To understand how continental sediments acquire their magnetic records through automized remanence anisotropy experiments
Final Report Abstract
We built a new and unique automated system to experiment on cylindrical specimens used in typical paleomagnetic investigations. Samples are inserted into cups and then placed on a 2-meter diameter circular track that passes below both a three-axis superconducting magnetometer and a home-made coil that facilitates the magnetization or demagnetization of up to 99 samples per loaded track. Telescoping arms lift the samples into an apparatus and rotate them about their body diagonal axes. Hence, the system performs stepwise alternating field demagnetization and routinely carries out ancillary protocols often ignored in paleomagnetic studies due to their time consuming nature when performed manually. The system, affectionately called the SushiBar due to its resemblance, enables researchers to explore the entire gamut of magnetic properties based on the acquisition of an anhysteretic remanent magnetization (ARM). This includes studies of magnetic viscosity and magnetic anisotropy in any coercivity window up to 95 mT with variable direct current bias fields to 0.17 mT. For example, partial ARM (pARM) spectra help characterize the magnetic grain size distribution in rocks. Rarely are pARM spectra measured because the complete curve for one sample takes about two hours to acquire manually (non-stop). The SushiBar achieves 99 such curves in slightly less than 100 hours. We applied the SushiBar to continental sediments collected from the Xishuigou section (Subei, western China). We confirm that the average grain size remains constant along the 2100 m-thick section and show that magnetic viscosity of ARM varies systematically from bottom to top of the section; samples with high magnetic viscosities also have higher proportions of non-viscous material on average. Anisotropy of ARM was measured in three discrete switching field windows. Principal axis directions from the lowest (20-0 mT) ARM switching fields correlate well with principal axis directions from anisotropy of magnetic susceptibility. Principal axis directions defined at higher switching fields systematically deviate from those at lower switching fields. Maximum principal axis directions measured using the highest switching fields appear to be biased toward the paleo-field direction, and hence, the technique can define the fabrics of the remanence carrying grains. Since routine operation began in March 2011, the SushiBar has made >100,000 automatic sample manipulations and readings from the magnetometer, saving >1000 hours of human working time.
Publications
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(2008), Towards a better understanding and correction of inclination shallowing, Castle Meeting, Bojnice, Slovakia, 22-28 June
Wack, M.
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(2009), A new software for the measurement of magnetic moments using SQUID and spinner magnetometers, AGU Fall Meeting, San Francisco, CA, USA, 14–18 December
Wack, M.
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(2010), A new software for the measurement of magnetic moments using SQUID and spinner magnetometers, Computers & Geosciences, 36(9), 1178-1184
Wack, M.
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(2010), An automated system to measure ARM anisotropy, pARM spectra, and high resolution AF demagnetization curves, AGU Fall Meeting 2010, San Francisco, CA, USA, 13-17 December
Wack, M. and Gilder, S.
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2011, 'An automated system to measure ARM anisotropy, pARM spectra and high resolution AF demagnetization curves', Congrès des Doctorants, Institut de Physique du Globe de Paris, Paris, France, 14-18 March
Wack, M.
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(2012), The SushiBar: An automated system for paleomagnetic investigations, Geochemistry, Geophysics, Geosystems, Q12Z38
Wack, M.R., and S.A. Gilder
