HR: 08:00h
AN: GP51A-01 INVITED    [Abstracts]
TI: What do the HIRM and S-ratio really measure in environmental magnetism?
AU: * Roberts, A P
EM: arob@noc.soton.ac.uk
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO143ZH, United Kingdom
AU: Liu, Q
EM: liux0272@yahoo.com
AF: National Oceanography Centre, University of Southampton, European Way, Southampton, SO143ZH, United Kingdom
AU: Torrent, J
AF: Universidad de Cordoba, Departamento de Ciencias y Recursos Agrícolas y Forestales, Edificio C4, Campus de Rabanales, Cordoba, E-14071, Spain
AU: Horng, C -
AF: Academia Sinica, Institute of Earth Sciences, P.O. Box 1-55, Nankang, Taipei, 115, Taiwan
AU: Larrasoana, J C
AF: Institut de Ciencies de la Terra ‘‘Jaume Almera'', CSIC, Lluís Solé i Sabarís s/n, Barcelona, E-08028, Spain
AB: The hard isothermal remanent magnetization (HIRM) and the S-ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. New results from synthetic Al-substituted hematite and goethite indicate that these minerals can have a wide range of coercivities, which significantly influences the HIRM and S- ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L-ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT (IRMAF300mT/IRMAF100mT). The newly proposed L-ratio is straightforward to measure on a wide range of instruments, including modern vibrating sample or alternating gradient magnetometers (equivalent parameters can also be measured on other instruments). Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L-ratio is relatively constant. Conversely, L-ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S-ratio variations should be interpreted with caution in such cases. Examples are shown where variable and constant L-ratios, respectively, provide significant new insights and reduce ambiguities associated with interpretation of the HIRM and S-ratio in environmental magnetism.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 1834 Human impacts
DE: 1861 Sedimentation (4863)
DE: 1890 Wetlands (0497)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting