CASE STUDY 1: SOILS 1. Iron minerals in soils: the iron cycle

Soils originate from the interaction of physical, chemical, and biological processes of weathering. When sediments and rocks are exposed at the land surface, physical and chemical weathering processes take place. The iron minerals borne by the parent sediment or rock are actively involved in the weathering process. Chemically unstable iron minerals oxidize to iron oxyhydroxides (FeOOH), or to hematite ( ). For example, olivine is weathered according to the reaction:

2 3Fe O

32 4 2 22Fe SiO H O O 2FeOOH + SiO+ + → 4

The weathering products accumulate in the so-called C horizon, which is the parent material from which a soil starts to form (Fig. 1). Weathering enables the dissolution of minerals in the rain water. These minerals are then available for plant growth. The photosynthetic activity of plants is responsible for the production of organic matter which accumulates in the upper part of the soil (the A horizon). The organic matter is oxidized and mineralized by microbes and small animals that live in the soil. The respiration of organic matter produces a local depletion of oxygen in the soil and enables the formation of a reducing environment. Under reducing conditions, (ferric ions) contained in weathering products (such as FeOOH), are reduced to (ferrous ions). Magnetite ( ) precipitates under reducing conditions and might be subsequently oxidized to maghemite ( ). The typical overall reaction is:

3Fe +

2Fe +

3 4Fe O

2 3-Fe Oγ

13 4 2 226FeOOH 2Fe O 3H O + O→ +

Fig. 1: A simplified soil profile. Capital letters designate the five most common soil horizons. O is a layer of accumulating orga-nic matter. The so-called topsoil (A) is a mixture of minerals and humified organic matter. The B horizon (also called sub-soil), is the layer where new minerals form and/or accumu-late. This is the layer where new magnetic minerals can form. The C horizon contain minerals from the parent rock or sedi-ment (R) and their weathering products. A schematic represen-tation of the typical chemical processes occuring in each layer is drawn on the right.

Fe

FeOOH

Fe O

Fe SiO

3

2

2 4

4

+

FeOOH

O

A

B

C

R

Fe O2 3

1

The precipitation of magnetic minerals can occur inorganically, but it has been shown that a class of so-called dissimilatory iron-reducing bacteria (such as Geobacter) is able to link the reduction of ferric iron to the oxidation of organic matter by inducing the precipitation of magnetite. The magnetite particles formed in this way are much smaller (typically 10-100 nm) than the detrital magnetite contained in rocks and their weathering products (Fig. 2).

Fig. 2: (a) Detrital magnetite crystals occur in varous shapes (an octaedron in the electron micrograph) and are typically >100 nm in size. (b) Pedogenic magnetite grains are much smaller (typically 10-100 nm). The difference in grain size is re-flected in many magnetic properties [from: B. Maher and R. Thompson, Quarternary climates, Environments and Magnetism, Cambridge University Press, 1999, 383 pp.). Oxidation of iron minerals by weathering and subsequent formation of magnetite by iron reduction in presence of organic matter are the two main processes that charac-terize the iron cycle in soils. Magnetic measurements allow the identification of dif-ferent iron minerals (typically detrital magnetite and hematite versus pedogenic magnetite) whose abundance and spatial distribution in the soil are indicators of the iron cycle.

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2. Applications in environmental magnetic studies

The production of pedogenic magnetite in soils is intimately related to the degrada-tion of organic matter, which in turn depends on the climatic conditions. Rainfall is the main factor that enhances the production of pedogenic magnetite up to a certain critical level. Above this level, soils become occasionally or permanently saturated with water, and excessively reducing conditions enable the dissolution of the fine grai-ned pedogenic magnetite. Magnetic measurements of soils formed from loess deposits have been successfully used as (paleo-) climatic proxies for rainfall. Magnetic suscep-tibility measurements of modern soils at different sites show a clear correlation with the measured annual rainfall (Fig. 3a). This correlation has been used to calculate the mean rainfall over the Chinese Loess Plateau during the last 150,000 years (Fig. 3b).

Fig. 2: (a) Present-day rainfall and magnetic susceptibility of modern soils at nine sites in the Chinese Loess Plateau. Rainfall values R represent 30-years averages (1951-1980). Magnetic values (χ in ) represent pedogenic susceptibility (i.e. the susceptibility of the B horizon minus that of the C horizon). The regression line with correlation coefficient 0.95 can be used to reconstruct the mean rainfall at the time of formation of fossil soils (paleosols) [Redrawn from B. Maher, R. Thompson, L. Zhou, Spatial and temporal reconstructions of changes in the Asian paleomonsoon: a new mineral magnetic approach, Earth and Planetary Science Letters, 125, 462-471, 1994]. (b) Paleoprecipitation at Lochuan (Chinese Loess Plateau) according to different authors. Bold numbers refer to the correspon-ding oxygen isotope stages [from M. Evans, F. Heller, Magnetism of loess/paleosol sequences: recent developments, Earth-Science Reviews, 54, 129-144, 2001.

8 310 m kg− 1−

10222 199 logR χ= +

3

Another environmental application of magnetic measurements on soils is the survey of polluted areas. Soils may become contaminated through the air or directly by dif-fusion of pollutants from waste deposits. Many industrial processes, such as the pro-duction of steel and cement, and the combustion of fossil fuels, generate abundant airborne magnetic particles. Magnetite spherules with a diameter of are ge-nerated during the combustion of fossil fuels from tiny amounts of iron sulphides ( ). These particles are transported hundres of km from the production point and are finally deposited in soils, where they can be detected with magnetic measu-rements (Fig. 4).

20 m≈ µ

2FeS

Fig. 4: Soil pollution in the Katowice Province (Poland), caused by 16 coal burning po-wer plants, 17 steel mills and 56 coal mines. (a) Dust fall and topsoil magnetic susceptibility along the west-east transect through Katowice Province. Both profiles show two peaks, each within a km from a large power station. (b) A typical fly-ash spherule produced by com-bustion processes. (c) Map of Upper Slesia (Poland) showing magnetic susceptibility of soils. Katowice Province is outlined. [(a,c) modified from F. Heller, Z. Strzyszcz, T. Magiera, Mag-netic record of industrial pollu-tion in forest soil of Upper Slesia, Journal of Geophysical Research, 103, 17767-17774, 1998)].

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3. Magnetic characterization of soils

The effective use of magnetic measurements as environmental proxies rely on the di-stinction among different magnetic minerals with different origins. This is an im-portant task in environmental magnetism and is called magnetic unmixing. Individual bulk magnetic measurements are not diagnostic: a combination of different measure-ments is always necessary to obtain useful information about the iron minerals in soils. For example, very high magnetic susceptibility values in a soil may be explained by (1) a strongly magnetic parent rock (such as basalt), (2) a strong pro-duction of pedogenic magnetite, due to a warm, humid climate, or (3) severe pollu-tion. Additional measurements are necessary to obtain the correct answer. For exam-ple, the frequency dependence of susceptibility is an indicator of very small grain sizes and might be used to discriminate (2) from (1) and (3), since magnetic grains in parent rocks and in polluted dusts are generally coarse. Cases (1) and (3) can be discriminated by comparing the magnetic properties of the C and the A horizons. This workshop is intended to give you a brief (and necessarily incomplete) overview of the different magnetic measurements that can be used to characterize a soil. Susceptibility Susceptibility measurements are fast and inexpensive, and can be performed on col-lected samples, as well as directly in the field with a so-called field sensor. Magnetic susceptibilty is sometimes difficult to interpret, since all kinds of magnetism (diamag-netism, parmamagnetism and ferrimagnetism) are sensed:

dia para ferχ χ χ χ= + + ri

Diamagnetism is very weak and is almost always negligible. The paramagnetic su-sceptibility can be calculated from the slope of a hysteresis loop at high fields (Fig. 5). Since most minerals in the parent rock or sediment are paramagnetic, can be used to track the loss of iron due to weathering processes.

paraχ

−2

1

2

−0.5 0.5

magnetic field, T

mag

net

izat

ion χpara

Fig. 5: Hysteresis loop of a sample taken at 65 cm depth in the St. Paul soil pit. The paramagnetic susceptibility

is simply the slope of the hysteresis loop at high fields. Most instruments for the masurement of hysteresis loops (such as the Micromag vibrating sample magnetome-ter) are supplied with a soft-ware that automatically cal-culates .

paraχ

paraχ

5

Frequency dependence of susceptibility Some susceptometers can measure the susceptibility at two different frequencies and and allow the calculation of the so-called frequency dependence of suscepti-bility, :

1f

2f

fdχ

1 2fd

1 2

( ) ( )100

( )log( / )f f

f f fχ χ

χχ

−=

1

2

with . The frequency dependence is simply the relative difference between two susceptibilities measured at different frequencies, expressed in percent. This parame-ter is sensitive to the presence of very fine (superparamagnetic) ferromagnetic parti-cles, and is therefore particularly suitable for the identitfication of pedogenic magne-tite in soils. The theoretical range of values for is between 0 and 50%. However a maximum value of 30% is observed in natural samples. The frequency dependence of well-developed soils may reach 20%, more common values are 5-10%. The frequency dependence of coarse (multidomain) magnetite is negligible.

1f f<

fdχ

Isothermal remanent magnetization (IRM) and magnetization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied. Each magnetic grain is a miniature compass needle with a permanent dipole moment. The minimum field required to re-verse the direction of such dipole is called the switching field. Each magnetic grain has a switching field that depends on its size, mineralogy and other physical pro-perties. An assemblage of magnetic grains, all with a different switching field, changes its remanent magnetization when a magnetic field is applied. The application and removal of successively increasing fields is used to measure a socalled magnetization curve. The derivative of such curve is the statistical distribution of switching fields, also called coercivity distribution (Fig. 6).

Fig. 6: A so-called backfield magnetization curve of a sa-mple taken at 65 cm depth in the St. Paul soil pit. Prior to measurement, a positive field of 1.0 T was applied to give the sample a saturation rema-nence . Then, successively higher fields where applied in the opposite direction, and the remanent magnetization was measured after each step, un-til the remanent magnetiza-tion of the sample was com-pletely reversed.

rsM

−0.5 −0.4 −0.3 −0.2 −0.1

−1.0

−0.5

0.0

0.5

1.0+

−

M

M

H

rs

rs

cr

magnetic field, T

mag

net

izat

ion

, µA

m2 kg

−1

6

The maximum possible remanent magnetization acquired by a sample is called the saturation remanence, . The field required to cancel a saturation remanence is the coercivity of remanence. It is the median of the distribution of switching fields. Magnetite and maghemite particles have maximum switching fields of 300 mT. This means that the curve in Fig. 6 would be perfectly horizontal for fields higher than 300 mT if the sample would contain only magnetite or maghemite. The fact that satura-tion is reached only at higher fields indicates that the sample contains some other ferromagnetic (s.l.) minerals, probably hematite or goethite. Minerals that can dis-play switching fields above 300 mT are called high-coercivity minerals. On the other hand, magnetite and maghemite are low-coercivity minerals. A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stands for saturation), defined as:

rsM crH

rs 300

rs

M IRMS

M−

=

where is the remanent magnetization acquired in a 300 mT field. indicates that there are no high-coercivity minerals. Other similar parameter have been defined empirically to discriminate between different minerals.

300IRM 1S =

Anhysteretic remanent magnetization (ARM) The ARM is a particular kind of magnetization acquired in an alternating field whose amplitude slowly decreases to zero. If the field is perfectly symmetric, the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized). However, if a small, constant bias field is added to the alternating field, the sample acquires a magnetization called ARM. This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence. An interesting property of the ARM is its extreme grain size selectivity: the highest ARMs are acquired by so called single-domain grains (which correspond to 30-100 nm grain size in magnetite). The efficiency of the ARM is eva-luated by normalizing it with the saturation remanence or with the susceptibility. Hence, the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators. High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles.

DCH

DC rs/( )ARM H M DC/( )ARM H χ

3DC rs/( ) 10 mAARM H M − −> 1

DC/( ) 5ARM H χ >

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of the fine grained pedogenic particles that become permanently magnetized below room temperature. High temperature measurements of soils are difficult, because the high surface-to-mass ratio of soils and the presence of organic matter accelerate the irreversible alteration of many magnetic minerals above 200°C.

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4. The St. Paul soil profile The goal of this case study is the characterization of the iron cycle in the example of a soil profile collected at the St. Paul campus of the University. The 1.1 m thick profile has been sampled every 3 cm in the uppermost 30 cm, and every 5 cm below. The soil developed from unconsolidated glacial till, which can be found at a depth of 1.1 m. Between 1.1 and 0.7 m depth, the finest fraction of the glacial till is mixed with clay minerals. The yellowish clay layer (C horizon) extends to a depth of 40 cm. Above, the dark brown B horizon can be clearly recognized. The uppermost 8 cm are rich in organic matter (A horizon). Soils are extremely heterogeneous: their physical and chemical properties change dra-stically even within the same layer on a cm scale. Therefore, large amounts of mate-rial (>100 g) have been collected from each layer. The material was dried and sieved to collect the fraction <63 µm. The sieving produre excludes large rock fragments, which occur randomly and may bias the measurements. The magnetic minerals have a much smaller grain size and pass through the sieve. Finally, the sieved, homogeni-zed material from each layer was packed in plastic boxes or gelcaps for magnetic measurements. The measurements that you are going to perform during this work-shop on a few samples are shown in Fig. 7-9 for the entire soil profile. We will now discuss the measurement in detail.

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0 1 2 3 4 5 6

0 1 2 3 4 5 6

susceptibility, µm3/kg

dept

h, c

m

paramegnetic susceptibility, nm3/kg frequency dependence, %

A

B

E

C

R

A

B

E

C

R

Fig. 7: Magnetic measurements on the St. Paul soil profile. Capital letters indicate the soil horizons. Left: magnetic low-field susceptibility, measured with a Kappabridge KLY2. Middle: paramagnetic susceptibility, calculated from the high-field slope of hysteresis loops measured with a Micromag Vibrating Sample Magnetometer. Right: frequency dependence measured with a Bartington MS2B.

8

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coercivity of remanence, Hcr (mT) ARM (0.1 mT DC), Am2/kg ARM/susceptibility2 3 4

1 52 3 4

A

B

E

C

R

Fig. 8: Magnetic measurements on the St. Paul soil profile. Capital letters indicate the soil horizons. Left: Coercivity of remanence, measured with a Micromag Vibra-ting Sample Magnetometer. Middle: ARM, imparted with a D-TECH D2000 AF sy-stem (max. alternating field: 200 mT, bias field: 0.1 mT) and measured with a 2G cryogenic magnetometer. The ARM ratio calculated from the ARM measurements plotted in this figure, and the susceptibility measurements of Fig. 7. Interpretation of the magnetic measurements All parameters that depend on the concentration of magnetic minerals (susceptibility, ARM, IRM) have a clear maximum in the B horizon, between 8 and 22 cm, and to a minor extent in a layer below the B horizon, called E (for eluvial). This indicates the formation of new magnetic minerals in the B horizon, a process called magnetic enhancement. A combination of magnetic parameters can be used to identify the mineralogy and, to some extent, the grain size of the newly formed minerals. The fre-quency dependence of susceptibility peaks in the B horizon, indicating the presence of superparamagnetic (i.e. very fine grained) minerals. Fine grained magnetic particles in the B horizon are also suggested by the higher values of the ARM ratio. On the other hand, both grain size indicators indicate the absence of fine particles in the parent sediment (the R horizon). The decrease of the coercivity of remanence in the B horizon (Fig. 8) is compatible with the presence of very small and round magnetite or maghemite particles (see Fig. 5b). The progressive transition between the typical magnetic parameters of the B and the R horizon suggests that the C horizon contains a mixture of parent material and of magnetic grains that have been formed above, in the B horizon. These grains were probably transported downwards by the percolation

9

of rain water. The difference between the IRM acquired in a 300 mT field and in a 500 mT field ( ) is equal to the magnetization of all magntic grains with switching fields between 300 and 500 mT. Since magnetite and maghemite are completely saturated above 300 mT, is an indicator of the concentration of hematite or goethite grains. As it can be seen from Fig. 9, hematite or goethite are formed in the B horizon as well.

500 300IRM −

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0 100 200 300 400 500

ARM (0.1 mT DC), µAm2/kg IRM (500 mT) − IRM (300 mT), mAm2/kg

A

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C

R

A

B

E

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R 100

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dep

th, c

m

Fig. 9: Comparison between the abundance of low-coerci-vity minerals (left), and high-coercivity minerals (right). Magnetic enhancement occurs for both types of minerals in the B horizon. Fluctuations of the signal in the C horizon in the right plot are due to mea-surement errors.

The fate of iron The measurement described above are relatively fast and easy to perform. They give a qualitative picture of what is happening in the soil. However, more detailed measu-rements are necessary to make a precise quantitative estimate of the iron budget in a soil. One conceptually simple technique to quantify the newly formed magnetic mine-rals by distinguishing them from those contained in the parent sediment consists in the measurement of the statistical distribution of coercivities of remanence. A set of magnetic particles with a common origin and “geochemical history” has a specific statistical distribution of switching fields that provides a unique magnetic signature. Magnetic grains that originated from the parent sediment (so-called detrital particles) have a distribution of switching fields that is different from grains that precipitated chemically in the soil (the so-called pedogenic component). These distributions of switching field can be used to quantify exactly the occurrence of the two groups of particles in a soil (see Fig. 10). Detailed measurements show that the abundance of ferrimagnetic detrital particles is constant along the entire soil profile above the R horizon. This means that the detrital ferrimagnetic grains in the soils are not destro-yed by weathering processes. This fact gives raise to following question: where does

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the iron necessary to the formation of the pedogenic minerals come from? A partial answer is provided by the measurment of the paramagnetic susceptibility (Fig. 7). Paramagnetic minerals are depleted in the B horizon to an extent which is proportional to the amount of newly formed minerals. We can therefore conclude that paramagnetic minerals (such as clays and biotite) lose part of their iron in the B hori-zon.

1 10 1000

4

2

modern soil(western ChineseLoess plateau)

AF peak field, mT AF peak field, mT

1 10 100

PD

L H

PD

D

4

2

0

modern soil(Minnesota)

(a) (b)

magnetic c

ontr

ibution

magnetic c

ontr

ibution

Fig. 10: Statistical distribution of switching fields in two modern soils collected in Minnesota (a) and on the Western Chinese Loess Plateau (b). The black curve in each plot is the derivative of a magnetization curve obtained by stepwise demagneti-zation of an ARM. This curve has been modelled with a set of statistical distributions (coloured curves), each representing the distribution of switching fields of a particular set of magnetic particles with a common geochemical origin. PD are pedogenic mag-netite or maghemite grains; D and L are low-coercivity minerals from the parent sediments (glacial till and loess, respectively); H is the contribution of a high-coercivi-ty component (hematite). Notice that the properties of PD are amazingly similar in the two soils, despite the difference in climate and parent material. This observation suggests that the mechanism of magnetic enhancement (but not the amount) is similar in a wide group of soils. Fig. 9 shows clearly that both magnetite and hematite or goetite form in the same layer of a soil. This is surprising at first sight, because the chemical conditions (pH and Eh) required for the formation of these minerals are very different. In fact, the mechanism of pedogenesis and the role played by some microorganisms is yet not completely clear. However, a possible explanation for the coprecipitation of such dif-ferent minerals relies on the heterogeneity of soils, which probably have very large chemical gradients on a sub-millimeter scale. The diffusion of ions toward more reducing or more oxidizing portions of the soil can lead to the formation of magnetite and hematite a few mm apart.

2Fe +

11