[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-02] Linearity of pseudo-Thellier plot and reliability of relative paleointensity estimation

Jiaxi Li ¹, *Toshitsugu Yamazaki¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:paleomagnetism, relative paleointensity, rock magnetism, magnetofossil

A serious problem of relative paleointensity (RPI) estimations from marine sediments is that magnetic mineral composition changes influence RPI, known as lithological contamination. Because RPI recording efficiency of magnetofossil is lower than that of detrital unprotected magnetite (e.g., Inoue et al., 2021; Li et al., 2022), a changing proportion of magnetofossil in magnetic mineral assemblages in sediments produces a negative correlation between RPI and the ratio of ARM susceptibility to SIRM (k_ARM/SIRM), which is a proxy for magnetofossil proportion. When the coercivity ranges of magnetofossil and detrital unprotected magnetite are different, NRM-ARM (or NRM-IRM) demagnetization diagram (pseudo-Thellier plot) shows a curvature. By calculating RPI from a coercivity window representing magnetofossil or detrital unprotected magnetite, a less contaminated RPI record may be obtained (Li et al., 2022; Yamazaki et al., 2023). However, in the case that the coercivity ranges of these two components are similar and the proportion of the two changes, a linear NRM-ARM demagnetization diagram and a negative correlation between RPI and k_ARM/SIRM happen. We show this is the case for a sediment core taken from the North Central Pacific Ocean based on rock-magnetic analyses including FORC diagrams and IRM decomposition. A linear NRM-ARM demagnetization diagram, which is sometimes used as a criterion of reliable RPI estimations, does not indicate little lithological contamination to RRI.