ENVIRONMENTAL MAGNETISM

Introduction:
Lake sediments consist of organic and non-organic components that are deposited continuously through time. Under optimal conditions deposition of this material occurs undisturbed and changes in the depositional system may be reconstructed by mapping compositional variations in organic and non-organic content.
A range of different methods may be used to assess compositional variations. We address the application of selected magnetic parameters that may assist in identifying composition, grain size and concentration of minerogenic components in sediments.

For example; a widely used parameter is magnetic susceptibility (MS). Magnetite has a positive and very high specific (volume or mass) MS, while quartz has a low and negative MS. Both are 'magnetic', but MS of these two minerals differs both in magnitude and sign. On the other hand, while magnetic can retain a magnetic signal (remanence), quartz cannot. Carriers of basic magnetic properties can be divided into three groups; ferrimagnetic (strong signal), paramagnetic (weak signal) and diamagnetic (negative and weak signal).

If we are able to determine variations in composition and concentration of different magnetic mineral phases, the fluxes, origins and sources of these minerals may be inferred and we can operate them as 'environmental proxies'. They can hence be interpreted in relation to the ever-changing climate. In this branch of the NORPEC-project we attempt to test the reach of environmental magnetism and have therefore chosen three very different lake systems.

Our main objective is to discriminate between individual lacustrine phases and evaluate processe(s) that may cause changes of these with time. This implies factors such as the local hydrological cycle and/or trends in the regional paleoclimate.

Mineral magnetic parameters
There are a number of parameters used to investigate mineral assemblages in lacustrine sediments (this also applies to solid rocks and all sediments in general).
The most important theoretical distinction stems from values which is obtained by a magnetisation (M) that is retained by the sample after the field is removed (remanent magnetisation) and vice versa (induced magnetisation).

Here follows a brief summary of relevant concepts:

1. Magnetic susceptibility (MS) is the ability of a given substance to become magnetised, i.e. give rise to an induced magnetisation that vanishes after the external field is removed.
MS is specified either by volume (k) or mass (c).
Frequency dependent MS (cfd) is a property of ultra-fine magnetite (<0.05m).

2. Isothermal Remanent Magnetisation (IRM); is the remanent magnetisation imposed by exposing a sample at constant temperature (i.e. room temperature) to a steady magnetic field.

3. Saturation Isothermal Remanent Magnetisation (SIRM) is the maximum IRM a sample can obtain. SIRM can only be determined by recording IRM in progressively higher magnetic fields(see Fig). IRM increases until saturation is obtained, i.e. IRM will not increase regardless of how strong the magnetic field is. The saturation field (HSAT) depends strongly on the type and to some degree on the grain-size distribution of magnetic mineral(s). For minerals like Hematite and Goethite magnetic fields larger than a few Tesla is required. A 1T field is not sufficient. Nevertheless laboratories with limited magnetic 'power' still call an IRM1T an SIRM.

4. Anhysteretic Remanent Magnetisation (ARM); is imposed by exposing a sample to a strong alternating (af) field which is gradually reduced to zero in the presence of a weak and steady magnetic field.

5. Magnetic Hysteresis; hysteresis loops is generated by exposing a sample to a strong field which forces it to wander from its maximum value, through zero until reaching its maximum negative value and then back again (see Fig).

6. Thermomagnetic analysis; determines the compositional diagnostic Curie temperature for different minerals by monitoring high-field induced magnetisation during heating to maximum 700°C (see Fig)..

7. Natural Remanent Magnetisation (NRM). Environmental magnetic studies do usually not involve determination of NRM. For the sake of completeness, it is included in this summary.
NRM is the magnetisation retained in a natural sample when it is measured for the first time.
NRM may hence consist of different magnetisation components imposed at different times.

NRM = STRM + DRM + CRM + VRM + IRM

7. Thermomagnetic remanent magnetisation (TRM) is imposed during cooling through the Curie-temperature of minerals. Major remanence acquisition process in igneous rocks. It is likely that mineral grains in sediments may carry TRM if derived from igneous rocks.

8. Detrital remanent magnetisation (DRM) is carried by detrital grains and is obtained during initial sediment deposition. Post-depositional processes can modify the DRM, which introduces a time lag between the age of the sediment and its magnetisation.

9. Chemical remanent magnetisation (CRM) is carried by precipitated magnetic minerals (F.ex. Goethite, Greigite etc).

10. Viscous remanent magnetisation (VRM) is a time-dependent magnetisation imposed at constant temperature. It may dominate NRM in certain rocks and sediments.

IRM: may be imposed by accidentally exposing samples to local strong magnetic fields, f.ex. magnetic fields surrounding strong direct currents, or small magnets.

Magnetic susceptibility
Kappabridge KLY-2 (Agico instrument) measures MS with a sensitivity of 4x10-8 SI.
As an example, a standard cubic sample box (6.2cc) filled with plain water gives a MS-reading of -200 on the most sensitive Range.The Kappabridge is also used to determine anisotropy of magnetic susceptibility (AMS). IMAGE

Bartington MS2. This highly successful design is the 'global' workhorse in mineral magnetic laboratories.
The sensitivity is 10-6 SI.

The MS2-instrument can be equipped with different sensors:

- The MS2B-sensor measures MS at low and high frequencies to determine the frequency dependent MS.
- Whole Core magnetic susceptibility (WCMS) is routinely measured on all cores using the inhouse-made automatic recording Conveyer-belt system (MkIII) controlled by the TRBand-program (developed by Jose Ojeda Åsheim).

- MS2E has been by far most successful sensor for our research. It measures onto the surface of split-cores and has a spatial resolution of only 2.8 mm, resulting in high-resolution records (typically every 5mm).

IRM - SIRM
A Digico slow-speed spinner magnetometer is used for measurements of artificially imposed magnetisations (IRM, SIRM, ARM) of soft sediments. The sensitivity is only 10-1 mAm-1.IMAGE
When better sensitivity is required, we cool down the 20 year old three-sensor Cryogenic magnetometer.
IRM is imposed in two steps; to 175 mT we use a solenoid controlled manually or by the WinIRMH-program (Walderhaug). Above 175 mT and to 4500 mT, IRM is imposed with a Redcliffe pulse magnetiser.

Thermomagnetic analysis
Image of Curie-balance - console and screenshot of curve
An automatic recording transversal Curie-balance constructed by Professor Nikolai Petersen (Münich).
Samples (5-250 mg) can be heated in atmospheric air or in a slow flow of Argon gas (minimizing oxidation).
Maximum magnetic field: 800 mT.
Heating/cooling rates: 2 to 50°C/min.
Thermal hysteresis (<5°C for 50°C/min).

http://www.geo.umn.edu/orgs/irm/index.html
http://www.igcworld.com/gm_glos.html
http://www.ngdc.noaa.gov/seg/potfld/geomag.shtml

LITERATURE

Dunlop, D.J. & Özdemir, Ö. (1997). Rock magnetism: fundamentals and frontiers. Cambridge University Press, UK.

McElhinny, M.W. & McFadden, P.L. (2000). Paleomagnetism - Continents and Oceans. International Geophysical Series, V.73.

Merrill, R.T., McElhinny, M.W. & McFadden, P.L. (1996). The Magnetic Field of the Earth: Paleomagnetism, the Core, and the deep Mantle. Academic Press, San Diego.

Tauxe, L. (1998). Paleomagnetic Principles and Practice. Kluwer Academic Publishers, Dordrecht.

Thompson, R. & Oldfield, F. (1986). Environmental Magnetism. Allen and Unwin.

Walden, J., Oldfield, F., & Smith, J. (eds.) (1999). Environmental Magnetism - A Practical Guide. Quaternary Research Association, Technical Guide No.6.