HR: 14:55h
AN: GP33E-06    [Abstracts]
TI: Possible overcompensation of non-interacting biogenic component by normalization with ARM in sedimentary paleointensity estimations
AU: * Yamazaki, T
EM: toshi-yamazaki@aist.go.jp
AF: Geological Survey of Japan, AIST, Higashi, Tsukuba, 305-8567, Japan
AB: In relative paleointensity studies, it has sometimes been reported that a significant coherence between the normalized intensity and the normalizer occurs when ARM is used as the normalizer, whereas this does not occur when SIRM is used. I conducted a rock magnetic study of deep-sea sediments from the Pacific Ocean to clarify the cause of the coherence. Characterization of magnetic grains in the sediments was carried out using first- order reversal curve (FORC) diagrams and iso-thermal remanent magnetization (IRM) acquisition curves. The FORC diagrams revealed that the magnetic grains consist mainly of a non-interacting single-domain (SD) component and an interacting SD component, and additionally of a multi-domain (MD) component. Relative abundance of these components were estimated semi-quantitatively by curve fitting of cross-sections along a line parallel to the axis of local interaction fields (Hu) and passes through a peak in the coercivity (Hc) with three components assuming a Gaussian distribution. The IRM acquisition curves could be described by two dominant components assuming a log-Gaussian distribution. The mean coercivities of the two components are ~40 and ~100 mT, respectively. It is estimated that the former component is carried by biogenic magnetite, and roughly corresponds to the non-interacting SD component derived from the FORC diagrams, and the latter is carried by eolian maghemite, corresponding to the interacting SD component. The ratio of anhysteretic remanent magnetization (ARM) to saturation IRM (SIRM) decreases with an increase in the proportion of the interacting SD component. This implies that the ARM/SIRM ratio is significantly affected by magnetostatic interactions, and does not necessarily reflect magnetic grain size although it is often used as such a proxy. In relative paleointensity estimations, ARM is often used as a normalizer to correct for the difference in magnetizability of sediments. The effect of magnetostatic interaction on detrital remanent magnetization (DRM) acquisition is not well understood, but if it is similar to that on SIRM, normalization by ARM may overcompensate the non-interacting SD component, and cause a significant coherence between the normalized intensity and the normalizer.
DE: 1505 Biogenic magnetic minerals
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting