[S-EM15] Geomagnetism and paleomagnetism

[SEM15-P02] Magnetic mineral assemblages of a marine sediment core subjected to reductive diagenesis and their contributions to paleomagnetic signals

*Jiaxi Li ¹, Toshitsugu Yamazaki¹, Masahiko Sato², Junichiro Kuroda¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Earth and Planetary Science, The University of Tokyo)

Keywords:sedimentary paleomagnetism, reductive diagenesis, paleomagnetic recording, silicate-hosted magnetic-mineral inclusions, Ontong-Java plateau

Diagenesis is ubiquitous in marine sediments, causing sedimentary iron-bearing minerals to undergo a series of redox reactions until they reach equilibrium with reactive chemical components in sediments. Paleomagnetic records in sediments subjected to severe diagenesis may be distorted or lost due to iron mineral dissolution. Some magnetic minerals like silicate-hosted magnetic inclusions can survive the diagenetic iron mineral dissolution and are widely found in marine sediments. Thus, they provide the possibility of preserving paleomagnetic records in sediments subjected to reductive diagenesis. To better understand this issue, we studied on a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. Rock magnetic measurements indicate that severe diagenetic iron-mineral dissolution occurred below about 6 m in depth in the studied sediments, where natural remanent magnetization (NRM) intensity is about 10% of that above 6 m. However, information on paleomagnetic declination and paleointensity could still be recovered. Silicate-hosted magnetic inclusions were separated from bulk sediments by chemical procedures. They contribute about 50% or more of saturation isothermal remanent magnetization (SIRM) in the reduced sediments. High coercivity hematite may be another major remanence carrier in the reduced sediments as indicated by rock magnetic and IRM decomposition results. Transmission electron microscope and scanning electron microscope observations provide direct evidence for the presence of magnetic inclusions and hematite in the reduced sediments. NRM acquisition efficiency of magnetic inclusions will also be discussed based on grain size analysis results.