Open AccessMaghemite‐to‐magnetite reduction across the Fe‐redox boundary in a sediment core from the Ontong‐Java Plateau: influence on relative palaeointensity estimation and environmental magnetic application
Author(s) -
Yamazaki Toshitsugu,
Solheid Peter
Publication year - 2011
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2011.05021.x
Subject(s) - geology , maghemite , terrigenous sediment , remanence , magnetite , magnetic mineralogy , redox , plateau (mathematics) , mineralogy , paleomagnetism , geophysics , geochemistry , magnetization , sedimentary rock , chemistry , paleontology , physics , inorganic chemistry , mathematical analysis , mathematics , quantum mechanics , magnetic field
SUMMARY During reduction diagenesis, production of dissolved Fe 2+ by reduction of ferric oxide starts at the Fe‐redox boundary. The associated magnetic property changes may influence palaeomagnetic and environmental magnetic records of marine sediments, however, this has not been evaluated thoroughly. In this study, using a gravity core from the Ontong‐Java Plateau, we document in detail rock magnetic changes across the Fe‐redox boundary, and investigate their influence on relative palaeointensity estimation and on a magnetic proxy for the proportion of terrigenous/biogenic magnetic minerals. The magnetic mineral assemblage above the Fe‐redox boundary is characterized by a component with a mean coercivity of ∼100 mT in isothermal remanent magnetization (IRM) component analyses and low S ‐ratios (S ‐0.1T ). Low‐temperature IRM measurements and Mössbauer spectroscopy indicate that the degree of maghemitization is higher above the Fe‐redox boundary. From these observations, we conclude that reduction of maghemite to magnetite occurs at the Fe‐redox boundary, and we infer that a maghemite skin on magnetic grains is lost across the boundary. Relative palaeointensity variations obtained by normalizing NRM intensity with SIRM agree well with regional and global palaeointensity stacks, which suggests that relative palaeointensity estimation is not significantly affected by the Fe‐redox boundary. Temporal variations of the ratio of anhysteretic remanent magnetization susceptibility and saturation IRM (χ ARM /SIRM) coincide with the regional pattern across the Ontong‐Java Plateau. It is also possible to estimate variations in the proportion of terrigenous to biogenic components using the χ ARM /SIRM ratio across the Fe‐redox boundary.