to magnetite. There must thus be instances when maghaemite does not fully oxidise and magnetite remains.
Mullins (1977) also highlighted the importance of the availability of oxygen in burning. This can be linked to aiding the burning process and oxidation-reduction. Soil iron is yet another significant factor and the resultant ferrimagnetic particles are also thought to be more evident in iron-enriched subsoil (Maher et al. 1994 cited in Dearing et al. 1996a).
Soils that are poor in iron gain minimal effects of enhancement by burning (Thompson &
Oldfield 1986). This is an important point to note as it has important implications for fieldwork attempting to distinguish the topsoil from the subsoil using frequency dependent magnetic susceptibility.
Other factors that affect burning are the intensity of the fire. Fires that occur in the canopy only or low-intensity fires on the surface are not sufficient to alter surface minerals (Thompson & Oldfield 1986). Thompson & Oldfield (1986) express their concern in attributing ferrimagnetic particle formation to burning or pedogenesis because of the lack of sufficient means to differentiate the product of these two processes.