The purpose of this exercise is to introduce you to how magnetic signals are derived and preserved in the natural environment and what they tell about soil development, and learn techniques to measure soil magnetic susceptibility, from soil cores in particular.
Upon completion of this exercise you should be able to:
- operate a Bartington MS2-C sensor to measure magnetic susceptibility of a soil core
- use Excel to create a magnetic susceptibility graph
- interpret a magnetic susceptibility graph
All material has a magnetic signal; this magnetic signal is a result of the spin of electrons, both around their axis and around the nucleus, within atoms. The degree of magnetism is controlled by the types and concentrations of atoms within a mineral structure and the arrangement of that structure. In soils, the magnetic signal is imparted by the soil’s parent material, which contains primary minerals, and by pedogenic weathering, which forms secondary minerals that produce magnetic enhancement.
Magnetic susceptibility is simply the measure of a material’s (i.e. soil’s) ability to become magnetized and is generally controlled by the iron-bearing minerals in a sample. Magnetic susceptibility is one measure for determining the mineralogy and geochemistry of samples, providing insight into provenance and environmental conditions during mineral formation. The Bartington MS2C core scanning sensor works by 1) creating a magnetic field, 2) detecting the magnetism of the sample, and then 3) creating a ratio between the two – magnetic susceptibility. This magnetic susceptibility value is on a volume basis, and thus, is referred to as volume susceptibility (κ), in contrast to mass-specific susceptibility, in which measurements from dried samples are adjusted for weight by volume.
Magnetic susceptibility offers a wide array of scientists a simple, inexpensive, and convenient tool for environmental research. There are several reasons, or advantages, for measuring magnetic susceptibility. The first, and most obvious, is that it is easy to use and fairly inexpensive; besides the initial purchase of equipment, there is virtually no expense since chemicals or other agents are not used. This method is also fast, non-toxic, and nondestructive to samples. Several hundred soil samples can easily be measured in a day, and, since no additional chemicals are used, samples can be reused for further study, such as x-ray diffraction, particle size analysis, and stable isotope analysis. Equipment is small, lightweight, and easily portable, so measurements can be made in the field or in a laboratory environment. In addition, due to its low cost, magnetic susceptibility is often coupled with additional environmental properties like stable carbon isotope, radioisotope, chemical, and microfossil analysis.
Partly due to its ease of operation and inexpensive nature and partly because of the ubiquity of a magnetic signal in the natural environment, magnetic susceptibility has wide-ranging applications in nearly every field of science. Here is a short list of some fields that commonly use magnetic susceptibility and associated applications:
Geology: field mapping, identifying rock type, identifying erratics, identifying minerals
Soils: field mapping, identifying provenance, identifying buried soils, highlighting slope processes, mapping patterns of sediment accumulation
Archaeology: locating occupation sites, identifying hearths, building materials, and foundations
Hydrology: tracing bedload movement, identifying sediment and pollution sources
The MS2C sensor is designed to make volume susceptibility measurements from soil cores collected in diamagnetic clear-plastic liners. The soil core is simply placed on the holding rack and incrementally slid through the MS2C sensor at intervals of 20 – 50 mm. Magnetic susceptibility readings are influenced by surrounding material up to one sensor diameter away, so a 72 mm diameter sensor actually measures a 144 mm section; thus, an individual measurement can be thought of as a 3-point moving average with one center position and two end points. As a result, the susceptibility signal degrades towards the end of the core, so measurements should not be taken within ½ core sensor diameter from soil core ends (i.e. with a 72 mm sensor measurements should not be taken within 36 mm of either end of the soil core). As discussed above, magnetic susceptibility in soils results from parent materials and pedogenic weathering; consequently, magnetic susceptibility measurements taken throughout a soil can reveal changes in parent material and environmental conditions, such as climate change. For example, deposited and reworked loess generally has a reduced magnetic susceptibility signal compared to underlying residuum while limestone bedrock has virtually no signal (Fig. 1).
Figure 1. Magnetic susceptibility and other data for a soil core (Bowen and Johnson, 2015).
- USB drive (supplied by student)
- Computer with Excel & internet access
- Bartington MS2C core logging system
- Soil cores
**NOTE** Before you begin, remove iPods (turn off also), cell phones (turn off also), keys, and any other metallic or electronic material on you (piercings are fine). Also, make sure no metallic material is on the table that the sensor and rack are sitting on.
- First, turn on the Bartington meter by turning the left-most knob to “SI”, make sure the right-most knob is set to “1”. Flip the silver toggle switch to the right to zero the sensor – every few seconds you should here a “beep” and the meter will quickly flash “00:0.0”. Allow sensor to zero for a minimum of ten minutes.
- If not on, turn on the computer and open the “304-MagSus_blank” Excel file.
- Save the 304-MagSus_blank Excel file as FirstnameLastname_MagSus. Notice on the Excel file that there are two worksheet tabs named Soil Core 1 and Soil Core 2 (see bottom left of Excel) – you need to analyze both soil cores. Within this file Depth and Tape Position values are already entered. Tape position is only used as a guide to position the bottom end of the core (i.e., the end with the black cap); you will not use tape position to make your graphs.
- After 10 minutes or more has elapsed, flip the silver toggle switch to the center position.
- Place the first core on the core rack and slide the end with the red cap through the sensor until the end of the black cap is aligned exactly at 122 cm.
- Take your first reading by pressing the “M” button. After the first reading appears, enter this value into the Excel spreadsheet in the “Mag Sus” column, advance the core 2 cm, and press “Measure” again. Repeat until the end of the black cap is aligned with the middle of the sensor. There is a piece of blue tape near the left end of the rack designating the 2 cm and 0 cm positions for your final two readings.
- Repeat steps 5 & 6 for the second soil core.
- Once both cores have been analyzed and saved, you need to create graphs of magnetic susceptibility variability by depth, as demonstrated in class. You should only create one file with two worksheets that include data and graphs. There is an example graph on the desktop of the Magnetic Susceptibility computer – MarkBowen_MagSusExample.
- Create an X-Y scatterplot with magnetic susceptibility on the x-axis and depth on the y-axis (see graph examples in Fig. 1 above).
- On each graph, you need to add horizontal lines across the graph denoting where you think horizon boundaries are located and label the horizons (see graph examples in Fig. 1 above).
- Save your Excel file as “FirstnameLastname_MagSus” and upload to D2L dropbox.