[S-EM15] Geomagnetism, paleomagnetism and rock magnetism

[SEM15-03] Research for improving relative paleointensity estimations: a case study on a sediment core from the Southeast Pacific off Chile

*Toshitsugu Yamazaki¹, Jiaxi Li ¹, Takaya Shimono², Toshiya Kanamatsu³ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Kaiyo Engineering Co., Ltd., 3.Japan Agancy for Marine-Earth Science and Techonology)

Keywords:paleomagnetism, relative paleointensity, biogenic magnetite

Progress of relative paleointensity (RPI) estimations using marine sediments have contributed to better understanding of the behavior of the geomagnetic field in the past, exemplified by the establishment of the stacked RPI curves such as Sint-2000 for the last 2 m.y. and PISO-1500 for the last 1.5 m.y. On the other hand, limitations of the current RPI estimations have become recognized. One of the significant problems is that magnetic-property variations of magnetic-mineral assemblages in sediments may contaminate RPI records (e.g., Yamazaki et al., 2013). For more reliable RPI estimations, the contamination problem should be understood and solved. Two major constituents of magnetic-mineral assemblages in marine sediments are biogenic magnetites and terrigenous magnetic minerals, and the latter consists of silicate-hosted magnetic inclusions and discrete magnetic minerals. It is considered that different RPI recording efficiencies among those components are responsible for the contamination, which is likely originated from their different acquisition efficiencies for natural remanent magnetization (NRM) and the normalizers usually used for RPI estimations, anhysteretic remanent magnetization (ARM) and iso-thermal remanent magnetization (IRM). We now promote research for understanding these differences. Using a sediment core from the Ontong-Java Plateau in the western equatorial Pacific, we revealed that RPI recording efficiency of biogenic magnetite is lower than the terrigenous component, and that increasing proportion of the biogenic component causes underestimation of RPI (Li et al., in revision). This conclusion is derived from the observations that RPI normalized by ARM is smaller for a higher-coercivity fraction than a lower-coercivity fraction, and that based on rock-magnetic investigations including FORC measurements the higher-coercivity fraction is mainly carried by biogenic magnetite whereas the proportion of terrigenous component increases in the low-coercivity fraction. Terrigenous magnetic minerals in this core are considered to be mainly of fluvial origin from New Guinea. Magnetic mineral compositions of sediments vary place by place depending on depositional environments, and hence we need to expand the research to global sediments for comprehensive understanding of this issue.

In this presentation, we show the results from sediment core (MR0806-PC3) from the southeast Pacific off Chile. The 19-m long core preserves a RPI record since ~1.5 Ma, which is correlative to the PISO-1500 stacked curve. In this core, a higher-coercivity fraction of ARM-normalized RPI is larger than a lower coercivity fraction, which is opposite to the results of the western equatorial Pacific sediments. In this core, however, rock-magnetic investigations including FORC measurements revealed that the lower-coercivity fraction is carried more by biogenic magnetite than the higher-coercivity fraction. Therefore, the idea that lower RPI recording efficiency of biogenic magnetite is valid for the sediments in this region.