Correcting for paleomagnetic inclination shallowing in magnetite-bearing clay-rich soft sediments with the aid of magnetic anisotropy and uniaxial compression experiments

Bradbury, Neil (2005) Correcting for paleomagnetic inclination shallowing in magnetite-bearing clay-rich soft sediments with the aid of magnetic anisotropy and uniaxial compression experiments. Masters thesis, Memorial University of Newfoundland.

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Abstract

The remanence inclination, I, of magnetite-bearing sediment can be shallower than the inclination of the Earth's magnetic field, IH, at the time of deposition due to burial compaction. Compaction also induces a magnetic anisotropy which can be used to correct for inclination shallowing. Theory predicts an approximately linear relation between tanI/tanIH and the sediment's remanence anisotropy parameter ARMmin/ARMmax (the ratio of intensities of anhysteretic remanence given identically perpendicular and parallel to the bedding plane of the sediment). The slope of this line depends on the average remanence anisotropy parameter of the sediment's magnetite particles, ARM⊥/ARM∥, (the ratio of intensities of anhysteretic remanence applied identically perpendicular and parallel to the long axes of the magnetite grains). For a suite of clay-rich magnetite-bearing sediments, we estimate ARM⊥/ARM∥ by making a composite sample, giving it an inclined anhysteretic remanence and then applying a uniaxial compression and observing the change in remanence inclination and ARMmin/ARMmax. This estimated ARM⊥/ARM∥ is used with the ARMmin/ARMmax and I from the natural specimens to estimate IH, corrected for inclination shallowing. This method was shown to succeed for our three suites of clay-rich soft sediments bearing pseudo-single-domain magnetite: two from the continental rise off Nova Scotia, Canada and one from the Shatsky Rise east of Japan. For compacted sediments with minimum (Kmin) and maximum (Kmax) magnetic susceptibility axes perpendicular and parallel to bedding respectively, theory also predicts (provided the magnetite grains are not single-domain) an approximately linear relation between tanI/tanIH and the susceptibility anisotropy parameter, Kmin/Kmax (which is easier to measure than ARMmin/ARMmax). This relation was observed to hold for our composite samples during uniaxial compression experiments and was used to successfully estimate IH, corrected for inclination shallowing. -- Burial compaction can also reduce remanence intensity which can affect the sediment's record of paleointensity changes in the Earth's magnetic field. We observed an approximately linear relation between remanence intensity and the remanence anisotropy parameter ARMmin/ARMmax during the uniaxial compression experiments on our composite samples. This is used to suggest a novel method of correcting for compaction-induced intensity decrease in magnetite-bearing soft sediments.

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